MICROSTRUCTURAL AND CORROSION CHARACTERISTICS OF PEO TREATED UNS A97075, A92099 and A92195 ALLOYS

*S. Shrestha and S. Hutchins

Keronite International Ltd Cambridge CB21 6GP

United Kingdom

ABSTRACT Microstructure characteristics of coatings prepared on some aerospace grade alloys UNS A97075, A92099 and A92195 by plasma electrolytic oxidation (PEO) have been investigated. Some comparisons have been made with a sulfuric acid hard anodized coating. Electrochemical corrosion plots showed the PEO coatings to be highly passive in salt solutions and are in agreement with standard salt fog exposure results. These results suggest that the PEO process can offer these high strength aluminum alloys with excellent and improved corrosion resistance. In addition, extreme hardness of the PEO ceramic coatings due the formation of crystalline Al2O3 phases have potentials to offer enhanced wear resistance and multifunctional characteristics required in extreme environments of aerospace and defense applications. The improved corrosion resistance of the PEO coating compared to a hard anodized coating is related to the formation of a highly inert oxide layer as well as its structure and integrity at corners. Keywords: plasma electrolytic oxidation, ceramic coating, aluminum alloys, corrosion

INTRODUCTION Hard anodizing of high strength aluminum alloys such as 2xxx, 7xxx series and others, including casting alloys with high Cu and Si content, presents considerable challenge. For these higher Si- and Cu-content alloys, the anodized layer tends to be highly porous and of low hardness. For example, the hard anodized coating on alloyed aluminium such as A92024/A97075 will have coating hardness of about 250-400HV compared to 700HV for the less alloyed materials [1, 2]. Thus, anodized high strength alloys of 2xxx and 7xxx series, which are of primary interest as structural materials in aerospace and defense applications, have poorer corrosion and wear (impact, fretting, and abrasion) resistance due to pores and defects in the anodized layer. For space applications, additional coating characteristics such as black color to achieve thermo-optical properties e.g. solar absorptance, thermal emittance and improved surface characteristics to resist against cold

1

welding are considered necessary. The black anodizing process is often used for space applications following the ESA PSS-01-703 [3]. However, this process is considered environmentally undesirable due to the use Co or Ni sulfide. In addition, the European Space Agency is currently having problems with these black anodized coatings due to their relatively poor performance [4]. Thus, there exists a strong need to identify new coating processes that can meet not only the environmental requirements but also the continual demand by aerospace and defense hardwares for improved coating performance with multifunctional properties to work under extreme environmental exposures (Fig.1).

Corrosion e.g. general, SCC, galvanic, fatigue

Wear e.g. fretting, impact, sliding,

friction etc Dielectric / Electrical conductance

Thermal conductance /barrier

Thermo-optical properties e.g. solar absorptance, thermal

emittanceEnvironmental

degradation e.g. humidity, thermal shocks, UV,

atomic oxygen in LEO

Debris and Outgassing leading to

contamination at sensitive and operational

surfaces

Aluminum alloys

Cold welding / seizure

Pre-treatment for paints / adhesives

Corrosion e.g. general, SCC, galvanic, fatigue

Wear e.g. fretting, impact, sliding,

friction etc Dielectric / Electrical conductance

Thermal conductance /barrier

Thermo-optical properties e.g. solar absorptance, thermal

emittanceEnvironmental

degradation e.g. humidity, thermal shocks, UV,

atomic oxygen in LEO

Debris and Outgassing leading to

contamination at sensitive and operational

surfaces

Aluminum alloys

Cold welding / seizure

Pre-treatment for paints / adhesives

FIGURE 1 - Multifunctional properties required from surface coatings on aluminum alloys for use in aerospace applications PEO is an emerging technology and offers versatility in terms of coating most aluminum alloys including high Cu, Zn and Si containing alloys and Al-MMCs in all forms such as cast, extrusions and sheets. Details of the PEO process are described elsewhere (5). While there are a number of published papers on PEO coatings on aluminum, reporting e.g. process characterization (5,6), physical and mechanical properties (7,8), tribological properties (9-14) and thermo-optical properties (15-16), there has not been any concerted attempt to compare PEO with hard anodized coatings in relation to corrosion resistance. The aim of this paper is to undertake corrosion tests on PEO coatings on high strength Al-Zn and Al-Li alloys by standard salt fog and electrochemical test methods and compare with a hard anodized coating.

