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
© Copyright 2012
by
Siobhan Fleming
All Rights Reserved
ii
CONTENTS
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT .................................................................................................. vi
ABSTRACT .................................................................................................................... vii
1. Introduction.................................................................................................................. 1
2. Methodology ................................................................................................................ 8
2.1
Review of Magnesium Alloys ............................................................................ 8
2.2
Review of coatings for corrosion protection ...................................................... 8
3. Results and Discussion .............................................................................................. 10
3.1
3.2
Magnesium Alloys ........................................................................................... 10
3.1.1
Alloys for Casting ................................................................................ 10
3.1.2
Alloys for Wrought Parts ..................................................................... 12
Coatings ........................................................................................................... 13
4. Conclusion ................................................................................................................. 20
5. References.................................................................................................................. 21
6. Appendices ................................................................................................................ 23
6.1
Appendix A: Alloying Element Effects ........................................................... 23
6.2
Appendix B: Magnesium Alloy Applications .................................................. 25
iii
LIST OF TABLES
Table 1: American Society for Testing Materials.............................................................. 2
Table 2: Select Magnesium Alloys and Characteristics2 ................................................... 5
Table 3: Mechanical Properties of Mg-9Al-1Zn19........................................................... 11
Table 4: General effects of elements used in magnesium alloys2 ................................... 24
Table 5: Proposed alloys for specific applications .......................................................... 25
iv
LIST OF FIGURES
Figure 1: Example of Hexagonal Close Packed Crystalline Structure .............................. 1
Figure 2: Magnesium die cast part..................................................................................... 4
Figure 3: Schematic of growth of anodizing coating11 .................................................... 14
Figure 4: Schematic of electroplating process11 .............................................................. 15
Figure 5: Galvanic Corrosion13 ........................................................................................ 18
Figure 6: Salt Spray Exposure13 ...................................................................................... 18
v
ACKNOWLEDGMENT
Type the text of your acknowledgment here.
vi
ABSTRACT
Magnesium is an excellent readily available light metal alloy for engineering
applications when weight is a critical design element. It is a strong and light material
that can be cast into thinner parts than aluminum. However, it can be volatile at high
temperatures and extremely corrosive in wet environments. Magnesium parts have
specific benefits that are required in design applications when magnesium is in alloyed
form and has a protective coating. Sometimes only a coating or alloyed magnesium is
needed in design, but both have benefits. When alloys and coatings are introduced into
the design there is more evaluation required by the designer. Coatings may not offer
enough protection depending on applications or may endanger the health and safety of
the part manufacturers or operators.
magnesium
parts
and
the
This paper evaluates different coatings for
advantages
vii
and
disadvantages
to
each
type.
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 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 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 should be handled very carefully in a
powder form.
Figure 1: Example of Hexagonal Close Packed Crystalline Structure
Pure magnesium is rarely used in 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 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.
1
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 it is light weight, low density (two thirds that of aluminum),
good high temperature mechanical properties and good to excellent corrosion resistance.
Magnesium alloys are good for engineering applications because they have good
strength, ductility and creep properties. Magnesium alloys have replaced engineering
plastics in mangy applications because it has a comparable density but is stiffer, more
recyclable and less costly to produce. Magnesium is strong and light, making it an
2
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), 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 semipermenant
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.
3
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. For example as mentioned above
Mg-Al-Zn is the most widely used alloy for castings; however these castings originally
suffered severe corrosion in wet or moist conditions. This corrosion susceptibility was
greatly reduced with the discovery that small additions (0.2%) of manganese gave
increased resistance. Each of the different alloys has specific characteristics that are
beneficial to different uses. 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
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
4
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
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
5
While the alloys provide a significant improvement to corrosion resistance an
additional way to protect the surface of magnesium and its alloys is coating 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 continues to use them for many different applications. Past
applications were commonly aircraft and vehicle structural platforms and lethality
applications, but not in personnel protection or armor 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”
proves 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.
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,
6
organics, plasma gel coating and cold spray. Another coating used specifically in the
aerospace industry is rockhard tow 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.
