Zinc phosphate coating process

In a continuous process for coating a zinc phosphate film on the surface of a metal article by spraying a zinc phosphate coating solution consisting essentially of nitrate ions, phosphate ions and zinc ions, onto the surface of the metal article, which metal article is one in a line of metal articles suspended and moving continuously or intermittently, at a station having a receptacle provided beneath the metal article, collecting the sprayed zinc phosphate coating solution in the receptable and circulating the collected zinc phosphate coating solution for spraying, the improvement wherein the zinc phosphate coating solution is controlled continuously throughout the coating process such that:

• 1. the phosphate ion concentration in the solution is at least 0.5% by weight;
• 2. nitrate ions are present in an amount such that the molar ratio of the phosphate ions to the nitrate ions in the solution is about 1:0.7 to 1:1.3;
• 3. the zinc ion concentration in the solution is at least 0.03% by weight;
• 4. the molar ratio of the zinc ions to the phosphate ions in the solution is less than 0.116:1;
• and
• 5. the pH of the solution is from 3.3 to 3.8.