EXPERIMENTAL Materials and Test Specimens Three high strength aerospace grade aluminum alloys: UNS A97075 (5.6Zn 2.5Mg 1.6Cu 0.3Cr Bal. Al), A92099 (2.5Cu 1.6Li 0.8Zn 0.3Mg 0.3Mn 0.1Ti Bal. Al) and A92195 (4Cu 1.0Li 0.4Mg 0.3Ag 0.15Zr Bal. Al) were chosen in this study as substrate materials. Specimens of either ∅25mm x 5mm thickness or 20mm x 20mm x 5mm were machined and wet ground to 600 grit finish prior to the surface treatment. The edge of the specimens was just slightly de-burred to remove any sharp edge. An M3 x 4mm deep threaded hole was prepared on the side face of each specimen for electrical connection required during the surface treatment process. The machined specimens were lightly degreased in a non-solvent based low-alkaline cleaner Gardoclean T5378 (Chemetall product) followed by rinsing for 30s in tap-water and 30s in de-mineralized water immediately prior to the surface treatment. Surface Treatment PEO coatings were prepared by Keronite International Ltd using commercial Keronite PEO equipment and proprietary low-concentration alkaline electrolytes, free of any heavy metals

2

and Cr+6. A schematic of the PEO process and a typical appearance of components during processing are shown in Fig.2. Nominal coating thicknesses were 50-60µm on the 7075 alloy and 30-40µm on 2099 and 2195 alloys. Hard-anodized coatings were prepared using the sulfuric acid hard-anodizing process. The coated specimens were immediately rinsed in cold water followed by hot water sealing. Nominal coating thickness of 50-60µm was prepared on the 7075 alloy. Typical process parameters during the PEO and hard anodizing / black anodizing of AA7075 alloy are given in Table 1.

b)a)

Modulated electrical voltage

Electrolyte

Pump Filter Cooler

SampleSpark

discharge

∼ Electro Electrolyte

Pump Filter Cooler

Sam

ple Spark

discharge

∼

Elec

trod

e

FIGURE 2 - Schematic of a PEO process (a); and plasma discharge (glow) surrounding the surface of a component during PEO processing (b) Coating Characterization A coated sample was sectioned to a size of 10x20mm using an abrasive cutting wheel and the cut specimen was mounted in cold mounting compound (VariDur 3000 from Buehler). Standard metallographic preparation techniques such as grinding on abrasive papers and polishing to 1µm diamond suspension were used to prepare the cross-section. The obtained cross-section was then examined under either an optical microscope or a scanning electron microscope (SEM). Determination of the various crystalline phases present in the coating was undertaken using the X-ray diffraction (XRD) technique. The XRD technique used CuKα radiation to collect spectra from the coating material. Coating surface roughness was measured using a Surfcom 130A profilometer using 0.25 cut-off and 10mm evaluation length. Coating hardness was measured on the polished cross section by Vickers micro-indentation with a load of 50g using the procedure described in ASTM E384-99. Salt Fog Testing For the salt fog tests, specimen sizes 20mmx20mmx5mm or ∅25mmx5mm were used depending upon the available feedstock material. The salt fog exposure was carried out following the guidelines described in the ASTM standard B117-2003 ‘Standard practice for operating salt-spray (fog) apparatus’ and using a CorrosionBoxe salt fog chamber manufactured by CO.FO.ME.GRA. srl, Italy. This test comprised of exposing the coating surface to a 5wt% NaCl solution atomized to create a fog within an enclosed chamber. The coating was supported 15-20° from the vertical. Tests were carried out for various exposure durations up to 360 hours. The chamber temperature was maintained at 35±1°C. Changes to the coating surfaces were recorded following periodic observations. Anodic Polarization Specimens were fixed to the bottom of the electrochemical test cell sealed with an ‘O’ ring with the exposed area being 2cm2. The threaded hole used for electrical connection during

