CORROSION BEHAVIOR OF ALUMINUM ALLOY AA7075 COLD SPRAYED COATINGS

Mr. Ozymandias B. Agar, The University of Alabama

Ms. A. Chappell Alex, The University of Alabama

Assoc. Prof. Luke N. Brewer, The University of Alabama

Keywords: cold spray deposition, corrosion repair, pitting, aluminum

ABSTRACT

This presentation will discuss the corrosion behavior of cold spray deposited AA7075 coatings. High strength alu-minum alloys such as AA7075 can suffer pitting, crevice, and galvanic corrosion around fasteners in marine envi -ronments. Components damaged by corrosion are increasingly repaired by removing the damaged material and then depositing new material by cold spray (CS) deposition. CS uses supersonic gas jets to deposit fine metallic powders (20 micron particle size) onto metallic substrates without melting the material. While cold spray deposi -tion is being successfully implemented for additive repair, it is important to examine the corrosion behavior of the repaired material. AA7075 deposits were produced using a high-pressure cold spray deposition system with he-lium gas. Electrochemical potentiodynamic tests were performed using MIL-STD-889C methods for measuring potential-current response behavior while immersed in artificial seawater. Fully dense, cold sprayed material was somewhat less active than AA7075-T651 plate but exhibited a higher corrosion rate. The cold sprayed materials exhibited less pitting in quiescent salt water exposures.

INTRODUCTIONCold-gas dynamic-spray, typically called cold spray (or “CS”) is an additive method that uses high pressure gas to accelerate micrometer-sized powders to supersonic velocities, as shown in Figure 1 (1). Upon impacting a substrate, these particles adhere to the surface in an event similar to explosive welding. Relatively thick deposits of several millimeters are readily achievable with a variety of feedstock powder and substrate material combinations.

2019 Department of De-fense – Allied Nations

Technical Corrosion Con-

Figure 1: A typical high pressure cold spray machine. (2)

A particularly interesting application for CS is for additive repair, especially of expensive parts made of alloys that are difficult to repair with traditional techniques, such as fusion weld repair. High strength aluminum alloys, such as AA7075 and AA2024, can suffer from solidification cracking and thermal softening in the heat affected zone, thus making any welding or high-temperature spray repairs problematic (3). However, as cold spray does not

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Mixer

Substrate

Deposit

Powder Stream

Supersonic Nozzle

Gas Controller

Electric Heater

Powder Feeder

Gas (N2 or He)

significantly raise the temperature of the powder or substrate, AA7075 can be deposited without deleterious effects from high temperature.

CS is already used in the repair of some aircraft parts, such as gearboxes. These parts were a natural choice for early application, as they can cost hundreds of thousands of dollars to replace, experience gradual damage, including from corrosion due to leaking fluid-lines or simple wear, and are dimensional rather than load-bearing repairs. An initial program funded by the U.S. military to use CS for gearbox repair yielded savings of about $17M for a cost of about $4M, with more savings expected in the future (4). When repairs can be accomplished with a faster turn-around, this shortened time allows for less down-time, keeping aircraft or ships mission-ready rather than off-line for maintenance. The potential savings from a more efficient use of existing assets can vastly outweigh the base cost of the saved parts and labor. These cost-savings all underscore the strong financial drive to develop and deploy CS technologies.

Evaluation of the potential for CS repairs requires understanding how the CS material will perform. Performance is less demanding for applications with relatively undemanding dimensional repairs, and only somewhat more demanding for mechanical evaluation of fastener repairs; here, the primary considerations are mechanical, such as sufficient bonding, hardness, yield strength, and perhaps fatigue (5). As the application becomes more critical, the evaluation becomes more complex and demanding, especially for load-bearing parts nearer their material yield engineering limits, and/or more complicated environments, especially including corrosion. Aluminum 7075 (AA7075) has relatively poor corrosion performance, but due to its high strength to weight ratio, it is commonly used in aircraft, and as such, it has been subject to research about corrosion behavior in seawater stretching back decades (6).

