| 摘要 |
PROBLEM TO BE SOLVED: To reduce the unevenness of treatment in a steel sheet even in the case of chromate treatment in which the unevenness of treatment is easy to be generated such as the case in which reaction type chromate treatment is executed after planting with a galvanizing soln. contg. a trace amt. of Ni ions in a plating soln., which Ni ions remain at the time of exchanging a planting soln. from zinc-nickel alloy plating to galvanizing. SOLUTION: The surface of a galvanized steel sheet is sprayed with an aq. soln. of chromate contg. Cr6+/(Cr3++Cr6+) in the range of 0.6 to 0.8, Cr3++Cr6+ by 4 to 6 g/l, SO42- by 1 to 8 g/l, NO3- by 8 to 12 g/l and Zn2+ by 3 to 8 g/l to form a chromate film of 20 to 50 mg/m2 chromium coating weight on the surface of the steel sheet. In the case a suitable amt. of SO42- is compositely added besides NO3- as anions, SO42- having a large ionic radius suppresses nonuniform etching reaction by NO3-, and uniform etching reaction can be executed, so that the unevenness in the treatment can be prevented.
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