3

the coating process was used also for electrical connection during electrochemical corrosion tests. The coatings were tested in as-prepared condition (i.e. without polishing or post-sealing) and only after degreasing with alcohol. Testing of the uncoated specimen was performed on a surface abraded with 600-grit sandpaper. Electrochemical corrosion tests were performed using a three-electrode electrochemical corrosion test cell arrangement attached to a computer-controlled potentiostat as shown in Fig.3. Experiments were carried out following the guidelines described in the ASTM standard G61. The electrolyte was a 5wt% NaCl solution (made from distilled water) with a pH value of 6.8-7.0 and solution conductivity of 85mS.cm-1 and in static condition. The temperature was maintained at 20±1°C. Platinum

auxiliary electrode (AE)

Coated sample exposing 2cm2 (Working electrode, WE)

Reference electrode (SCE)

Electrical connection

Electrolyte solution 4L

FIGURE 3 - Schematic diagram of three-electrode corrosion test cell The specimens were electrically connected to the Gamry potentiostat. The coated surface was exposed to the electrolyte and the rest potential ‘Ecorr’ (also known as free corrosion potential) was stabilized for 15 minutes prior to the anodic polarization. This potential was measured using a reference saturated calomel electrode (SCE). The area exposed to the electrolyte was then anodically polarized from its rest potential to more positive potential at a rate of 10mV.min-1. This was continued until a current density of 1mA.cm-2 or potential of 1000mV wrt to Ecorr was measured in the external circuit. A plot of the corrosion current density as a function to the polarization potential was recorded. The collected polarization plots were used to compare the corrosion behavior of the alloys in the uncoated condition and with the coating. The following features were used to compare the corrosion behavior:

• Breakdown potential ‘Eb’ – At potentials above Eb, the current rapidly increases with further increase in potential. The constant current region below Eb is referred to as a passive region where little or no corrosion occurs. At potentials above Eb, rapid corrosion is occurring. The presence of a passive region and a more positive value for Eb indicates better resistance to corrosion attack.

• Rest potential ‘Ecorr’ – a more negative Ecorr representing greater susceptibility to

corrosion attack.

4

RESULTS AND DISCUSSION Visual Surface Appearance Visual examination of the PEO coatings on 7075, 2099 and 2195 showed uniform and smooth appearance with no visible defects and irregular features. The appearance was dark grey / light black. Such darker appearance comes from Cu content in the alloys. The hard anodized coating also displayed a uniform and smooth appearance with no visible irregular features. Surface roughness (Ra) measured on the as-prepared surface for the 7075, 2099, 2195 and the hard anodized 7075 were 0.9, 0.7, 0.8 and 0.5±0.1µm respectively with hard anodized surface showing the lower roughness values. When a light polish was given to the PEO layers, surface roughness < 0.3µm with further polishing to 0.04±0.01µm was achieved.

b)a)

c)

FIGURE 4 - SEM images of PEO coatings on: a) 7075 b) 2099 and c) 2195 showing abundance of smaller irregular and larger pancake shaped alumina particles of various sizes corresponding to plasma discharges. These particles show central discharge channels and shrinkage cracks typical from the PEO process. Microstructure Characteristics Secondary electron images of as-prepared surface of the PEO coatings on the three alloys are shown in Fig.4 that show abundance of spherical and irregular shaped particles-like structures that were formed during the PEO process. The presence of a number of small holes of corresponding discharge channels and shrinkage cracks of larger pancake shape particles are also visible from the surfaces. Although it is difficult to differentiate the three coating surfaces from the SEM examination, it is clear that the PEO coating on the 7075 alloy with least Cu content has abundance of smaller irregular features on the surface. The

5

Al-Li alloys with increasing Cu content, i.e. from 2099 to 2195, show increasing abundance of larger pancake shaped features. These features have not yet been analyzed in this work but would be interesting to investigate if these correlate to the alloying variations and subsequent PEO process.

a) b) Keronite

FIGURE 5 - Cross sectional micrographs of: a) PEO coating on A97075 showing a uniform coating coverage and good edge retention b) Hard anodized coating on A97075 showing extensive truncated V-shaped through

thickness cracks extending down to the substrate at corners.

a) b)

FIGURE 6 - Cross sectional micrographs of: a) PEO coated A92099 showing denser microstructure b) PEO coated A92195 showing more open microstructure Optical micrographs of the cross sections of the PEO coating and the hard anodized coatings in Fig.5 show very distinct edge characteristics of the coatings. The PEO coating displays a highly uniform coverage and no evidence of through thickness cracks despite of the presence of micro-scale porosity, which is not seen as interconnected. The hard