The corrosion performance of cold sprayed AA7075, especially in relation to the traditionally processed materials, is critical to their performance, and a current subject of investigation (7). Generally, an exact match of corrosion performance between the substrate to be repaired and the material used for the repair is desirable. If the repair material is more noble than the substrate, then it will, over time, create a ring of galvanically-accelerated damage outside the repair, causing the repair region to grow over time. A less noble repair will cause the repair region be preferentially attacked, quickly degrade, and require frequent repairs. As repairs need to remove slightly more material than they are replacing, this anodic attack will also cause a growing repair region. The closer the repair’s corrosion performance to that of the base material, the less problematic these issues will be.

Galvanic corrosion of aluminum structures and mechanical fastener is a particularly important driver for aircraft. High strength aluminum components are often fastened together using stainless steel, titanium, and other relatively noble materials. The potential galvanic coupling between these materials can generate substantial corrosion damage in the plate around the fastener and down the fastener hole. The impact of different material combinations can be assessed by the comparison of anodic and cathodic branches from polarization data. While this data exists for many plate materials and fastener types, it is not currently available for cold sprayed high strength aluminum alloys.

This paper will test and describe the polarization behavior for CS-AA7075 and wrought AA7075-T651, using MIL-STD-889C to obtain results suitable for corrosion prediction (8) (9). While Ngai et al. recently published initial polarization data for cold sprayed AA7075, these tests were performed using ASTM-G5 with sodium chloride solutions. For better prediction of galvanic coupling, the community needs data for cold sprayed materials following MIL-STD-889C in synthetic sea water (ASTM D1141).

EXPERIMENTAL PROCEDURE

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Wrought 7075-T651 and cold sprayed AA7075 samples were tested in an immersive seawater environment prepared to ASTM D1141 as shown in Table 1 (10). Sample preparation and potentiodynamic tests were per the MIL-STD-889C 2018-289 standard. Corrosion testing equipment consisted of Gamry Interface 1010 potentiostats, paracells, and Ag/AgCl reference probes. A graphite block was used as the counter electrode.

Table 1: D1141 composition and resulting concentration for artificial seawater, without heavy metals

Compound Concentration (g/l)

NaCl 24.53

MgCl2 5.2

Na2SO4 4.09

CaCl2 1.16

KCl 0.695

NaHCO3 .201

KBr .101

H3BO3 .027

SrCl2 .025

NaF .003

Materials PreparationFor the CS-AA7075 samples, the deposits were made as follows. Inert gas atomized AA7075 powder from Valimet Inc. was dried for two hours at 1000C. This powder was sprayed using a VRC Metal Systems Generation III high pressure cold spray system onto AA7075-T6 plate which had been sanded, cleaned, and dried. Spray conditions are in Table 2. Three layers were deposited, for a total deposit thickness of 2mm.

Table 2: Spray Conditions:

Spray gas: Helium

Standoff distance: 20mm

Pressure: 500 psi (3.45 MPa)

Gun Temperature: 4150C

Traverse Speed 100 mm/s

Powder feed rate 6 RPM

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Prior to corrosion testing, samples were prepared per the NAVAIR PR 2018-289 standard. This procedure includes grinding sample surfaces to a P800 finish, ultrasonication in acetone and IPA/Ethanol for ten minutes each, and then humidification at 250C and at least at least 80% RH humidity for 24 hours before the corrosion experiment.

ASTM D1141 sea water was prepared one day in advance (10). This solution was thoroughly aerated for ten minutes prior to testing, and adjusted to pH 8.2 by addition of HCl or NaOH solution as necessary.

Corrosion TestingTests were performed at 25 +/- 30C. Samples were left in an OCP condition for four hours prior to any polarization testing. The OCP was recorded for at least the last 30 minutes of those four hours. For potentiodynamic tests, either an anodic or cathodic branch was taken on an individual sample, but not both. Scans started at the OCP, and proceeded in the desired polarization direction at a rate of 0.2mV/s until either the current density reached 10mA/cm2 or the voltage reached +0.7V vs OCP (anodic branch) or -1.4V vs reference electrode (cathodic branch). The sampling period was 1s.

