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Recent work at the National Bureau of Standards has revealed the effects of acid pretreatment of porcelain enamels upon their acid and abrasion resistance. Studies were made to compare the relative effects of hydrochloric, acetic, butyric, and citric acids on a numberof enamels. These studies indicate that acetic acid is much less corrosive than hydrochloric and citric acids; moreover, treatment with acetic or butyric acid, which produces only minor visible attack, strongly inhibites further attack when the specimens are subsequently treated with citric acid. By comparison, citric acid severely attacked the untreated areas of the same enameled specimens. In other tests, the resistance of certain enamels to abrasion was appreciably reduced after they had been treated with any of the acids.
Both effects, the increase in acid resistance and the decrease in abrasion resistance, may be explained by the hypothesis that a silicarich film is formed on the enameled surface by the acetic and butyric acids. This layer is then resistant to further attack even by citric or stronger acids. However, it is less resistant to abrasion than the original surface.
The increased use of titanium-type enamels and the sensitivity of these enamels to attack by abrasion after treatment with acid, as compared with antimony-type enamels, indicates that an abrasion treatment should be incorporated in the standard test for acid resistance.
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