Evaluation of Coating Methods for Corrosion Protection of Magnesium
Castings
by
Siobhan Fleming
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
August, 2012
1
© Copyright 2012
by
Siobhan Fleming
All Rights Reserved
2
CONTENTS
LIST OF TABLES ............................................................................................................. 4
LIST OF FIGURES ........................................................................................................... 5
ACKNOWLEDGMENT ................................................................................................... 6
ABSTRACT ...................................................................................................................... 7
1. Introduction.................................................................................................................. 8
2. Methodology .............................................................................................................. 16
2.1
Review of Magnesium Alloys .......................................................................... 16
2.2
Review of coatings for corrosion protection .................................................... 16
3. Results and Discussion .............................................................................................. 18
3.1
3.2
Magnesium Alloys ........................................................................................... 18
3.1.1
Alloys for Casting ................................................................................ 18
3.1.2
Alloys for Wrought Parts ..................................................................... 20
Coatings ........................................................................................................... 21
4. Conclusion ................................................................................................................. 28
5. References.................................................................................................................. 30
6. Appendices ................................................................................................................ 32
6.1
Appendix A: Alloying Element Effects ........................................................... 32
6.2
Appendix B: Magnesium Alloy Applications .................................................. 34
6.3
Appendix B: Coating Types ............................................................................. 35
3
LIST OF TABLES
Table 1: American Society for Testing Materials .............................................................. 9
Table 2: Select Magnesium Alloys and Characteristics2 ................................................. 13
Table 3: Mechanical Properties of Mg-9Al-1Zn19........................................................... 19
Table 4: General effects of elements used in magnesium alloys2 ................................... 33
Table 5: Proposed alloys for specific applications .......................................................... 34
4
LIST OF FIGURES
Figure 1: Example of Hexagonal Close Packed Crystalline Structure .............................. 8
Figure 2: Magnesium die cast part................................................................................... 11
Figure 3: Schematic of growth of anodizing coating11 .................................................... 22
Figure 4: Schematic of electroplating process11 .............................................................. 23
Figure 5: Galvanic Corrosion13 ........................................................................................ 26
Figure 6: Salt Spray Exposure13 ...................................................................................... 26
5
ACKNOWLEDGMENT
I’d like to thank Professors Marcin and Donachie for introducing me to the fascinating
world of materials science especially the use of magnesium for aerospace and
automotive applications. I would also like to thank Professor Gutierrez-Miravete for his
support and guidance in completing this project. Finally I’d like to thank my family for
their support and encouragement in completing this final step towards my master of
engineering degree.
6
ABSTRACT
Magnesium is the lightest of all light metal alloys and therefore is an excellent choice for
engineering application when weight is a critical design element. It is strong, has good
heat dissipation, good damping and is readily available. The use of pure magnesium is
rare due to its volatility at high temperatures and it is extremely corrosive in wet
environments. Therefore the use of magnesium alloys when designing aerospace and
automotive parts is critical. Specific alloys are better for certain applications and often
also need a coating to provide the longest life of the part. This paper details specific
alloys used for certain aerospace and automotive applications. Additionally there is a
review of coatings for magnesium alloys and an analysis of alloys and coatings. Finally
it recommends an option for a future coating that may prove to be the best coating for
long lasting corrosion resistant parts.
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1. Introduction
Magnesium is an excellent metal as it is readily available commercially and it is the
lightest of all the structural metals having a density of 1.7g/cm3; it also has good heat
dissipation, good damping and good electro-magnetic shield. It is most commonly
found in the earth’s ocean. At room temperature magnesium and its alloys are difficult
to deform due to the crystal structure which is hexagonal close packed (Figure 1). This
structure restricts its ability to deform because it has fewer slip systems at lower
temperatures. Magnesium has a moderately low melting temperature making it easier to
melt for casting.
Additionally it is relatively unstable chemically and extremely
susceptible to corrosion in a marine environment. It is thought that the corrosion is due
more to impurities in the metal versus an inherent characteristic. Finally magnesium
powder ignites easily when heated in air and must be handled very carefully in a powder
form.
The rest of this section will review the advantages and disadvantages to
magnesium use in engineering applications. In addition, alloy types and an introduction
to coating protections will be discussed.
Figure 1: Example of Hexagonal Close Packed Crystalline Structure
Pure magnesium is rarely used in the manufacturing of aerospace and automotive
parts. In order to be used in manufacturing, it is alloyed with other metals. Some of the
most common alloyed elements in commercial alloys are: aluminum, zinc, cerium,
silver, thorium, yttrium and zirconium. In order to name magnesium alloys, the
American Society for Testing Materials developed a method for designating the alloys.
The first two letters indicate the principal alloying elements according to the code listed
8
in Table 1. The one or two letters are followed by numbers which represent the elements
in weight % rounded to the nearest whole number. For example AZ91 indicates the alloy
Mg-9Al-1Zn.
Code Letter
Alloying Element
A
Aluminum
B
Bismuth
C
Copper
D
Cadmium
E
Rare Earth
F
Iron
G
Magnesium
H
Thorium
K
Zirconium
L
Lithium
M
Manganese
N
Nickel
P
Lead
Q
Silver
R
Chromium
S
Silicon
T
Tin
W
Yttrium
Y
Antimony
Z
Zinc
Table 1: American Society for Testing Materials code for designating magnesium alloys
Magnesium can also be alloyed with rare earth elements, which increase the
strength of magnesium especially at high temperatures.
