GB944897A - Improvements in or relating to the formation of chemical conversion coatings upon metal surfaces - Google Patents

Abstract

Phosphate coatings are formed on ferrous metal surfaces by a process comprising precleansing, coating and after-rinsing wherein during, (i) precleansing, the metal surface is cleaned with an alkaline solution having a minimum pH of 11 and a minimum pointage of 1.5 (for definition see Specification), (ii) coating, the cleaned surface is treated with aqueous acid phosphate solution at 54-79 DEG C. containing not less than 0.07% by weight zinc ion, calcium ion in weight ratio to zinc ion of from 0.5-8.0, phosphate ion associated in conventional amount with the zinc and calcium ions, nitrate ion (calculated as NO3) in weight ratio to phosphate ion (calculated as PO4) of from 0.2 to 5.0 and an oxidizing agent capable of oxidizing ferrous ion to ferric ion in amount equivalent to between 0.01 and 0.2 gms. of nitrite (calculated as NO2) per litre ; and during (iii) after-rinsing, the coated metal is rinsed with a dilute aqueous acidic solution wherein free acid is absent but hexavalent chromium is present in an amount of 0.01-1.0 gms./litre (calculated as CrO3). There is no upper limit for the pH of the cleaning solution and no critical upper limit for the pointage, although a pointage of 75 should not in general be exceeded. The alkaline cleaning solutions may be composed from alkali metal silicates, carbonates, phosphates or hydroxides or mixtures thereof, and may also incorporate a surface active agent, e.g. ethoxylated alkyl aryl phenols or "Triton" (Registered Trade Mark), ethoxylated alkyl alcohols or "Surfonic" (Registered Trade Mark). Coatings formed by the process of the invention range in weight only from 150-300 mgs. per sq. foot and the coating temperature may range from 54 to 79 DEG C., preferably 68 to 73 DEG C. The precleaning is operated at about the same temperature as coating. The nitrate ion may derive from any soluble nitrate salt provided the metallic cation of the salt is not detrimental, e.g. alkali metal salts such as sodium nitrate. The nitrite ion may derive from any water-soluble salt provided the metallic cation is not detrimental, e.g. alkali metal nitrites such as sodium nitrite. A preferred range of nitrite concentration is 0.05-0.16 gms. (calculated as NO2) per litre. Other oxidizing agents may be employed in equivalent amounts, the preferred alternative being per-oxygen compounds i.e. hydrogen, metal and organic peroxides. The pH of the coating solution ranges from 2.4-2.9. The free acidity (for definition see Specification) is between 0.4 and 1.5 and the ratio of total to free acidity is preferably between 6 and 25. The pH of the rinsing solution may vary from about 2.4 to 3.5 and may be prepared from soluble alkali metal or alkaline earth metal salts of any oxychromium acid, e.g. lithium, sodium, potassium, calcium, magnesium or ammonium chromates or dichromates or mixtures thereof ; and may also contain a minor amount, 0.0015-0.01 gms./litre, of phosphate ion (calculated as PO4) devoid of any free acidity.