There are references claiming that during the cataphoretic deposition of organic coatings, if the substrate metal is amphoteric (Al, Zn), it will corrode due to very alkaline pH near the cathode. But I am asking, since we provide very negative voltage to the substrate, will it not be cathodically protected from corrosion?
It depends on how negative a potential you are applying, both Zn and Al are active metals that will disolve in basic conditions. It also depends on what the organic molecules are as some organics can speed up or catalyze the corrosion.
Check the Pourbaix diagrams for a starting point.
The ohmic drop through the coating shall cancel most of the applied potential thus, if some alkaline electrolyte remains trapped at the metal-coating interface, corrosion can occur.
We apply very negative potential, because the cell voltage is 250V. The potential is so negative that water reduction occurs, and pH near cathode goes to 12. But yes, organic coating is very good insulator, so voltage drops very fast, and probably in coating pores metal can dissolve. We deposit epoxy resin.
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