RESULTS and DISCUSSIONThe CS-AA7075 sprayed well, as seen in Figure 2. The porosity is low, and the few pores are not interconnected, and are oriented generally parallel to the outer material surface. This is because the main spraying defects are from poor bonding at deposition layer interfaces. As such, they represent little risk of significantly accelerate corrosion attack. Considering AA7075 tends to fail from pitting damage, any pit deep enough to reach a significant pore would likely already be deep enough to have considerably accelerated corrosion at the pit’s bottom, and thus necessitate repair.

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Figure 2: Optical micrograph taken at 100x of the cross section of a CS-AA7075 sample, showing the cold spray deposit and substrate. The deposit is 2 mm thick.

The CS-AA7075 material was more noble than AA7075-T651 as shown in the OCP results in Figure 3. Note that although the OCP is fairly noisy; average OCP readings, which are 10 mV apart, can be used to identify one material temper versus another. Figure 3 shows the OCP varying by almost 10mV in both samples. This noise may be due to rapid localized pitting corrosion events starting and stopping. The CS-AA7075 is less noisy due to fewer pits, but experiences greater drift, indicating that the pits are active for longer.

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Substrate

Deposit

0 200 400 600 800 1000 1200 1400 1600 1800-0.730

-0.725

-0.720

-0.715

-0.710

-0.705

-0.700

Representative 7075 OCP (last 30 minutes)

CS-70757075-T651

t (s)

E (V

vs A

g/Ag

Cl)

Figure 3: Last 30 minutes of a 4 hour OCP for representative samples of CS-AA7075 (top, red) and AA7075-T651 (bottom, blue).

Visual inspection of the CS-AA7075 after electrochemical testing showed much less pitting than for the wrought AA7075-T651, as illustrated in Figure 4. This lack of pitting is likely due to the fact that the CS coating does not have large constituent particles, which can form initiation sites for pitting nucleation on the material surface. Fewer active pits in CS-AA7075 versus a large and relatively consistent number of active pits in AA7075-T651 supports the pitting explanation for the OCP behavior shown in Figure 3. This relative pitting behavior does, however, have an important implication since AA7075 corrosion failure is largely in the pitting mode. Avoiding pitting may be preferable, even if it comes at the cost of slightly faster uniform corrosion rate.

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Figure 4: Optical images taken after anodic polarization per the 2018-289 standard. Left: AA7075-T651. Right: CS-AA7075.

The representative potentiodynamic curves for AA7075-T651 and CS-AA7075 in Figure 5 show that while CS-AA7075 is less active, it is more reactive with a higher icorr. The increase of both Ecorr and icorr of the CS-AA7075 with respect to the AA7075-T651 suggests an increase in cathodic reaction kinetics on the cold sprayed surface. There is minimal risk indicated of galvanic action between the repair and the original part; in fact, the intersection occurs at a lower corrosion current density than CS-AA7075’s icorr. Overall, the CS-AA7075 is a potentially good match for the AA7075-T651 from a galvanic perspective.

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5.0E-08 5.0E-05-1.400

-1.300

-1.200

-1.100

-1.000

-0.900

-0.800

-0.700

-0.600

-0.500

-0.400Representative AA7075 Potentiodynamic Curves

7075-T651 Anodic7075-T651 CathodicCS-7075 AnodicCS-7075 Cathodic

i (A/cm^2)

E (V

vs

Ag/

AgC

l)

Figure 5: Potentiodynamic curves for AA7075-T651 and CS-AA7075. These are median results from at least 3 samples per material condition. The anodic and cathodic branches were measured on separate samples and then combined on this plot.