The key properties of
magnesium alloys are that they are light weight, with low density (two thirds that of
aluminum), and have good high temperature mechanical properties with good to
excellent corrosion resistance.
9
Magnesium alloys are good for engineering applications because they have good
strength, ductility and creep properties. Magnesium alloys have replaced engineering
plastics in many applications because they have a comparable density but are stiffer,
more recyclable and less costly to produce. Magnesium is strong and light, making it an
excellent choice for aerospace applications. Magnesium is also used in a number of
other products such as hand-held devices (chain saws, power tools, hedge clippers), in
automobiles (steering wheels and columns, seat frames, transmission cases, crank case,
camshaft sprocket, gearbox housings), and in audio-video-computer-communications
equipment (laptop computers, camcorders, TV sets, cellular telephones). In particular
cast magnesium alloys have specific design and manufacturing advantages:
1. Castings can be made with thinner walls than aluminum (1-1.5mm versus 22.5mm).
2. Castings cool more quickly due to a reduced latent heat of fusion per unit
volume.
3. High gate pressures can be achieved using moderate pressures due to the low
density of magnesium.
4. Iron from casting dies has low solubility in magnesium alloys, which reduces any
tendency to die soldering.
Magnesium alloy components can be successfully produced with nearly all of the
conventional casting methods. These methods are sand, permanent and semi-permanent
mold and shell, investment and die casting. In early castings it was found that the grain
size tended to be large and variable, which often resulted in more mechanical properties
and microporosity. Not all alloys are suitable for production by all casting methods.
Sand castings are generally used in aerospace applications because they have a clear
weight advantage over aluminum and other metals. Permanent mold casting can also be
used with similar alloys used for sand casting. The advantage over sand casting is the
mold or die can be used repeatedly, but the initial cost of the die is expensive so the
number of parts to be made must be high. Die-casting is ideally suited for high-volume
production parts and typically uses the Mg-Al-Zn type alloys.
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Figure 2: Magnesium die cast part
The disadvantage to using pure magnesium is that it is extremely susceptible to
corrosion. When alloyed, the corrosion resistance is improved, but specific alloys have
been proven to be more corrosion resistant than others. Magnesium is susceptible to
different types of corrosion one type, galvanic corrosion, can sometimes be designed out
of the part. Two ways to protect from galvanic corrosion are: (1) to minimize the
chemical potential difference between the magnesium/magnesium alloys and the
dissimilar materials and (2) maximize the circuit resistance.
This corrosion
susceptibility was greatly reduced with the discovery that small additions (0.2%) of
manganese gave increased resistance. There are also metallurgical factors that affect the
corrosion performance of a magnesium part which are composition and its
corresponding microstructure and the alloy temper/heat treatment. Each of the different
alloys has specific characteristics that are beneficial to different uses. Some alloys such
as AZ91E, WE43B and Elektron 21 are corrosion resistant alloys. Incorporating these
into the design is beneficial for having a part with a longer life.
A selection of
magnesium alloys and characteristics are described in Table 2.
Alloy
Characteristics
AZ63
Good room temperature strength and ductility
AZ81
Tough, leaktight castings with 0.0015 Be, used for pressure diecasting
AZ91
General-purpose alloy used for sand and diecastings
AM50
High-pressure diecastings
11
AM20
Good ductility and impact strength
AS41
Good creep properties to 150ºC
AS21
Good creep properties to 150ºC
AE42
Good creep properties to 150ºC
ZK51
Sand castings, good room temperature strength and ductility
ZK61
As for ZK51
ZE41
Sand castings, good room temperature strength, improved
castability
ZC63
Pressure-tight castings, good elevated temperature strength,
weldable
EZ33
Good castability, pressure-tight, weldable, creep resistant to
250ºF
HK31
Sand castings, good castability, weldable, creep resistant to
350ºC
HZ32
As for HK31
QE22
Pressure tight and weldable, high proof stress to 250ºC
QH21
Pressure-tight, weldable, good creep resistance and proof stress
to 300ºC
WE54
High strength at room and elevated temperatures
WE43
Good corrosion resistance, weldable
M1
Low-to medium- strength alloy, weldable, corrosion resistant
AZ31
Medium-strength alloy, weldable, good formability
AZ61
High-strength alloy, weldable
AZ80
High-strength alloy
ZM21
Medium-strength alloy, good formability, good damping
capacity
ZK30
High-strength alloys
ZK60
Good formability
ZMC711
High-strength alloy
HK31
High creep resistance to 350ºC, weldable
12
HM21
High creep resistance to 350ºC, short time exposure to 425ºC,
weldable
WE43
High temperature creep resistance
WE54
High temperature creep resistance
LA141
Ultra-light weight
Table 2: Select Magnesium Alloys and Characteristics2
While the alloys provide a significant improvement to corrosion resistance, an
additional method to protect the surface of magnesium and its alloys is to coat the
magnesium part. This is specifically beneficial in cases where the part is in contact with
other metal parts and could cause galvanic corrosion. Some examples of protective
coatings are fluoride anodizing, chemical treatments, electrolytic anodizing, sealing with
epoxy resins, standard paint finishes, vitreous enameling, electroplating and cold spray.
While there are a number of advantages and disadvantages to using magnesium
alloys the military has continued to use them for many different applications. Past
applications were commonly aircraft and vehicle structural platforms and lethality
applications. In World War II magnesium was heavily used in aircraft components.
