The Passivity Produced by Chromic Acid on 18‐8 Chromium‐Nickel Alloy (1931)

A comparison of the corrosion resistance toward ferric chloride solution, calcium chloride solution and sodium chloride salt spray of untreated metal and metal anodically treated in chromic acid showed that anodic treatment greatly increases the corrosion resistance. It was observed that the local action on areas which had been subjected to cutting and punching stresses could be avoided by anodic treatment of this material. A comparison of the corrosion resistance toward ferric chloride solution of samples of the untreated metal, samples treated in chromic acid without the use of electric current and samples treated anodically in chromic acid showed that the anodic treatment produced a greater increase in corrosion resistance than treatment without electric current, and that the untreated samples and samples treated without electric current corroded to a large extent by local action, whereas the anodically treated samples underwent uniform general corrosion over the entire surface, but much less in degree and generally without pitting. It was shown that during anodic treatment there was produced a decided loss of weight, whereas treatment without the use of electric current produced practically no loss in weight and also that the anodic treatment loss increased as the temperature was increased above 25° C. It was shown that an increase in anodic treatment loss was accompanied by a decrease in subsequent corrosion loss. Since microscopic examination showed that cross sections of the alloy have practically the same structures throughout, the improvement in corrosion resistance produced by anodic treatment cannot be accounted for on the grounds that the surface of the metal was of a more easily corrodible structure than the interior of the sheet, and that the removal of the surface by anodic treatment left exposed the more corrosion resistant interior. Since it was shown that anodic treatment greatly increased the corrosion resistance of unpolished samples the increase in the corrosion resistance of anodically treated polished samples cannot be accounted for entirely on the grounds that the surface film produced by polishing has been removed in anodic treatment. When potential measurements were carried out by means of a small capillary at various points on the surface of anodically treated and untreated samples which had previously been subjected to a treatment to free them from oxygen, it was found that the treated samples showed a more uniform potential than the untreated samples. The above observations point to the belief that anodic treatment tends to equipotentialize the surface by selectively dissolving the more anodic areas, thereby leaving the more cathodic areas to be coated with oxide. The final result is to form an oxide film over the entire surface of the sample, which has been, to some extent, equipotentialized. Such a surface should be more resistant to corrosion than one consisting of an oxide coating over a surface of non‐uniform potential, for once the oxide coating is removed in subsequent corrosion the equipotentialized surface should present much less opportunity for local action and general corrosion than the other. The passivity produced by anodic treatment in chromic acid of 18 per cent chromium, 8 per cent nickel alloy is brought about by (1) the equipotentialization of the surface and (2) the coating of the surface with an oxide film.