Note the partially passive behavior in the CS-AA7075 anodic branch near Ecorr. This means it is less sensitive to small positive increases in potential. It also indicates that the Ecorr for the CS-AA7075 may be below the pitting potential, agreeing with the lesser amounts of pitting observed on CS-AA7075 samples compared with the AA7075-T651. The AA7075-T651 shows no such passivation,

Also note that anodic and cathodic branches were taken on different days, using re-finished samples, different batches of artificial seawater, and slightly varying temperatures. The good agreement between E corr for a given sample (CS-AA7075 or AA7075-T651) show that despite the issues with noisy OCP, a combination of the potentiodynamic testing standard and careful experimental controls allows for highly repeatable results, as is demonstrated in Figure 6. When inspecting Figure 6, remember that the noise in the OCP causes difficulty resolving the region around the OCP, and variation especially in the initial response.

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5.00E-08 5.00E-05-0.750

-0.700

-0.650

-0.600

-0.550

-0.500Reproducibility of AA7075 Potentiodynamic Curves

CS-7075 Sample 1CS-7075 Sample 2CS-7075 Sample 3CS-7075 Sample 47075-T651 Sample 17075-T651 Sample 27075-T651 Sample 37075-T651 Sample 4

i (A/cm^2)

E (V

vs A

g/Ag

Cl)

Figure 6: Potentiodynamic plots of multiple CS-AA7075 and AA7075-T651 samples showing reproducibility of the data.

The icorr values for the AA7075-T651 and CS-AA7075 were calculated using Tafel fitting. The results are given in Table 3, as are some literature values for AA7075 in natural seawater (11) and natural Red Sea water (12).

Table 3: Tafel-extrapolated electrochemical characteristics of AA7075-T651 and cold sprayed AA7075 from representative curves and selected literature values for comparison.

7075-T651 CS-7075 Kim et al. (11) Al-Moubaraki et al. (12)

Ecorr (mV vs Ag/AgCl) -724 -700 -739 -732

icorr (uA/cm2) 2.14 3.58 20.5 5.78

Tafel constants (βA, 12.5, 260 47.7, 244 Not reported Not reported

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βC; mV/decade)

The small βA implies that a small change in potential in the anodic direction from the OCP results in a large change in current, in other words, that the reaction is highly polarizable. Similarly, the larger βC and high curvature on the cathodic branch imply that the sensitivity to potential changes in that direction is much smaller. This is the case for both CS-AA7075 and AA7075-T651. However, note in that the βA for the AA7075-T651 is significantly smaller compared to that of CS-AA7075. This means that CS-7075 is less sensitive than AA7075-T651 to small positive potential shifts. This may be related to the relatively lesser pitting observed in CS-7075 compared to AA7075-T651. Future work including longer term exposure tests can provide a more definitive answer to the question as to whether the slightly faster, but more uniform corrosion, seen in the CS-AA7075 is preferable to the slightly lower mass loss but more significant pitting experienced by the wrought AA7075.

These corrosion currents compare favorably with the previous literature values in Table 3, and are within the bounds for round-robin results done per the new Potentiodynamic testing standard for MIL-STD-889C at various universities and government laboratories (13). An inspection of the curve intercepts shows that the galvanic interaction is relatively small; while testing and interface microscopy will be necessary, it seems unlikely to cause an issue. However, it should be noted that if processing were to further increase the OCP of the CS-AA7075, making it more noble, then it is possible for deposit-substrate galvanic interactions to become significant.

The effect of spray quality on corrosion properties necessitates corrosion testing on future improved CS materials. Spray parameters, including powder velocity, and resulting deposit quality can have significant effects on corrosion properties. Prior work by Ngai et al. has shown that using nitrogen as the spraying gas may result in a much more active CS-deposit, due to interconnected porosity within the coating (7). Spencer and Zhang varied powder particle size which caused more than a 250mV difference in OCP for 316L stainless steel (14). Furthermore, the microstructure of AA7075 powder is significantly different from that of typical wrought material. Heat-treated AA7075 powders have shown considerable reduction in the formation of eta phase precipitates in the powder particle microstructure. This reduction in eta phase precipitates comes with advantages in deposition efficiency. Whether a heat-treated powder approach will improve corrosion resistance is an open question and will be investigated in future work.

