Patent GB-632102-A (GB)
Title: Production of coatings on aluminium and its alloys
IPC: C23C22/38
Filed: 1947-09-09
Abstract: Protective and decorative coatings are produced on aluminium and aluminiferous alloys by subjecting the metal to the action of an aqueous acid solution containing phosphate, fluoride and dichromate ions, the phosphate ion content being between 2 and 285 grams per litre, the ratio of fluorine to chromate ions being between 0.135 and 0.405 fluorine ions per unit CrO3 ion, and the total acidity of the solution not exceeding that of a 3 normal acid.  The solution preferably has an apparent pH value of 1.6 to 2.2 when measured by a commercial glass electrode pH meter as the lowest value indicated within the first ten minutes of immersion of the glass electrode in the solution.  The solution may contain per litre 0.9 to 12.5 (preferably 2 to 6) grams of fluoride ions, 3.5 to 60 (preferably 6 to 20) grams of CrO3 ions, and 2 to 285 (preferably 20 to 100) grams of phosphate ions.  The solution adhering to the surface after coating may be allowed to dry thereon, and may then be rinsed with water and dried, or the coated surface, before or after drying, may be rinsed with a solution containing 4 to 60 grams of free chromic acid per 100 litres of water and the surface dried.  Paint or other organic finish may be applied to the coated and dried surface.  A heat treatment may be applied to the surface.  The solution may be prepared from a concentrate having a ratio of F to CrO3 to PO4 of 1.0 to 2.47-7.4 to 2-70 and preferably 1 to 2.88-5.55 to 7-50 and may be replenished by a concentrated admixture having a ratio of F to CrO3 to PO4 of 1 to 0.7-1.4 to 0.5-1 and which may also contain free acid in the proportion of 0.06 to 0.14 gram-equivalent of acid to one gram F. Specified reagents are phosphoric acid, monosodium dihydrogen phosphate, sodium fluoride, potassium fluoride, aluminium fluoride, hydrofluoric acid, chromic acid, potassium bichromate, sulphuric acid and hydrochloric acid.ALSO:Protective coatings are produced on aluminium and aluminiferrous alloys by subjecting the metal to the action of an aqueous acid solution containing phosphate, fluoride and dichromate ions, the phosphate ion content being between 2 and 285 grams per litre, the ratio of fluorine to chromate ions being between 0.135 and 0.405 fluorine ions per unit CrO3 ion and the total acidity of the solution not exceeding that of a 3 normal acid.  The solution preferably has an apparent pH valve of 1.6 to 2.2 when measured by a commercial glass electrode as the lowest value indicated within the first ten minutes.  The solution may contain per litre 0.9 to 12.5 (preferably 2 to 6) grams of fluoride ions, 3.5 to 60 (preferably 6 to 20) grams of CrO3 ions and 2 to 285 (preferably 20 to 100) grams of phosphate ions. The solution remaining after coating may be dried, and rinsed with water and dried, or the coated surface, before or after drying may be rinsed with a solution containing 4 to 60 grams of free chromic acid per 100 litres of water and the surface dried. The coated surface may be heated.  The solution may be prepared from a concentrate having a ratio of F to CrO3 to PO4 of 1.0 to 2.47-7.4 to 2-70 and preferably 1 to 2.88-5.55 to 7-50 and may be replenished by a concentrated admixture having a ratio of F to CrO3 to PO4 of 1 to 0.7-1.4 to 0.5-1 and which may also contain free acid in the proportion of 0.06 to 0.14 gram-equivalent of acid to one gram F. Specified reagents are phosphoric acid, monosodium dihydrogen-phosphate, sodium fluoride, potassium fluoride, aluminium fluoride, hydrofluoric acid, chromic acid, potassium bichromate, sulphuric acid and hydrochloric acid.