Surface-treated steel sheet coated with chromate film for fuel tanks and method of fabricating the same

A chromate solution used for preparing a surface-treated steel sheet for a fuel tank of an automobile comprising (a) a main body solution and (b) a hardener aqueous solution in an amount ranging from 10 to 60% by weight of the main solution, wherein the main solution comprises:

• a chrome aqueous solution where the concentration of the chrome is in the range of 7 to 50 g/l, and a ratio of trivalent chrome to the chrome content is in the range of 0.4 to 0.8;
• phosphoric acid in an amount ranging from 50 to 80% by weight with respect to the chrome content;
• fluoric acid in an amount ranging from 20 to 40% by weight with respect to the chrome content;
• sulfuric acid in an amount of 5 to 15% by weight with respect to the chrome content;
• colloidal silica in an amount ranging from 300 to 2000% by weight with respect to the chrome content;
• and
• sodium phosphate in an amount of 25 to 40% by weight with respect to the chrome content;
• and
• the hardener aqueous solution comprises epoxy-based silane or glycidyl silane in an amount ranging from 2 to 10% by weight with respect to the hardener aqueous solution and has a pH of 2 to 3.