The Electrochemical Corrosion Behavior And Mechanism Of Microarc Oxidation Coatings On Magnesium Alloys

Micro-arc oxidation (MAO) is a new surface treatment technology developed on the basis of the conventional anodizing. The surface of valve metals, such as Al, Mg, Ti and their alloys, can convert to the ceramic coatings in situ under the conditions of plasma chemical, electrochemical, thermal-chemical effects. The oxide coatings prepared by this technique have a dense structure, high adhesion and excellent integrated mechanical properties. In this work, A ceramic coating with thickness of30±2μm was obtained on AZ91D magnesium alloy in alkaline silicate electrolyte by a micro-arc oxidation (MAO) technique. The coating consisted mainly of MgO and Mg2SiO4, which distributed into two layers, a porous outer layer and a dense inner layer. The corrosion behavior of the MAO coating on magnesium alloys was studied in different environments by electrochemical and surface analysis method,. Appropriate equivalent circuits were proposed. And the corrosion models were also established, which provide an experimental and theoretical basis for the further application of MAO coated on magnesium alloys.The corrosion behavior of the MAO coating on magnesium alloys in NaCl solution was studied. The results showed that the value of B in this system was0.04V. And the degradation of the MAO coating immersion in3.5%NaCl solution can be identified with three continuous steps. In the initial stage, NaCl aqueous solutions was penetrated into the MAO coating and infiltrated through the micropores or microcracks in the out porous layer by diffusion and reached the interface of out/inner layer quickly; As corrosive ions, chloride ions were absorbed preferentially and replaced partial oxygen site, which led to formation of some soluble species.Whereafter a lot of metastable pits appeared. In the middle stage, the hydration products Mg(OH)2hardly filled the metastable pits. It was "self-sealing" process and prevented the exposure of magnesium alloys substrate. In the later stage, several metastable pits developed horizontally and vertically in the MAO coating and finally joined together to form a larger pit, which became macroscopic pores at last. The MAO coating on magnesium alloys had a better corrosion protection in alkaline NaCl solution. And in acidic NaCl solution, the MAO coating provided no corrosion protection to the metal substrates. The main form of corrosion failure was localized corrosion for the MAO coated immersed in higher concentration NaCl solutions (1.0%,3.5%and5.0%), while it was general corrosion in dilute NaCl solutions (0.1%and0.5%). The corrosion rates of the MAO coating on magnesium alloys increased with increasing chloride ion concentration in NaCl solutions. A mathematical model was established as follows:Coefficients G and H related to the concentration of corrosive ions (Cl) and f(k) is about the process of preparation.The MAO coating on magnesium alloys was found having better corrosion resistance in neutral0.1mol/L Na2SO4solution. The corrosion rate of the MAO coating increased with increasing chloride ion concentration. Generalized corrosion was apparent on the MAO coating at immersion initial stage. Localized corrosion occurred when the samples were immersion in0.1mol/L Na2SO4+3.5%NaCl solution for more than120h. The MAO coating was found having a excellent corrosion resistance in neutral0.1mol/L NaNO3solution. The MAO coating was found having a much superior corrosion resistance in alkaline solutions. More ethylene glycol adsorbed in the MAO coating and the charge transfer resistance Rct increased with increasing concentration of ethylene glycol, and the corrosion resistance of MAO coating is improved. Corrosive ions (Cl-and SO42-) coexisting in the solution enhance corrosivity of the MAO coating. The corrosion of MAO coated magnesium alloys is effectively inhibited by addition of0.2mol/L NaF. The inhibition effect of the fluoride could be ascribed to the formation of a protective fluoride-containing crystal which fills the porous layer in the MAO coating. With the temperature increasing, more ethylene glycol molecules desorbed from the MAO coating. Chloride ions replaced ethylene glycol and led to severe corrosion. The main form of corrosion was localized corrosion for the MAO coated magnesium alloys immersed in Hank’s solution influenced by chloride ion.The effects of six anions, WO42-、MoO42-、F-、SiO42-、PO43-and H2PO4-on corrosion behavior of MAO coating on magnesium alloys were investigated in3.5%NaCl solution by electrochemistry measurements. The results showed that F-、SiO42-、PO43-and MoO42-could retard the pit propagation. The inhibition decreased in the order PO43-> F-> SiO42-> MoO42-. While MoO42-and H2PO4-stimulated the pit growth, and the order is MoO42-<H2PO4-. This could be explained by the competitive adsorption of anions on the active site of the MAO coating with chloride ions.Six MAO coatings with two kinds of thickness,25μm and40μm, were prepared on magnesium alloys under the constant voltage craftwork regarded positive voltage as key parameter. And their corrosion behavior were studied immersion in3.5%NaCl solution. The results showed that corrosion rate of the MAO coating decreased with increasing voltage and had less affected by the MAO coating thickness. The measured data indicated that the mathematical model of Icorr～t was accurate and had universality.