A PROCESS LFOR ENHANCING THE RESISTANCE TO DARKENING OF HOT DIP GALVANIZED SHEET, CHROMATING BATH AND RESULTED CHROMATED GALVANIZED SHEET THEREOF

摘要

The invention relates to a process for enhancing the resistance to darkening of hot dip galvanized sheet comprising the steps of chromatising the galvanized sheet in a chromating bath solution at 25degreeC-60degreeC having Cr+6 ion of 2.5 to 4.5 gm/I and Cr+3 ion of 2 to 4 gm//I, having PH of 1.65 wherein in the bath nitric acid is added drop by drop until PH of the lbath reaches 1.00 to 1.60; followed by addition of cobalt nitrate in the bath; chromating the galvanized sheet in the bath for 0.5-4 seconds and drying the sheet after chromating in forced hot air at 75degree-150degreeC. The invention also relates to a chromating bath solution for treatment of GA steel sheet and the chromatized GA sheet resulted thereof.