Application of Aluminum Coatings for the Corrosion Protection of Magnesium by Cold Spray

8/8/2019 Application of Aluminum Coatings for the Corrosion Protection of Magnesium by Cold Spray

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APPLICATION OF ALUMINUM COATINGS FOR THE CORROSION PROTECTION OF

MAGNESIUM BY COLD SPRAY

Brian DeForce, Tim Eden, and John Potter

Applied Research Laboratory

The Pennsylvania State UniversityP.O. Box 30

State College, PA 16804-0030

Victor Champagne, Phil Leyman, and Dennis Helfritch

US Army Research Laboratory

Materials Analysis Group, AMSRL-WM-MDBuilding 4600

Aberdeen Proving Ground, MD 21005-0569

ABSTRACT

Magnesium alloys are frequently used in the fabrication of aircraft components due to their good

mechanical properties and low density. Due to poor corrosion resistance, magnesium alloys require

multi-layer hazardous coating systems and frequent repair or replacement. A low-cost environmentallyfriendly solution is the application of an aluminum barrier coating to the magnesium by the Cold Spray

(CS) process. CS is a low temperature coating technology in which small metal particles (1-50 m) are

injected into a high velocity gas stream to form a dense coating upon impact with a substrate.

In the present study, Cold Spray coatings of aluminum alloys (commercially pure Al (99.5%),

high purity Al (99.95%), Al 5356, and Al 4047) were applied to ZE41A-T5 magnesium. The coatingswere evaluated for corrosion resistance by electrochemical testing to determine pitting potential, ASTM

B117 Salt Spray, and ASTM G71 Galvanic corrosion. Coating adhesion was evaluated by ASTM C633.

The coatings were demonstrated as effective corrosion barriers to salt water corrosion. HP Al coatingsprovided the best galvanic compatibility with Mg, showing essentially no galvanic effect. Coatings of

CP Al, 5356 Al and 4047 Al resulted in galvanic currents almost 50 times greater.

Keywords: magnesium, aluminum, cold spray, coating, corrosion, galvanic, pitting


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INTRODUCTION

Magnesium alloys are frequently used in the fabrication of aircraft components due to their good

mechanical properties and low density.1, 2 Magnesium alloys are attractive for the manufacture of

complex shapes such as transmission gearboxes, in that they are readily cast and machined.2

However,the corrosion resistance of Mg alloys is very poor in wet salt-laden environments.3, 4 Furthermore, it is

highly susceptible to galvanic corrosion when coupled to another metal.1, 5 The undesirable corrosion

properties of Mg alloys are due to magnesium being the most electrochemically active structural metaland the detrimental effect of many alloying elements on the corrosion resistance of Mg.

1, 6The

corrosion resistance of magnesium alloys often results in high maintenance costs or a limited lifecycle

for many aircraft system and exclusion from other potential applications.

Environmental and Safety Issues

Due to the poor corrosion resistance of magnesium alloys, chemical pretreatments are often

employed to improve the performance. Common pretreatments include hard anodizing, chromateprimers, and epoxy or resin sealants.1, 5, 7 Application of these pretreatments involves hazardous

chemicals (chromates, fluorides, heavy metals) and present environmental, health, and safety problems.

Regardless of the pretreatment used, the corrosion resistance of magnesium alloys is still unsatisfactoryfor many applications.8

Helicopter Transmission Gearboxes

The transmission gearboxes of the UH-60 Seahawk are made from ZE41A-T5 magnesium alloy.

The surface of the gearbox is hard anodized, followed by a resin coating, a chromate primer, and epoxypaint. During use, the magnesium alloy is subject to corrosion and significant material loss can occur.

When the damage reaches critical levels, the component must be replaced with a new or refurbished

component. Corrosion problems result in high costs of the inspection, repair, and replacement of Mg

gearboxes. Additionally, aircraft availability is impacted. It is anticipated that the need for repair will begreater in the MH-60S Seahawk since it operates in more environmentally challenging environments.