Specifically the B-36 incorporated 8,620Kg of magnesium: 5,555Kg of sheet, 700Kg of
forgings and 300Kg of castings. In 1951 the Sikorsky H-19 “Chicasaw” had the highest
percentage by weight of magnesium castings and sheet of any aircraft then in service at
17%. The M274 “Mechanical Mule” proved that magnesium is a strong metal even
though it is light weight; the cargo carrier weighed only 870lbs and could transport up to
1000lbs for 90-150 miles. Present applications in the military are vehicle and helicopter
transmission housings such as the UH60 Blackhawk transmission. There is still no use
in current lethality or armor applications, but systems are being developed which could
allow for use in those applications. In the future new ground and air vehicle structural
applications should be created, but modern tools need to be used to address the
significant scientific challenges which have prevented prior use.
Some of these
challenges are similar to disadvantages of using magnesium already discussed:
1. High maintenance intervals and long product lifetime are unfavorable due to
corrosion behavior.
13
2. Coated or treated parts can still corrode due to wear, abrasion and mechanical
damage which can initiate corrosion.
3. Joining of dissimilar metals and exposure to moisture due to poor engineering
design.
Some of the coating solutions described by the military include: electrochemical plating,
conversion coatings, anodizing, gas phase deposition, laser surface alloying/cladding,
organics, plasma gel coating and cold spray. Another coating used specifically in the
aerospace industry is rockhard two pack cold cure and single pack stoving epoxy. These
provide maximum corrosion protection to magnesium. They are also formulated free
from heavy anti-corrosive pigments which results in a lower film density than most
protective paints. Finally, due to their high corrosion resistance fewer coats are required,
a single coat instead of two or two instead of three which allows the weight to be
minimized; a critical design component in the aerospace industry.
The biggest
disadvantage to current coatings is they are hazardous to the environment. Common
coatings that fall into the wet methods (conversion film, electrochemical plating,
anodizing, painting and sol-gel) are less expensive but often contain chromates and
cyanide among other toxic carcinogens. Other common coatings that fall into the dry
methods (thermal spray, laser surface alloy, physical or chemical deposition, and solid
diffusion) have less environmental impact but often require special apparatuses that are
expensive. Therefore, while there are numerous coating types and application processes
available, there are significant disadvantages to all existing coatings and no current
coating allows for 100% corrosion protection. Through literature reviews and new
testing there maybe new coatings available that will provide this required corrosion
protection.
Magnesium is an important metal for light weight applications. However, due to
some of the disadvantages previously mentioned it cannot be used without being alloyed,
and alloys still do not eliminate all of the concerns. This paper documents some of the
developments that have been made in making alloys better for aerospace and automotive
applications. As mentioned above the military finds magnesium alloys to be very
important to new product development especially if coatings can eliminate the concern
of corrosion and lead to longer life. Many technical papers have proven one alloy or one
14
coating to be the best for a specific application. This review illustrates why these alloys
have been chosen, and the advantages and disadvantages of each alloy. The results are
summarized in a table in Appendix B. An additional table (Appendix C) summarizes the
advantages and disadvantages to coatings used to protect magnesium parts.
15
2. Methodology
Magnesium alloys are beneficial for use in light weight applications. The purpose of this
paper is to review aerospace and automotive applications for magnesium alloys; focus
on the disadvantage of corrosion and determine how to prolong the life of magnesium
components with coatings. In completing the review, this paper only focuses on the
automotive and aerospace industries because of their need for light weight, strong parts.
There are other areas that magnesium alloys are beneficial, but due to the overwhelming
use in these two industries and the amount of detailed information that was found it was
determined that reviewing and compiling data for magnesium alloy types and coating
types to prevent corrosion for the automotive and aerospace industries would be most
beneficial.
In order to determine the best alloys and coatings for automotive and
aerospace applications materials textbooks and technical journal articles were reviewed.
The review was broken down into two parts to allow detailed study and provide better
conclusions to the best alloys and coatings for specific applications.
In order to
determine conclusions for best materials the results and discussion was formed directly
from the literature review. To expand further it would be beneficial to complete some
experimental studies on alloy and coating type in a salt fog chamber or set up for
galvanic corrosion to further test the theory of best alloy, best coating conclusions.
2.1 Review of Magnesium Alloys
The review of magnesium alloys was completed by reviewing several texts specifically
Light Alloys from Traditional Alloys to Nanocrystals. This provided a detailed list of
many alloys that were later reviewed in technical journal articles for specific industry
applications. The list of alloys and characteristics reviewed can be found in Table 2.
Additionally, Appendix A on page 32 lists alloying elements and their effects on
magnesium. This was specifically used in the review of magnesium alloys that are
particularly good for specific industries.
2.2 Review of coatings for corrosion protection
The Cole Library provided numerous technical articles for review of magnesium alloy
coatings. These were specifically helpful in developing conclusions for industries as
16
they provided specific test results that could not be completed with the resources for this
review. Detailed review was covered on cold spray which is a relatively new promising
coating method for magnesium castings.
17
3. Results and Discussion
The results section is broken into two sections one discussing the most relevant
magnesium alloys for aerospace and automotive applications and the other discussing
the coatings used for the same applications. This allows separation between two critical
design components. Suggestions for best combinations are made in the conclusion
section and documented in table form in Appendix B; therefore separating the results
into the two sections allows for further research to be completed more easily by allowing
one to pick different alloys and coatings from the separate sections to design an
experiment for testing.
