Hydrophilic surface treatment agent for aluminum-containing metal heat exchangers having excellent drainage

A hydrophilic surface treatment agent for an aluminum-containing metallic heat exchanger, the agent obtained by mixing:

• a polyvinyl alcohol or a derivative thereof (A);
• colloidal silica (B);
• an organo alkoxy silane and/or a hydrolysate thereof (C);
• a cross-linking agent (D) capable of forming cross-linkage with the polyvinyl alcohol or a derivative thereof (A);
• and
• water (E),
• wherein the ratio {(B)+(C)}/{(A)+(B)+(C)+(D)} is 0.1 to 0.5 in terms of solid content ratio (mass ratio), the ratio (C)/(B) is 1.0 to 3.0 in terms of solid content ratio (mass ratio),
• the polyvinyl alcohol or a derivative thereof (A) is selected from the group consisting of partial saponification products or complete saponification products of polyvinyl acetate, partial saponification products or complete saponification products of copolymers of vinyl acetate with other monomer, and modified products of polyvinyl alcohol,
• the cross-linking agent (D) is different from the organo alkoxy silane and/or a hydrolysate thereof (C), and contains two or more carboxyl groups per one molecule,
• the colloidal silica (B) has an average particle size in the range of 1 nm to 100 nm, and
• the organo alkoxy silane and/or the hydrolysate thereof (C) has a glycidyl group.