Transpassive corrosion of galvanic couple (Mg-alloy/A380)

[sansei10]

Mg-alloy plate was fastened to Al 380 plate by Zn-plated steel bolt without washer. The above system was subjected to SAE J2334 corrosion test. After approximately 25 cycles I noticed signs of corrosion on the aluminum side instead of Mg-alloy.
I was told that this phenomenon is due to transpassive effect…
Since I'm not a guru in corrosion field I would like to know the nature of this effect.

 

[CoryPad]

The ASM Handbook defines transpassive region as:

The region of an anodic polarization curve, noble to and above the passive potential range, in which there is a significant increase in current density (increased metal dissolution) as the potential becomes more positive (noble).

and transpassive state as:

(1) State of anodically passivated metal characterized by a considerable increase of the corrosion current, in the absence of pitting, when the potential is increased. (2) The noble region of potential where an electrode exhibits a higher than passive current density.

Transpassive behavior is when a metal oxide is converted into metal ions, for example:

M_xO_y --> M⁽ⁿ⁺¹⁾⁺

Regards,

Cory

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