3.1 Magnesium Alloys
Some of the magnesium alloys reviewed for this paper are documented in Table 2. The
specific alloys reviewed are detailed in section 3.1.1. Each alloy has specific advantages
and disadvantages depending on the application it will be used for. The results from this
review are specific to automotive and aerospace applications and are further broken
down into alloys for casting and alloys for wrought applications. Some of the parts
overlap and in Appendix B on page 34 some alloys described in the below sections are
listed in table form with the best applications for those alloys.
3.1.1
Alloys for Casting
For castings AZ91 is the most widely used magnesium alloy.
From the naming
convention in Table 1 this is the alloy Mg-Al-Zn. This alloy can be used in both
automotive and aerospace applications and is used specifically for its good casting
qualities and generally satisfactory resistance to corrosion. Additionally it is less costly
in comparison to other magnesium alloys available on the market. The aluminum in the
alloy causes an increase in the tensile strength and hardness of the alloy to a temperature
of 120ºC and improves castability. The disadvantages to this alloy are its susceptibility
to creep at temperatures above 120ºC and that the corrosion resistance is impacted by the
presence of cathodic impurities such as iron and nickel.
18
Alloy
Mg-9Al-
YS
YS
UTS
UTS
Elongation Elongation
(MPa)
(MPa)
(MPa)
(MPa)
(%) 20ºC
(%) 180ºC
20ºC
180ºC
20ºC
180ºC
94
72
157
138
4
14
150
121
250
212
5
11
1Zn (ascast)
Mg-9Al1Zn (T6)
Table 3: Mechanical Properties of Mg-9Al-1Zn19
In order to improve the corrosion resistance higher-purity versions of AZ91 have been
formed and have comparable corrosion rates in testing to some aluminum casting alloys.
For automotive applications where greater ductility and fracture toughness are
required magnesium alloys such as AM60, AM50 and AM20 are used. These are high
purity alloys with reduced aluminum contents and are used in the following automotive
applications: wheels, seat frames and steering wheels.
If silicon is introduced into the Mg-Al alloys, creep properties can be improved.
Two such alloys used in automotive applications are AS41 and AS21, while AS21
performs better with less aluminum AS41 is easier to cast with better fluidity. An
application specific to these alloys was the use in the rear engine of the Volkswagen
Beetle. These alloys were used to replace the cast iron crank case and transmission
housing saving nearly 50Kg in weight. This weight savings was critical for the road
stability of the vehicle.
Alloys that are specifically used in aerospace industry include AZ31 which was diecast for the military Falcon GAR-1 stabilizer fins. Another alloy found in aircraft
landing wheels, gearbox housings, and helicopter rotor fittings is QE22. This alloy has
superior tensile properties over most magnesium alloys which are maintained to 250ºC.
However, this alloy is relatively expensive due to the silver used to make it; attempts
have been made to replace silver with copper with some success although no practical
alloys have been found thus far.
Another alloy widely used in aircraft and automotive industries is Mg-5Y-4RE-Zr.
Some parts cast from this alloy are gear housings of helicopters and parts of engines for
19
racing cars. Similarly to QE22 it is one of the most expensive magnesium alloys due to
the presence of expensive materials in the alloy, which in this case is yttrium versus the
silver in QE22.
3.1.2
Alloys for Wrought Parts
Due to the hexagonal crystal structure of magnesium it has fewer slip systems than face
centered cubic aluminum which restricts its ability to deform; therefore wrought
magnesium alloy products are normally carried out by hot working.
Additionally
extrusion speeds are five to ten times slower than is possible with aluminum alloys.
Instead of describing the specific parts in automotive or aerospace application the best
way to detail the results of the literature review of magnesium alloys is by describing the
wrought product formed.
Sheet and plate alloys are most commonly AZ31 which is the most widely used
magnesium alloy for applications at or slightly above room temperature. Sheets made
from AZ31 have been used for prototype testing for automotive sheet panels, but as the
cost of these panels is very high they are not seen often in cars; however it could offer
unique opportunities in the future.
The strongest alloy for extrusion is AZ81, but the most common general purpose
extruded alloy is AZ61. Magnesium must be extruded five to ten times slower than a
typical aluminum alloy and is therefore, more costly. Similarly to sheet alloys if the cost
of manufacturing can be brought down in the future there may be more opportunities for
use.
Magnesium forgings can only be fabricated from alloys with fine grained
microstructures. They tend to be made from AZ80 and ZK60 for parts that will be used
as ambient temperatures; WE43 is used for forging parts for use at elevated
temperatures. Forgings are important for manufacturing parts that have an intricate
shape and must have strength higher than can be achieved with castings.
It is important to know the capabilities of wrought alloys because future
development could make these parts very important to automotive and aerospace
applications. As noted above both sheet and extruded alloys could very easily be made
into automotive parts but the cost inhibits the ability to be used.
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3.2 Coatings
There are numerous coatings that provide corrosion protection for magnesium parts.
Some that were reviewed and detailed in this section are electrochemical plating,
conversion films, anodizing, gas-phase deposition, laser surface alloying/cladding and
organic coatings. Additionally a typical aerospace coatings process is also discussed.
Appendix C documents all coatings discussed in table form with advantages and
disadvantages. For many applications a single coating process is successful enough to
protect the magnesium alloy from corrosion, but for aerospace application a combination
of coatings is required.
Additionally, cold spray using aluminum particles, a new
coating technique that is still in development, but proving to be very successful for
protection of magnesium parts is discussed. This new coating technique may be the best
new coating for magnesium alloys that is environmentally friendly and long lasting.
