| 摘要 |
PURPOSE:To develop the title Zn-Al alloy hot-dip plated steel sheet with less elution of hexavalent Cr and having excellent resistance to blackening and corrosion by spraying the aq. soln. of a specified metal salt onto the surface of a Zn-Al alloy hot-dip plated steel sheet, and further chromating the surface. CONSTITUTION:The aq. soln. of a salt such as the nitrate and oxide of Co or Fe is sprayed onto the surface of the steel sheet hot-dip plated with a Zn-Al alloy consisting, by weight, of 4-53% Al, 0-0.5% Mg, 0-10% Si, 0.002-0.3% Pb, and the balance Zn while the plating surface is still kept at a high temp. of >=170 deg.C, and thermally decomposed by the temp. of the steel sheet to form the film of the oxides of Co, Fe, etc. A chromate film is formed at the Cr deposition amt. of 30-120mg/m<2> by the processing soln. contg. SiO2 as the silica sol in 1-4 molar ratio of Si/Cr<3+>, expressed in terms of Si, and PO in 0.02-0.38 molar ratio of P/total Cr, expressed in terms of P, and wherein the molar ratio of Cr<3+>/total Cr is controlled to 0.40-0.60. The film is not washed with water but heated at 150-250 deg.C, and dried. |
