PRIMARY COATING CHROMATE COMPOSITION FOR PRECOATED STEEL SHEET AND TREATMENT THEREFOR

摘要

PURPOSE: To provide a chromate treating technique using a chromium composition, constituted of a specified compsn. such as 2C2B coating. ;CONSTITUTION: The surface of a galvanized steel sheet is coated with a chromate composition in which the total chromium concn. is regulated to 5 to 30 (g/l), the chromium reduction rate to 5 to 30 (%), the quantity of a linear silica sol to the Cr³⁺ content in the chromium reduction rate × 0.1 to 3.0 (gel) and the grain size ratio in the linear silica sol into 2 to 100 by length/diameter, by 10 to 150mg/m² expressed in terms of metallic chromium to form a solid film. Immediately, it is charged to a drying furnace and is dried while regulating the maximum sheet thickness on the outlet side to 60 to 150°C. In the coating chromate treating technique, its corrosion resistance, the adhesion of coating material and the uniform coating suitability of the film are maintained in high quality, and particularly, its coating corrosion resistance can remarkably be improved.;COPYRIGHT: (C)1994,JPO&Japio