A Study of Zinc-Nickel as an Alternate Coating to Cadmium
for Electrical Connector Shells Used in Aerospace Applications
By
Odunayo Ogundiran
A Thesis Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
In partial fulfillment of the
Requirements for the degree of
Master of Science in Mechanical Engineering
Approved:
_________________________________________
Ernesto Gutierrez-Miravete: Thesis Adviser
Rensselaer Polytechnic Institute
Hartford, CT
April, 2011
© Copyright 2011
by
Odunayo Ogundiran
All Rights Reserved
ii
TABLE OF CONTENTS
LIST OF KEYWORDS .................................................................................................... vi
LIST OF SYMBOLS ....................................................................................................... vii
LIST OF TABLES .......................................................................................................... viii
LIST OF FIGURES .......................................................................................................... ix
ACKNOWLEDGMENT .................................................................................................. xi
ABSTRACT .................................................................................................................... xii
1. Introduction.................................................................................................................. 1
1.1
Overview of Circular Connectors. ..................................................................... 1
1.2
Corrosion on Circular Connectors ..................................................................... 2
1.3
Electromagnetic Interference Shielding ............................................................. 3
1.4
Project Background ............................................................................................ 5
1.5
Overview of Project Goals ................................................................................. 5
2. Coatings for Circular Connectors ................................................................................ 7
2.1
2.2
Existing finishes and their applications ............................................................. 7
2.1.1
Class A finish: Cadmium over nickel. Light Gold Color ...................... 7
2.1.2
Class F & G finish: Electroless Nickel .................................................. 7
2.1.3
Class W finish: Cadmium over nickel. Olive Drab Color ..................... 8
2.1.4
Class K finish: Passivate. ....................................................................... 8
2.1.5
Class L finish: Electrodeposited Nickel ................................................. 9
Currently identified alternate finishes to Cadmium ........................................... 9
2.2.1
Class P finish: Electrodeposited Aluminum. ......................................... 9
2.2.2
Class T finish: Nickel fluorocarbon polymer. ........................................ 9
2.2.3
Class Z finish: Zinc Nickel .................................................................... 9
2.3
How Circular Connectors Corrode .................................................................. 10
2.4
Cadmium Protection ........................................................................................ 11
2.5
Zinc Nickel finish ............................................................................................. 12
iii
2.5.1
Electrodeposition of zinc nickel alloy Coating Deposits on Circular
Connectors ........................................................................................... 16
2.5.2
Properties of Cadmium and zinc nickel of Interest in this Application 17
3. Experimental .............................................................................................................. 19
3.1
Test Unit Identification .................................................................................... 19
3.1.1
Test Unit Set-up ................................................................................... 20
3.1.2
Mounting bracket ................................................................................. 31
3.2
Shield Braid Termination. ................................................................................ 32
3.3
Salt Spray Test ................................................................................................. 33
3.4
3.5
3.3.1
Test Apparatus ..................................................................................... 33
3.3.2
Salt Composition .................................................................................. 34
3.3.3
Procedure.............................................................................................. 35
Electrical Continuity Test ................................................................................ 36
3.4.1
Equipment ............................................................................................ 36
3.4.2
Procedure.............................................................................................. 36
Coupling Torque Test ...................................................................................... 38
3.5.1
Equipment ............................................................................................ 38
3.5.2
Procedures ............................................................................................ 38
3.6
Visual Inspection .............................................................................................. 38
3.7
Test Sequence................................................................................................... 39
4. Results and Discussion .............................................................................................. 41
4.1
4.2
Pre-Salt Fog Exposure Readings ...................................................................... 41
4.1.1
Coupling Torque at 0 Hours ................................................................. 41
4.1.2
Electrical Continuity Readings at 0 Hours ........................................... 41
Post-Salt Fog Exposure Readings .................................................................... 43
4.2.1
Electrical Continuity Readings at 48 Hours ......................................... 43
4.2.2
Electrical Continuity Readings at 96 Hours ......................................... 45
iv
4.3
4.2.3
Electrical Continuity Readings at 500 Hours ....................................... 47
4.2.4
Coupling Torque at 500 hours.............................................................. 50
Discussion ........................................................................................................ 51
4.3.1
Recommendations for Additional Testing ........................................... 56
5. Conclusion ................................................................................................................. 57
6. References.................................................................................................................. 59
7. Appendix.................................................................................................................... 60
7.1
Test Unit 1 ........................................................................................................ 60
7.2
Test Unit 2 ........................................................................................................ 62
7.3
Test Unit 3 ........................................................................................................ 64
7.4
Test Unit 4 ........................................................................................................ 66
7.5
Test Unit 5 ........................................................................................................ 68
7.6
Test Unit 6 ........................................................................................................ 70
7.7
Test Unit 7 ........................................................................................................ 71
7.8
Test Unit 8 ........................................................................................................ 73
7.9
Test Unit 9 ........................................................................................................ 75
7.10 Mounting Bracket Detailed Design .................................................................. 77
v
LIST OF KEYWORDS
Circular Connector
Part of a harness assembly used to terminate wires and provide
connection between harnesses or a harness and avionics
equipment on an aircraft.
Receptacle
Type of connector that is attached to a bulkhead of an aircraft or
avionic equipment on an aircraft.
Backshell
A connector accessory that provides protection to the wire
terminations on the connector
Strain Relief
A connector accessory mounted to the rear of the backshell to
keep the harness secure thus preventing the wires from being
pulled out of the connectors
Finish
A protective layer of coating meant for corrosion resistance of the
substrate to which it is applied.
Bulkhead:
The sheet metal structure of an aircraft to which receptacles are
mounted.
Zinc Nickel finish
Newly identified alternate finish to cadmium for circular
connectors
Cadmium finish
Widely used finish on electrical connectors today.
Plug
Type of connector that is free moving and mates with a receptacle.
Harness
An assembly of a group of wires that transmits data or current
bound together by string ties, clamps or electrical tape.
vi
LIST OF SYMBOLS
Tn/Zn
-
Tin plated finish on Zinc Nickel finish
Tn/Cd
-
Tin plated finish on Cadmium finish
Cd/Zn
-
Cadmium finish on Zinc Nickel finish
Zn/Zn
-
Zinc Nickel finish on Zinc Nickel finish
Cd/Cd
-
Cadmium finish on Cadmium finish
En/Zn
-
Electroless Nickel finish on Zinc Nickel finish
CdC/Zn
-
Composite Cadmium finish on Zinc Nickel finish
EnC/Zn
-
Composite Electroless Nickel finish on Zinc Nickel finish
SS/Zn
-
Stainless steel passivate finish on Zinc Nickel finish
Zn
-
Zinc Nickel finish
Cd
-
Cadmium finish
En
-
Electroless Nickel finish
SS
-
Stainless steel passivate finish
Sn
-
Tin
Co
-
Cobalt
Ni
-
Nickel
NaCl
-
Sodium Choride
Al
-
Aluminum
C
-
Composite
vii
LIST OF TABLES
Table 1: Summary of Properties of Cadmium, Zinc Nickel and Pure Zinc Coatings [4].
......................................................................................................................................... 18
Table 2: Matrix of existing materials and finishes .......................................................... 20
Table 3: Test Unit Finish and Material Matrix (Al – Aluminum. C – Composite) ......... 21
Table 4: Test Unit 1 Parts List ......................................................................................... 22
Table 5: Test Unit 2 Parts List ......................................................................................... 23
Table 6: Test Unit 3 Parts List ......................................................................................... 24
Table 7: Test Unit 4 Parts List ......................................................................................... 25
Table 8: Test Unit 5 Parts List ......................................................................................... 26
Table 9: Test Unit 6 Parts List ......................................................................................... 27
Table 10: Test Unit 7 Parts List ....................................................................................... 28
Table 11: Test Unit 8 Parts List ....................................................................................... 29
Table 12: Test Unit 9 Parts List ....................................................................................... 30
Table 13: Salt Spray Test intervals. ................................................................................. 36
Table 14: Electrical Continuity Measurement Conditions. ............................................. 37
Table 15: Measurement Points/Surfaces on test units ..................................................... 37
Table 16: Maximum and Minimum torques for Shell Size 19 Connectors. .................... 38
Table 17: Initial Mate Torque Values .............................................................................. 41
Table 18: Initial Electrical Continuity Reading (Millivolts) ........................................... 42
Table 19: 48 Hour Electrical Continuity Reading (Millivolts)........................................ 44
Table 20: 96 Hour Electrical Continuity Reading (Millivolts)........................................ 46
Table 21: 500 Hour Electrical Continuity Reading (Millivolts)...................................... 49
Table 22: Unmating Torque Values after 500 Hours ...................................................... 50
viii
LIST OF FIGURES
Figure 1: View of a Mated Plug and Receptacle Assembled with Connector Accessories.
........................................................................................................................................... 1
Figure 2: Cutaway view of a Circular Connector Plug and Receptacle [13]..................... 2
Figure 3: Connector and Accessory Assembly for EMI shielding [10] ............................ 4
Figure 4: Cadmium Plated Aluminum Receptacle and Plug ........................................... 12
Figure 5: Average Corrosion Rates on Zinc Nickel Alloy Deposits by Nickel Content
[15]................................................................................................................................... 14
Figure 6: Zinc Nickel Phase Diagram [16] ...................................................................... 15
Figure 7: Micrograph of Zinc Nickel Coatings before and after 48 hours Immersion in
NaCl Environment [16] ................................................................................................... 16
Figure 8: Zinc Nickel Plated Aluminum Receptacle and Plug ........................................ 17
Figure 9: Test Unit 1 ........................................................................................................ 22
Figure 10: Test Unit 2 ...................................................................................................... 23
Figure 11: Test Unit 3 ...................................................................................................... 24
Figure 12: Test Unit 4 ...................................................................................................... 25
Figure 13: Test Unit 5 ...................................................................................................... 26
Figure 14: Test Unit 6 ...................................................................................................... 27
Figure 15: Test Unit 7 ...................................................................................................... 28
Figure 16: Test Unit 8 ...................................................................................................... 29
Figure 17: Test Unit 9 ...................................................................................................... 30
Figure 18: Test bracket set up. ......................................................................................... 31
Figure 19: Shield Braid Termination [10]. ...................................................................... 32
Figure 20: Sample Composition of Salt Spray Test Apparatus [9] ................................. 34
Figure 21: Salt Solution Reservoir showing positioning of filter [11] ............................ 35
Figure 22: Measurement Points on test units................................................................... 37
Figure 23: Test Sequence................................................................................................. 40
Figure 24: 48 Hour Maximum Electrical Continuity Readings Per Interface on Test
Units ................................................................................................................................. 43
ix
Figure 25: 96 Hour Maximum Electrical Continuity Readings Per Interface on Test
Units ................................................................................................................................. 45
Figure 26: Minumum and Maximum Electrical Continuity Readings Per Interface on
Test Units ......................................................................................................................... 48
Figure 27: Comparison of Torque values at 0 and 500 Hours ......................................... 50
Figure 28: Test Unit 1 Backshell with Cadmium Finish on Receptacle after 48 hours .. 51
Figure 29: Test Unit 3 Backshell with Zinc Nickel Finish on Receptacle after 48 hours 51
Figure 30: Test Unit 1 Backshell and Plug with Cadmium Finish after 48 hours ........... 52
Figure 31: Test Unit 3 Backshell and Plug with Zinc Nickel Finish after 48 hours........ 52
Figure 32: Test Unit 1 Backshell and Receptacle with Cadmium Finish after 96 hours. 53
Figure 33: Test Unit 1 Backshell and Plug with Cadmium Finish after 96 hours ........... 53
Figure 34: Test Unit 3 Backshell and Receptacle with Zinc Nickel Finish after 96 hours
......................................................................................................................................... 53
Figure 35: Test Unit 3 Backshell and Plug with Zinc Nickel Finish after 96 hours........ 53
Figure 36: Test Unit 1 Backshell and Receptacle with Zinc Nickel Finish after 500 hours
......................................................................................................................................... 55
Figure 37: Test Unit 1 Backshell and Plug with Zinc Nickel Finish after 500 hours...... 55
Figure 38: Test Unit 3 Backshell and Receptacle with Zinc Nickel Finish after 500 hours
......................................................................................................................................... 55
Figure 39: Test Unit 3 Backshell and Plug with Zinc Nickel Finish after 500 hours...... 55
x
ACKNOWLEDGMENT
I would like to acknowledge the harness engineering department at Sikorsky
Aircraft Corporation for providing the funding for this study. I would also like to
acknowledge the materials laboratory personnel for their efforts in this study.
