Deterioration of mechanical properties for pre-corroded AZ31 sheet in simulated physiological environment

The evolutions of uniaxial tension properties and the ratcheting fatigue behavior of AZ31 magnesium alloy sheet pre-corroded in the phosphate buffered solution (PBS) stimulated physiological environment with increasing immersion time were investigated in this study. The corrosion behavior of AZ31 sheet was also studied. It was found that the corrosion rate decreased exponentially with immersion time until stabilized at about 0.05g/m²h after 28 days immersion. The corrosion behavior was characterized by pitting corrosion and progressive pitting corrosion in the direction of pits depth was gradually inhibited. The Young's modulus and the elongation of AZ31 sheet decreased exponentially with increasing immersion time and became low levels with reductions of around 10% and 20% respectively at 28 days immersion. The ultimate strength, however, decreased no more than 5%. With increasing immersion time, both the ratcheting strain and ratcheting strain rate in the stable stage of ratcheting strain evolution tended to increase. 28 days degradation decreased the fatigue lives by a factor of about 2.5-5. The pre-corroded damage defined as life reduction after degradation on the basis of the Miner's linear damage rule (LDR) was found to increase exponentially with immersion time regardless of the loading conditions. A ratcheting fatigue life prediction model based on the Miner LDR, the Smith-Watson-Topper (SWT) parameter and the Basquin's equation was proposed and yielded good prediction for the pre-corroded AZ31 sheet.