Cold Spray

A process is needed that can apply a coating which will provide protection against corrosion and

pitting but does not adversely affect the base material. Magnesium alloys, such as ZE41A-T5, are very

susceptible to damage from excess heat. Additionally, magnesium readily reacts with molten materialdeposited by conventional thermal spray methods. A low-cost environmentally friendly solution is the

application of an aluminum barrier coating by Cold Spray (CS),9 Cold Gas Dynamic Spraying,10 High

Velocity Particle Consolidation (HVPC),11

and Supersonic Particle Deposition (SPD).12

In this process,

coatings are applied in the solid state at a greatly reduced temperature, as compared to plasma spray.

In the CS process, a carrier gas, usually nitrogen (N2) or Helium (He), at pressures as high as 3.4MPa and temperatures up to 800C, is expanded to supersonic speed through a converging diverging

nozzle. Particles are introduced in the gas flow at the inlet of the nozzle. The particles are accelerated

through the nozzle and leave with speeds as high as 1100 m/s. Particle velocity is a function of particle

size, density, carrier gas type, pressure, temperature, and nozzle design. The particles impact a substratelocated approximately 25 mm from the exit of the nozzle. Upon impact, the particles undergo plastic

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deformation at very high strain rates. The impact process produces a bond between the particles and the

substrate and between particles. The process continues until the desired coating thickness is realized.

This process is ideal for applying coating to heat-sensitive materials such as magnesium alloys.

Objective

The objectives of this project were to apply a corrosion resistant barrier coating of aluminum to

ZE41A-T5 Mg using CS and to evaluate the corrosion resistance and adhesion strength of the coating.

The proposed concept to prevent and permit repair of the magnesium is anticipated to have no negativeimpact of the function of the base magnesiums ability to react to static and fatigue applied loads whenin service.

EXPERIMENTAL

Coating Application

Materials. Aluminum coatings were applied to 10.16 x 15.24 x 6.35 cm plates (4 x 6 x 0.25

inch) of cast ZE41A-T5 Mg by CS. The composition of the Mg plate is listed in Table 1. Prior to

application of the coating, the panels were grit blasted and degreased with ethanol. Powders from four

aluminum alloys were used to create the coatings in this study: commercially pure (CP) Al (99.5%),high purity (HP) Al (99.95%), 5356 Al (5 wt. % Mg), and 4047 Al (12 wt. % Si). The compositions are

provided in Table 2. Alloy 5356 Al was selected based on its compatibility with Mg. Alloy 4047 Al

was selected for potential enhanced wear properties. Particle size distribution was determined for eachpowder with a particle size analyzer. The powders were examined with a scanning electron microscope

(SEM) and micrographs were obtained.

Cold Spray System. The CS system is shown schematically in Figure 1. Nitrogen or helium is

used as the carrier gas. A 3000 gallon nitrogen tank is used to store bulk liquid nitrogen before it isdiffused through fin vaporizers. It enters the lab at approximately 1.24 MPa. Inside the building, a

compressor is used to boost nitrogen pressure to 3.45 MPa inside a 400 gallon tank. High pressure

helium is supplied in 12-packs at 2400 psi (16.54 MPa). The high pressure nitrogen (or helium) is thencontrolled by a gas control cabinet.

Prior to coating application, bulk metallic powders are dried in an oven at 150F (66C) for a minimum

of 24 hours. The powders are then placed into a powder feeder mounted on a scale interfaced to a feed-

rate controller. The powder feed line is routed through the heater box to the prechamber,converging/diverging (De Laval) nozzle, and heater box. The heater box is an electrical resistance

heater. A power source provides voltage and current managed by a temperature controller. The nozzle

position is controlled by an inverted robotic arm (Figure 2).

Table 1 - Composition of Mg ZE41A-T5 (wt. %) (From vendor supplied materialcertification)

Element Mg Zn Rare

Earths

Zr Mn Cu Ni

Wt. % Remainder 4.19 1.25 0.79 0.02 < 0.01


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Table 3 Cold Spray process parameters

Parameter aluesV

Powder size -42 m6

Carrier gas type gen, HeliumNitro

Gas pressure 0-350 psi (2.030 7-2.41 MPa)

Gas temperature 5C, N2: 30He: 2 0-425C

Powder feed rate m2-6 rp

Coating thicknes inch (0-1s 0-0.045 .14 mm)

Nozzle traverse r m/sate 50, 100 m

Nozzle stand-off (6distance 0.25-1.00 inch .35-25.4 mm)

Index 1 mm-2 mm

Coating Evaluation

Coatings applied by CS were evaluated for thickness, hardness, adhesion strength, and visualappearance of the cross-section. The coating evaluation was used as a screening tool for more

comprehensive corrosion testing; therefore, not all coating conditions were evaluated.