xi
ABSTRACT
This report presents results of a study aimed to investigate the ability of zinc nickel
coatings to provide adequate corrosion protection and electromagnetic interference
shielding for circular connectors used in aerospace applications and in comparison to
cadmium coatings. The effects of aqueous corrosion in circular connectors and
connector
accessories
were
investigated.
Substrate
material
degradation
and
electromagnetic interference concerns are the main reasons protective finishes are used
on circular connectors.
Several finishes used today provide adequate protection for circular connectors.
Cadmium is the most widely used finish in this application. However, the hazardous
nature of these finishes has long driven the search for an alternative. Zinc nickel has
been identified as an alternative to cadmium on circular connectors; however there are
no studies of how zinc nickel finishes couple with the materials that are used today on
aerospace circular connectors, connector accessories, and mounting hardware. The need
to know more about the service behavior of zinc nickel coatings provided the motivation
for this study.
Tests in this study include a salt spray test, an electrical continuity test, a coupling
test, and visual inspection of the test units. Initial inspections after exposure to salt fog
showed rapid formation of oxide films on the surface of aluminum and stainless steel
connectors and backshells coated with the zinc nickel finish. Electrical continuity
measurements taken at predetermined intervals confirmed the presence of oxide films as
voltage across the measurement points increased drastically from the first set of
readings.
At 500 hours, which was the pass or fail point for this study, heavy corrosion was
noticed on most test units regardless of the finish. Continuity measurements at the 500
hour interval ranged from values lower than the recommended <5.0 millivolts to much
higher values. However, the electrical path between the shield braid and the ground was
shown not to have been compromised. The measured torques required to unmate the test
units after the 500 hours increased from the initial values but were below the baseline
value in most cases. No significant difference in continuity readings collected was
xii
determined between connectors with zinc nickel finish mated with connectors with the
same finish and connectors with zinc nickel finish mated with connectors with other
finishes. Although the zinc nickel finish did not perform as well as the cadmium finish in
this study, it met the industry requirement for finishes on circular connectors and should
be seriously considered as cadmium replacement.
xiii
1. Introduction
1.1 Overview of Circular Connectors.
Circular connectors are common components of wire harnesses typically used in
aerospace applications for terminating wires that transmit data or distribute current.
They can also be found attached to a piece of equipment. Circular connectors are usually
found at the ends of a harness where the wires terminate. Circular connectors can be one
of two kinds; a plug which is attached to the harness alone and is free moving; or a
receptacle which is not only attached to the harness, but also to a frame or structure to
keep it stationary. Figure 1 shows a mated plug and receptacle on two harnesses as they
are mounted on an aircraft.
Figure 1: View of a Mated Plug and Receptacle Assembled with Connector
Accessories.
Receptacles are also attached to avionic equipment as they are the only type of
connectors used on this equipment. These two different types of connectors are designed
to mate with one another. Circular connectors consist of a shell which could be of
different materials e.g. aluminum, composite, stainless steel, or cold rolled steel. The
shell provides protection against abrasion and contact with contaminants that could
1
damage the mating contacts. They also provide shielding for the conductors in the
harness against unwanted electromagnetic and radio frequency interference. Another
component of the connector is an insulator placed inside the shell. It contains inserts for
contacts and a seal for wires. The contacts serve as terminations in the connector for the
wires in the harness. Due to the environments in which aircraft operate, circular
connectors are specifically designed to withstand high shock and vibration forces. Figure
2 shows cutaway views of the two types of connectors and some of their components. It
shows how the wire from the harness is connected to the contacts and how this
connection is encapsulated within the insulator. One can also see from the figure that the
contacts in the plug are different from the contacts in the receptacle and that they are
designed to mate with one another.
Plug
Receptacle
Figure 2: Cutaway view of a Circular Connector Plug and Receptacle [13]
1.2 Corrosion on Circular Connectors
Circular connectors used in the aerospace industry need adequate protection from
corrosion due to the environment in which the aircraft operates. Specifically, the effect
of long term exposure to salt atmosphere environments is a major concern. The mating
of connectors of different materials and finishes also poses a concern due to the
2
electrochemical action of two dissimilar metals in contact with each other. This can
cause galvanic corrosion given the presence of a conductive path and an electrolyte. This
is a primary problem for electrical connectors in the environment in which the aircraft
operates because moisture could serve as a medium for current flow.
In connectors, galvanic compatibility is required between the base material or the
shell and the coating for conduction. Conduction of the coating is required on connectors
because this is what prevents electromagnetic interference from disruption of the flow of
data in the harness, by providing a path to ground. Cadmium is a widely used finish in
this application for corrosion resistance and electrical conductivity over long term
exposure to salt atmosphere environments.
1.3 Electromagnetic Interference Shielding
Electromagnetic Interference (EMI) can result in the disruption of transmitted
data or performance of an electrical circuit within an aircraft system. This could be due
to electromagnetic conduction or radiation produced from an external source. A number
of sources can produce electromagnetic interference (EMI) in an aircraft system. Coils,
electromagnets, electric motors, and transformers that can be found in a number of
locations in an aircraft are well known generators of electromagnetic interference.
Others are high power radars, and broadcast stations that are not necessarily associated
with the aircraft but are part of the environment in which the aircraft operates. Because
the conductors in the cables can act as antennas and pick up radiated signals, the
reception of the interference can lead to inaccurate or distorted data being transmitted.
The examples stated are usually unintentional sources of EMI. However, due to
electronic warfare, other sources that are used to intentionally jam radio signals can also
affect aircraft systems.
The conductive property of plated connectors and backshells helps prevent the
interruption of communication in an aircraft system by electromagnetic interference
(EMI). It allows for the harmless passage of the interference from a harness shield braid
to a grounding point through the backshell and connector. A backshell is a connector
accessory that is used to shield the wire connection points when the harness is attached
to the connector. The harness is shielded by grounding a conductive cable over the
3
harness to the backshell and connector. To shield against electromagnetic interference,
the harness is enclosed in a tightly woven metal braid often referred to as the shield
braid. This braid is terminated at the backshell and at the end of the cable. This serves as
a path for the unwanted EMI from the harness to aircraft electrical ground. Figure 3
shows a breakaway assembly of a connector and its accessories as they are assembled on
a harness. It shows the connection from the shield braid to the connector. Termination of
shield braids will be discussed further in the experimental section of this study. The
strain relief that prevents the wires from being pulled out of the connector is mounted to
the rear of the backshell and also secures the backshell termination rings. The
termination rings are used to secure the shield braid to the backshell in order to provide a
grounding path. The backshell is mounted on the rear of the connector thus providing a
ground path from the shield braid to the connector. Since the connector is mounted on
the aircraft bulkhead, a full path to the aircraft electrical ground is established. All these
components and their finishes are electrically conductive.
Figure 3: Connector and Accessory Assembly for EMI shielding [10]
4
1.4 Project Background
The search for a suitable replacement for cadmium as a protective finish for the
circular connectors used in aircraft systems has been going on for quite some time now.
Cadmium is a generally preferred finish in this application because of its corrosion
resistance, conductivity, lubricity, and low cost. However, cadmium is a heavy metal
and it has been identified as a hazardous material now listed by OSHA and on the
Restriction of Hazardous Substances (RoHS) document as a workplace hazard [12]. This
has caused a ramp up in the search for a replacement for cadmium. Zinc nickel has been
identified as an effective finish in protecting circular connectors from the environment
[14]. It has been added to military specification MIL-DTL-38999 as one of the available
finishes on connectors used in the aerospace industry [14]. In addition to nickel
fluorocarbon polymer, and electrodeposited aluminum, zinc nickel finish has also been
identified as a potential alternative to cadmium in this and other types of applications.
The existing test data regarding zinc nickel finish on circular connectors in MILDTL-38999 states it’s successful passage of the 500 hour dynamic salt spray test [14].
However, concerns still exist regarding the behavior of zinc nickel finishes on circular
connectors if introduced into the field. This is due to the lack of extensive testing of the
effects of combining different materials and existing finish combinations of backshells,
connectors, shield braids, and mounting hardware. The requirement for prevention of
connectors from long term corrosion degradation in this application when the finish is
combined with different materials and existing finishes demonstrates the need to
perform a corrosion performance study. Maintaining the shell to shell continuity in order
to keep the integrity of electromagnetic wiring shielding is also required.
1.5 Overview of Project Goals
The purpose of this thesis is to provide insight into the ability of the zinc nickel
finish on the shell of circular connectors to provide adequate protection from corrosion
when combined with the different materials and finishes used today in aerospace
applications. In addition to this, the ability of zinc nickel finish to maintain the required
electrical conductivity and electromagnetic interference shielding integrity when
5
combined with different materials and finishes existing today will be examined. The
observed behavior of the zinc nickel finish will be compared to cadmium finish that is
widely used today in the same application. The results from this study would reveal the
electrical and physical properties, and corrosion resistance level of a connector shell
with zinc nickel finish after it has undergone a salt fog test for an extended period of
time when mated with connectors with the same and other finishes that exist today. The
results will be used to conclude whether or not zinc nickel finish is an acceptable
replacement for cadmium finishes in this application.
