Cirrus Hybrid™ Coating Technology – a New, Environmentally-Friendly Approach to Functional Surfaces on Light Metals

Cirrus Hybrid™ coating technology – a new, environmentally-friendly approach to functional surfaces on light metals.

Fengyan Hou Cirrus Materials Science Limited Auckland, NEW ZEALAND Email: [email protected]

& Chris Goode Cirrus Materials Science Limited Auckland, NEW ZEALAND Email: [email protected]

& Junzhe Dong Department of Chemical and Materials Engineering The University of Auckland, NEW ZEALAND Email: [email protected] Abstract As the second most widely used metal group worldwide, light metals including aluminium, titanium, and magnesium alloys are essential to many industries and applications. All light metals require coatings to provide significant functional properties, especially corrosion resistance, however, many current coatings solutions are neither cost effective nor environmentally friendly. Cirrus patented Hybrid™ coating technology is an extremely thin, low cost alternative that offers excellent corrosion resistance along with superior electrical and mechanical performance. Cirrus Hybrid™ coatings and their production process are environmentally friendly, with no toxic consumables or by products such as Cr6+, Cd. Hybrid™ coating technology has a great compatibility with existing plating processes including Ni, Cu, Zn-Ni, Ag, Au, which offers wide commercial applications and this paper introduces the fundamentals of this new coating technology.

Introduction Light metals, including aluminium, titanium, magnesium, and their alloys have been widely applied in various industrial applications due to their low density, excellent strength to weight ratio, and good machinability.1,2 Unfortunately,, light metal surfaces require additional protection in practical applications due to the high chemical reactivity that leads to poor corrosion resistance which in turn deteriorates tribological properties.3, 4

Various techniques have been employed to prevent corrosion and improve the surface properties of light metals. These can be divided into two categories: the application of a protective coating, or direct modification of the surface itself.5,6 Deposition of a desired material is an effective way to protect the light metal substrate by providing a thin barrier layer to inhibit corrosive chemical attack7. Examples include physical vapour deposition (PVD), chemical conversion, electro or electroless plating, anodization, and organic coating.8-12 Although each of these methods can be effective in improving corrosion resistance, they have drawbacks, such as incompatibility between different protective layers,9 difficult surface preparation for direct plating due to the presence of an oxide layer13, or the use of harmful chemicals such as chromium or fluorine,3, 12. Ultimately, none of these techniques avoid eventual cathodic corrosion14.

Anodizing has been extensively reported as a useful technique to improve the surface properties of light metals, especially in high wear and heavily corrosive environments15,16. Hoche et al. found that anodized magnesium alloys only showed slight surface damage after 120h salt spray test17. Electrochemical anodization not only provides a corrosion barrier, but also creates a suitable base for post anodising surface treatment due to the unique adsorption properties of the porous or tubular anodic morphology.5 The most common post-treatment methods to improve the corrosion inhibiting properties of the anodic layer is sealing18. Nickel fluoride sealing and steam sealing have been shown to limit hardness and wear resistance reduction 14 while significantly improving corrosion properties. Anodized magnesium alloys with Al2O3 deposited by PVD exhibited better corrosion behavior than those treated with PVD CrN.17

In this paper, we present a newly developed, patented, light metal coating technique (Cirrus Hybrid™ coating technology) that combines anodization and electroplating to increase the corrosion resistance of light metals. The Hybrid coatings obtained have excellent platability which may be used as substrate for further electroplated layers to provide unique mechanical, optical or electrical properties. This method is environmentally friendly, cost effectively and produces versatile functional and structural materials capable of meeting a variety of needs.

Experimental Detail 6061 alloy aluminium substrates 3 cm x 5 cm were used for all experiments. The substrates initially prepared by grinding with increasingly fine emery paper from 600 grit to 1200 grit; samples were then pre-treated as outlined in Table 1. Thorough rinsing between each pre-treatment and process step was performed in running tap water. Constant voltage anodising was performed in a bath containing a combination of phosphoric, sulphuric and oxalic acids to produce a thin anodised film. The anodised film was activated for electroplating in a very dilute hydrogen fluoride solution. Initial plating was performed using a combination of constant voltage and constant current and using a current profile to ensure that the anodising pores were filled before a plated film was developed across the anodised surface. A second coating was deposited over the initial electroplated layer to provide functional surface properties.

Table 1:Experimental Details

Processing Step Parameters Comment Alkaline Cleaner 60 , 8 min Chemetall MG19-PA-A Acid Etch 25℃, 2 minutes PPS Probright-AL Desmut 25℃, 30 secs 50% Nitric acid 21 ℃ To create a film thickness Anodising Constant voltage at 60V of approximately 2 microns 10℃ mins Activate 25 , 60 secs Dilute HF solution 58℃ 5 microns including 2 Semi-bright Ni Ramp current 30 mins microns interlocked into Constant℃ current 12 mins the anodised surface Bright Ni Constant current 8 minutes 2 microns

