Synergistic action of fatigue and corrosion during crack growth in the 2024 aluminium alloy

Abstract

This study was undertaken with the aim of identifying the domains where synergistic interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024, as well as the rate controlling factors. Corrosion fatigue crack propagation tests have been carried out at different frequencies and/or waveforms, grain orientations and tempers, in air, distilled water and 3.5% NaCl solution at free potential. In saline solution in the S-L orientation, crack growth rates are shown to decrease with decreasing frequency. In addition, at high frequencies, these growth rates are substantially higher than in air and in distilled water. This enhancement is accompanied by a typical change in the shape of the dadN curve. It is demonstrated that this enhancement, that is only observed in metallurgical conditions of high sensitivity to stress corrosion cracking, is controlled by the loading rate, and not by the duration of exposure to the corrosive medium. The behaviour is discussed on the basis of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. It is finally proposed that the fatigue crack growth enhancement observed is related to a crack-tip embrittlement mechanism by hydrogen produced by anodic dissolution in relation with film rupture periodicity.