Method for producing rust-proof member

Abstract

A reactive chemical conversion treatment liquid capable of forming a Si-containing chemical conversion film is provided. The chemical conversion treatment liquid contains colloidal silica, a trivalent chromium-containing water-soluble substance, and a high pKa organic acid-containing water-soluble substance such as a hydroxymonocarboxylic acid and may further contain a film-formable metal-containing water-soluble substance in a preferred embodiment, and the pH of the chemical conversion treatment liquid is 3.0 or more.

1 . A method for producing a rust-proof member, comprising
 a chemical conversion treatment by forming a chemical conversion film on a member to be treated containing zinc on its surface, the chemical conversion treatment comprising bringing a chemical conversion treatment liquid into contact with the member to be treated and then washing the member,   wherein the chemical conversion treatment liquid comprises:
 colloidal silica, 
 a trivalent chromium-containing water-soluble substance that is an ionic substance comprising trivalent chromium, and 
 a high pKa organic acid-containing water-soluble substance that is a water-soluble substance comprising a high pKa organic acid that is an organic acid having a lowest pKa of no lower than 3.5, 
   the chemical conversion treatment liquid has a pH in a range from 3.0 to 4.5 or less in a state before contacting with the member to be treated, and   the high pKa organic acid comprises glycolic acid, except for a case that the high pKa organic acid comprises at least one material selected from the group consisting of allylamine, polyallylamine, aromatic sulfonic acid, aromatic sulfonic acid-formaldehyde condensate, and derivatives thereof.   
     
     
         2 . The method for producing a rust-proof member according to  claim 1 , wherein in the chemical conversion treatment liquid,
 the colloidal silica has a content in a range from 2 g/L to 25 g/L,   the trivalent chromium-containing water-soluble substance has a trivalent chromium equivalent content in a range from 1 g/L to 6 g/L, and   a molar ratio of a high pKa organic acid equivalent content of the high pKa organic acid-containing water-soluble substance to the trivalent chromium equivalent content of the trivalent chromium-containing water-soluble substance is in a range from 0.2 to 2.   
     
     
         3 . The method for producing a rust-proof member according to  claim 1 , wherein the chemical conversion treatment liquid further comprises a film-formable metal-containing water-soluble substance that is a water-soluble substance comprising metal ions capable of forming a film by interacting with oxygen. 
     
     
         4 . The method for producing a rust-proof member according to  claim 1 , wherein in the chemical conversion treatment liquid, a ratio of a content of the colloidal silica in a unit of g/L to a high pKa organic acid equivalent content in a unit of g/L of the high pKa organic acid-containing water-soluble substance (colloidal silica/high pKa organic acid) is within a range from 0.5 to 10. 
     
     
         5 . The method for producing a rust-proof member according to  claim 1 , wherein the chemical conversion treatment liquid further comprises a low pKa organic acid-containing water-soluble substance that is a water-soluble substance containing a low pKa organic acid that is an organic acid having a lowest pKa in a range from 1.27 to less than 3.5, and a molar ratio of a low pKa organic acid equivalent content of the low pKa organic acid-containing water-soluble substance to a high pKa organic acid equivalent content of the high pKa organic acid-containing water-soluble substance is 1 or less. 
     
     
         6 . The method for producing a rust-proof member according to  claim 1 , wherein the member to be treated is obtained by forming a zinc-based plating layer on a base material. 
     
     
         7 . The method for producing a rust-proof member according to  claim 6 , wherein the base material is a cast product.