6
2. Coatings for Circular Connectors
2.1 Existing finishes and their applications
According to MIL-DTL-38999, there are 22 finishes existing today for MILDTL-38999 circular connectors applied over the different types of shell material and
connector series [14]. All these finishes are rated per period of exposure to salt fog and
have been qualified based on material, finish color. and connector series. Finishes
provide corrosion resistance and electrical conductivity for shells on circular connectors.
Without the finish, the bare metal would come in contact with the environment and rapid
degradation would occur. Once this happens, reduction in conductive properties and
strength of the shell begin which may result in electromagnetic interference in the wiring
interconnect system. Described below are some of the finishes that exist today as listed
in MIL-DTL-38999 [14].
2.1.1
Class A finish: Cadmium over nickel. Light Gold Color
This finish has a light gold color and is only available on connector shells made
with aluminum. An electrodepositing process is used to plate this finish on the connector
shell. A nickel plate is applied on the shell of the connector followed by a cadmium
plate. The final finish is electrically conductive and also provides corrosion resistance
for the shell. It is currently rated for up to 48 hours in a salt fog test and is inactive for
new designs.
2.1.2
Class F & G finish: Electroless Nickel
This finish is applied from an aqueous acidic solution to the connector shell
using an autocatalytic process operating at elevated temperatures. The autocatalytic
process is a process where the product of the reaction is also the catalyst for that
reaction. In this case, the deposited nickel alloy is the catalyst and the product in the
process. This finish is applied on aluminum connector shells. The final finish is
electrically conductive and also provides corrosion resistance for the shell. The same
7
finish is available for composite shell material, however it is identified as a class M
finish. Class M finish provides electrical conductivity only as corrosion is not a concern
in composite materials. This finish is currently rated for up to 48 hours in a salt fog test
for aluminum shells and 2000 hours for composite shells and is in use on some avionic
equipment.
2.1.3
Class W finish: Cadmium over nickel. Olive Drab Color
This finish has an olive drab color and is available on connector shells made with
aluminum. The same finish is available for composite shell material, however it is
identified as a class J finish. An electrodepositing process is used to plate this finish on
the connector shell. A nickel plate is applied on the shell of the connector followed by a
cadmium plate. The final finish is electrically conductive and also provides corrosion
resistance for the shell. Class J finishes provide electrical conductivity only as corrosion
is not a concern in composite materials. It is currently rated for up to 500 hours in a salt
fog test and 2000 hours for composite shells (class J finish). The Class W finish is the
most commonly used finish on circular connectors and backshells. This is because it
provides ultimate corrosion protection. It continues to provide corrosion protection when
the finish is scratched. It also has good electrical conductivity properties and low contact
resistance. A combination of connectors and backshells with Class W finish is used as
the baseline for the tests performed in this study.
2.1.4
Class K finish: Passivate.
This finish is available on connector shells made with stainless steel. Connectors
with this finish are used in harnesses exposed to high temperature such as on the engine
firewall. The passivate finish uses an oxidant such as nitric acid solution to enhance the
spontaneous formation of protective passive films. These film helps to remove
contamination that can be potential corrosion sites. It is currently rated for up to 500
hours in a salt fog test.
8
2.1.5
Class L finish: Electrodeposited Nickel
This finish is available on connector shells made with stainless steel. Similar to
class K finishes, connectors with this finish are found in areas where high temperature is
a concern. The electrodeposited nickel finish helps seal up contaminations that can
become potential corrosion sites. An electrodepositing process is used to plate this finish
on the connector shell. It is currently rated for up to 500 hours in a salt fog test.
2.2 Currently identified alternate finishes to Cadmium
2.2.1
Class P finish: Electrodeposited Aluminum.
This finish has a non-reflective color and is available on connector shells made
with aluminum. An electrodepositing process is used to plate this finish on the connector
shell. The final finish is electrically conductive and also provides corrosion resistance
for the shell. It is currently rated for up to 500 hours in a salt spray chamber. The Class P
finish is one of the newer finishes approved MIL-DTL-38999 as a replacement for
cadmium.
2.2.2
Class T finish: Nickel fluorocarbon polymer.
This finish has a gun-metal gray non-reflective color and is available on
connector shells made with aluminum. An electrodepositing process is used to plate this
finish on the connector shell. A nickel plate is applied on the shell of the connector
followed by a composite coating of electroless nickel phosphorus and polytetrafluoroethylene (PTFE). The final finish is electrically conductive and also provides
corrosion resistance for the shell. It is currently rated for up to 500 hours in a salt spray
chamber. The Class T finish is one of the newer finishes approved for MIL-DTL-38999
as a replacement for cadmium.
2.2.3
Class Z finish: Zinc Nickel
This finish has a black color but can be found in other colors depending on the
conversion process used and is available only on connector shells made with aluminum.
An electrodepositing process is used to plate this finish on the connector shell. The final
9
finish is electrically conductive and also provides corrosion resistance for the shell. It is
currently rated for up to 500 hours in a salt spray test. The Class Z finish is one of the
newer finishes approved for MIL-DTL-38999 as a replacement for cadmium [14]. It is
also seen as a cost effective alternative to cadmium. This finish is further discussed later
in this study.
2.3 How Circular Connectors Corrode
Corrosion is an electrochemical process that involves the degradation of a metal
due to electrochemical reactions it undergoes within its environment. The process of
corrosion takes place at the most basic molecular level which is the atom and in most
cases driven by electricity. Corrosion does not only occur in the presence of a liquid, it
can also occur as a result of the presence of hot gasses. In this application, hot gases are
derived from fumes caused by fuel combustion in an aircraft.
Most metals are unstable in their metallic states. Metals used for circular
connector shells are refined from raw materials and ores in order to preserve their
chemical stability when in corrosive environments. Aluminum is the most common
material used on circular connectors and it is processed from raw material or ores. [1]
Problems associated with corrosion on circular connectors are made worse
because of the need for electrical conductivity and the ability to provide adequate
environmental protection. [1]
Migration of electrons occur between the anode and the cathode along a metallic
path because of the electrical potential between them. The combination of positively
charged atoms produced at the anode and negatively charged ions from the environment
form rust which is a product of corrosion. In aerospace applications, the source of ions is
the atmosphere. The conductor of the electrical current is the mating point for most of
the components such as the threads of the plug and receptacle when mated. The metallic
path for the current flow is between the shell and its plating when the finish is broken.
An unbroken plating prevents the connection of the electrolyte to the cathode and anode
elements thereby preventing ionic current flow. In aerospace applications, a sacrificial
protection method known as cathodic protection for corrosion control is deployed. [1]
10
2.4 Cadmium Protection
Cadmium plating has historically been the ideal material finish for corrosion
protection on electrical systems in aerospace applications. It acts as a sacrificial coating
when used as a plating over more noble metals such as nickel [1]. It has superior
environmental resistance; corrosion resistance properties. In corrosive environments,
cadmium dissolves thereby leaving the substrate metal intact. In cases where the finish is
broken, the surrounding finish still protects the substrate. It also serves as a lubricious
coating to reduce wear and thread galling when mated with other connectors because of
its softness and durability. It is also a very good electrical conductor thereby providing
electromagnetic compatibility and resistance to electromagnetic interference. Cadmium
currently has an advantage over other finishes in terms of cost because of its current
high production use and plating method available from numerous sources. It’s ease of
manufacturing and repair make this possible. Cadmium is solderable which is very
important in this application. Cadmium also provides a low resistant electrically
conductive surface which allows for part marking to be accomplished. The amount of
deposit thickness is easy to control within .0010 to .00030 inches making it suitable for
threaded components and components in which close tolerance is a critical requirement
[1]. It can operate in a temperature range of -65C to 175C and has been tested to provide
over 500 hours salt fog protection on aluminum shells.
However, Cadmium salts are toxic. Because of its salts, there is an increased risk
of failure of steel components due to hydrogen embrittlement, and there are health
hazards from exposure to cyanide baths when plating cadmium on a substrate material.
It is one of the most dangerous occupational hazards when the fine dusts or fumes are
inhaled or soluble compounds of the metal are ingested. Hence there is a powerful
motivation to look for alternative coatings for this application. Figure 4 shows a plug
and receptacle with cadmium coating.
11
Receptacle
Plug
Figure 4: Cadmium Plated Aluminum Receptacle and Plug
2.5 Zinc Nickel finish
The performance features expected of the thin film alloy deposits include
corrosion sacrificially to substrate, stability of corrosion byproducts, coating adherence
to non-conversion films, and low dissolution rates in corrosive atmosphere [3]. Because
the coating must be sacrificial in relation to its substrate, the positioning of the
substitution metals in relation to the substrate in the corrosion potential scale in a highly
corrosive environment is important [7].
In a previous study performed, substrates were protected by the deposition of
zinc and zinc alloys in order to test their performance in corrosive environments. The
study revealed that electroplated zinc alloy deposits are superior and achieve a consistent
and stable corrosion resistance performance over the substrate alloy [7].
Zinc electrodeposits are widely used for corrosion protection in place of
cadmium due to cadmium’s highly toxic nature and the environmental hazard it
represents. Similar to cadmium, zinc deposits are effective in isolating a substrate from
the environment and provide sacrificial protection to the substrate in a case where the
coating is damaged [4]. However, in long term cases, pure zinc coatings offer
insufficient corrosion resistance. The industry requirement to reduce coating thickness
makes it difficult to plate the required thickness for optimal corrosion resistance [5].
12
Improvements to the corrosion resistance of zinc can be achieved by alloying it with
some more noble metals such as tin, cobalt, and nickel.
Zinc coatings alloyed with nickel have generated the highest interest in the
aerospace industry because they meet most of the benchmarks set by cadmium. Zinc
coatings also have good corrosion resistance properties, superior formability and
improved welding characteristics [6]. It has been stated in several studies that the
corrosion resistance of zinc nickel alloy coatings can be 5 to 6 times higher than that of
pure zinc and that higher corrosion resistance is seen in zinc nickel coatings with smaller
grain size, higher uniformity or grain distribution, and higher number of lattice
imperfections in their microstructure [5]. This is due to the protective properties of the
oxide layers formed on the surface of the zinc nickel coatings during corrosion [5].
Previous studies indicate that zinc nickel alloys containing 10-15wt% nickel
possess the highest corrosion resistance of the possible alloy composition however, they
have more negative potential because of the high zinc content thereby forming oxides
instantaneously in corrosive environments [6]. Nickel slows down the dehydration of
zinc hydroxide into zinc oxide. Zinc hydroxide is a byproduct of corrosion. Since the
hydroxide has a lower level of electrical continuity than the oxide, the gain in electrons
is weaker thus reducing the rate of corrosion [7]. Figure 5 is a chart showing the average
corrosion rates from a study previously conducted on zinc nickel deposits in a 3.5%
NaCl solution. As the nickel content of the alloy composition increases, the alloy
becomes more noble thus becoming less sacrificial to the substrate material. This is most
likely due to the overall surface enrichment of nickel by dezincification [15].