6

anodized coating clearly shows truncated ‘V’ opening cracks that extend down to the substrate. These features of the hard anodized coatings are well known but rarely presented in publications. Cross sectional images of the PEO coating on the 2099 and 2195 substrates in Fig.6 clearly show an increasingly open coating microstructure with higher Cu content although a highly dense PEO coating that could be achievable on even higher Cu containing 2219 alloy had been reported in the past [15]. Figure 7a shows coating hardness values measured by Vickers indentation method. All the PEO coatings had hardness in excess of 1400HV, which is more than an order of magnitude of the base alloy hardness. The hard anodized coating on the 7075 alloy had hardness measured at about 400HV compared to 1620HV of the PEO coating on the same alloy. XRD patterns collected from the PEO and hard-anodized coatings on the 7075 alloy are shown in Fig.7b. Sharp peaks of crystalline γ-Al2O3 were detected from the PEO coating. The hard-anodized coating shows a small ‘hump’ between 25 to 35 degrees 2θ near the base line trace, which is typical of amorphous material. It is believed that the aluminum peaks are from the underlying substrate while the anodized coating was an amorphous alumina. The presence of such crystalline alumina phases explains the high hardness of the PEO coating. Although not measured here, the phase contents of the PEO coatings on the Al-Li alloys, various reports [6-8, 15, 16] suggest that these do contain similar crystalline structures that are responsible for such high hardness values.

0

500

1000

1500

2000

A92195 A92099 A97075

Vic

kers

har

dnes

s, H

V 0.0

5

PEO coatingHard anodised coating

a) b)

FIGURE 7 - Micro-indentation hardness of PEO and hard anodized coatings (a) and XRD spectra collected from PEO and hard anodized coatings on 7075 (b). The PEO coating on 7075 alloy shows the presence of crystalline γ-Al2O3 and hard anodized coating shows the presence of amorphous alumina Anodic Polarization Anodic polarization plots from the electrochemical tests taken on flat faces of the uncoated 7075, the PEO coated 7075 and the hard anodized 7075 alloy are shown in Fig.8a. The uncoated 7075 alloy displayed an immediate rapid increase of the current density on slightly increasing the potential from its rest potential ‘Ecorr’. This immediate rise of the anodic current density over a small positive potential range from ‘Ecorr’ is indicative of very rapid corrosion occurring at the surface of the uncoated alloy. Both the PEO-coated and the hard anodized 7075 alloy displayed a large potential range about 500mVsce (passivity) positive from the Ecorr, where the current density was stable and recorded low at <1μA.cm-2. This is indicative of very little corrosion occurring on the coating/substrate system and a good level of barrier protection afforded to the underlying substrate. Until about –200mVsce,

7

both the PEO and the hard anodized specimens displayed rather similar behavior albeit up to this potential the PEO-specimen displayed somewhat better corrosion resistance as indicated by the polarization trend with reduced corrosion current. Above –200mVsce, higher currents are recorded by the PEO coated specimen possibly due to now larger pores, opening via shrinkage cracks and discharge channels resulting in corrosion initiation by pitting mechanism. Meanwhile, very fine-scale columnar cracks in the hard anodized coating may now have been further sealed by the corrosion product itself. It is noteworthy that the PEO coating on this instance was not sealed whereas the hard-anodized coating had already been sealed. Significantly measurable differences in the coating/substrate system were observed in Fig.8b when the corners of the coated specimens were involved during the corrosion tests. Exposure of such corners and edges would be inevitable in real component situations. The PEO coated system exhibits a small initial passivity of about 60mVsce from the Ecorr while the anodized system shows no initial passivity at all. After the initial breakdown ‘Eb’ of the unsealed PEO surface, there was a rapid increase in the corrosion current until (although at very low level) about 150mVsce above the Ecorr when the corrosion began to slow down indicating semi-passivation and filling of the pores by the corrosion product. The sealed hard anodized coating in Fig.8b shows rapid increase in the corrosion current until it reached approximately 100µA/cm2 and the potential of 200mVsce above the Ecorr. After this, passivation started probably after filling of the large cracks by the corrosion product. Such changes in the corrosion performance due to the exposure of the corners, particularly in the hard anodized system can be explained due to the presence of the through thickness opening cracks described above in Fig.5b. a) b)

