Corrosion protection of thickness reduced plasma electrolytic layers on AZ31

Abstract

In the present study plasma electrolytic oxidation of magnesium in fluoride free alkaline solution were used to form thin oxide layers with high corrosion protection. The relation between potassium hydroxide and sodium silicate was varied in the electrolyte. The plasma electrolytic process was performed as current controlled unipolar pulse technique. The microstructure of the formed layers were investigated by X-ray diffraction and scanning electron microscopy. The electrochemical behavior was studied by linear sweep voltammetry and impedance spectroscopy. The thickness of the formed oxide layers was in the range of 1–2 µm. The surface morphology strictly depends on the electrolyte composition. Oxide layers formed in an electrolyte with the lowest relation KOH: Na2SO3 show the best performance in view of corrosion protection

Nano‐sized zeolite particles as inhibitor carrier in plasma electrolytic oxide layers on AZ31

“…Sirivasan et al concluded that thin but compact coatings formed on magnesium at low current densities during the PEO process offer a better corrosion protection. This conclusion is confirmed by Schneider et al, which have recently studied the electrochemical behavior of plasma electrolytically formed oxide layers on AZ31 with a thickness of d ≈ 2 μm. The corrosion current densities in aerated 0.01 M NaCl solution amount to j corr ≈ 0.03 up to 0.1 μA/cm 2 and the passive current densities to j p ≈ 0.08 and 1.1 μA/cm 2 .…”

“…Figure shows the open circuit potential in aerated 0.01 M NaCl over 180 min. The open circuit potential of the PEO anodized samples is only slightly more positive than the ocp measured on bare AZ31 under the same conditions (−1.6 V SCE ) and the difference between the layers formed in a bath with and without suspension of inhibitor‐loaded zeolites is marginal. More interesting is the comparison of the linear sweep voltammograms as shown in Figure .…”

“…Besides the expected elements Mg and O (from the layer and the substrate) significant signals of Ce and Cl are detectable. Since the accumulation of chloride seems to confirm the mechanism of pitting corrosion it is assumed that the corrosion attack on PEO layers starts on preferred sites, e.g., cracks or especially structural pores …”

“…Based on the literature (e.g.,) and own earlier experiments the authors decided to use an anodizing bath consisting of 10 g/L KOH and 10 g/L Na 2 SiO 3 in aqueous solution (pH 14) with and without an inhibitor containing additive. The concentration of the additive suspension in the anodizing bath amounts to 1.3 g/L cerium containing zeolite‐X ().…”

The electrochemical behavior of Mg–9Al–0.5Zn, Mg–9Al–0.7Zn, and Mg–9Al–1.0Zn in a NaCl solution

“…Therefore, Mg–H 2 O 2 semi‐fuel cells have been applied as attractive submarine power sources . However, because pure Mg as the anode material naturally causes self‐discharge reactions and lag effects, it has the following disadvantages: (1) low‐current efficiency, (2) weak‐corrosion resistance, and (3) long‐occurrence time . Considering that the corrosion process is controlled by the pitting corrosion rate, these problems can be solved by doping other elements (such as zinc, aluminum, and manganese) into the magnesium alloys …”