13
Figure 5: Average Corrosion Rates on Zinc Nickel Alloy Deposits by Nickel
Content [15].
The corrosion products of zinc nickel coatings greatly affect the corrosion
behavior of the substrate. The initial stage of corrosion of zinc nickel coatings results in
instant growth of an oxide film [5]. In the presence of chloride or sulphate ions, zinc
hydrochlorides or hydrosulphates with water molecules could be formed [5]. In a
previous study under accelerated corrosion conditions, zinc hydroxychloride
Zn5(OH)8Cl2 H2O was the main corrosion byproduct formed on the zinc nickel surface
[5]. Smaller quantities of sodium hydroxysulphate NaZn4(SO4) Cl(OH)6 H2O were also
identified in the byproducts [5].
The composition of zinc nickel on circular connectors consists of 5-15wt%
nickel and the balance is zinc in an aqueous solution that can be deposited from both
acid and alkaline processes. The alkaline process is easier to control and provides a more
consistent coating composition [2]. Current studies have shown that paint adhesion and
hydrogen embrittlement tests conducted on zinc nickel coatings were successful [1].
Paint adhesion is preferred on circular connectors because of part marking. Studies have
also shown that corrosion resistance of zinc nickel on circular connectors is less than
cadmium baselines, however an increase in the thickness of the coating and selection of
an appropriate conversion coating will improve results [2]. Thicker zinc nickel coatings
14
provide enhanced corrosion resistance. The form and fit of the component might be
affected but dimensions can be modified. Because the corrosion performance of zinc
nickel plating film is a function of the nickel content, the corrosion potential for zinc
nickel deposits with a high nickel content -0.9 to -1.0V is closer to being noble than
basic. This slows down the dissolution rate of zinc nickel coating [3].
Zinc nickel alloys have different phases depending on the nickel content of the
composition. Zinc nickel alloys with 10-17% nickel usually have a single γ-phase in
electrodeposited coatings. Yield strength of zinc nickel alloys in this phase is reported to
be 260MN m-2 [15]. Hardness values range from approximately 260 to 400HV [15].
Anti-scratch performance is preferred in circular connectors because of rough handling
during harness assembly and installation. Nickel contents greater than 17% have a
mixture of γ and α-phase [15]. Figure 6 is the phase diagram for zinc nickel alloys.
Evaluation in a previous study of zinc nickel on steel sheets concluded that no
deterioration in yield strength, tensile strength or elongation properties of the substrate
material was observed [17].
Figure 6: Zinc Nickel Phase Diagram [16]
Figure 7 shows the microstructure of zinc nickel coatings deposited from an
alkaline bath before and after a 48 hour corrosion test in a sodium chloride environment.
15
The alloy had a nickel content of 6-10% thus presenting a two phase structure of γ and δ
with γ-phase being predominant [16].
Before Test
After 48 hrs
Figure 7: Micrograph of Zinc Nickel Coatings before and after 48 hours Immersion
in NaCl Environment [16]
Zinc nickel alloys produce zinc chloride in the presence of salts which is a very
stable byproduct in corrosive environments. The basic zinc chloride that is produced at
the surface corrosion site acts as a barrier film, however it has high electrical resistance
and a reduced dissolution rate [3]. Due to the nickel content, zinc nickel is harder and
thus provides superior anti-scratch performance. As stated earlier, this is a favorable
characteristic because of the rough handling of the connectors when the harness is
assembled and installed.
2.5.1
Electrodeposition of zinc nickel alloy Coating Deposits on Circular
Connectors
There is one class of zinc nickel coating identified by ASTM B841. It consists of
5-12wt% of nickel with the balance of zinc. There are 5 types of conversion coatings
that can be applied, however type D which is black chromate conversion coating is what
is identified in MIL-DTL-38999 for coatings on circular connectors. The chromate
16
conversion coating helps inhibit dezincification of the alloy. Manufacturers are allowed
to select which conversion coatings to use. The coating is produced from an aqueous
electroplating system. This aqueous system can either be alkaline or acidic as stated
previously, but alkaline is preferred. As part of the coating requirements, the substrate
must be free of defects that are detrimental to the zinc nickel coating. Application of
procedures such as cleaning, pickling, and electroplating occurs after all base metal heat
treatments have been performed. This is required in order to ensure satisfactory adhesion
and corrosion resistance performance of the coating. After electrodeposition, the coating
must be adherent and free of blisters, pits or discontinuities, and also free of cracks. The
coating allows for normal handling storage conditions to occur without chipping, flaking
or other coating damage [8]. Figure 8 shows zinc nickel finish with black conversion
coating applied over an aluminum MIL-DTL-38999 plug and receptacle.
Receptacle
Plug
Figure 8: Zinc Nickel Plated Aluminum Receptacle and Plug
2.5.2
Properties of Cadmium and zinc nickel of Interest in this Application
Table 1 shows a comparison between cadmium coatings, electrodeposited zinc
nickel coatings, and pure zinc coatings based on performance in properties of interest in
the application of this study. These properties are preferred in coatings for circular
connectors.
17
Properties of Interest
Cadmium
Zinc Nickel
Zinc
Corrosion Resistance
Good
Good
Good
Sacrificial Protection
Good
Good
Good
Galvanic Compatibility with Aluminum
Good
Good
Poor
Friction Coefficient
Low
Low
Intermediate
Electrical Conductivity
Good
Good
Fair
Solderability
Good
Good
Poor
White Rust Formation
Low
Low
High
Uniform Deposition
Good
Good
Good
Adhesion
Good
Good
Good
Table 1: Summary of Properties of Cadmium, Zinc Nickel and Pure Zinc Coatings
[4].
18
3. Experimental
The goal of this study was to provide insight into the ability of the zinc nickel
finishes on circular connectors to adequately provide protection from corrosion and
maintain the required electrical conductivity and electromagnetic interference shielding
integrity, when combined with connectors and connector accessories with the same and
other finishes that exist today. In addition to these combinations, zinc nickel’s
compatibility with the tin plated shield braid, which is a key component of the ground
path from the wires to the bulkhead, will be examined. The different terminations of the
shield braid used in this study will be discussed. The study was conducted using the
following tests.
•
Salt spray test
•
Electrical Continuity Test
•
Coupling torque Test
•
Visual Inspection.
All testing was carried out per industry accepted specifications. ASTM B117 for
the salt spray test, EIA-364-83 for the electrical continuity test, and EIA-364-13D for
the coupling torque test. Data collected at periodic intervals during the test, between 0
and 500 hours, will be used to determine the ability of zinc nickel to provide adequate
protection needed.
3.1 Test Unit Identification
There are 10 test units used in this study, however only 9 of them are applicable
to this study. This is because the tenth test unit does not simulate any condition that a
connector or connector accessory with zinc nickel finish might come in contact with.
Test unit 10 has a nickel Teflon backshell on an electroless nickel connector mated with
a nickel Teflon backshell on a cadmium connector, all with aluminum shells. The test
unit combinations were arrived at by identifying the materials and finishes currently
used in today’s aircraft on connectors, connector accessories, shield braids, screws,
washers and mounting flanges. Table 2 shows the materials and finishes that were
identified.
19
Connectors
Material
Finish
Aluminum
Cadmium over Nickel
Electroless Nickel
Composite
Cadmium over Nickel
Electroless Nickel
Stainless Steel
Passivate
Aluminum
Cadmium over Nickel
Composite
Cadmium over Nickel
Composite
Electroless Nickel
Shield Braids
Copper
Tin
Screws
Carbon Steel
Cadmium
Washers
Aluminum
Anodized
Mounting Flanges
Aluminum
Epoxy Polyamide Primer
Nuts
Steel
Cadmium
Backshells
Table 2: Matrix of existing materials and finishes
Introducing zinc nickel finish into the industry today will involve the possible
combination of the materials and finishes listed in table 2 depending on what part of the
interconnect system is being replaced. The test units simulate different potential
conditions that arise in practice.
3.1.1
Test Unit Set-up
Described are the test units simulating the different combination of materials and
finishes if zinc nickel finish is introduced into the field. The parts list and figure of the
test units applicable to this study is depicted in the description of the test units. Table 3
shows a matrix of the test units based on connector and backshell materials and finishes.
20
Test Unit
Backshell
Plug
Receptacle
Backshell
1
Cadmium (Al)
Cadmium (Al)
Cadmium (Al)
Cadmium (Al)
2
Cadmium (Al)
Cadmium (Al)
Zinc Nickel (Al)
Zinc Nickel (Al)
3
Zinc Nickel (Al)
Zinc Nickel (Al)
Zinc Nickel (Al)
Zinc Nickel (Al)
4
Zinc Nickel (Al)
Zinc Nickel (Al)
Electroless Nickel (Al)
N/A
5
Zinc Nickel (Al)
Cadmium (Al)
Zinc Nickel (Al)
N/A
6
Zinc Nickel (Al)
Zinc Nickel (Al)
Passivate (SS)
N/A
7
Cadmium (C)
Cadmium (C)
Zinc Nickel (Al)
Zinc Nickel (Al)
8
Electroless Nickel (C)
Electroless Nickel (C)
Zinc Nickel (Al)
Zinc Nickel (Al)
9
Zinc Nickel (Al)
Electroless Nickel (C)
Zinc Nickel) (Al)
Cadmium (C)
Table 3: Test Unit Finish and Material Matrix (Al – Aluminum. C – Composite)
21
3.1.1.1 Test Unit 1 (Test Baseline)
Plug Side
Receptacle
Mounting Hardware
Component
Material and Finish
Connector
Aluminum, Cadmium finish
Backshell
Aluminum, Cadmium finish
Shield Braid
Copper, Tin plated
Connector
Aluminum, Cadmium finish
Backshell
Aluminum, Cadmium finish
Shield Braid
Copper, Tin plated
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 4: Test Unit 1 Parts List
Figure 9: Test Unit 1
Test Unit 1 simulates a condition that currently exists on many aircraft where
cadmium plated connectors and backshells are mated with other cadmium plated
connectors. The degradation of cadmium finish and the interface between the shield
braid and the rings will be reviewed. The results gathered from this test unit will serve as
a baseline for this test.
22
3.1.1.2 Test Unit 2
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Aluminum, Cadmium finish
Backshell
Aluminum, Cadmium finish
Shield Braid
Copper, Tin plated
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 5: Test Unit 2 Parts List
Figure 10: Test Unit 2
Test Unit 2 simulates the introduction of a connector and backshell with zinc
nickel finish into an existing aircraft today where one side of the interconnect system is
replaced. This test unit will establish the corrosion resistance of zinc nickel finish
connectors when mated with cadmium plated connectors. In addition the degradation of
the zinc nickel finish and the interface between the shield braid and backshell rings will
be reviewed. The possibility of this test unit in actual use in the industry is high if zinc
nickel finish is introduced.
