Electrochemical methods study on corrosion of 5% Al-Zn alloy-coated steel under thin electrolyte layers

The corrosion behavior of a 5% Al-Zn alloy (GF) coated steel was investigated under cyclic wet dry condition using electrochemical techniques. The wet dry cycle was conducted by exposure to alternate condition of 1 h immersion in seawater and 7 h drying at ambient temperature. The polarization resistance, R(p) of the coating was monitored during the wet dry cycles by two points AC impedance method and the corrosion potential, E(corr) was measured only when the coating was immersed in seawater. Simultaneously, the electrochemical impedance spectroscopy (EIS) of the coating was obtained after it was immersed in different cycles of wet dry condition. The results obtained by two points AC impedance method had good agreement with those achieved from EIS technique, which proved that the two points AC impedance method was correct and an effective method for atmospheric corrosion study. The monitoring results indicated that the corrosion rate of GF coating firstly increased, then decreased slowly with time, and at last reached a relative steady state with local corrosion under the cyclic wet dry alternate condition.