tin - free steel sheets exhibiting improved retorting resistance with respect to paint adhesion are produced by chromium plating a steel sheet through cathodic electrolysis in a chromium ion - containing solution , reversely electrolyzing the sheet by a successive anodizing treatment in said solution , and subjecting the sheet to an electrolytic treatment in an aqueous chromate solution . the so 4 -- concentration of the chromate solution used in the electrolytic chromate treatment should be limited to the range of 0 . 01 to 0 . 10 gram per liter of the solution .