HIGHLY CORROSION-RESISTANT, SURFACE-TREATED STEEL SHEET.

摘要

Steel sheet has three coatings: (1) a Zn or Zn alloy deposit as an under coating deposit, (2) a chromate coating and (3) a coating of resin compsn. (R) in this order. (R) comprises a base resin (A), polyisocyanate cpds., silica, and a chrome cpd. (B) having a low solubility. (A) is obtd. by combining at least one basic nitrogen atom and at least two prim. hydroxy gps. to the thermal of an epoxy resin. The wt. ratio of the base resin (A) to silica lies between 99:1 and 30:70 (pref. 90:10 and 50:50, best 80:20 and 60:40). The wt. ratio of (A) to (B) lies between 99:1 and 60:40 (pref. 97:3 and 65:35, best 75:25). The wt. ratio of (A) to the total wt. of silica and (B) is 75:25 to 50:50 (pref. 70:30-55:45). The wt. ratio of silica to (B) is 37:3-20:20 (pref. 35:5-25:15). The wt. ratio of (A) to the polyisocyanate cpds. lies between 100:5 and 100:80 (pref. 100:10 and 100:50). The silica is specified to be hydrophobic silica. The base resin (A) is specified to be a solvent type. The chrome cpd. (B) is specified to be BaCrO4.