CONCLUSIONSThe anodic and cathodic polarization behavior of cold sprayed AA7075 and wrought AA7075-T651 plate were tested per MIL-STD-889C. Representative potentiodynamic curves were presented, as were a number of anodic potentiodynamic curves as an example of typical reproducibility and variation. Photographs and microscopic images of the CS-AA7075 and AA7075-T651 surfaces were shown for comparison of pitting damage.

Potentiodynamic analysis of CS-AA7075 vs wrought AA7075-T651 showed close matches for immersion, corrosion performance. CS-AA7075 was slightly more noble than the wrought material, but more reactive with a higher icorr, CS-AA7075 was also less sensitive to small positive potentials, with a βA nearly four times that of the AA7075-T651. These results were supported by visual inspection of the samples and the microscopic surface images.

CS-AA7075 is a highly promising candidate for corrosion repair, not just in being able to repair or replace damaged locations on an AA7075 structure, but in matching the original performance of the AA7075.

REFERENCES

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1. Barnes, John, Champagne, Victor and al., et. Mechanical and Mictrostructural Effects of Cold Spray Aluminum on Al 7075 Using Kinetic Metallization and Cold Spray Processes. s.l. : Air Force Research Laboratory, January 2007.

2. Arbegast Materials Processing and Joining Laboratory. South Dakota School of Mines. [Online] https://www.s-dsmt.edu/Research/Research-Laboratories/AMP/Capabilities/Cold-Spray/.

3. Liquation Mechanisms in Multicomponent Aluminum Alloys during Welding. Huang, C. and Kou, S. October 2002, Welding Journal, pp. 211-222.

4. Story, William. Dissertation Defense Presentation. Tuscaloosa : University of Alabama, August 24, 2018.

5. An investigation into microstructure and mechanical properties of cold sprayed 7075 Al deposition. Rokni, M., et al. Pages 19-27, s.l. : Materials Science and Engineering: A, 2015, Vol. 625.

6. Davis, J.R. Corrosion of Aluminum and Aluminum Alloys. s.l. : ASM International, 1999.

7. Saltwater corrosion behavior of cold sprayed AA7075 aluminum alloy coatings. Ngai, S., et al. January 2018, Corrosion Science, Vol. 130, pp. 231-240.

8. Rodriguez-Santiago, V. and Safigan, A. Best Practices for Polarization Data Acquisition: Data Collection Guide for MIL-STD-99C Technical Revision, NAVAIR Public Release 2018-289. s.l. : Naval Air Systems Command, 2018.

9. Defense, Department of. MIL-STD-889C: Dissimilar Metals. s.l. : DoD, 2016.

10. ASTM. Standard Practice for the Preparation of Substitute Ocean Water, D1141-98(2013). s.l. : ASTM, 2013.

11. Electrochemical Properties of Al and Al Alloys Relevant to Corrosion Protection in Seawater Environments. Kim, S-J and Ko, J-Y. 5, 2006, Korean J. Chem. Eng., Vol. 23, pp. 847-853.

12. The Red Sea as a Corrosive Environment: Corrosion Rates and Corrosion Mechanism of Aluminum Alloys 7075, 2024, and 6061. Al-Moubaraki, A. and Al-Rushud, H. 2018, International Journal of Corrosion, Vol. 2018, p. Article ID 2381287.

13. Victor Rodriguez-Santiago, Anna Safigan. Galvanic Compatibility Assessment: New Methodology and Stan-dardization. ASETS Defense Workshop 2018. s.l. : NAVAIR, 2018. Public Release 2018-631.

14. Optimization of Stainless Steel Cold Spray Coatings Using Mixed Particle Size Distributions. Spencer, K. and Zhang, M. 2011, Surf. Coat. Technol., Vol. 205, pp. 5135-5140.

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