As mentioned above there are numerous coatings that have been used to protect
magnesium alloys. These include conversion films, electrochemical plating, surface
coatings and multiple surface treatments.
conversion film.
The first to be discussed is chemical
These are superficial films of substrate metal oxides, chromates,
phosphates or other compounds. These are produced by chemical or electrochemical
treating on a metal surface. The films are then chemically bonded to the metal surfaces.
Chromate conversion coat is the most effective and mature process and is used most
commonly due to its excellent adhesion and corrosion resistance. The downside is that
the Cr6+ in chromate bath is a highly toxic carcinogen and is gradually facing restrictions
preventing its use. An alternative is phosphate treatments. A chromate-free phosphatefluoride conversion film was invented to improve the corrosion rate and compactness of
phosphate films. The challenges with phosphate film are that the grains are coarse and
cracks can occur due to the high activity of magnesium alloy and heavy metal ions in
phosphate solution can cause environmental pollution.
Anodizing is another successful technology for corrosion protection of magnesium
alloys. This is an electrolytic process producing a thick and stable oxide film on the
part. This can also be used to improve paint adhesion to metals or as a passivity
treatment. The two types of anodizing are oxygen precipitation and film forming. The
anodized coat is formed in three stages show in Figure 3. The first is the forming of the
21
compact layer followed by the porous layer and finally the growth of the porous layer.
The properties of the coating depend on various parameters such as electrolyte
composition, voltage and time. One type of anodize treatment is Dow 17 which was
invented by Dow Chemicals. However this contains toxic chromate and therefore the
application has been limits. One disadvantage to anodic coatings on magnesium is the
electrochemical inhomogeneity due to the phase separation in alloys.
Another
disadvantage is that the coatings are brittle and are prone to cracking or shedding after
collision.
Figure 3: Schematic of growth of anodizing coating11
Another successful coating method is plating of magnesium alloys. Plating can
be divided into two categories: electroplating (Figure 4) and electroless plating. The
process is that a metal salt in solution is reduced to its metallic form and deposited on a
part. The difference between electroplating and electroless are that in electroplating the
electrons are supplied by an external power source versus a chemical reducing agent in
the solution. The process for electroplating is:
1. Cations are gathered at the cathode surface by concentration diffusion.
2. Displacement reaction occurs and the cathode and the cations are consumed.
3. Film is formed by the deposition of metal crystal from the displacement reaction
on the substrate surface.
22
Figure 4: Schematic of electroplating process11
The downside to plating is that due to the high chemical activity of magnesium the
plating films have a weak adhesion to magnesium alloys. In order to improve the
adhesion a pretreatment of Cr plating was invented. This was successful, but the Cr
plating solution contains toxic substances. Another difficulty that occurs with plating
that magnesium is very prone to galvanic corrosion. In particular Ni as an impurity in
Mg alloys reduces corrosion resistance severely and is a disastrous element to corrosion
resistance of Mg alloys yet most coatings contain Ni, which must be carefully removed
when parts are recycled.
Sol-gel is another coating method. It is often used instead of electroless plating as
electroless plating can only achieve a relatively uniform metallic layer where sol-gel is
an advanced technique that synthesizes high quality oxide thin films and powders. In one
test TiO2 was applied using sol-gel on a magnesium alloy. This film was chosen because
it has good physical properties, chemical stability, low toxicity and low cost. After
testing it was shown that it could be applied successfully using the sol-gel method and
that the coating can improve the interfacial bonding strength between the matrix and
magnesium alloy which provides a higher efficiency of load transfer from the matrix and
higher mechanical properties of the composite. Finally the flexural strength and flexural
modulus were improved.
The problem with a number of coatings as previously mentioned is their
environmental impact. The chromates found in most of the common chemical methods
23
are environmentally hazardous. One coating that is not hazardous is TAGNITE, which
is a chromate-free, anodic electrodeposition surface treatment. It has been proven useful
in touch up areas on helicopters and in testing comparison with HAE and Dow 17 it has
been shown to have significantly better abrasion resistance, wet paint adhesion, impact
resistance and fatigue properties.
In the testing it was also shown that there is a
significant advantage in salt spray corrosion. While it may be possible to use TAGNITE
as a complete protection it has been proven most useful in fine tolerance areas such as
liner bores and faying surfaces, where organic finishes are prohibited or must be very
thin.
Using TAGNITE improves corrosion resistance and eliminates the use of
environmentally hazardous chromates.
Sometimes more than one surface treatment technology is needed for successful
corrosion protection.
One example is to apply an oxide film by anodic oxidation
followed by a thermosetting resin film and finally a metallic conductive film is formed
using vapor deposition.
This enhances surface characteristics such as corrosion
resistance and conductivity and is similar to what is done in aerospace to provide the
longest lasting life possible with the best corrosion resistance.
A typical coating procedure for an aerospace part would be fluoride anodizing,
pretreatment by chromating or anodizing, sealing with epoxy resin, followed by
chromate primer and top coat. Fluoride anodizing involves using alternating current
anodizing at up to 120V in a bath of 25% ammonium biflouride. The film is then
stripped in boiling chromic acid before further treatment as it does not allow for
adhesion to organic treatments. Electrolytic anodizing deposits a hard ceramic-like
coating which offers some abrasion resistance; examples include Dow 17 and HEA.