23
3.1.1.3 Test Unit 3
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 6: Test Unit 3 Parts List
Figure 11: Test Unit 3
Test Unit 3 simulates a situation where all connectors and backshells used have a
zinc nickel finish. This unit will establish the corrosion protection strength of zinc nickel
finishes on connectors when mated with one another and also the degradation of the zinc
nickel finish and the interface between the shield braid and backshell rings will be
reviewed. A part of this test unit has been investigated by previous studies and results
have been deemed favorable.
24
3.1.1.4 Test Unit 4
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Connector
Aluminum, Electroless Nickel finish
Backshell
N/A
Shield Braid
N/A
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 7: Test Unit 4 Parts List
Figure 12: Test Unit 4
Test Unit 4 simulates a situation where a zinc nickel finish connector with a zinc
nickel finish backshell is mated with an electroless nickel finish connector without a
backshell. Since electroless nickel finish connectors are mostly found on equipment, a
backshell is not required in this simulation. This test unit will be reviewed to establish
the corrosion protection strength of zinc nickel finishes when mated with electroless
nickel plated connectors.
25
3.1.1.5 Test Unit 5
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Aluminum, Cadmium finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Connector
Aluminum, Zinc Nickel finish
Backshell
N/A
Shield Braid
N/A
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 8: Test Unit 5 Parts List
Figure 13: Test Unit 5
Test unit 5 simulates a situation where a backshell with a cadmium finish is
replaced with one with a zinc nickel finish without a change to the connector on one side
of the interconnect system. This case is likely to exist if only a connector swap is
needed. This test unit will be used to examine the interface between the connector and
backshell of the different finishes. The other side of this test unit is a zinc nickel finish
connector.
26
3.1.1.6 Test Unit 6
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Connector
Stainless Steel, Passivate finish
Backshell
N/A
Shield Braid
N/A
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Nuts
Steel, Cadmium finish
Table 9: Test Unit 6 Parts List
Figure 14: Test Unit 6
Test Unit 6 simulates a situation where a zinc nickel finish connector is mated
with a stainless steel connector with a passivate finish. These types of connectors can be
found in areas of high temperature and when high strength is required. This unit will be
used to establish the corrosion protection strength of zinc nickel finishes when mated
with passivate finishes on stainless steel connectors.
27
3.1.1.7 Test Unit 7
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Composite, Cadmium finish
Backshell
Composite, Cadmium finish
Shield Braid
N/A
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 10: Test Unit 7 Parts List
Figure 15: Test Unit 7
Test Unit 7 is similar to Test Unit 2 but the substrate material for the cadmium
finish is composite. Results from this test unit should be similar to Test Unit 2 because
of the arrangement of the finishes on the components of the test unit. The possibility of
this configuration in actual use in the industry is high if zinc nickel finish is introduced.
28
3.1.1.8 Test Unit 8
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Composite, Electroless Nickel
Backshell
Composite, Electroless Nickel
Shield Braid
N/A
Connector
Aluminum, Zinc Nickel finish
Backshell
Aluminum, Zinc Nickel finish
Shield Braid
Copper, Tin plated
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 11: Test Unit 8 Parts List
Figure 16: Test Unit 8
Test Unit 7 is similar to Test Unit 4 but the substrate material for the electroless
nickel is composite. The electroless nickel connector has an electroless nickel backshell
that is also composite. Results from this test unit should be similar to Test Unit 4
because of the arrangement of the finishes on the test unit components.
29
3.1.1.9 Test Unit 9
Plug
Receptacle
Mounting Hardware
Part Number
Material and Finish
Connector
Composite, Electroless Nickel
Backshell
Aluminum, Zinc Nickel alloy
Shield Braid
Copper, Tin plated
Connector
Aluminum, Zinc Nickel finish
Backshell
Composite, Cadmium finish
Shield Braid
N/A
Screw
Carbon Steel, Cadmium finish
Washer
Aluminum, Anodized
Mounting Flange
Aluminum, epoxy polyamide primer
Table 12: Test Unit 9 Parts List
Figure 17: Test Unit 9
Test unit 9 comprised of 4 finishes examined in this study. The substrate of the
electroless nickel and cadmium finishes is composite. This test unit will be used to
examine the corrosion resistance of a zinc nickel backshell on an electroless nickel finish
30
composite connector and cadmium finish composite backshell on a zinc nickel finish
connector.
It is noted that the combination of the tin plated copper shield braid and the zinc
nickel backshell occurs in several of the test units set up. This allows for more data on
this interface to be collected because this interface is of major concern in this study. This
also applies to the interface between the zinc nickel backshell and connector.
3.1.2
Mounting bracket
The mounting bracket in this study was used to hold the test units in place and
also to simulate the bulkhead in an aircraft. It was fabricated from Al 7076 and is 0.063
inches thick. The bracket surface was chemically heat treated and primed before the test
units were assembled. Chemical surface treatment is the application of alodine on
aluminum and aluminum alloys where maximum corrosion protection and electrical
conductivity is required. The surface area where the receptacles interfaced with the
mounting bracket were masked off prior to the application of primer in order to allow
electrical continuity through the bracket. This is the same practice used on aircraft. (See
appendix 7.10 for detailed design of the bracket)
Figure 18: Test bracket set up.
31
3.2 Shield Braid Termination.
As stated earlier in the report, a shield braid is a tightly woven braid in which the
harness is enclosed and is part of the conductive path from the wires to the grounding
point. The shield braid, made of copper material and plated with tin, is applied over the
harness. The shield braid is terminated at the backshell. Any unwanted EMI travels from
the harness to the shield braid, to the backshell, to the connector, and to the bulkhead
which is attached to the aircraft ground. Termination of the shield braid is important so
as not to cause a break in this path. There are different termination styles depending on
the backshell, but the one used in this study has a ring termination style. Two or three
rings are utilized for grounding the shields. However, two are used in this study because
the third ring is meant for termination of the individual wire braids that don’t apply to
this study. The shield braid is woven between the rings and is secured at the end of the
backshell by the strain relief. This allows contact between the shield braid and the
backshell thereby providing a conductive path. The interface between the zinc nickel
finish and the shield braid is important for the integrity of the conductive path. The
effects of corrosion at this interface will be examined.
Figure 19: Shield Braid Termination [10].
32
3.3 Salt Spray Test
This test involves exposing test units to a fine mist of salt solution in order to
establish an evaluation of the corrosion resisting properties of various materials and
protective coatings. Electrolytic corrosion is the failure mechanism induced by the salt
spray on the test units. The test provides an avenue to assess the effects of salt-laden
atmosphere on electrical connectors, connector accessories, finishes, and components
through electrical continuity readings and visual inspections. The apparatus, salt
composition and procedure of the salt spray test will be discussed.
3.3.1
Test Apparatus
The apparatus required for this test consists of a salt fog chamber, a salt solution
reservoir, a supply of suitably conditioned compressed air, atomizing nozzles, supports
for test units, and temperature control provisions.
Figure 20 shows a salt spray test apparatus consisting of the salt fog chamber
which is the container in which the test fixture is placed. It is made of materials such as
glass, hard rubber and plastic that cannot be affected by the salt fog. The chamber is
constructed in a way that allows free circulation of the salt fog to the same degree on all
the test units. It also allows for no direct encroaching of the salt fog on the test units or
condensation dripping on the units. The salt chamber also contains the supports for the
test units made of a material that cannot be affected by the salt solution. The temperature
of the exposure zone of the salt spray chamber is maintained at 35 ± 2ºC. The atomizers
provide a finely divided dense fog to the test units. The nozzles delivers the fog and are
made of materials that cannot be affected by the salt solution. The compressed air
entering the atomizers is free from impurities such as oil and dirt and is passed through a
filter. Air pressure is used to produce the suitable and finely divided dense fog in
conjunction with the atomizers. Humidity and temperature of the compressed air is
controlled in order to meet the operating conditions. In order to protect the atomizers and
nozzles from clogging by salt deposition, the relative humidity of the air at the point of
release from the nozzle is 95-98%.
An air saturation tower is used to meet the
requirements for the pressure and humidity. Air pressure varies based on the temperature
of the water used to control the humidity and salt fog. The salt solution reservoir
33
contains the salt solution that will be discussed in the next section of this report. It is a
separate piece from the salt chamber in order to provide ease of maintainability. The part
of the reservoir’s construction that comes in contact with the salt solution is of a material
that cannot be affected by the salt solution. The temperature of the chamber is
maintained at 35 ± 2ºC in order to keep the salt solution at the required temperature.
During the testing, the salt solution is drawn from the reservoir by the chamber’s
peristaltic pump through a filter unit. This filter unit removes any salt crystals that are
not dissolved and debris that are contained in the salt solution. Figure 21 shows the
placement of the filter in the reservoir.
Figure 20: Sample Composition of Salt Spray Test Apparatus [9]
3.3.2
Salt Composition
The salt solution used in this study is prepared by dissolving 5 ± 1 parts mass of
sodium chloride in 95 parts mass of water thereby creating a 5% sodium chloride salt
solution. Distilled water is used for this solution and does not contain more than 50 parts
per million of total solids. The solution is kept free of solids by a filter that is located in a
34
reservoir (figure 21) as stated earlier in the report. The chemical content of the salt does
not include more than 0.3% by mass of total impurities. The pH value of the salt solution
is maintained at a value between 6.5 and 7.2 when measured at a temperature range
between 34ºC and 36ºC.
Figure 21: Salt Solution Reservoir showing positioning of filter [11]
3.3.3
Procedure
The test fixture in this study consists of the test units mounted on a bracket (see
figure 18). The test fixture is suspended in the chamber from the top using plastic hooks
with the test fixture vertical thereby making the test units horizontal in order to simulate
the conditions of the aircraft. The plastic hooks used are fabricated from a material that
is non-reactive to the salt solution. The test units are free of oil, dirt, and grease and
cleaned as necessary before they were put into the chamber. As stated earlier, the test is
conducted at a temperature maintained at 35ºC ± 2ºC in the exposure zone. The salt
solution deposited on the test unit in the exposure zone is about 0.5 milliliter to 3.0
milliliters of solution per hour. The length of the test is 500 hours but data is collected at
the intervals indicated. Table 13 shows the test intervals when the exposure to salt fog
will be interrupted and inspection and data collection will occur.
35
Test Intervals
Length of exposure in the Salt Spray Chamber
A
48 Hours
B
96 Hours
C
500 Hours
Table 13: Salt Spray Test intervals.
At each test interval, the test fixture was dipped in running tap water at a
temperature of 39ºC for 5 minutes and dried for 16 hours in a circulating air oven at a
temperature of 38ºC ± 3ºC. Once this was completed, examination and measurement
taking was conducted which will be discussed further in the following sections.