Thickness. Coating thickness was determined by measuring the panel thickness before and after

coating application at four specific locations with a 1-inch micrometer. The coating thickness was

averaged from measurements at the locations.

Hardness. Vickers scale hardness (100 gm. load) was measured along the cross-section of the

aluminum coatings. Measurements were obtained at 0.007 inch (0.178 mm) from coating interface with

0.015 inch (0.381 mm) spacing between measurements. Each reported value is the mean of 15measurements. Coatings examined were CP Al (18 m), CP Al (42 m), 4047 Al, and 5356 Al.

Coating Adhesion (Bond) Strength. Coating adhesion strength using ASTM C633 was

determined for three coating materials (CP Al 18 m, 5356 Al, 4047 Al). Test samples were preparedby applying an aluminum coating of approximately 0.030-0.040 inch (0.76-1.02 mm) thickness to a 1

inch (25.4 mm) diameter cylindrical magnesium test slug. The coating was machined to a uniform

thickness of 0.020 inch (0.51 mm). The coated slug was then degreased and bonded to an uncoatedmagnesium slug using adhesive film. Testing was performed with an automated tensile testing system

using 0.030 in/min (0.76 mm/min) cross-head travel rate applying increasing tensile load at constant

rate. Five samples were tested for each coating evaluated.

Microscopy. Samples were prepared for optical microscopy examination by mounting, cross-

sectioning and polishing. An optical microscope was used to examine the coating interface andmicrographs were obtained. Coatings examined were CP Al (18 m), CP Al (42 m), 4047 Al, and

5356 Al.

Corrosion Testing

The corrosion resistance of the coatings was evaluated by electrochemical polarization, open

circuit galvanic coupling, and accelerated salt spray corrosion testing.

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Potentiodynamic Polarization. Samples measuring 1.0 inch2 ( 6.45 cm2) were cut from the CS-

coated panels. A coated copper wire was attached to the uncoated side of the samples. Each sample was

masked with two coats of an organic maskant. The CS aluminum coating was exposed by removing2.79 cm2 of the maskant. The corrosion resistance was evaluated in deaerated 3.56% NaCl. Polarization

was performed with a potentiostat in a Greene cell (600 ml solution) with two graphite counter

electrodes and a saturated calomel reference electrode placed in a Luggin probe. After 10 minutes insolution, samples were polarized from open circuit to a maximum of 500 mVSCE. The polarization was

stopped if the current exceeded 1 mA/cm2. The pitting potential was defined as the potential at which

the current equaled 0.1 mA/cm2. The corrosion current was estimated from the polarization curve.

Galvanic Corrosion (ASTM G71). Coatings of CP Al (18 m), HP Al (23 m), 4047 Al, and

5356 Al were evaluated for galvanic compatibility with ZE41A-T5 Mg. Coating thickness ranged from

0.432-0.533 mm (0.017-0.021 in). Samples were prepared as described for the potentiodynamicpolarization testing with the exception that the exposed area of the cathode was 3.17 cm2 and the anode

(uncoated Mg) was 2.79 cm2. The two electrodes were connected in solution using the Gamry Series G

300 potentiostat/zero resistance ammeter and the current and potential were measured for 7200 seconds.

Salt Spray (ASTM B117). Accelerated salt spray corrosion testing was used to evaluated CS CP

Al coatings of varying thicknesses (0.001-0.045 inch, 0.025-1.143 mm) (HP Al, 5356 Al, and 4047 Al

coatings will be evaluated by ASTM B117 in future work). Samples measuring 3 x 4 inch (7.62 x 10.16cm) were cut from CS coated panels. The uncoated magnesium alloy (back and sides) was masked with

a non-chromate paint primer (MIL-PRF-23377NC). The masked samples were placed in a corrosion

chamber and exposed to a 5% NaCl fog per ASTM B117. Samples were examined periodically forsigns of corrosion and removed from the chamber when the Mg was observed to corrode through the Al

coating.