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
coating to be the best for a specific application. This review illustrates why these alloys
have been chosen, what the advantages and disadvantages are and summarizes results in
a table in Appendix B.
7
2. Methodology
Magnesium alloys are beneficial for use in light weight applications and the purpose of
this paper was 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 the automotive and
aerospace industries were focused on 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 23 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
8
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.
9
3. Results and Discussion
The results section will be broken into two sections one discussing the most relevant
magnesium alloys for aerospace and automotive applications and one discussing the
coatings used for the same applications. This will allow separation between two critical
design components. Suggestions for best combinations will be made in the conclusion
section and document in table form in Appendix B; therefore separating the results into
the two sections will allow 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 25 all alloys described in the below sections are
listed in table form with proposed applications.
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.
10
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 are comparable with corrosion rates in testing for 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.
11
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
racing cars. Similarly to QE22 it is one of the most expensive magnesium alloys due to
the presence of yttrium.
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 and WE43 is used for 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 as future development
could make these parts very important to the automotive and aerospace applications. As
12
noted above both sheet and extruded alloys could very easily be made into automotive
parts but the cost discourages their use.
3.2 Coatings
There are numerous coatings that provide corrosion protection for magnesium parts.
Some that were researched are electrochemical plating, conversion films, anodizing, gasphase deposition, laser surface alloying/cladding and organic coatings. Additionally a
typical aerospace coatings process is also discussed. 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 a new
coating that is still in development, but proving to be very successful for is discussed.
This new coating technique called cold spray using aluminum particles 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
13
anodized coat is formed in three stages show in Figure 3. The first is the forming of the
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 called 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.
14
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 as 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.
Some additional coating technologies are thermal spraying, overlaying welding and
hot-dipping. The layers formed using these methods can be metal alloys, ceramics and
15
paints. More details on these coating methods will be discussed with the cold spray
discussion.
Sometimes more than on 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, some examples include Dow 17 and
HEA. These offer little protection in an unsealed state. 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.
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 UH-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.
16
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 commercial pure Al suffered galvanic corrosion when cold sprayed onto magnesium
test pieces the values tested were roughly 50 times greater than the current Mg-Mg
couple (See Figure 5).
17
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
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 on what is the best aluminum coating to use.
Using a non compatible coating could introduce new corrosion issues on a magnesium
part.
18
There are numerous somewhat successful technologies for improving corrosion
resistance of magnesium alloys, but there is currently no single techniques 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 which 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.
19
4. Conclusion
Magnesium is a critically important metal in design of aircraft and automotive parts
because of its desirable mechanical properties and low density. This literature review
allowed for some judgments to be made based on past testing of best alloys for specific
applications. There are numerous combinations to review when choosing an alloy and
determining how the product will be made as many alloys are best used in castings and
various others are better in a wrought product. Once the alloy has been chosen and the
part made one must determine if the environment of use requires specific coatings.
Based on the corrosion properties and use of most magnesium alloys in corrosive
environments some form of coating is required. A list of potential alloy combinations,
applications and advantages and disadvantages is listed in Appendix B.
This list
provides a useful baseline for alloy choice based on an in-depth literature review. This
review proved that the developments of cold spray as a coating are very important to
using this as the main coating for aerospace applications in the future. By using cold
spray you gain the benefit of using light weight magnesium with the corrosion resistance
of aluminum. If the galvanic corrosion issues can be eliminated by using a more
common aluminum alloys 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 there is constant development
on new alloys and new coatings it is possible in the future that a better coating or better
alloy will be designed to eliminate any of the previous issues seen with the alloys and
coatings reviewed in this paper.
20
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.
21
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.
22
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.
Improves creep
properties.
Solid solution
hardener at ambient
temperatures,
reduces density,
enhances ductility.
Increase creep
resistance.
Corrosion behavior
I/M produced
Detrimental
influence on
corrosion behavior
Minor influence
Detrimental
influence on
corrosion behavior
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
23
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
24
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
25