3.4 Electrical Continuity Test
This test involves taking electrical continuity measurements between shield
braids and backshells, backshells and connectors, mated plugs and receptacles, and
between receptacles and the mounting bracket. The test equipment and procedure of the
test units will be discussed.
3.4.1
Equipment
The shell to shell conductivity test equipment uses a voltmeter capable of
measuring voltage within ±2% and an ammeter capable of measuring applied current
within ±1%. A regulated current power supply delivering up to 1.0 ± 0.1 Amps was also
used in this test. The test probes have spherical ends of 1.27 mm min radius and are used
to take voltage measurements on the bonding points.
3.4.2
Procedure
A 1.0 amp DC current is passed through the two surfaces that are being checked.
The test probes are placed on the shells in a way that doesn’t cause any damage to the
coating of the connector. Voltage drop across the two surfaces is measured from any
point on the surfaces. Measurements are conducted and recorded at the test intervals
listed in table 14.
36
Test Intervals
Length of exposure to salt spray in chamber
A
0 Hours
B
48 Hours
C
96 Hours
D
500 Hours
Table 14: Electrical Continuity Measurement Conditions.
As stated earlier, electrical continuity measurements are conducted on the
surfaces of the test units to measure the electrical continuity per interface. Table 15
shows the different interfaces that are studied in this report as referenced in figure 22
except the mounting bracket. Figure 18 shows the complete assembly of the test units
mounted on the bracket.
Measuring Point/Surface 1
Measuring Point/Surface 2
Shield Braid
Backshell
Backshell
Receptacle/Plug
Receptacle
Plug
Receptacle
Mounting bracket
Table 15: Measurement Points/Surfaces on test units
Figure 22: Measurement Points on test units
37
Per MIL-DTL-38999, the voltage drop across the test units should be a maximum
of 2.5 millivolts with 100% increase after 500 hours of salt fog exposure.
3.5 Coupling Torque Test
This test involves taking measurements of force required to mate and unmate
plugs and receptacles. The test equipment and procedure of the test units will be
discussed.
3.5.1
Equipment
The equipment for this test is a torque gauge. When measurement are taken,
readings are between 25% and 75% of the scale with nominal full scale accuracy of
+2%.
3.5.2
Procedures
The mating connectors are brought to the position where mechanical mating
begins and the torque gauge is set to zero. The connectors are then fully mated at a rate
of 25.4 millimeters/minute until peak torque is achieved. The reading is recorded at this
point. The same applies when unmating the connectors as the peak torque used to
unmate the connectors is recorded. Table 16 shows the maximum and minimum torques
for mating and unmating shell size 19 connectors used in this study per MIL-DTL-38999
[14].
Maximum Engagement and Disengagement
Minimum Disengagement
28 Inch Pounds
3 Inch Pounds
Table 16: Maximum and Minimum torques for Shell Size 19 Connectors.
3.6 Visual Inspection
This involves examining the connectors for exposure of base metals, pitting and
porosity of finishes. Cracking and delamination of the connector and coatings is also
examined. Abnormal nicks, cracks or scratches on the finished surfaces indicate that the
normal protective coating is removed. White rust indicates corrosion in the finish and
38
successful protection of the substrate to the corrosive conditions. Red rust could indicate
that the substrate has been exposed to the corrosive environment and degradation has
begun to take place. This inspection is conducted at the test intervals indicated in table
14.
3.7 Test Sequence
The study is performed following the test sequence described in figure 23 until it
reaches the total salt fog exposure time of 500 hours at the completion of the test. After
assembling the test units onto the test fixture, a visual inspection of the test units is
performed in order to check for damage to their finishes. The connectors are then
torqued to values below the maximum engagement value specified in table 16. Once this
is completed, electrical continuity measurements are collected. Since this is done prior to
exposing the test fixture to salt fog, the readings are consistent across all test units. At
this point, the test fixture is exposed to 48 hours of salt fog in the salt spray chamber.
The test fixture is rinsed for a maximum of 5 minutes under continuously running water
colder than 38°C in order to remove the salt deposits on the test units thus reducing the
chances of confusing salt deposits with oxide film formation. In order to remove
moisture from the test fixture after it has been rinsed, it is placed in a circulating air oven
with dry air cooler than 38°C for a maximum of 16 hours. Visual inspection is conducted
at this time on the test units at room temperature and electrical continuity measurements
are collected. This sequence is repeated at 96 hours and 500 hours with the addition of
the unmating torque measurements after electrical continuity measurements have been
recorded at 500 hours.
39
Figure 23: Test Sequence
40
4. Results and Discussion
Readings from the electrical continuity test are collected at 0, 48, 96 and 500
hours of the test fixture’s exposure to salt fog and recorded. Coupling torque
measurements are taken at 0 and 500 hours only.
4.1 Pre-Salt Fog Exposure Readings
Before the test fixture was exposed to salt fog, a set of readings were collected in
order to determine the effect of the exposure to salt fog on the test units compared to no
exposure at all. Coupling torque measurements and electrical continuity measurements
were recorded. The values recorded are within the specified range that MIL-DTL-38999
recommends for these test units.
4.1.1
Coupling Torque at 0 Hours
As stated earlier, the connectors are torqued to values within the range
recommended by MIL-DTL-38999 of 3 – 28 inch pounds (See table 17) [14] before
exposure to salt fog. Unmate torque values are recorded after 500 hours of exposure.
This maximum unmating force is however allowed to increase 100% after 500 hours of
salt spray test [14].
Test Unit
Mate Torque
Test Unit
Mate Torque
1
8” inch lbs.
6
3”inch lbs.
2
8” inch lbs.
7
8”inch lbs.
3
8”inch lbs.
8
5”inch lbs.
4
7” inch lbs.
9
5”inch lbs.
5
10”inch lbs.
Table 17: Initial Mate Torque Values
4.1.2
Electrical Continuity Readings at 0 Hours
As stated earlier, the electrical continuity readings recorded prior to the
beginning of the test are consistent and below the maximum voltage of 2.5 millivolts
recommended by MIL-DTL-38999 (See table 18). This maximum voltage is however
allowed to increase 100% after 500 hours of salt spray test [14].
41
Test Hour: 0 Hours
Test Unit 1
Test Unit 2
Test Unit 3
Test Unit 4
Test Unit 5
Test Unit 6
Test Unit 7
Test Unit 8
Test Unit 9
P-Side
J-Side
Shield Braid to
Backshell to
Plug to
Shield Braid to
Backshell to
Mounting Plate to
Backshell
Plug
Receptacle
Backshell
Receptacle
Receptacle
Reading 1
1.6
1.6
1.6
1.6
1.6
1.5
Reading 2
1.6
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
1.6
1.6
1.6
Reading 2
1.6
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
1.6
1.6
1.6
Reading 2
1.6
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
1.6
1.6
1.6
N/A
N/A
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
1.6
1.6
1.6
N/A
N/A
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
1.6
1.6
1.6
N/A
N/A
1.6
Reading 1
N/A
1.6
1.6
1.6
1.6
1.6
Reading 2
N/A
1.6
1.6
1.6
1.6
1.6
Reading 1
N/A
1.6
1.6
1.6
1.6
1.6
Reading 2
N/A
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
N/A
1.6
1.6
Reading 2
1.6
1.6
1.6
N/A
1.6
1.6
Table 18: Initial Electrical Continuity Reading (Millivolts)
42
4.2 Post-Salt Fog Exposure Readings
4.2.1
Electrical Continuity Readings at 48 Hours
The first set of readings recorded after exposing the test fixture to salt fog was at
48 hours. The electrical continuity measurement readings in table 19 show that all the
test units except the ones with zinc nickel finish on the connector or backshell remained
the at same values when compared to the initial readings. The highest readings were
recorded between the shield braid and the zinc nickel backshell and between the
connectors and backshells with zinc nickel finish. These values were unexpectedly high
for this test interval. It was expected that values would be closer to the 2.5 millivolts
maximum potential drop because all but one of the connectors used in this study are
rated for 500 hours in the salt fog test. Values less than the maximum voltage drop of 2.5
millivolts were achieved when the probes were moved to other parts of the backshell
surface. The readings recorded stayed constant for the continuity measurements between
the receptacles and the mounting bracket. See figure 24 for maximum continuity
readings recorded per interface after 48 hours of salt exposure. Figure 26 shows the
minimum reading collected across per interface for all the test intervals.
Figure 24: 48 Hour Maximum Electrical Continuity Readings Per Interface on Test
Units
43
Test Hour: 48 Hours
Test Unit 1
Test Unit 2
Test Unit 3
Test Unit 4
Test Unit 5
Test Unit 6
Test Unit 7
Test Unit 8
Test Unit 9
P-Side
J-Side
Shield Braid to
Backshell to
Plug to
Shield Braid to
Backshell to
Mounting Plate to
Backshell
Plug
Receptacle
Backshell
Receptacle
Receptacle
Reading 1
1.6
1.6
1.6
1.6
1.6
1.5
Reading 2
1.6
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
1.6
1.6
1.6
Reading 2
1.6
1.6
1.6
3.8
1.6
1.6
Reading 1
1.6
1.6
1.6
1.6
1.6
1.6
Reading 2
36.1
4.1
1.8
22.5
38.2
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
12.0
5.8
1.8
N/A
N/A
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
14.2
10.5
1.8
N/A
N/A
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
12.5
1.6
1.7
N/A
N/A
1.6
Reading 1
N/A
1.6
1.6
1.6
1.6
1.6
Reading 2
N/A
1.6
1.6
13.5
1.7
1.6
Reading 1
N/A
1.6
1.6
1.6
1.6
1.6
Reading 2
N/A
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
N/A
N/A
1.6
Reading 2
11.4
6.8
1.9
N/A
N/A
1.6
Table 19: 48 Hour Electrical Continuity Reading (Millivolts)
44
4.2.2
Electrical Continuity Readings at 96 Hours
As stated earlier, the second set of readings after exposing the test fixture to salt
fog were collected at 96 hours. The electrical continuity measurement readings in table
20 show an increase in overall values when compared to the initial and 48 hour
measurements. All the test units except the ones with zinc nickel finish on the connector
or backshell remained below the recommended values. The measurements recorded
between the shield braid and the zinc nickel backshell in the test units showed values
higher the maximum 2.5 millivolts potential drop when measured on some parts of the
surfaces. Figure 25 shows the maximum values recorded per interface in this test. The
values below the recommended maximum value were achieved when the probes were
moved to other parts of the surface of the backshell. A slight increase in values was
recorded for continuity measurements between the receptacles and the mounting bracket
and a value above the recommended maximum voltage in test unit 7. See figure 26 for
minimum continuity readings recorded per interface after 96 hours of salt exposure.
