Six stages of crevice corrosion

1.Formation of crevice by crevice former which can be either man made by design or by nature -oversight.
Some examples of man made ( desigh basis) crevice formers are : stationary "O" ring in "O" ring groove,Gasket surfaces,Tube to tube sheet joint of heat exchangers etc.
Some examples of man made by oversight are: poor root pass of pipe weld joint ,roughened surfaces,natural sediments and deposits.
2.Oxygen depletion. It has been recognized that oxygen is depleted in well shielded crevices creating an oxygen concentration cell between crevice and area out side the crevice.
3.Hydrolysis of metal ions and decrease in pH. The few metal ions entering the moist environment of crevice hydrolyze depleting the hydroxyl ions.The pH decreases.
4.Migration of chloride ions in to the crevice from outside of crevice region to balance the charge resulting from depletion of hydroxyl ions.
5.Initiation of crevice corrosion if pH decreases sufficiently and chloride ion concentration increases sufficiently to a critical values (Threshhold value) at which film break down  and corrosion initiation will occur
6.Propagation.