Abstract
In order to elucidate the correlation between the durability of coating against chlorine penetration and the composition of Zn-Fe alloy coating, the corrosion mechanism of coating at under-film corrosion tip was investigated.
The measurement of couple initial current as original method was devised for the purpose of simulating the reaction of coating corrosion. The couple initial current corresponds to the charge transfer controlled corrosion rate of the coating.
The couple initial current with equivalent two coated steel sheets gradually increases with the increase of Fe content of coating, while the couple initial current with coated and cold rolled steel sheets abruptly decreases with the increase of that.
Furthermore, the better correlation between couple initial current with equivalent two coated steel sheets and under-film corrosion distance after atmospheric exposure test on Zn-Fe alloy coated steel sheets was recognized.
On the basis of the knowledge obtained by the laboratory investigation, it was concluded that i) under-film corrosion of Zn and Zn-Fe alloy coatings in atmospheric exposure environment proceeds by charge transfer controlled mechanism and ii) corrosion of Zn-Fe alloy coatings is controlled by corrosion cell on coating iii)corrosion of Zn coating is controlled by corrosion cell between coating and iron substrate.