These offer little protection in an unsealed state and thus the next step would be to seal
with an epoxy resin. This requires the part to first be heated to 200-220ºC to remove
moisture and then after cooling the part is dipped in the resin solution. In order to build
up the desired coating, heat treatment can be repeated once or twice. After the part is
prepared a standard paint, finish can be applied. The paint should be a chromateinhibited primer followed by a good quality top coat. This is the standard procedure for
aerospace parts and some or all are also used on automotive parts. However, if any of
the coating is damaged in building or in use, it provides no corrosion protection.
24
Therefore it is desirable to have a different coating that provides better, longer lasting
protection.
Cold-sprayed aluminum coatings are being studied in detail as the next best
coating for aerospace applications. For a MH-60 Seahawk that spends a significant
amount of time in an extremely corrosive environment on the deck of a ship, it is critical
that the transmission gearbox housing can stand up to the environment. While it may
theoretically be better to build the transmission housing out of a better material such as
aluminum with better corrosion resistance, the weight of aluminum inhibits this choice.
Therefore, the next best thing is to adhere an aluminum coating using cold-spray to the
entire magnesium housing so that the part now reacts to the environment the same way
as a housing made from aluminum.
Cold spray is also known as cold gas dynamic spraying, high-velocity particle
consolidations and supersonic particle deposition. Coatings are applied in the solid state
at a much lower temperature than plasma spray, which avoids the common problems
associated with traditional thermal-spray methods such as oxidation, evaporation,
melting, crystallization, residual stresses, debonding and gas release problems. In the
cold spray process a carrier gas (N2 or He) is expanded to supersonic speed and sent
through a converging/diverging nozzle. Particles are introduced to the gas flow at the
nozzle inlet and accelerated through the nozzle. Once the particles from the nozzle
impact the part being cold sprayed the particles undergo plastic deformation at very high
strain rates.
Cold spray can be used for numerous different metals, but the most
experimented and best use for magnesium alloys is using aluminum.
Testing has been completed on commercially pure Al, high purity Al, AA5356 and
AA4047. In cases of galvanic corrosion high purity Al performed the best with no
galvanic corrosion when it was cold sprayed onto test pieces of ZE41. Al 5356, Al 4047
and commercially pure Al suffered galvanic corrosion when cold sprayed onto
magnesium test pieces the results were roughly 50 times greater than the current Mg-Mg
couple (See Figure 5).
25
Figure 5: Galvanic Corrosion13
Commercially pure Al and high purity Al were also tested in a salt fog chamber for
28 days and reviewed every 7 days. Again the high purity Al performed the best with
less than 5% weight loss versus nearly 50% weight loss (See Figure 6).
Figure 6: Salt Spray Exposure13
Cold spray has performed well in testing.
The hardness values have been
comparable to the commercial aluminum alloys and much greater than commercially
pure and high purity aluminum. The coating adhesion has been shown to be better when
applied with helium as the gas versus nitrogen. The coating- substrate interface appears
without defects which leads to the conclusion that the coating is expected to perform as
an effective corrosion barrier. Salt fog testing has proved to be successful in one test
26
with an Al-5%Mg coating applied to 0.380mm thickness a 1000hr test was completed
without failure. The only test that has been shown to not be completely successful is
galvanic corrosion.
There has been some improvement depending on the coating
applied, but this appears to be the major concern holding back cold spray from being
widely used.
Cold sprayed aluminum will greatly reduce any other magnesium corrosion issues,
but more work needs to be completed to determine the best aluminum coating to use.
Using a non compatible coating could introduce new corrosion issues on a magnesium
part.
There are numerous somewhat successful technologies for improving corrosion
resistance of magnesium alloys, but there is currently no single technique that can meet
the industry requirements for Mg alloys in different service conditions. There are two
specific types of coating types: dry and wet methods. Dry methods include thermal
spray, laser surface alloy or cladding, physical or chemical deposition, and solid
diffusion. The wet methods are conversion film, electrochemical plating, anodizing or
plasma oxidizing, painting or organic/polymer coating, and sol-gel.
Typically dry
methods are environmentally friendly and are suitable for treating precision. However,
they often require specific special apparatuses that are often very expensive.
Wet
methods are less expensive and suitable for complex and large components used in the
automotive and aerospace industries, but require great effort for waste disposal as many
of the elements are toxic (chromium and cyanide). The future of coatings may be in
more research for cold spray if galvanic corrosion can be avoided and it can be produced
at a low cost. It is critical that the next best coating be low cost, pollution-free, and easy
to recycle.
27
4. Conclusion
Magnesium is a critically important metal in design of aerospace and automotive parts
because of its desirable mechanical properties. The low density, good heat dissipation,
good damping and good electro-magnetic shield all make it a top choice for design of
aerospace and automotive parts. Pure magnesium has a number of good properties, but
it has a lack of corrosion resistance. Magnesium is a top choice for light weight aircraft
parts; however the varying operational environments require a better material.
Therefore, magnesium is alloyed with other materials (metals and rare earth elements) to
provide the best material for the different application.
There are numerous magnesium alloys available and new ones are being tested to
provide the best combinations of properties for specific applications. The selection of an
alloy type depends on how the part will be made (cast or wrought), the strength required
and the operational environment. There are other considerations made in designing each
specific part to help select between several very similar alloys. This paper documents a
number of different alloys that can be used for aerospace and automotive applications
and provides some specific proven alloys for certain uses in Appendix B. There are
many other alloys available as shown in Table 2; however the most commonly proven
good alloys for specific aerospace and automotive applications are available in Appendix
B.