-1000

-800

-600

-400

-200

0

200

1.0E-05 1.0E-03 1.0E-01 1.0E+01 1.0E+03

Current density, µA/cm2

Pot

entia

l, m

Vsc

e

Unsealed PEO coated 7075

untreated 7075

Sealed hard anodised 7075

-1000

-800

-600

-400

-200

0

200

1.0E-05 1.0E-03 1.0E-01 1.0E+01 1.0E+03

Current density, µA/cm2

Pot

entia

l, m

Vsc

e

Unsealed PEO coated 7075 with corner

Sealed hard anodised 7075 with corner

FIGURE 8 - Anodic polarization plots of uncoated UNS A97075, unsealed PEO and sealed sulfuric acid hard anodized coatings on UNS A97075 in 5 wt% NaCl solution

a) flat surfaces showing passive performance b) with corners exposed showing a small passive region for the PEO and a highly active

performance shown by the hard anodized system.

The anodic polarization plots of the PEO-coated 2099 and 2195 alloys are presented in Fig.9 both of which again demonstrate the passivity offered by the PEO ceramic coatings. Both uncoated alloys show actively corroding surface in the salt solution. A large range of passivity indicated that the PEO coatings are capable of offering significant corrosion protection to these high strength alloys.

a) b)

8

-800

-600

-400

-200

0

200

400

1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04

Current density, µA/cm2

Pot

entia

l, m

Vsc

ePEO treated 2099

untreated 2099

-800-600-400-200

0200400600800

1.0E-05 1.0E-03 1.0E-01 1.0E+01 1.0E+03

Current density, µA/cm2

Pot

entia

l, m

Vsc

e

PEO treated 2195

untreated 2195

FIGURE 9 - Anodic polarization plots in 5 wt% NaCl solution:

a) Uncoated UNS A92099 and PEO coated A92099 b) Uncoated UNS A92195 and PEO coated A92195

-1000-800-600-400-200

0200400600800

2195

2195

+PE

O

2099

2099

+PE

O

7075

7075

+PE

O

7075

+Har

dan

odis

ing

Pot

entia

l (E

), m

Vsc

e

Ecorr

Eb

Eb

Ecorr

Pas

sivi

ty

Eb Eb

Ecorr

FIGURE 10 - Free corrosion potential ‘Ecorr’, typical breakdown potential ‘Eb’ and passive range of un-coated, PEO coated and hard anodized alloys in 5 wt% NaCl solution. The Ecorr and Eb values for the tested coating / substrate systems are presented in Fig.10. It was noticed that the 7075 alloy had a more negative Ecorr than 2099 followed by 2195 alloys. This can be explained by the presence of higher amount of elements such as Zn and Mg in the 7075 alloy that have lower electrode potentials. As such, poorer corrosion resistance of this alloy would be expected in the uncoated condition. Coatings on all substrates appeared to have resulted in the Ecorr moving towards a more positive value which clearly indicates the improvement in corrosion initiation (susceptibility). While a large Eb-Ecorr range indicated that these coatings have improved resistance to corrosion attack. One common feature with the PEO coated and the hard anodize coated systems when excluding corners (Fig. 10) is that they all appear to have similar passivity range. A more interesting observation is that the Ecorr for the higher-Cu/lower-Li containing alloy such as 2195 appeared to have ennobled to higher Ecorr values perhaps indicating improved resistance to corrosion initiation while at the same time with the higher Eb it can be well susceptible to localized pitting initiation and further propagation. Detailed further study on the phase composition with the alloying of Li would be required to better understanding these electrochemical parameters.

9

The PEO coated 7075 specimens after 360 hrs of salt fog exposure are presented in Fig.11. As discussed above, the more negative Ecorr of the Zn, Mg containing alloy appeared to show extensive general corrosion of this alloy (Fig.11a) although this would be difficult to compare with other PEO coated alloys in Fig.12 which have done only 106 hrs. However, it is clear from the salt fog tests that general mechanism of corrosion of the PEO coated 7075 was by minor pitting attack as seen in Fig.11b. The more active polarization plot with the presence of corners on hard anodized coating is further substantiated by Fig.11c which shows enhanced corrosion attack at edges after the salt fog exposure.

