Surface-treated steel sheet for fuel tanks and method of fabricating same

A method of fabricating a surface-treated steel sheet, the method comprising the steps of:

• electroplating a cold-rolled steel sheet with zinc or zinc-based alloy;
• and
• coating a chromate film on the zinc or zinc-based alloy plating layer, the chromate film being formed from a chromate solution, the chromate solution comprising a) a subject solution and b) an aqueous silane solution in an amount ranging from 5 to 50% by weight of the subject solution, wherein the subject solution comprises
• i) a chrome aqueous solution comprising trivalent chrome, the chrome having a concentration ranging from 5 to 50 g/l , wherein the ratio of trivalent chrome to the chrome content is in the range of 0.4 to 0.8, and
• ii) Phosphoric acid in an amount ranging from 20 to 150% by weight with respect to the chrome content, fluoric acid in an amount ranging from 10 to 100% by weight with respect to the chrome content, colloidal silica having pH of 2-5 in an amount ranging from 50 to 2000% by weight with respect to the chrome content, and sulfuric acid in an amount ranging from 5 to 30% by weight with respect to the chrome content;
• and
• the aqueous silane solution comprises Epoxy-based silane in an amount ranging from 2 to 10% by weight and has a pH of 2-3.

A surface treatment solution for use in manufacturing fuel tanks, the surface treatment solution comprising:

• a chromate solution comprising a) a subject solution and b) an aqueous silane solution in an amount ranging from 5 to 50% by weight of the subject solution, wherein the subject solution comprises
• i) a chrome aqueous solution comprising trivalent chrome, the chrome having a concentration ranging from 5 to 50 g/l , wherein the ratio of trivalent chrome to the chrome content is in the range of 0.4 to 0.8, and
• ii) Phosphoric acid in an amount ranging from 20 to 150% by weight with respect to the chrome content, fluoric acid in an amount ranging from 10 to 100% by weight with respect to the chrome content, colloidal silica having pH of 2-5 in an amount ranging from 50 to 2000% by weight with respect to the chrome content, and sulfuric acid in an amount ranging from 5 to 30% by weight with respect to the chrome content;
• and
• the aqueous silane solution comprises Epoxy-based silane in an amount ranging from 2 to 10% by weight and has a pH of 2-3.