What Does Transpassivation Mean?
Transpassivation is a phenomenon in which a passivated metal starts rapid dissolution (increase in corrosion rate) if the metal's electrode potential becomes too positive. Transpassive dissolution of metals is closely related to passive film breakdown and localized corrosion.
The level of solution oxidizing power that causes transpassivation is known as the transpassive region.
Corrosionpedia Explains Transpassivation
If a metal is immersed in an acid solution and oxidizing power of this solution is increased, the corrosion rate of metals continuously increases.
A slight increase in the solution's oxidizing power causes a corresponding rapid increase in the corrosion rate. But, as more oxidizing agent is added, the corrosion rate shows a sudden decrease, due to the formation of a highly protective but very thin film on the metal surface, making it noble. The film is non-porous, insoluble and self-healing in nature. This corresponds to the beginning of the passive region.
At very high concentrations of oxidizers, or in the presence of very powerful oxidizers, the corrosion rate again increases with increased oxidizing power. This region is known as the transpassive region, where the thin film will dissolve and the corrosion rate increases again.
Metals showing passivity exhibit outstanding corrosion resistance in oxidizing environments. Some of the metals showing passivity include titanium, aluminum and chromium.