Pin point corrosion

Corrosion at the edge

Sealed hard anodised Unsealed PEO Uncoated alloy

a) b) c)

FIGURE 11 -Surface of: a) uncoated; b) PEO coated and c) hard anodized 7075 alloy after 360 hrs of salt fog exposure

Uncoated 2099 PEO coated 2099 Uncoated 2195 PEO coated 2195

a) b)Pittingcorrosion

FIGURE 12 - Surface of: uncoated and PEO coated specimens after 106 hrs of salt fog exposure:

a) 2099 b) 2195

The nobler Ecorr of the Li alloys are shown to have reduced general corrosion attack on the uncoated surfaces. A slightly reduced corrosion of the 2099 alloy could be due to its less Cu / higher Li contents. While on the coated (unsealed) surfaces, this was also reflected with the presence of a corrosion pit. General Discussion The results presented above clearly show that PEO ceramics in general tend to have their unique structures formed during high voltage plasma discharge events and by subsequent formation of molten pool of alumina, and rapid quenching in cooled electrolyte. These pore features can be easily sealed to achieve significantly improved corrosion protection [15, 16]. In addition, the low roughness and very high hardness of these coatings can offer potential benefits in extreme environments where very high corrosion together with high wear

10

resistance are required. Furthermore, components having difficult shapes and corners can be uniformly coated using the PEO process, thus making this process attractive for highly demanding applications where hard anodized coatings would struggle to meet requirements e.g. resistance to fretting and impact wear occurring e.g. in a space environment [15, 17], high hardness and corrosion protection and the requirements of thermo-optical properties [15, 16].

CONCLUSIONS The following main conclusions can be drawn from the present work:

1. The surface morphology of PEO coatings is characterized by existence of circular pancake and smaller structures, the center of each structure representing the discharge channel surrounded by circular rings of rapidly solidified molten pool of alumina. The resulting surface is relatively smooth with Ra < 1µm in as- prepared condition, which can be further polished if required down to Ra 0.05µm.

2. The PEO process gives particularly good coverage of edges and corners compared

to a hard anodizing process. The PEO process is suitable for the surface modification of Cu and Li containing high-strength aluminum alloys.

3. Such surface conversion process involving plasma discharges result in the formation

of crystalline alumina with very high hardness of minimum 1400HV on A97075, A92099 and A92195 alloys which is about 3.5 to 4 times harder than a hard anodized coating.

4. Anodic polarization confirmed the PEO coatings to maintain high passivity as shown

by their high breakdown potential ‘Eb’ and ennobled ‘Ecorr’ even when unsealed, thus these coatings minimizing susceptibility to corrosion initiation and offering extremely good resistance to corrosion attack of the underlying high strength Al-Zn and Al-Li alloys. This was further demonstrated by exposures to salt fog environment.

5. Anodic polarization was able to indicate the weaknesses at corners in particular of

hard anodized coating which, even after hot water sealing, allowed immediate ingress of salt solution to the substrate thus displaying an active behavior and this was further supported by the presence of corrosion attack accentuated at corners after the salt fog exposure.

ACKNOWLEDGEMENTS

The authors would like to thank Dr V. Samsonov for the preparation of coated specimens. Dr B D Dunn from European Space Agency is acknowledged for the supply of Al-Li alloys.

REFERENCES 1. Wang H W, Skeldon P, Thompson G E and Stevens K, ‘Hard anodising of aluminium

alloys’, Proc. Aluminium surface science and technology, May 2000, 588-593. 2. Shrestha S, Dunn B D, ‘Plasma electrolytic oxidation vs anodising of Al alloys -

spacecraft applications’, Submitted in 2009 to Surface Engineering for Light Alloys - H Dong ed. Abington Publishing.

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3. ECSS-Q-ST-70-03C, July 2008, Black anodising of aluminium with inorganic dyes, ESA Requirements and Standards Division.