Results and Discussion The schematic of the preparation process for the Cirrus Hybrid™ coating is shown in Figure 1(a). Well- aligned uniform metal oxide nanotube arrays (NTs) or nano-porous (NP) morphology is produced after the electrochemical anodizing process. Precisely controlled anodizing parameters allow the oxide barrier layer at the bottom of NTs that contact directly with metal substrate to be adjusted to a thickness in the order of a hundred nanometres. Consequently, the interlock layer metal plating process tends to commence at the NT base, where electric resistance is low compared with tube wall region, and plated layer grows from the bottom-up. As shown in Figure 1(b), regular Al2O3 NTs with a pore size of ~70 nm and length of 1.9 μm were produced via electrochemical anodization. After electrodeposition of nickel, small shiny nanoparticles randomly adorned the nanotube wall and a solid nickel nanorod grew up inside the tube as shown in Figure 1(c). The EDS spectra of cross- sectional region, shows an obvious Ni peak as well as Al and O, indicating the successful deposition of Ni into the tubular morphology. Comparing the concentrations of the elements to the tube geometry suggests that the nickel is completely filling the nanopore array. The image of cross- section area also demonstrates the coverage of Ni layer on the anodic Al2O3 NTs.

Figure 1: (a) Schematic of Cirrus Hybrid™- patented coating process; (b) surface view of Al2O3 nanotube arrays after anodization; (c) cross-sectional view of anodic Al2O3 nanotube arrays filled with bright nickel nanoparticles after electrodeposition

Salt spray testing demonstrates that a thin hybrid Al2O3 coating interlocked with a semi-bright Ni, and plated with a bright Ni overlay, significantly improves the corrosion resistance of the surface. As illustrated in Figure 2, there were no corrosion pits or obvious damage to the coating surface even after exposure to neutral salt spray for 102 h. The pre- and post-salt spray conductivity of the Hybrid coating is an indirect proof of the absence of corrosion with a constant value of less than 0.14 mΩ·cm-1 in both cases.

Figure 2: 6-micron Hybrid Al2O3 with semi-bright and bright Ni after 102 Hours of Salt Spray Testing

Adhesion is a principal factor that determines the stability and practicality of prepared coatings. As

shown in Figure 3(a) and (b), thin (6-micron) hybrid Al2O3 coatings with a semi-bright nickel interlock layer and a bright nickel functional layer did not peel off either after salt spray testing or after thermal shock testing, indicating that there is excellent adhesion between the hybrid coating and the metal substrate. Coating adhesion was verified using a standard crosshatched scribe test. Thermal shock was performed by subjecting samples to 5 cycles of rapid temperature change from 175 to - 65 with 30 minutes dwell time at the temperature extremes. ℃ ℃

Figure 3: 6-micron Hybrid Al2O3 with semi-bright and bright Ni scratch test after (a) salt spray and (b) thermal shock. While the information provided above concentrates on the creation of a hybrid coating on an aluminium substrate with a nickel electrodeposited layer(s), other research has shown that the hybrid technology is applicable to titanium substrates and that a variety of metals can be induced to coat in the anodising pores including, copper, silver, tin, and black nickel. Cirrus has also tried many different plated functional layers, some of which are discussed in the applications section below.

Thus, it may be concluded that the Cirrus Hybrid™ coating process is a novel method to provide light metal substrate with excellent corrosion resistance, high stability and good adhesion, which makes it suitable for structural materials or as a substrate for further coatings.

Application of Hybrid Coatings Hemispherical surface morphology. The selection of a semi—bright nickel as the interlock layer provides a level surface on which additional functional layers may be deposited; however, if we selected bright- nickel as the interlock layer, we can develop a novel hemispherical surface morphology (HSMC). Figure 4 is a SEM cross section view of a bright-nickel hybrid coating showing the surface morphology; as may be observed, after the pores are filled with nickel plating, hemispherical surface features with diameter of ~20μm will form and cover the anodising layer. The unique hemispherical shape of the nickel Figure 4: Hemispherical (HMSC) Coating Cross Section surface significantly reduces the coefficient of friction of the HMSC Hybrid coating (Figure 6) when compared to a standard bright nickel coating on aluminium (Figure 5).

Figure 5: Bright Nickel Coefficient of Friction

Figure 6: Hybrid Bright Nickel Coefficient of Friction

Figure 7: Wear Track Comparison - HMSC Hybrid (left), Bright Nickel (Right) As would be expected from the CoF results the wear track width for a HMSC Hybrid coating is much narrower than a conventional bright nickel coating (Figure 8).

Light absorption improvement. Figure 4 depicts the light absorption properties of a hybrid coating. In this case a black nickel bath (instead of a semi-bright Ni) was used to deposit an interlock layer into the anodising nanotubes. As may be observed, the light absorption in the range from UV to infrared is dramatically improved when compared with traditional black Ni on Al alloy. This improvement is as a result of reflectivity of the nanorod structure formed on the hybrid Al2O3 substrate.

Figure 8: UV-vis infrared light absorption of Hybrid™ Black Ni Functional Layers. The Cirrus Hybrid™ coating forms an outstanding substrate for the deposition of functional coatings. Coatings of nickel, tin, copper, silver, gold and a variety of alloys have been deposited on the hybrid coating substrate. These functional layers impart important decorative or protective attributes to the coating system, including hardness, wear resistance, conductivity, corrosion resistance, or antimicrobial properties.

