Electrochemical corrosion monitoring of galvanized steel under cyclic wet-dry conditions

The corrosion behavior of a galvanized steel was investigated under cyclic wet-dry environments using electrochemical techniques. The wet-dry cyclic was conducted by exposure to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution (or Na2SO4 solution) and 7 h-drying at 60% RH. The polarization resistance R-p of the galvanized steel was monitored during the wet-dry cycles by AC impedance method, simultaneously, the corrosion potential E-corr was measured only when the specimen was immersed in the solution. The corrosion current density i(corr) of the zinc coating was estimated from the monitored R-p using Stern-Geary equation. The corrosion mass loss Delta M, which was obtained from the i(corr) vs time curve, was plotted vs the wet-dry cycle number. The average corrosion rate per cycle starts to decrease and the E-corr shifts in the noble direction, immediately before the red rust (FeOOH) appears on the coating surface. It seems that the underlying steel corrosion commences when the zinc coating does not act as sacrificial anode due to accumulation of zinc corrosion products on the coating surface. The electrochemical methods, such as impedance and corrosion potential measurements, are an effective tool for monitoring the degradation of metallic coating under atmospheric environments. (C) 1999 Elsevier Science Ltd. All rights reserved.