Simulated Material Restoration

To simulate material loss due to corrosion of Mg, material was removed from a ZE41A-T5 Mg

plate (4 x 6 inch, 10.16 x 15.24 cm) using multiple drill bits (4, 6, 10, 12 mm) to create holes 4 mm

deep. Prior to deposition, the edge of the holes were rounded slightly to improve fill efficiency.Multiple passes of CS CP Al (18 m) was then used to fill the holes. The excess material from the

coating build-up was mechanically removed to expose the Mg and the filled holes were examined.

RESULTS

Powder Analysis

A graph of the particle size analysis results is shown in Figure 3 and the mean particle size of

each powder is listed in Table 4. A visual inspection of the graphs shows the distribution for each

powder approximates a guassian distribution. Powders ranged in size from 6 to 42 m (mean diameter).

SEM images for the powders are shown in Figures 4a-h. The particle shape for all powders is sphericalwith the exception of the 26 m Al CP and the 42 m CP Al which have an elliptical shape.

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Particle Size Distribution - Al Powders

0

2

4

6

8

10

12

14

1 10 100 1000

Particle Diameter (microns)

Volum

ePercent

CP Al (Al 101)

5356 Al

CP Al (13/325)

HP Al (9-40 HP)

CP Al (H-12)4047 Al

HP Al (-20)

CP Al (H-5)

Figure 3 Graph of particle size distributions of aluminum powders

Table 4 Measured mean particle size of Al powders

Material Product

Identifier

Mean Particle

Size (m)

CP Al AL-101 42

CP Al 13/325 26

CP Al H-12 18

CP Al H-5 6

HP Al 9-40 HP 23

HP Al -20 HP 8

Al 5356 5356 29

Al 4047 S-8 13

Figure 4b Commercially pure 18 m

aluminum powder, H-12 (1000x)Figure 4a Commercially pure 6 m aluminum

powder, Al-168 (1000x)

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Figure 4c Commercially pure 26 m

aluminum powder, -325 mesh (1000x)

Figure 4d Commercially pure 42 m

aluminum powder, (1000x)

Figure 4e 5356 Al powder, 29 m (1000x)

Figure 4f 4047 Al powder, 13 m S-8 (1000x)

Figure 4g HP Al, 8 m (1000x)

Figure 4h HP Al, 23 (1000x)

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Cold Spray Parameters

A representative CS coating is shown in Figure 5. A SEM image of the coating is shown inFigure 6. Coatings were produced for all of the powders evaluated with the exception of the 6 and 8 m

powders. The small particle size resulted in powder flow limitations that were not resolved with

mechanical vibration. Nitrogen was used to create CP and HP Al coatings. However, no depositionoccurred when spraying 4047 Al and 5356 Al with nitrogen. For these powders, coating deposition

required He as the carrier gas. Since the alloyed powders are harder than pure Al, a higher particle

velocity is required to achieve deposition. Attempts were made to increase the particle velocity of 5356by reducing the mean particle diameter by sieving. The particle size was reduced from 29 to 14 m, yetdeposition did not occur with the reduced powder size.

Figure 5 Example of cold spray Al (CP Al 18m) ZE41A-T5 Mg panel (2x3 inch, 5.1x7.6 cm

section)

Figure 6 SEM of CP Al (18 m) cold spray coatingon ZE41A-T5 Mg

Hardness

Results of the hardness testing are listed in Table 5. Increasing the alloying content yielded anincrease in hardness as is typical with aluminum. The HP Al coating had the lowest hardness at HV0.100

= 48.0. The value for the 42 m CP Al (HV0.100 = 54.6) powder is less than the 18 m powder (HV0.100

= 63.2) which is indicative of the greater porosity. The coatings from the alloyed powders are much

harder than the pure Al coatings (4047 Al: HV0.100 = 128.2, 5356 Al: HV0.100 = 140.7). This isconsistent with the increased hardness of aluminum alloys over pure aluminum in wrought and cast

alloys. For comparison, the HV values are 72.5 for the Mg substrate and 26 for wrought 1100 Al (99%)13

. The greater hardness of the CS Al coatings over the bulk alloy is expected due to the strainhardening during the coating process.