The use of Cirrus Dopant™ is an additive to standard commercial plating baths that creates a stable suspension of nanoparticles and prevents particles agglomeration. The suspended nanoparticles co- deposit and act as a grain refining and dispersion strengthening agent, which increases the hardness of the coating. The use of Cirrus Dopant™ as the functional layer is well known and produces reliable results.

Cirrus have determined that doped plating baths can also be deposited directly into an anodized substrate as an interlock layer. Addition of a dopant to a soft coating provides the advantage of improving the wear resistance of the coating. This has been demonstrated with black nickel and with copper Hybrid coatings. Figure 5 compares the wear properties of Hybrid Black Nickel with a Cirrus Doped Hybrid black nickel.

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 The frictionThe of coefficient -0.8 -1.0 -1.2 -1.4 0 2 4 6 8 10 time min

Figure 9: Coefficient of Friction for Cirrus Hybrid Black Nickel (left) and Cirrus Dopant Enhanced Hybrid Black Nickel (right) Conclusion The Cirrus Hybrid™ coating process offers a new and advantageous approach to providing high performance thin coatings on aluminium and titanium substrates. Hybrid coatings have excellent adhesion, high conductivity and can provide many other important properties depending on the plating bath chosen for the interlocking plated layer. Among these are improved light absorption. The Hybrid coating also provides an excellent substrate for secondary plated coatings including those doped with Cirrus dopant.

1. Song, G. L.; Atrens, A., Corrosion mechanisms of magnesium alloys. Advanced engineering materials 1999, 1, (1), 11-33. 2. Svenningsen, G.; Larsen, M. H.; Walmsley, J. C.; Nordlien, J. H.; Nisancioglu, K., Effect of artificial aging on intergranular corrosion of extruded AlMgSi alloy with small Cu content. Corrosion science 2006, 48, (6), 1528-1543. 3. Song, Y.; Shan, D.; Han, E., High corrosion resistance of electroless composite plating coatings on AZ91D magnesium alloys. Electrochimica Acta 2008, 53, (5), 2135-2143. 4. Song, G.; Bowles, A. L.; StJohn, D. H., Corrosion resistance of aged die cast magnesium alloy AZ91D. Materials Science and Engineering: A 2004, 366, (1), 74-86. 5. Shi, Z.; Song, G.; Atrens, A., Influence of the β phase on the corrosion performance of anodised coatings on magnesium–aluminium alloys. Corrosion Science 2005, 47, (11), 2760-2777. 6. Witte, F.; Kaese, V.; Haferkamp, H.; Switzer, E.; Meyer-Lindenberg, A.; Wirth, C. J.; Windhagen, H., In vivo corrosion of four magnesium alloys and the associated bone response. Biomaterials 2005, 26, (17), 3557-3563. 7. 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. 8. Chen, F.; Zhou, H.; Yao, B.; Qin, Z.; Zhang, Q., Corrosion resistance property of the ceramic coating obtained through microarc oxidation on the AZ31 magnesium alloy surfaces. Surface and Coatings Technology 2007, 201, (9), 4905-4908. 9. Gadow, R.; Scherer, D., Composite coatings with dry lubrication ability on light metal substrates. Surface and coatings technology 2002, 151, 471-477. 10. Rudd, A. L.; Breslin, C. B.; Mansfeld, F., The corrosion protection afforded by rare earth conversion coatings applied to magnesium. Corrosion science 2000, 42, (2), 275-288. 11. Lian, J.; Li, G.; Niu, L.; Gu, C.; Jiang, Z.; Jiang, Q., Electroless Ni–P deposition plus zinc phosphate coating on AZ91D magnesium alloy. Surface and Coatings Technology 2006, 200, (20), 5956-5962. 12. Huo, H.; Li, Y.; Wang, F., Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer. Corrosion Science 2004, 46, (6), 1467-1477. 13. Lee, M. H.; Bae, I. Y.; Kim, K. J.; Moon, K. M.; Oki, T., Formation mechanism of new corrosion resistance magnesium thin films by PVD method. Surface and Coatings Technology 2003, 169, (Supplement C), 670-674. 14. Song, Y.; Shan, D.; Han, E., Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy. Materials and Corrosion 2007, 58, (7), 506-510. 15. Hoche, H.; Scheerer, H.; Probst, D.; Broszeit, E.; Berger, C., Development of a plasma surface treatment for magnesium alloys to ensure sufficient wear and corrosion resistance. Surface and coatings Technology 2003, 174, 1018-1023. 16. Dolan, S. E., Light metal anodization. In Google Patents: 2005. 17. Hoche, H.; Scheerer, H.; Probst, D.; Broszeit, E.; Berger, C., Development of a plasma surface treatment for magnesium alloys to ensure sufficient wear and corrosion resistance. Surface and Coatings Technology 2003, 174, (Supplement C), 1018-1023. 18. Hao, L.; Cheng, B. R., Sealing processes of anodic coatings—past, present, and future. Metal Finishing 2000, 98, (12), 8-18.