CORROSION BEHAVIOR OF 55Al-Zn COATED STEEL UNDER WET-DRY CYCLING CONDITIONS

摘要

The degradation of 55%Al-Zn coated steel exposed to alternate conditions of 1h immersion and 7h drying in NaCl solution at 60% RH at different temperatures and pH values has been monitored using AC impedance and potential measurements. The corrosion rate of the coated steel increases with increasing the temperature and lowering the pH of NaCl solution. Also the effects of ammonium sulfate and the specimen orientation on the corrdability of the coated steel were investigated. The reciprocal of polarization resistance was measured continuously under wet-dry cycling conditions. The corrosion rate of the coated steel increases rapidly at the initial stage and then decreases slowly at the last stage. Also the corrosion rate of the specimen exposed at 30° to horizontal is lower than that exposed horizontally facing upwards. A mechanism was proposed to describe the successive stages of the corrosion process under wet-dry cycling environment.