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Table 5 - Vickers hardness (HV0.100) results for cold spray Al coating on ZE41A-T5 Mg

Coating Mean Std. Dev. 95% Confidence

CP Al (18 m) 63.2 3.7 (61.2, 65.3)

CP Al (42 m) 54.6 4.5 (52.1, 57.1)

HP Al (23 m) 48.0 2.7 (46.0, 49.9)

4047 Al 128.2 4.9 (125.5, 131.0)

5356 Al 140.7 5.6 (137.6, 143.8)

Substrate (Mg) 72.5 3.5 (72.5, 78.4)

Microscopy

The micrographs of the CS aluminum coatings are displayed in Figure 7. From these images it is

apparent that the CP Al 18 m and the 4047 Al (13 m) coatings have little porosity. The 5356 Al (29m) coating contains voids CP Al 42 m coating has significant porosity. The porosity in this case is

probably due to the large particle sizes, which leads to a lower particle velocity during spraying.

Figure 7 Micrographs of cross-section of cold spray aluminum coatings on ZE41A-T5

Mg (coating on top), A - CP Al 18 m, B CP Al 42 m, C 4047 Al (Helium), D

5356 Al (Helium)

Adhesion Strength

The mean bond strength for the coatings tested is listed in Table 6. The average bond strength in

all coatings tested exceeded 5 ksi. The value exceeds the minimum requirement of 2 ksi (13.8 MPa) ofMIL-STD-2138A Metal Sprayed Coatings for Corrosion Protection Aboard Naval Ships. The average

bond strength in all coatings tested exceeded 5 ksi (34.47 MPa). However, in all samples tested, the

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adhesive film (between the coating and the second bond slug) failed. No failures occurred between the

HPVC coating and the Mg substrate. Additional work has been performed to optimize the adhesion of

the FM1000 and future work is planned to repeat the adhesion testing to determine the actual adhesionstrength of the CS coatings.

Table 6 Adhesion strength of cold spray coatings on ZE41A-T5 Mg per ASTM C633

Powder Max Stress

(ksi)

Std. Dev. (ksi) 95%

Confidence

Failure Mode

CP Al 18 m 6.265 0.323 5.865, 6.664 Epoxy5356 Al 5.136 0.769 4.181. 6.092 Epoxy

4047 Al 5.481 0.983 4.890, 6.655 Epoxy

Corrosion Testing

Electrochemical. Typical polarization curves are shown is Figure 8. A decrease in current of

two orders of magnitude is observed when comparing the CP Al coated Mg (thickness 0.406 mm, 0.016inch) to bare Mg. The variation in corrosion current and pitting potential with coating thickness for

HPVC CP Al (18 m) on Mg ZE41A-T5 is shown in Figure 9 (data listed in Table 7). The current

decreases and the pitting potential increases with increasing thickness until 0.279-0.406 (0.011-0.016inch). This suggests the minimum coating thickness to provide barrier protection is approximately 0.406

mm (0.016 inch). A variation in corrosion current with aluminum particle size was observed as listed in

Table 8. The current for CP Al - 18 m coated panel is an order of magnitude less than 42 m, which ispresumably due to the lower porosity of the 18 m as discussed previously. The variation in corrosion

current with aluminum powder type is also apparent (Table 8). The corrosion current for CP Al is an

order of magnitude less than the coatings from alloy powders. This is expected as any alloy additions

tend to decrease the corrosion resistance of pure Al.

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

1.00E-10 1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00

Current Density (A/cm2)

Potential(V

SCE)

No coating

0.0041 in

0.0160 in

Figure 8 Representative polarization curves for cold spray CP Al (18 m) coatings of

varying thickness on Mg ZE41A-T5 plate in 3.56% NaCl

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0.01

0.1

1

10

100

1000

0 5 10 15 20 25

Coating Thickness (x 100 in.)

CorrosionCurrent,i

(microA/cm2)

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Pitting

Potential,Ep(VSCE)

i

Ep

Figure 9 Variation in corrosion current and pitting potential with coating thickness for

cold spray CP Al (18 m) on Mg ZE41A-T5

Table 7 - Variation of Corrosion Current and Pitting Potential with Coating Thickness forCP Al (18 m)

Coating Thickness Corrosion Current

(A/cm2)

Pitting Potential (VSCE)

No Coating 1000 -1.5 (no passivation)

0.0002 526 -1.5 (no passivation)

