Effect of oxidation time on black MAO coatings

Abstract

A highly corrosion-resistant black coating was successfully fabricated on the surface of LA103Z magnesium-lithium alloy utilizing micro-arc oxidation (MAO) technology. The novel black coating was developed by employing Cu₂P₂O₇ and Co₂SO₄ as colorants for the first time, with the optimal process parameters determined through the adjustment of oxidation time. The phase composition, morphological characteristics, and elemental composition of the ceramic coatings were comprehensively analyzed using SEM, XRD, and XPS. To evaluate the corrosion resistance of the ceramic coatings, immersion experiments and electrochemical tests were conducted in simulated seawater. The results revealed that the MAO ceramic coatings were predominantly composed of MgO and Mg₂SiO₄, with CuO and Co₃O₄ contributing to their black appearance. When the oxidation time is 10 min, the coating has an excellent apparent quality and is shown to be in its darkest state. Electrochemical tests demonstrated that the 10-min MAO coating exhibited the highest corrosion potential −1.601 V and the lowest corrosion current 9.885 × 10⁻⁷ A/cm² compared to coatings produced at other oxidation times. This finding indicates a strong correlation between oxidation time and corrosion resistance. Furthermore, immersion tests in simulated seawater for 168 h showed significantly less corrosion product formation on the 10-min MAO coating. This observation provided additional evidence of the superior protective properties of the 10-min MAO coating in simulated seawater environments.