CORROSION PRODUCTS ON CUT EDGE SURFACE OF ZINC-COATED STEELS IN HUMIDIFIED AIR IN THE PRESENCE OF NaCl PRECIPITATE BY RAMAN SPECTROSCOPY

摘要

Corrosion products on zinc- and zinc alloy-coated steels by atmospheric corrosion in the presence of NaCl precipitate was investigated by in-situ Raman spectroscopy. A model cut edge surface a half of which consists of the coated surface and the other consists of the bare steel was exposed to humidified air at 84% relative humidity (RH) at 25°C. The galvanic coupling between the zinc or zinc alloy coating layer and the bare steel on which NaCl particles precipitate accelerated corrosion of the zinc coating. On the zinc or zinc alloy coating at the model cut edge surface, aqueous layer containing dissolved NaCl and ZnCl_2 initially formed. After the saturation of ZnCl_2, precipitate of simonkollite (zinc hydroxy-chloride) was formed. On the bare steel part at the model cut edge surface, zinc hydroxy-carbonate was formed immediately after the exposure to humidified air. The corrosion rate of the coatings under the galvanic coupling is the following order;;pure Zn> Zn-12%Ni = Zn-5%A1 > Zn-55%A1.