塗装鋼板における塗膜下腐食と物質移動

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  • Mass Trnsport and Delamination of Coated Steels During Under-Film Corrosion
  • トソウ コウハン ニ オケル トマクカ フショク ト ブッシツ イドウ

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Transport of water and ionic species through coating films during blistering of epoxycoated steels under electric current was studied in a sodium chloride solution by a mass gain method and a chemical analysis. Areas of cathodic delamination were compared with each other in some alkali and alkali earth metal chloride solutions. In the presence of an anodic current, the apparent transport numbers across the film were determined as 0.5 for both Cl- and Fe2+ ions. Mass increase of the specimen, due to the penetration of water, corresponded to a water transport of 1 to 3mol/F. On the other hand, in the presence of a cathodic current, the apparent transport number for Na+ was 0.5, and the residual electricity would be transported by H+ and/or OH- ions. The mass increase of the specimen corresponded to the water penetration of 20 to 60mol/F. These water penetrations were attributed to an osmotic pressure of concentrated FeCl2 and NaOH solutions in the blisters, and the degree of water penetration depended upon the solubilities of salts and hydroxides. Because of high solubility of NaOH, the area of cathodic delamination was always several ten times larger than that of anodic one. Alkali metal chlorides accelerated the cathodic delamination, since the solubilities of their hydroxides are usually very large and the pH of the solutions are very high.

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