Invited
IMPEDANCE STUDY OF PROTECTIVE PROPERTIES OF CLEAN CONVERSION COATINGS, PLASMA ELECTROLYTIC OXIDATION AND PAINT APPLIED ON AZ31 Mg ALLOY

For magnesium, corrosion resistance is a concern and has to be fully addressed. Conversion coatings have been developed using substitutes to chromates with non carcinogenic Cr^VI free substances in order to have a low environmental impact.

Due to the very negative value of Ecorr for Mg, reduction of water and oxygen must be considered.

The effect of the conversion layer Phosphate –Permanganate for example (like Chromate) is found to inhibit the cathodic reaction of water reduction to hydrogen.

For stannate conversion coating in alkaline solution, pH increase provokes a potential increase due to passivation and anodic control dominates. This is the case also of anodic films obtained by plasma electrolytic oxidation when low concentration of stannates is added to the anodizing solution.

Analysis of impedance data allows us to obtain fast and sensitive data including mechanistic information concerning the action of the protective films. This technique is very efficient in determining defective areas in the coating and is applied with particular success in forecasting early deterioration of highly resistive surfaces.

Impedance spectra obtained for bare (polished) and protected AZ31 alloy by a conversion layer or by anodization and the equivalent electrical circuits used for numerical fitting of EIS data will be discussed. The most directly interesting impedance parameters from the point of view of corrosion, protection, diffusion and absorption are detected.

In the case of paint (epoxy) layer applied on the AZ31 substrate, the general model for a porous coating has been observed to be satisfactory.

The use of the conversion or the anodic oxide films as interlayer for a metallic or polymer coatings will be discussed.