For a certain coating on Mg alloys I m getting more negative potential but improved corrosion rate than the bare Mg alloys. what makes this condition possible?
If you assume hydrogen evolution reaction is the cathodic reaction, and the exchange current density (i0) for this reaction on the coating is much smaller than that on the bare Mg-alloy surface, and apply mixed potential theory then you will notice that the Ecorr becomes more negative and icorr be smaller than that of the bare alloy (similar Tafel slopes in both conditions). Similar observation could be made by making the Tafel slope of the cathodic reaction for the coated surface steeper than that of the bare alloy (for a similar exchange current density) .
In brief, the cathodic reaction on the coated surface is slower than that on the bare alloy. Other scenarios are also possible. I am giving just one example. I assume that the anodic reaction to be oxidation of Mg, which need not be the case for the coating.
Hi
This could happen due to corrosion activity is taking place at the interface between the coating and the Mg-alloy, and in actuality, there is no apparent of corrosion. There is always a present of coating holiday. So, try to inspect for coating holiday by conducting a coating holiday test after applying the coating on the Mg-alloy. Refer to the following link:
https://www.youtube.com/watch?v=dFupMZL-YzY
an increase in corrosion potential can be attributed to a decrease in the anodic reaction with the growth of a passive film or the increase in the cathodic reaction with an increase in dissolved oxygen. A decrease in corrosion potential can be attributed to an increase in the anodic reaction or a decrease in the cathodic reaction
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