While choosing an alloy that has the best properties for a specific application
improves the life of a magnesium part, coatings are also a critical part of design.
Numerous different coatings are explored in this paper. All provide good corrosion
resistance, but have varying advantages and disadvantages which are documented in
Appendix C.
The biggest disadvantage for most coatings is that they are not
environmentally friendly because they contain chromates. Not only is this coating
difficult to dispose of and hazardous to the health of employees working with it, but
there is an increase in restrictions for use of these materials by the government.
Therefore, it is critical that a new coating be tested and proven successful. This coating
should improve the corrosion resistance of magnesium alloyed parts and be inexpensive
to apply.
28
This paper explores the possibility of using cold sprayed aluminum alloys for
coating magnesium parts.
However, there is inadequate research in cold sprayed
aluminum alloys. In order to use cold sprayed aluminum on flight critical parts it needs
to be well proven. Tests have shown that the corrosion resistance improves significantly
with high purity aluminum. However, the material used for coating galvanic corrosion
remains an issue. The significant advantage to using cold-sprayed aluminum on a
magnesium alloy part is that the low density properties of magnesium are retained and
the corrosion resistance of aluminum is gained. This combination could be extremely
successful for transmission housing for helicopters. If the galvanic corrosion issues can
be eliminated by using a more common aluminum alloy than high purity aluminum, this
will be the most successful combination for aerospace applications.
In order to further this study, an experiment could be designed to test the
combinations in similar environments and prove that they are the best combinations for
specific use. Additionally, further testing of cold sprayed aluminum alloys on different
magnesium alloys to demonstrate galvanic corrosion resistance of those alloys with the
magnesium part and would allow for added trust in using cold sprayed aluminum alloys
on critical parts resulting in parts with ultimate longer life. Continuing development of
new alloys and new coatings will serve to enhance the ability to use magnesium for more
applications allowing designers the choice of an excellent long lasting light weight metal
for automotive and aerospace applications.
29
5. References
1. Made-in-China.com
http://www.made-in-china.com/showroom/yuanlongjason/productdetailIblESqjdSMRB/China-Magnesium-Alloy-Die-Casting.html
2. Polmear, I. Light Alloys from Traditional Alloys to Nanocrystals. Amsterdam:
Elsevier, 2006.
3. Magnesium Alloys – An Introduction,
http://www.azom.com/article.aspx?ArticleID=355
4. ASM Handbook. Volume 15 Casting. Materials Park: ASM International, 2008.
5. Shackelford, James. Introduction to Materials Science for Engineers. Upper
Saddle River: Pearson Prentice Hall, 2005.
6. Callister Jr., William D. Materials Science and Engineering An Introduction.
New York: John Wiley & Sons Inc. 2007.
7. Hexagonal Close Packed Structure.
http://www.miniphysics.com/2010/12/hexagonal-close-packed-structure.html
8. Ying-Liang, Cheng. Comparison of corrosion behaviors of AZ31, AZ91, AM60
and ZK60 magnesium alloys. Transactions of Nonferrous Metals Society of
China: v. 19, pg 517-524. 2009.
9. Li, Juanguo; Xia, Canjuan; Zhang, Yijie; Wang, Mingliang; Wang, Howei.
Effects of TiO2 coating on microstructure and mechanical properties of
magnesium matrix composite reinforced with Mg2B2O5w. Materials and Design,
v. 39, pg 334-337. 2012.
10. Bu, Hengyong; Yandouzi, Mohammed; Lu, Chen; Jodin, Bertrand. Effect of heat
treatment on the intermetallic layer of cold sprayed aluminum coatings on
magnesium alloy. Surface and Coatings Technology, v. 205, pg 4665-4671.
2011.
11. Wu, Chao-yun; Zhang, Jin. State-of-art on corrosion and protection of
magnesium alloys based on patent literatures. Transactions of Nonferrous Metals
Society of China, v. 21, pg 892-902. 2011.
12. Bierwagen, Gordon; Brown, Roger; Battocchi, Dante; Hayes, Scott. Active
metal-based corrosion protective coating systems for aircraft requiring. Progress
in Organic Coatings v. 68, pg 48-61. 2010.
13. DeForce, Brian. Materials Performance: Cold Sprayed Aluminum Coatings for
magnesium aircraft components. Materials Performance, v. 48, pg 40-44. 2009.
14. DeForce, Brian. Cold Spray Al-5%Mg Coatings for the Corrosion Protection of
Magnesium Alloys. Journal of Thermal Spray Technology, v. 20, pg 1352-1358.
2011.
15. Norton, Brian. Transactions of the Institute of Metal Finishing: Aerospace
coatings –A specialist field. Transactions of the Institute of Metal Finishing, v.
84, pg 277-278. 2006.
16. Arruebarrena, G. Materials Science & Technology Conference proceedings:
Weight reduction in aircraft by means of new magnesium castings. Materials
Science and Technology, v. 3, pg 13-20. 2005.
30
17. Duffy, Laurence. Magnesium Alloys: The Light choice for Aerospace. Materials
World, v.4, pg 127-130. 1996.
18. Mathaudhu, Suveen. Magnesium technology: Magnesium alloys in U.S. military
applications: Past, Current and future solutions. Magnesium Technology, pg 2730. 2010.
19. Kiebus, Andrzej. Microstructure and properties of sand casting magnesium alloys
for elevated temperature applications. Diffusion and defect data, solid state data.
Part B, Solid state phenomena, v. 176, pg 63-74. 2011.
