Yoshikazu Kondo

Effects of Surface of Steel Substrate on Appearance of Steel Treated in Dilute Chromic Acid Solution

The surface film, produced by cathodic treatment of steel sheets in a dilute chromic acid solution containing small amounts of sulfuric acid and a fluoride, consisted of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium. Low carbon steel sheets of various surface states specially prepared were subjected to cathodic treatment in the dilute chromic acid solution, and then, coated with clear epoxy-phenolic lacquer. The examinations of surface roughness, reflectivity, and lightness were made on the steel substrates, cathodic-treated sheets, and lacquered sheets. The results of examinations were as follows. The surface roughness of cathodic-treated sheets was almost the same as that of original steel substrates. The reflectivity of cathodic-treated sheets largely depended upon that of original steel substrates and the former value was lower than the latter value. The reflectivity of the lacquered steels merely depended upon that of original sheet substrates. The lightness of the cathodic-treated sheets depended upon that of original steel substrates; that is, the former linearly increased with the increase of the latter. The lightness of the lacquered sheets also depended upon that of original steel substrates, and the relation between the both was expressed by a linear function.

Effects of Amount of Cathodic Film on Appearance of Steel Treated in Dilute Chromic Acid Solution

Samples of low carbon steel sheets containing various amounts of metallic chromium and hydrated chromium oxide were prepared by cathodic treatment in a dilute chromic acid solution. The samples obtained were then baked and coated with a clear lacquer. The examinations were made for the effects of amounts of metallic chromium and chromium oxide on reflectivity, lightness, and chromaticity of the cathodic-treated sheets, baked sheets, and lacquered sheets. The results were summarizedas follows: The metallic chromium had no effects on the reflectivity of cathodic-treated sheets. However, it had effects on the lightness of cathodic-treated sheets in the range of not higer than 0.3-0.4mg/dm2 of chromium oxide. The reflectivity of cathodic-treated sheets decreased with the increase of chromium oxide in the cathodic film; however, it remained unchanged when the amount increased to 0.5mg/dm2 or higher. The lacquered sheets showed a constant value of reflectivity regardless of the amount of chromium oxide. The lightness of cathodic-treated sheets decreased with the increase of chromium oxide however, it remained unchanged when the amount increased to 0.4-0.5mg/dm2 or higher. The lacquered sheets showed a constant value of lightness regardless of the amount of chromium oxide; however, in the lacquered sheets of high lightness, their values of lightness depended on the amount of chromium oxide. The surface chromaticity of cathodic-treated sheets showed “a” values of -0.6-+0.7 and “b” values of -0.5- -0.6, which indicated that the hue was pale blue with low saturation. The above values of “a” and “b” were obtained at the chromium oxide amounts of not higher than 0.2-0.25mg/dm2 showing bluish luster undiscernible with naked eyes. The hue of lacquered sheets slightly shifted to the yellow-green zone and remained constant regardless of the amount of chromium oxide. After the cathodic-treated sheets were baked at 210°C for 20min., their bluish hue slightly faded.