Microbial influenced corrosion behavior of micro-arc oxidation coating on AA2024

Aluminum alloy is widely used in engineering and aerospace applications, and is prone to microbiological influenced corrosion (MIC). Micro-arc oxidation (MAO) has previously been shown to be an effective method to resist corrosion. The aim of this study is to investigate the MIC behavior of MAO coatings on AA2024 in the present of sulfate reducing bacteria (SRB). SEM and epifluorescent microscopy were applied to show the surface of the MAO coatings and distribution of the SRB biofilms. The open circuit potentials (OCP) and potentiodynamic polarization curve indicated that the ceramic coatings effectively inhibited the corrosion reaction of the AA2024 in culture medium inoculated with SRB and without SRB. Electrochemical impedance spectroscopy (EIS) was used to further illustrate the electrochemical characteristics of MAO coatings. The results indicated that the outer layers were defective and the inner layers were relatively compact and intact. We suppose the outer layer of 80 μm MAO coating was so porous that allowed the transportation and localization of the SRB biofilm and/or its metabolite, as well as the corrosive medium, which damaged the protective ability of the inner MAO layer. The MIC behavior of AA2024 with 30 μm MAO coating was a diffusion controlled electrochemical process. Compared with the 80 μm MAO coating, the 30 μm MAO ceramic coating provided more effective protection for the substrate.

► The MAO coating on AA2024 contained a compact inner layer and a porous outer layer.
► The outer layers were defective and the inner layers were compact and intact.
► The outer layer allowed the transportation and localization of aggressive substances.
► The MIC behavior of AA2024 with 30 μm MAO coating was diffusion controlled.
► The 30 μm MAO coating provided more effective protection for the substrate.