Micro-Arc Oxidation on Ti and Al Alloys: Basic Aspects

Micro-arc oxidation has been a hot topic in the field of surface treatment of aluminum, magnesium and titanium for about twenty years. It consists in a high-voltage oxidation beyond the dielectric breakdown occurring at the interface metal/oxide/electrolyte. Due to the high temperature reached in the resulting plasma, the grown coatings are dry ceramic layers, exhibiting interesting corrosion and wear resistance. This process is generally performed in alkaline media containing additives favoring both the growth rate and the corrosion resistance of the coated materials such as aluminates, phosphates or silicates.

In this study, low energy-consuming micro-arc oxidation treatments were successfully performed on aluminum alloys and titanium in KOH-based electrolytes by using a simple galvanostatic regime (with low current density: 10 mA cm^-2) in order to get basic knowledge on this high potential anodizing process.

The first steps of anodizing were investigated by combining in-situ electrochemical methods and post-treatment characterizations performed by FEG-SEM observations coupled with EDX analyses, grazing angle X-Ray diffraction and µ-Raman analyses. The dielectric breakdown of the interface, corresponding to the initiation to the micro-arc regime, appears around 230 V on titanium and 280 V on aluminum alloys. It occurs after a first “classical” anodizing step [1], inducing the formation of a first insulating layer with a thickness in the range of 150 to 250 nm, mainly constituted of anatase on titanium and amorphous alumina on aluminum alloys. The growth mechanism and kinetic as well as the physico-chemical properties of this inner layer determine the possible initiation of the micro-arc regime [2-3]. It depends on both the electrolyte composition and the nature and composition of the substrate. One of the key parameters is the pH of the electrolytic bath. On the other hand, the dielectric properties of this “classically anodized” layer are modified by the incorporation of elements from the electrolytic bath. In the specific case of copper-containing aluminum alloys, extensively used in the aircraft industry, the presence of copper-enriched intermetallic phases results in the presence of copper nanoparticules encapsulated in the anodic layer, catalysing water oxidation at the substrate’s interface The anodizing yield is therefore decreased and the dielectric breakdown is delayed.

In terms of corrosion resistance, the electrochemical behaviour of short-time anodized samples (only “classically” anodized) and longer-time anodized samples (micro-arc anodized) was investigated by stationary methods such as open-circuit measurements and potentiodynamic polarizations as well as electrochemical impedance spectroscopy. The corrosive media were chosen regarding the potential applications of these materials: Na₂SO₄0.1M for aluminium alloys destined for aircraft industry and acidified artificial saliva for titanium, materials of choice for dental implants. In both cases, the improvement of the corrosion behaviour is due to the enhancement of the charge transfer resistance through the thin inner layer grown before the initiation of the micro-arc regime.

[1] TH The, A Berkani, S. Mato, P. Skeldon, GE. Thompson, H. Habazaki, K. Shimizu, Corros. Sci.45, 2757-2768 (2003)

[2] D. Veys-Renaux, E. Rocca, G. Henrion, Electrochem. Comm. 31, 42-45 (2013)

[3] D. Veys-Renaux, E. Rocca, J. Martin, G. Henrion, Electrochim. Acta 124, 36-45 (2014)