Module Six of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

The chronology of a Crevice

Stage Three, Obstruction and Electromigration

Stage three of a crevice formation

In stage three of the crevice development a few more accelerating factors fully develop:

• The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products. The pH in a crevice can reach very acidic values, sometimes equivalent to pure acids.

• The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. See, for example, how the corrosion of steel is affected as a function of water pH.

• The corrosion products seal even further the crevice environment.

• The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in the environment, such as chlorides and sulfates, that can be corrosive in their own right.