Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454-H32 was studied under laboratory air and 3% NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3% NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long-life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long-life (low-stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in A1 5454-H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long-cycle region.