Figure 25: 96 Hour Maximum Electrical Continuity Readings Per Interface on Test
Units
45
Test Hour: 96 Hours
Test Unit 1
Test Unit 2
Test Unit 3
Test Unit 4
Test Unit 5
Test Unit 6
Test Unit 7
Test Unit 8
Test Unit 9
P-Side
J-Side
Shield Braid to
Backshell to
Plug to
Shield Braid to
Backshell to
Mounting Plate to
Backshell
Plug
Receptacle
Backshell
Receptacle
Receptacle
Reading 1
1.6
1.6
1.6
1.6
1.6
1.6
Reading 2
1.6
1.6
1.6
1.6
1.6
1.6
Reading 1
1.6
1.6
1.6
1.6
3.9
1.6
Reading 2
1.6
1.6
1.6
9.7
8.3
1.6
Reading 1
4.6
2.3
14.6
2.0
1.6
1.7
Reading 2
6.5
3.0
7.3
3.4
1.8
1.7
Reading 1
1.6
2.3
1.6
N/A
N/A
1.6
Reading 2
19.4
1.9
9.0
N/A
N/A
1.6
Reading 1
2.2
5.2
1.6
N/A
N/A
1.6
Reading 2
12.2
4.6
1.6
N/A
N/A
1.7
Reading 1
3.2
2.0
1.6
N/A
N/A
1.6
Reading 2
4.1
3.7
1.6
N/A
N/A
1.6
Reading 1
N/A
1.6
1.6
6.7
1.6
3.2
Reading 2
N/A
1.6
1.6
1.7
2.9
1.6
Reading 1
N/A
1.6
1.7
1.6
1.6
1.6
Reading 2
N/A
1.6
1.6
2.7
1.6
1.6
Reading 1
N/A
11.8
1.6
N/A
1.6
1.6
Reading 2
N/A
1.7
1.6
N/A
1.6
1.6
Table 20: 96 Hour Electrical Continuity Reading (Millivolts)
46
4.2.3
Electrical Continuity Readings at 500 Hours
The last set of readings recorded after exposing the test fixture to salt fog was at
500 hours. The electrical continuity measurements in table 21 show an additional
increase in overall values when compared to the initial, 48, and 96 hour measurements.
The measurements recorded between the shield braid and the zinc nickel backshell in the
test units showed values higher than the 5.0 millivolts recommended maximum voltage
on some parts of the surface. The range of values recorded between the shield braid and
the zinc nickel backshell were from 1.6 to 29.7 millivolts depending on where on the test
unit’s surface the voltmeter probes were placed. Compared to cadmium which recorded
a range from 1.6 to 2.8 millivolts, this range is much wider. The readings between the
zinc nickel backshell and the cadmium connectors showed a similar range of values but
with a maximum voltage of 21.3 millivolts. The readings between the zinc nickel
backshell and the zinc nickel connectors showed a maximum value of 8.9 millivolts. The
readings for continuity between the zinc nickel and cadmium connectors showed a
maximum value at 10 millivolts which was less than that recorded for the zinc nickel
finish plugs mated with zinc nickel finish receptacles. The readings for the continuity
between the zinc nickel connector and the mounting bracket exhibited a maximum value
of 7.5 millivolts which is higher than the continuity measurement on the same interface
with cadmium, electroless nickel, and stainless steel finishes.
Overall, the maximum values recorded were a lot higher than the recommended
maximum voltage of 5 millivolts after 500 hours of salt fog exposure. The minimum
values on the other hand remained at the same potential drop values recorded during the
initial readings. In some cases, values as low as 1.2 millivolts were recorded on the
voltmeter. This signifies that a good grounding path existed at the interfaces examined in
this study. Figure 26 shows charts with the minimum and maximum values recorded
after 500 hours of exposing the test units to salt fog per interface.
47
Figure 26: Minumum and Maximum Electrical Continuity Readings Per Interface
on Test Units
48
Test Hour: 500 Hours
Test Unit 1
Test Unit 2
Test Unit 3
Test Unit 4
Test Unit 5
Test Unit 6
Test Unit 7
Test Unit 8
Test Unit 9
P-Side
J-Side
Shield Braid to
Backshell to
Plug to
Shield Braid to
Backshell to
Mounting Plate to
Backshell
Plug
Receptacle
Backshell
Receptacle
Receptacle
Reading 1
1.7
1.6
1.6
1.7
1.6
1.6
Reading 2
2.8
6.9
1.6
1.6
1.6
1.6
Reading 1
1.6
1.2
1.6
3.5
1.6
1.6
Reading 2
1.7
1.7
1.7
11.0
7.2
1.8
Reading 1
3.9
1.6
1.6
5.6
2.3
1.6
Reading 2
9.6
1.6
21.2
9.6
7.3
5.2
Reading 1
6.8
1.4
2.7
N/A
N/A
1.6
Reading 2
29.7
8.9
16.9
N/A
N/A
1.7
Reading 1
16.0
1.9
1.6
N/A
N/A
1.6
Reading 2
25.0
21.3
1.8
N/A
N/A
2.1
Reading 1
9.1
1.5
1.6
4.8
2.5
1.7
Reading 2
14.6
3.9
9.8
11.2
6.1
5.3
Reading 1
N/A
2.9
1.6
4.8
2.5
1.7
Reading 2
N/A
6.6
5.3
11.2
6.1
5.3
Reading 1
N/A
1.6
1.7
8.1
1.6
1.6
Reading 2
N/A
1.7
7.5
9.7
3.5
4.1
Reading 1
N/A
2.2
1.6
N/A
2.5
1.6
Reading 2
N/A
10.5
5.1
N/A
3.4
7.5
Table 21: 500 Hour Electrical Continuity Reading (Millivolts)
49
4.2.4
Coupling Torque at 500 hours
Test Unit
Unmate Torque
Test Unit
Unmate Torque
1
65” inch lbs.
6
> 300” inch lbs.
2
45” inch lbs.
7
80” inch lbs.
3
60” inch lbs.
8
50” inch lbs.
4
51” inch lbs.
9
60” inch lbs.
5
45” inch lbs.
Table 22: Unmating Torque Values after 500 Hours
Figure 27: Comparison of Torque values at 0 and 500 Hours
Table 22 shows the values recorded to unmate the connectors after the 500 hours
of salt spray. The values recorded are significantly higher than the values the connectors
were torqued to initially. Test units 1, 3, 6, 7 & 9 recorded unmating torques higher than
the maximum disengagement force required after 500 hours of salt spray. The maximum
disengagement torque recommended by MIL-DTL-38999 is 56 inch pounds after 500
hours of salt spray [14]. Initial mate and final unmate torque values are compared in
figure 27. Surprisingly, test unit 1, which is the baseline of this test and is used widely
today, recorded a higher value than all the test units except test units 6 and 7. Test unit 6
did not unmate as the plug was stuck to the receptacle. This could be due to galvanic
50
corrosion. The potential for significant corrosion buildup between the two materials of
the plug and receptacle will cause this to occur. Test unit 6 had an aluminum plug mated
to a stainless steel receptacle.
4.3 Discussion
The test units all performed well at the first set of measurements after exposure
to salt solution. Increase in electrical continuity was seen in the test units with zinc
nickel plating on the connectors or backshell.
Figure 28: Test Unit 1 Backshell with
Figure 29: Test Unit 3 Backshell with
Cadmium Finish on Receptacle after 48
Zinc Nickel Finish on Receptacle after
hours
48 hours
Figure 29 shows a white residue on the zinc nickel backshell that was seen after
48 hours of salt exposure. Earlier in the report, it was indicated that zinc nickel alloys
produce oxide films as a byproduct and the oxide films have high electrical resistance
but acts as a barrier film which reduces dissolution rate. This can be an explanation for
the high continuity measurements that were recorded after 48 hours of salt fog exposure.
Since the oxide films act as a layer between the test probes and the backshell surface,
51
high measurement values were achieved. The non-uniformity of the oxide films allows
for parts of the surface to be free of the film thereby allowing for the lower readings to
be achieved. The lower readings were also achieved when the probes were pressed
harder into the shell thereby cutting through the oxide film. This confirms that even
though the higher continuity measurements were achieved, the substrate was still
protected from the salt environment and the grounding path remained intact. Figure 28
shows no residue on the cadmium backshell which confirms the continuity measurement
that was recorded for test unit 1.
Figure 30: Test Unit 1 Backshell and
Figure 31: Test Unit 3 Backshell and
Plug with Cadmium Finish after 48
Plug with Zinc Nickel Finish after 48
hours
hours
Figures 30 and 31 show the plug side of test units 1 and 3 compared earlier.
Figure 31 shows that the zinc nickel finish backshell and the plug both produced the
white residue earlier discussed. This was the same for the rest of the test units that had
zinc nickel backshells or connectors. The cadmium finish on the other hand still shows
no sign of corrosion. See appendix for figures of all the test units after 48 hours.
52
After 96 hours, there was an overall increase in the continuity readings on
connectors and backshells with the zinc nickel finish from the 48 hour readings. Also
seen was an increase in the white residue on the test units with zinc nickel finish.
Figure 32: Test Unit 1 Backshell and
Figure 34: Test Unit 3 Backshell and
Receptacle with Cadmium Finish after
Receptacle with Zinc Nickel Finish after
96 hours
96 hours
Figure 33: Test Unit 1 Backshell and
Figure 35: Test Unit 3 Backshell and
Plug with Cadmium Finish after 96
Plug with Zinc Nickel Finish after 96
hours
hours
53
Figures 32 through 35 show test units 1 and 3 compared earlier. Figure 34 and 35
show the zinc nickel finish backshell and the connector both produced additional white
residue when compared to 48 hours in figures 29 and 31. This was the same for the rest
of the test units that had zinc nickel backshells or connectors. The cadmium finish on the
other hand began to show a little corrosion near the strain relief clamp, however
continuity measurements were still below the maximum recommended voltage. See
appendix for figures of all the test units after 96 hours. No apparent effects had been
seen at this point of the test at the interface between the zinc nickel finish on connectors
and the mounting hardware. Readings from test unit 3 mating a connector with zinc
nickel finish with another connector with zinc nickel finish show similar values to
connectors with zinc nickel finish mated with connectors with the other finishes in test
units 2, 4, 5, 6, 7, 8 & 9.
After 500 hours of exposing the test fixture to salt fog, significant changes in the
test units were noticed from a visual inspection. The buildup of oxide layers still existed
on most of the connectors and backshells with zinc nickel finish. Through visual
inspections, corrosion was seen on test units with aluminum shell connectors and
backshells with cadmium, electroless nickel finish, and the stainless steel connector with
passivate finish. There was presence of heavy corrosion on the finish and the substrate of
the strain relief clamp on backshells with cadmium and zinc nickel finish. This heavy
corrosion was however not seen on any other backshells and connectors in the study.