0.0041 80 -1.4

0.0073 2.50 -0.735

0.0106 0.310 -0.727

0.0163 0.066 -0.723

0.0211 0.307 -0.722

Table 8 - Variation in Corrosion Current and Pitting Potential with Aluminum ParticleSize (Coating thickness = 0.017 in, 0.432 mm)

Particle Size (m) Corrosion Current

(A/cm2)

Pitting Potential (VSCE)

18 0.217 -0.723

42 2.30 -0.728

Table 9 - Variation in Corrosion Current with Aluminum Powder Type (Coating

thickness = 0.017-0.021 in, 0.432-0.533 mm)Aluminum Alloy Corrosion Current

(A/cm2)

Pitting Potential (VSCE) for reference

CP Al (18 m) 0.1 -0.722

5356 2.5 -0.742

4047 3.0 -0.717

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Galvanic. The galvanic currents from an Al coated Mg/bare Mg couple are shown below (Figure

10, Table 10). There was no statistical difference in the currents from the Mg couples of CP Al, 5356

Al, and 4047 with the average current densities ranging from 2.03 to 2.33 mA/cm2. These values are

roughly 50 times greater than the current a Mg-Mg couple. The current from the HP Al-Mg couple is

significantly less than the other coatings, yielding a current that is comparable to the rate of the Mg-Mg

couple. In other words, no galvanic corrosion occurred when HP Al was coupled to Mg. Thecompatibility of CS HP Al with Mg is consistent with the work of Bothwell 14 on Al cladding. Bothwell

observed rapid corrosion of AZ31A Mg clad with 99% Al, whereas negligible corrosion occurred when

99.99% Al was used as cladding.

Galvanic Corrosion - Al-Mg Couple

0.01

0.1

1

10

None (Bare

Mg)

HP Al (29

m)

5356 4047 CP Al (18

m)

Coating Material

Current(A

/cm2)

Figure 10 - Galvanic Corrosion Rate of Cold Spray Coated Sample coupled to UncoatedMg Sample (Coating thickness = 0.017-0.021 in, 0.432-0.533 mm)

Table 10 - Galvanic Corrosion Rate of Cold Spray Coated Sample coupled to Uncoated

Mg Sample (Coating thickness = 0.017-0.021 in, 0.432-0.533 mm)

Coating Current (mA/cm2) Std. Dev. 95% CI

None (Bare Mg) 0.046 0.03 0.00, 0.09

HP Al (29 m) 0.029 0.01 0.00, 0.04

5356 2.03 0.34 1.61, 2.46

CP Al (18 m) 2.33 0.38 1.86, 2.814047 2.24 0.14 1.89, 2.58

Salt Spray. For coatings with thicknesses less than 0.010-0.015 inch (0.254-0.381 mm), the

aluminum coatings did not provide adequate barrier protection. Within 24 hours of exposure the Mgsubstrate was corroding through large pits in the aluminum coating. The onset of Mg corrosion is very

obvious with the rapid formation of dark corrosion products. For coating thickness greater than 0.015

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inch (0.381 mm) effective barrier protection was achieve with the coating exceeding 500 hours in the

ASTM B117 Salt Spray test without any sign of Mg corrosion. The critical thickness of approximately

0.015 inch (0.381 mm) is consistent with the electrochemical results discussed above.

Simulated Material Restoration

Figure 11 shows the restored panel after deposition and machining of the coating. For

comparison, the as-drilled panel is also displayed. The holes have been filled with CS CP Al (18 m) by

repeat spray passes until the holes appeared to be filled. The coating thickness was 6.8 mm (measuredfrom the substrate surface). The 6 and 10 mm holes were completely filled and no evidence of theoriginal hole was visually apparent. The 12 mm hole had several small voids along the surface edge and

the 4 mm hole shows a large void along the majority of the surface edge. As mentioned previously,

prior to application of the coating, the edges of the holes were rounded slightly by a grinding process.The presence of the surface voids suggests additional work is needed to determine the minimum edge

radius and aspect ratio for restoration work.

A

B

Figure 11 Material restoration example. A - ZE41A-T5 Mg panel with machined holes

prior to application of cold spray coating. B Panel after application of CP Al coating to

fill holes and subsequent machining to expose substrate surface.

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Cost

Coatings produced with 4047 Al and 5356 Al powders required He as a carrier gas whichsignificantly increases the cost of applying the coating. The cost of He gas is 32 times that of N2.