20. Wendt, Achim. Magnesium castings in aeronautics applications – Special
requirements. Magnesium technology, pg 269-273. 2005.
21. Guo, Kelvii Wei. A Review of Magnesium/Magnesium Alloys Corrosion and its
Protection. Recent Patents on Corrosion Science, pg 13-21. 2010
22. Hawkins, James H. Assessment of Protective Finishing Systems for Magnesium.
International Magnesium Association, Pg 1-13. 1993.
31
6. Appendices
6.1 Appendix A: Alloying Element Effects
Alloying
Element
Melting and Casting
Behavior
Ag
Al
Improves castability,
tendency to microporosity
Be
Significantly reduces
oxidation of melt surface
at very low concentrations,
leads to coarse grains.
Effective grain refining
effect, slight suppression
of oxidation of the molten
metal.
System with easily
forming metallic glasses,
improves castability.
Ca
Cu
Fe
Magnesium hardly reacts
with mild steel crucibles
Li
Increases evaporation and
burning behavior, melting
only in protected and
sealed furnaces.
Mn
Control of Fe content by
precipitating Fe-Mn
compound, refinement of
precipitates.
System with easily
forming metallic glasses.
Ni
Mechanical and
technological
properties
Improves elevated
temp tensile and
creep props. In the
presence of rare
earths.
Solid solution
hardener,
precipitation
hardening at low
temps.
Corrosion
behavior I/M
produced
Detrimental
influence on
corrosion behavior
Improves creep
properties.
Detrimental
influence on
corrosion behavior
Solid solution
hardener at ambient
temperatures,
reduces density,
enhances ductility.
Increase creep
resistance.
Minor influence
Detrimental
influence on
corrosion behavior,
limitation
necessary.
Detrimental
influence on
corrosion behavior,
limitation
necessary.
Decreases corrosion
properties strongly,
coating to protect
from humidity is
necessary.
Improves corrosion
behavior due to iron
control effect.
Detrimental
influence on
32
Rare Earth
Si
Th
Y
Zn
Zr
Improve castability, reduce Solid solution and
microporosity.
precipitation
hardening at
ambient and
elevated temps;
improve elevated
temp tensile and
creep properties.
Decreases castability,
Improves creep
forms stable silicide
properties.
compounds with many
other alloying elements,
compatible with Al, Zn,
and Ag, weak grain
refiner.
Suppresses microporosity. Improves elevated
temp tensile and
creep properties,
improves ductility,
most efficient
alloying element.
Grain refining element
Improves elevated
temp tensile and
creep properties.
Increases fluidity of the
Precipitation
melt, weak grain refiner,
hardening, improves
tendency to microscopy.
strength at ambient
temps, tendency to
brittleness and hot
shortness unless Zr
refined.
Most effective grain
Improves ambient
refiner, incompatible with temperature tensile
Si, Al, and Mn, removes
properties slightly.
Fe, Al, and Si from the
melt.
Table 4: General effects of elements used in magnesium alloys2
33
corrosion behavior,
limitation
necessary.
Improve corrosion
behavior.
Detrimental
Influence.
Improves corrosion
behavior.
Minor influence,
sufficient Zn
content
compensates for the
detrimental effect of
Cu.
6.2 Appendix B: Magnesium Alloy Applications
Alloy
Application
Notes
AZ91
Cast Helicopter Transmission Less costly, good tensile
Housings
strength, very susceptible
to creep above 120ºC
AM60/AM50/AM20 Automotive
(wheels,
frames, steering wheels)
AS41
Automotive
(crank
transmission housing)
QE22
seat Greater
ductility
and
fracture toughness.
case, Easier than AS21 to cast
with better fluidity
Aerospace (landing wheels, Superior
tensile
gearbox housings, helicopter properties, Expensive due
rotor fittings)
to silver
Table 5: Proposed alloys for specific applications
34
6.3 Appendix B: Coating Types
Coating Type
Advantages
Disadvantages
Chemical Conversion
Most effective and mature Cr6+ in chromate bath is a
process
highly
toxic
carcinogen
facing restrictions with use.
Phosphate Treatments
Chromate Free
Coarse grains can cause
cracks; heavy metal ions in
solution
can
cause
environmental pollution.
Anodizing
Can improve paint adhesion Coatings are brittle and
to metals,
prone
to
cracking
or
shedding after collision.
Plating
electroless)
(electro
and Can
improve
resistance
corrosion Plating films have a weak
depending
on adhesion
to
magnesium
what material is plated on alloys. Issues with galvanic
surface.
corrosion
depending
on
type of metal used.
Sol-Gel
Can achieve better layering Potential
than
electroless
issues
plating. galvanic
with
corrosion
Some testing has proved depending on material used.
very successful using TiO2
TAGNITE
Chromate
free,
abrasion
resistance,
paint
better Has not been proven as a
adhesion,
resistance
wet complete
impact Useful more in touch up
and
fatigue conditions
properties.
Paint
protection.
where
fine
tolerances are required.
Provides a final protective Only provides additional
coating
when
combination
used
of
coating techniques.
35
in protection when built up
other correctly
with
other
coatings. Does not provide
protection when chipped or
cracked.
Cold Spray
Adheres well with fewer Still a new technology, not
issues than plasma spray. widely tested.
Concerns
Testing
corrosion
has
improved
proved with
galvanic
corrosion depending
resistance when high purity material used.
aluminum
is
sprayed material.
36
used
as
on
coating