Concurrent changes in constituent and B type-like crack of PEO-treated Al-oxide layer by static post-annealing

Abstract

The concurrent changes of both constituent and critical defect in the Al-oxide layer formed by plasma electrolytic oxidation (PEO) down to 268 K, at which the occurrence of discharge channel would be inhibited effectively, were investigated with respect to post-treatment at annealing temperatures ranging from 473 to 673 K. With an increase in annealing temperature, the appreciable defect-channels of relatively dense PEO-treated layer was detected at 673 K since the dehydration reaction from α- and γ-Al(OH)3 to γ-Al2O3 gave rise to the change in lattice volume. Such defect was defined as B type-like channel that would accelerate the corrosion failure by harmful Cl− ions. In contrast, the corrosion potential of PEO-treated layer increased by lowering the Al(OH)3/γ-Al2O3 ratio due to thermal transformation. Hence, the excellent corrosion resistance was attained on the PEO-treated sample annealed at 573 K where the concurrent evolutions of constituent compounds and B type-like crack were in balance.