Permanganate-Fluorozirconate conversion coating as precursor for Flash-PEO on AZ31B magnesium alloy

Abstract

The corrosion resistance of AZ31B magnesium alloy was evaluated after a two-step surface treatment consisting of a permanganate–fluorozirconate conversion layer followed by Flash-PEO in a silicate–fluoride (SiF) electrolyte. Electrochemical tests after 24 h immersion in 0.5 wt% NaCl demonstrated that the structured coating reduced the corrosion rate when compared to the reference Flash-PEO SiF coating ((3.4 ± 1.0) × 10−6 mm yr−1 versus (1.3 ± 0.8) × 10−5 mm yr−1). After 72 h, this trend persisted, with impedance values at low frequency one order of magnitude higher than its counterpart without precursor layer ((1.4 ± 1.0) × 107 Ω cm2 versus (2.6 ± 1.0) × 106 Ω cm2). Notably, Flash-PEO coatings in both cases preserved electrochemical integrity even in the presence of pre-existing surface defects. The permanganate–fluorozirconate precursor layer promoted, after Flash-PEO processing, a distinctive “coral reef-like” morphology with reduced interconnected porosity, thereby enhancing barrier properties by limiting substrate exposure to the corrosive environment. Complementary characterization confirmed the incorporation of zirconium and manganese species, along with compounds typically derived from the Flash-PEO SiF electrolyte.