Pitting and Stress Corrosions of Aluminum Alloys; Correlation between Them

It is found that pitting corrosion evolves solely in solutions bearing monovalent anions. This fact is associated with the specific features of one-step dissolution. The pitting corrosion of aluminum typically evolves over its (100) crystallographic planes. Such crystallographic type of corrosion gives way to other types with an increase in the pitting initiation potential and the content of alloying elements. The obtained curves for the pitting initiation potential as a function of the solution strength show regions of passivity, a linear dependence, and the transition to the value of the pitting initiation potential measured in pure water. Common features in pitting and stress corrosions evolving from passive state are revealed. However, for stress corrosion, the factors which accelerate depassivation are dislocation piles-up at grain boundaries and hydrogen, which arises by corrosion reactions and weakens intergranular bonds. Under conditions when pitting corrosion shows up as local tunnels directed toward phase inclusions, transgranular corrosion cracking is observed; whereas, under conditions when pitting corrosion evolves on grain boundaries, intergranular cracking is observed.