4. Goueffon Y, Arurault L, Mabru C, Tonon C and Guigue P, J of Materials Processing

Technology, 209 (2009) 5145-5151. 5. Suminov I V, Apelfeld A V, Ludin V B, Krit B L and Borisov A M, Microarc oxidation -

theory, technology and equipment, Moscow Ecomet 2005, ISBN 5-89594-110-9. 6. Yerokhin A L, Nie X Leyland A, Matthews A and Dowey S J, Surf. & Coat. Technology,

1999, 122, 73-93. 7. Curran J A and Clyne T W, Surf. & Coat. Technology, 2005, 199, 168-176. 8. Curran J A and Clyne T W, Acta Materialia, 54 (2006), 1985-1993. 9. Dearnley P A, Gummersbach J, Weiss H, Ogwu A A and Davies T J, Wear, 1999, 225–

229, 127-134. 10. Dahm K L, Black A J, Shrestha S and Dearnley P A ‘Plasma Electrolytic Oxidation

treatment of aluminium alloys for lightweight disc brake rotors’, submitted to Braking 2009, 9-10 June 2009, IMechE.

11. Mistry K, Priest M and Shrestha S, ‘The potential of plasma electrolytic oxidised eutectic

Al-Si alloy as a cylinder wall surface for lightweight engine blocks’, IMechE Proceedings, Tribology 2008: Surface engineering of automotive powertrains for environmentally friendly transport, July 2008.

12. Rama Krishna L, Sudha Purnima A and Sundararajan G, Wear 261, (2006), 1095-1101. 13. Choppy K J, Kovar R F and Cushman B M, ‘Fretting wear-resistant micro-arc oxidation

coatings for aluminium and titanium alloy bearings’, AFRL-ML-WP-TP-2007-443. 14. Rao V D N, Cikanek H A, Boyer B A, Lesnevsky L N, Tchernovsky N M and Tjurin N V,

Friction and wear characteristics of micro-arc oxidation coating for light weight, wear resistant, powertrain component application, SAE Technical Paper Series 970022.

15. Shrestha S, Merstallinger A, Sickert D and Dunn B D, ‘Some preliminary evaluations of

black coating on aluminium AA2219 alloy produced by plasma electrolytic oxidation (PEO) process for space applications’, Proc. 9th Intl. Symposium on Materials in a Space Environment, 16-20 June 2003, The Netherlands, 57-65.

16. Shrestha S, Shashkov P and Dunn B D, ‘Microstructural and thermo-optical properties of

black Keronite PEO coating on aluminium alloy AA7075 for spacecraft materials applications’, Proc. 10th Intl. Symposium on Materials in a Space Environment, Collioure, France, 19-23 June 2006 (ESA SP-616).

17. Merstallinger A, Semerad E and Dunn B D, ‘Influence of coatings and Alloying on Cold

Welding due to Impact and Fretting’, Proc. 9th European Space Symposium on Material in a Space Environment, ESTEC Noordwijk (NL), June 2003.

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TABLE 1 TYPICAL PROCESS PARAMETERS DURING PEO AND HARD ANODIZING

Coating process

Alloy UNS

Pre-treatment Coating electrolyte

Total salt content

(%)

Typical pH

Nominal thickness

(µm)

Coating rate

(μm/min)

Voltage

(V)

Current density(A/dm2)

Processtemp (°C)

Post-treatment

Coating formation method

Surface appearance

PEO (AC)

A97075

A92195

A92099

Degrease in alkaline solution

Proprietary

alkaline free of Cr, V, Ni or other heavy

metals

< 4

10-12

50-55

30-40

30-40

1 - 2

300 - 900

Up to 30

14 to 20

Silicate sealing

None

None

Plasma oxidation

Grey to charcoal

black

Hard Anodizing (DC)

A97075

Degrease and chemical clean

in alkaline solution

Sulfuric acid

H2SO4

15 - 20

<3

50-60

0.8 - 1

45 - 50

1-2

-10 to 0

Hot water sealing

Oxidation without plasma

Bluish black

Black Anodizing (DC)

A97075

Vapor degrease in C2HCl3, etch in Na3PO4 +

Na2CO3 at 90-95°C, rinsing, deoxidising in

HNO3, rinsing

Sulphuric acid

H2SO4

12-18

< 3

25-35

< 1

< 50

1-2

25±1

Cobalt acetate or

Nickel acetate

solutions

Oxidation without plasma

Black

obtained by inorganic

dye

13