Corrosion degradation of AZ31 magnesium alloy coated by plasma electrolytic oxidation

Corrosion Behaviour of Preserved PEO Coating on AZ31 Magnesium Alloy

“…The electrochemical characteristics obtained by their analysis are shown in Table 3 and the graph of evolution of the R p values obtained on the sealed PEO coating after individual exposure times in 0.1M NaCl solution is shown in Figure 5. After 1 hour of exposure to the electrolyte, the sealed PEO surface reached more than four times higher R p value compared to the simple PEO layer [23], which fully corresponds to the results of kinetic corrosion parameters evaluation within the PDP measurements. This improvement is primarily caused by the physical barrier effect of the added preservative in the initial stages of exposure.…”

“…The SEM image of the PEO coating prepared on tested AZ31 magnesium alloy in accordance to authors' experience from previous studies [21][22][23] is presented in Figure 2. The PEO layer exhibits a porous structure typical for the PEO process.…”

“…Evaluation of the PEO surface treatment effect on the corrosion resistance of magnesium alloy AZ31 in 0.1M NaCl solution, using PDP and EIS tests, was already performed in authors' previous studies [21][22][23]. Some of these results are used in the following discussion.…”

“…As can be seen from Figure 5, the decrease in the total polarization resistance was very sharp in the first stages of exposure up to 12 hours, which was mainly related to weakening of the barrier effect of the preservative and PEO coating, saturation of the surface system with electrolyte and degradation of the soluble MgO (its dissolution process in NaCl solution is described elsewhere [35][36]) representing the substantial part of created PEO coating [37]. Subsequently, the decrease was significantly inhibited to near saturation trend, which was caused by action of the corrosion inhibitors retarding the corrosion processes at the base material / electrolyte interface and also by sealing the critical areas by corrosion products forming a certain barrier to the corrosion processes (an effect similar to that observed on unprotected material during longterm exposure [23].) This theory is supported by values and trends of individual partial electrochemical corrosion characteristics.…”

“…The process of PEO preparation was based on authors' experience from previous studies [21][22][23]. The plates of AZ31 (5 mm thick) were ground by an emery paper p1000 to provide the same roughness across the treated surface.…”