Effects of Prior Corrosion and Stress in Corrosion Fatigue of Aluminum Alloy 7075-T6

The nucleation of fatigue cracks from corrosion pits was investigated by evaluating the effects of two variables on the fatigue life of dog-bone specimens of aluminum alloy 7075-T6 (UNS A97075). The specimens were exposed to different levels of corrosion in an acidified saline solution of 3.5% sodium chloride (NaCl). In addition, the specimens were exposed to concomitant fatigue and corrosion exposure during fatigue until failure by fracture occurred. Scanning electron microscopy (SEM) analysis indicated that fatigue cracks formed/nucleated from each pit, and subsurface mechanisms of degradation were identified associated with the pitting nucleation sites including subsurface pitting, cracking, tunneling, and intergranular attack. Failure data was analyzed using analysis of variance (ANOVA) methods, and three transformations were evaluated to minimize the variance, including natural log, inverse square root, and power with a lambda of 1/3. Contour and surface plots were developed to show how these variables impact the response of cycles to failure for the conditions evaluated. Results indicate that p values for these models are statistically significant. The effect of stress appears to be more detrimental than corrosion time on the fatigue life of the specimens for the values previously defined by the matrix.