Effect of Plasma Electrolytic Oxidation on the Short-Term Corrosion Behaviour of AZ91 Magnesium Alloy in Aggressive Chloride Environment

Abstract

In order to increase the corrosion resistance of magnesium alloy AZ91 in corrosion environments containing chlorides, the alloy surface has been modified by plasma electrolytic oxidation (PEO). The chemical composition of electrolyte in the PEO process consisted of 12 g/L Na3PO4·12 H2O and 1 g/L KOH, and a direct current was applied to the sample. The corrosion resistance of PEO coating and as-cast AZ91 (sample without PEO coating) was assessed using two different electrochemical methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP) in 0.1 M NaCl at laboratory temperature. In addition to the electrochemical methods, the morphology of the oxidic coating was observed in the cross-sectional and top surface view by using the SEM technique. For better determination of the microstructure and PEO coating, chemical composition EDX analysis was used. The results of the experiments show that the formation of the PEO coating on AZ91 alloy has a more positive effect on the corrosion resistance in 0.1 M NaCl based on electrochemical methods than in the case of the formed coating on AZ31 alloy from the previous study. Based on electrochemical measurements in the selected environment, the formation of PEO coating on AZ91 was accompanied by a significant increase in polarisation resistance after short-term exposure compared to the as-cast surface. The EIS results showed a 73 times higher Rp value for PEO coated AZ91 when compared to the as-cast AZ91. Correspondingly, a 27 times lower icorr value was observed for PEO coated AZ91 than in the case of substrate AZ91 in 0.1 M NaCl. At the same time, the typically porous and inhomogeneous structure of the formed PEO coating on the magnesium alloy AZ91 was demonstrated.

Corrosion Behavior of Sensitized AISI 304 Stainless Steel in Acid Chloride Solution

“…The potentiodynamic polarization curves were recorded at the sweep rate of 1 mV/s [ 26 , 27 ], a potential scan range was applied between −0.3 and 0.9 V vs. open circuit potential (OCP). At least three experiment repeats were carried out for each specimen and the representative curve was selected.…”

The Effect of Mechanical Pretreatment on the Electrochemical Characteristics of PEO Coatings Prepared on Magnesium Alloy AZ80

“…However, the largest pores are typical for sites consisting of intermetallic phases, which are associated with a higher occurrence of discharges. On the other hand, the presence of molten oxides in the cold electrolyte and the gas bubbles generated during discharges cause the formation of micropores and microcracks [ 4 , 17 ].…”

“…To avoid possible errors in the interpretation, all experiments were repeated three times. Corrosion parameters were determined using Nyquist diagrams, which were quantitatively evaluated using the equivalent circuit technique with EC-Lab V11.10 software [ 5 , 17 , 18 ]. The schematics of circuits used are shown in Figure 2 .…”

The Effect of Yttrium Nitrate Content on the Microstructure and Properties of a Micro-Arc Oxidation Coating Prepared on a ZK61M Magnesium Alloy

“…It is widely employed in the manufacture of engine casings, radar shells, notebook shells, satellite brackets, wing skins for anti-aircraft missiles, and rudders. Furthermore, it has numerous applications in the fields of automotive and high-speed rail transportation [1][2][3][4]. However, the inferior chemical stability, corrosion resistance, and wear resistance of magnesium are significant limitations that must be addressed to enable its expanded use in industrial applications [5,6].…”