| 摘要 |
PURPOSE:To prolong the period providing a darkish discoloration inhibiting effect by spraying an electroless plating bath containing Co, Ni, and Fe ions and hypophosphite in the course of the cooling period of a plating layer of Zn, Al, or an alloy thereof applied to a steel sheet by means of hot dipping. CONSTITUTION:After a steel sheet is hot-dipped with Zn, Al, or Zn-Al alloy, coating weight is regulated, and, in the course of the cooling of the plating layer from the molten state down to 170 deg.C, a solution is sprayed on the above steel sheet. In the above method, as the above solution, an electroless plating bath containing one or more kinds among Co, Ni, and Fe ions and hypophosphite is used and sprayed. By this method, a darkish discoloration inhibiting effect can be provided over a long period as compared with the case using the conventional method in which an aqueous solution containing Co salt and/or Fe salt is sprayed, and further, the machining resistance of a darkish discoloration inhibiting film and also its chemical conversion treating property can be improved. |
