What are the Causes for the Passivation of MMO Coated Titanium Anodes?

What are the Causes for the Passivation of MMO Coated Titanium Anodes?

Passivation failure is caused by the formation of a nonconductive oxide layer at the substrate/ coating interface resulting in increased resistance of the electrode and passivation of the interface.
There are several causes for the passivation of MMO coated titanium anode. For porous coatings, oxygenated species are able to penetrate to the underlying titanium substrate causing the formation of TiO2 at the substrate/coating interface. This causes an ohmic drop at the interface thereby increasing resistance at the interface. Factors governing coating porosity include film thickness, composition, and method of coating preparation.

The substrate/coating interface can also be passivated. The TiO2 usually present on the titanium substrate becomes doped by the RuO2(or IrO2) during the coating process. If the doping is insufficient due to low RuO2 (IrO2) content, then the potential drop that occurs during current flow across TiO2 causes it to grow thereby passivating the interface. Passivation of the substrate/coating can be minimized by etching the Ti substrate with about 60% H2SO4 to form titanium hydride at the titanium surface.

Failure can be caused by the deposition of foreign materials (MnO2, Fe2O3, oxysalts, etc) on the surface of the coating. This leads to blinding of the active coating area, poisoning, or passivation of the coating. The effective surface area of the coating under consideration is reduced thus increasing the effective current density for the active coating. This method of coating deactivation can be common in industrial practice. This can be mitigated by periodically cleaning the electrode.