This can be attributed to the possibility of inadequate coating of the internal threads in
the clamps of the strain relief. Another observation that brings us to this conclusion is
the fact that cadmium finish, which is known for its high performance in this
environment, showed heavy corrosion on the clamps of the strain relief. This heavy
corrosion on the strain relief clamps was seen on test units 1 and 3 shown in figures 36
through 39. The backshell on the receptacle on test unit 8 did show much more corrosion
of the finish when compared to the other test units (See appendix 7.8). This happened
only on this test unit. Based on visual inspection, one cannot determine how much
degradation had occurred on the substrate, if any, in test unit 8.
54
Figure 36: Test Unit 1 Backshell and
Figure 38: Test Unit 3 Backshell and
Receptacle with Zinc Nickel Finish after
Receptacle with Zinc Nickel Finish after
500 hours
500 hours
Figure 37: Test Unit 1 Backshell and
Figure 39: Test Unit 3 Backshell and
Plug with Zinc Nickel Finish after 500
Plug with Zinc Nickel Finish after 500
hours
hours
55
Overall, continuity measurements, coupling torque measurements and visual
inspections indicate that the requirement of protecting the substrate from corrosion while
keeping the electrical grounding path intact has been demonstrated by zinc nickel
finishes after 500 hours exposure to salt fog. The measurements taken to examine the
interaction of zinc nickel finishes with the different materials and finishes that exist
today are not significantly different from the measurements taken between zinc nickel
finishes and zinc nickel finishes except for the coupling measurements on the stainless
steel connector and the unexplained higher torque value achieved on test unit 7.
4.3.1
Recommendations for Additional Testing
Below are some observed points and recommendations for continued testing that
were not performed in this study.
•
Additional testing can be performed on these test units to collect data at failure
point. This will help determine the limits to which zinc nickel can provide
corrosion protection before the substrate component begins to corrode
•
Test unit 6 was simulated for connectors on engine firewalls where high
temperature might be of a concern. This study was done at a temperature of 35 ±
2ºC. The effect of high temperature is not seen on this test unit. Further testing at
higher temperatures is recommended for this test unit.
56
5. Conclusion
In this report, results of a study undertaken to investigate aerospace circular
connectors and connector accessories, their means of corrosion, their electromagnetic
interference and shielding integrity behavior are presented. As background and for
completeness, a description of existing finishes and proposed alternate finishes to
cadmium is included. Since zinc nickel finishes have long attracted interest in the
industry, they were selected for this study. A summary review of the electrodeposition of
zinc nickel is included. The rapid formation of oxide films was anticipated on the zinc
nickel finish and this is precisely what was observed during testing.
Testing in this study included a salt spray test, an electrical continuity test, a
coupling test and the careful visual inspection of the test units. The electrical continuity
measurements taken at the various time intervals confirmed the presence of oxide films
on the zinc nickel backshells and connectors in this study as voltage across the
measurement points increased from the first set of readings with respect to the value
observed in the untested specimens. At 500 hours, which is the pass or fail point for this
study, heavy corrosion was noticed on some of the test units regardless of the finishes on
the connectors. Continuity measurements at the 500 hour interval ranged from values
lower than the recommended <5.0 millivolts to as high as 29.7 millivolts. Because of
values as low as 1.2 millivolts recorded, a conclusion can be made that the grounding
path between the shield braid and the backshell, the backshell and the plug, the plug and
the receptacle, and the receptacle and the mounting bracket had not been compromised
in all cases. The coupling torque values measured after the 500 hours showed a delta
from the initial values that were significantly higher and in some test units above the
recommended range, one of the test units did not unmate. The upside to this is that
values outside of the recommended range were also recorded for the baseline of the test.
From this study, no significant differences between readings collected at the interface
between zinc nickel finish and itself and the interface between zinc nickel finish and
other finishes were determined.
Based on this, one can conclude that although the zinc nickel finish on aluminum
connectors and backshells did not perform as well as the cadmium finish in this study by
57
not meeting all the benchmarks, it achieved the industry requirement for finishes on
circular connectors and can be used in combination with the different materials and
finishes that exist today.
58
6. References
[1]
Bert Bergsrud. Glenair Inc. “Replacing Cadmium surface plating in electrical systems: the
research and development of Alternate Finishes” Aerospace Electrical Interconnect System
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[2]
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59
7. Appendix
7.1 Test Unit 1
Test Unit 1 Backshell and Receptacle
Test Unit 1 Backshell and Plug with
with Cadmium Finish before exposure
Cadmium Finish before exposure
Test Unit 1 Backshell and Receptacle
Test Unit 1 Backshell and Plug with
with Cadmium Finish after 48 hours
Cadmium Finish after 48 hours
60
Test Unit 1 Backshell and Receptacle
Test Unit 1 Backshell and Plug with
with Cadmium Finish after 96 hours
Cadmium Finish after 96 hours
Test Unit 1 Backshell and Receptacle
Test Unit 1 Backshell and Plug with
with Cadmium Finish after 500 hours
Cadmium Finish after 500 hours
61
7.2 Test Unit 2
Test Unit 2 Backshell and Receptacle
Test Unit 2 Backshell and Plug with
with Zinc Nickel Finish before exposure
Cadmium Finish before exposure
Test Unit 2 Backshell and Receptacle
Test Unit 2 Backshell and Plug with
with Zinc Nickel Finish after 48 hours
Cadmium Finish after 48 hours
62
Test Unit 2 Backshell and Receptacle
Test Unit 2 Backshell and Plug with
with Zinc Nickel Finish after 96 hours
Cadmium Finish after 96 hours
Test Unit 2 Backshell and Receptacle
Test Unit 2 Backshell and Plug with
with Zinc Nickel Finish after 500 hours
Cadmium Finish after 500 hours
63
7.3 Test Unit 3
Test Unit 3 Backshell and Receptacle
Test Unit 3 Backshell and Plug with
with Zinc Nickel Finish before exposure
Zinc Nickel Finish before exposure
Test Unit 3 Backshell and Receptacle
Test Unit 3 Backshell and Plug with
with Zinc Nickel Finish after 48 hours
Zinc Nickel Finish after 48 hours
64
Test Unit 3 Backshell and Receptacle
Test Unit 3 Backshell and Plug with
with Zinc Nickel Finish after 96 hours
Zinc Nickel Finish after 96 hours
Test Unit 3 Backshell and Receptacle
Test Unit 3 Backshell and Plug with
with Zinc Nickel Finish after 500 hours
Zinc Nickel Finish after 500 hours
65
7.4 Test Unit 4
Test Unit 4 Receptacle with Electroless
Test Unit 4 Backshell and Plug with
Nickel Finish before exposure
Zinc Nickel Finish before exposure
Test Unit 3 Receptacle with Electroless
Test Unit 4 Backshell and Plug with
Nickel Finish after 48 hours
Zinc Nickel Finish after 48 hours
66
Test Unit 4 Receptacle with Electroless
Test Unit 4 Backshell and Plug with
Nickel Finish after hours
Zinc Nickel Finish after 96 hours
Test Unit 4 Backshell and Plug with
Test Unit 4 Receptacle with Electroless
Zinc Nickel Finish after 500 hours
Nickel Finish after 500 hours
67
7.5 Test Unit 5
Test Unit 5 Receptacle with Zinc Nickel
Test
Unit
5
Zinc
Nickel
Finish
Finish before exposure
Backshell with Cadmium finish Plug
before exposure
Test Unit 5 Receptacle with Zinc Nickel
Test
Finish after 48 hours
Backshell with Cadmium finish Plug
Unit
after 48 hours
68
5
Zinc
Nickel
Finish
Test Unit 5 Receptacle with Zinc Nickel
Test
Finish after 96 hours
Backshell with Cadmium finish Plug
Unit
5
Zinc
Nickel
Finish
after 96 hours
Test Unit 5 Receptacle with Zinc Nickel
Test
Unit
5
Finish after 500 hours
Backshell with Cadmium finish Plug
after 500 hours
69
Zinc
Nickel
Finish
7.6 Test Unit 6
\
Test Unit 6 Zinc Nickel Backshell and
Test Unit 6 Zinc Nickel Backshell and
plug
plug
mated
with
stainless
steel
mated
with
stainless
steel
receptacle before exposure
receptacle after 96 hours
Test Unit 6 Zinc Nickel Backshell and
Test Unit 6 Zinc Nickel Backshell and
plug
plug
mated
with
stainless
steel
receptacle after 48 hours
mated
with
stainless
receptacle after 500 hours
70
steel
7.7 Test Unit 7
Test Unit 7 Zinc Nickel Backshell and
Test Unit 7 Composite Cadmium finish
Receptacle before exposure
Backshell and Plug before exposure
Test Unit 7 Zinc Nickel Backshell and
Test Unit 7 Composite Cadmium finish
Receptacle after 48 hours
Backshell and Plug after 48 hours
71
Test Unit 7 Zinc Nickel Backshell and
Test Unit 7 Composite Cadmium finish
Receptacle after 96 hours
Backshell and Plug after 96 hours
Test Unit 7 Zinc Nickel Backshell and
Test Unit 7 Composite Cadmium finish
Receptacle after 500 hours
Backshell and Plug after 500 hours
72
7.8 Test Unit 8
Test Unit 8 Zinc Nickel Backshell and
Test Unit 8 Composite Electroless
Receptacle before exposure
Nickel finish Backshell and Plug before
exposure
Test Unit 8 Zinc Nickel Backshell and
Test Unit 8 Composite Electroless
Receptacle after 48 hours
Nickel finish Backshell and Plug after
48 hours
73
Test Unit 8 Zinc Nickel Backshell and
Test Unit 8 Composite Electroless
Receptacle after 96 hours
Nickel finish Backshell and Plug after
96 hours
Test Unit 8 Zinc Nickel Backshell and
Test Unit 8 Composite Electroless
Receptacle after 500 hours
Nickel finish Backshell and Plug after
500 hours
74
7.9 Test Unit 9
Test Unit 9 Composite Electroless
Test Unit 9 Zinc Nickel finish Backshell
Nickel finish backshell on Zinc Nickel
on
finish Receptacle before exposure
exposure
Test Unit 9 Composite Electroless
Test Unit 9 Zinc Nickel finish Backshell
Nickel finish backshell on Zinc Nickel
on Cadmium finish Plug after 48 hours
finish Receptacle after 48 hours
75
Cadmium
finish
Plug
before
Test Unit 9 Composite Electroless
Test Unit 9 Zinc Nickel finish Backshell
Nickel finish backshell on Zinc Nickel
on Cadmium finish Plug after 96 hours
finish Receptacle after 96 hours
Test Unit 9 Composite Electroless
Test Unit 9 Zinc Nickel finish Backshell
Nickel finish backshell on Zinc Nickel
on Cadmium finish Plug after 500 hours
finish Receptacle after 500 hours
76
7.10 Mounting Bracket Detailed Design
Crossed hatched areas were free of primer in order to allow electrical conductivity between the receptacle and the bracket.
77