| 摘要 |
PROBLEM TO BE SOLVED: To improve the white rust resistance of a steel sheet without using hexavalent chromium and requiring special waste water discharging treatment by bringing a galvanizing layer into contact with a treating soln. of specified temp. and pH contg. trivalent chromium. SOLUTION: At least one side of a steel sheet is applied with galvanizing and is thereafter brought into contact with a treating soln.contg. trivalent chromium, of 70 to 90 deg.C temp. and pH 1.5 to 3. In the case the temp. of the treating soln. exceeds 90 deg.C, the formed chromium film is made nonuniform by the evaporation of water as a solvent to reduce its white rust resistance. In the case the pH does not lie in the range of 1.5 to 3, the film forming rate of chromium is made slow, and moreover, its white rust resistance reduces. As the trivalent chromium compd., chromium sulfate, chromium chloride, or the like, can be used. The steel sheet is brought into contact with the treating soln., is thereafter water-washed and is dried under heating. As to the film thus formed, the coating weight of the trivalent chromium is preferably controlled to 10 to 300 mg/m2.
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