Stress corrosion cracking behaviour of some aluminium-based metal matrix composites

This paper deals with the study of the corrosion behaviour of MMCs with AA 6061 or AA 2014 aluminium alloys as matrixes, under the combined action of a sustained elastic tensile stress and a corrosive environment, namely 3.5% NaCl solution. Such behaviour was compared with that exhibited by the corresponding matrixes. Gravimetric corrosion rates were evaluated after alternate exposures to 3.5% NaCl salt spray or to 3.5% NaCl solution. Micrographs of specimen sections obtained by the scanning electron microscope after the alternate exposure tests indicated the onset of stress corrosion cracking on both the AA 6061 matrix and the Al2O3p-reinforced AA 2014 composite. Electron Probe Microanalysis on the crack and pit surfaces and on the material bulk gave a contribution to determine the cause of the corrosion damage. An attempt was also done to apply the electrochemical noise analysis to evaluate the susceptibility of the various materials to stress corrosion cracking, during continuous immersions in 3.5% NaCl solution, in the presence of stress applied. The results were compared with those obtained with unstressed specimens. The tensile stress produced an increase in the corrosion rate which was clearly detected by the noise analysis. However, the concomitant presence of pitting (and in some cases of intergranular corrosion) and stress corrosion made difficult to distinguish between specimens suffering from the various localized corrosion forms.