めっきの塗膜下腐食先端における Zn の腐食反応

概要

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The study on corrosion reaction of zinc at under-film corrosion tips of coatings was carried out. It was found that the following coating corrosion reaction in wet & dry and wet environments occured. (1) Anodic reaction : Zn→Zn^<2+>+2e^- (2) Cathodic reaction : 1/2O_2+H_2O+2e^-→2OH^- (3) Formation of simonkollite (ZnCl_2・4Zn(OH)_2) : 5Zn^<2+>+2Cl^-+80H^-→ZnCl_2・4Zn(OH)_2 (4) Overall reaction : 5Zn+5H_2O+5/2O_2+2Cl^-→ZnCl_2・4Zn(OH)_2+2OH^- On the other hand, it was made clear that the layer of ZnCl_2・4Zn(OH)_2 formated by the coating corrosion retains weak alkali (pH=8∿9) environment with in-situ pH measurement. The zinc phosphate film on the ZnCl_2・4Zn(OH)_2 layer exists as undissolved and stable layer with EPMA analysis. It is considered that the ZnCl_2・4Zn(OH)_2 layer is the corrosion protective film with respect to the corrosion of steel substrate because steel substrate has no corrosion under the ZnCl_2・4Zn(OH)_2 layer.
社団法人日本鉄鋼協会の論文
1992-10-01