However, the cost of He will be offset somewhat by the high deposition efficiency when spraying with

He and can be further reduced with the implementation of a He gas recycling system. Use of HP Alover CP Al increases the powder cost by roughly four times. Considering the galvanic compatibility of

HP Al with Mg, this increase in cost may be justifiable for many applications.

CONCLUSIONS

Cold spray was successfully employed to create a corrosion resistant barrier coating of aluminum

on a ZE41A-T5 Mg substrate and CS was demonstrated as a method of material restoration for Mgpanels. Coatings of CP Al, HP Al, 5356 Al and 4047 Al were produced.

From electrochemical polarization and accelerated salt spray testing (ASTM B117), theminimum coating thickness of CP Al (18 m) required to provide an effective barrier was determined to

be approximately 0.015 inch (0.38 mm).

HP Al coatings provided the best galvanic compatibility with Mg, showing essentially nogalvanic effect. Coatings of CP Al, 5356 Al and 4047 Al resulted in galvanic currents almost 50 times

greater.

FUTURE WORK

Optimization of the cold spray process parameters will continue. Additional corrosion testingwill be performed to determine the minimum thickness of HP Al to provide barrier protection and to

evaluate the crevice corrosion resistance of the CS coatings. Adhesion testing will be repeated withimproved application of the film adhesive. Mechanical testing of the coatings will be performed to

include a duplication of previous adhesion testing (with improved film adhesion) as well as tensile and

fatigue testing. Finally, investigation of CS as a restoration tool will continue to determine the minimumedge radius and aspect ratio for restoration work and CS will be performed on helicopter components.

ACKNOWLEDGEMENTS

This research was sponsored by the United States Navy Manufacturing Technology (ManTech)

Program, Office of Naval Research, under Navy Contract N00024-02-D-6604. Any opinions, findings,

conclusions, or recommendations expressed in this material are those of the authors and do notnecessarily reflect the view of the U.S. Navy.

REFERENCES

1. Shaw, B. A.; Wolfe, R. C., Corrosion Resistance of Magnesium Alloys. ASM International:Materials Park, 2005; Vol. 13B, p 205-227.

2. Polmear, I. J.,Introduction to Magnesium ASM International: Materials Park, 1999; p 3-11.

3. Song, G. L.; Atrens, A., Corrosion mechanisms of magnesium alloys.Advanced engineering

materials 1999, 1, (1), 11-33.

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4. Ghali, E.,Magnesium and Magnesium Alloys. Second ed.; John Wiley & Sons, Inc.: New York,

2000; p 793.

5. Gray, J. E.; Luan, B., Protective coatings on magnesium and its alloys - A critical review.

Journal of alloys and compounds 2002, 336, (1), 88-113.

6. Hanawalt, J. D.; Nelson, C. E.; Peloubet, J. A., Corrosion Studies of Magnesium. Trans AIME

1942, 147, 273-299.7. Hawkins, J. H., Assessment of Protective Finishing Systems for Magnesium. In 50th Annual

World Magnesium Conference, Washington, D.C., 1993.

8. Makar, G. L.; Kruger, J., Corrosion of magnesium.International Materials Reviews 1993, 38,(3), 138-153.9. Dykhuizen, R. C.; Smith, M. F., Gas dynamic principles of cold spray.Journal of Thermal SprayTechnology 1998, 7, (2), 205-212.

10. Tokarev, A. O., Structure of aluminium powder coating produced by cold gas dynamicdeposition.Metallovedenie i Termicheskaya Obrabotka Metallov 1996, (3), 36-39.

11. Amateau, M. F.; Eden, T., High Velocity Particle Consolidation Technology. iMast Quarterly

2000, (No. 2), 306.12. Champagne, V., Helfritch, D., Leyman, P., Grendahl, S., Klotz, B., Interface Material Mixing

Formed by the Deposition of Copper on Aluminum by Means of the Cold Spray Process. Journal ofThermal Spray Technology 2005, 14, (3), 330-334.

13. Matweb Material Property Data. In.14. Bothwell, M. R., Galvanic Relationships between Aluminum Alloys and Magnesium Alloys II.

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Application of Aluminum Coatings for the Corrosion Protection of Magnesium by Cold Spray

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