Patent GB-642228-A (GB)
Title: Improvements in process for enameling aluminium-rich alloys and products produced thereby
IPC: C23D3/00
Filed: 1948-07-01
Abstract: 642,228. Enamelling aluminium rich alloys. DU PONT DE NEMOURS &; CO., E.I. July 1, 1948, No. 17752. Convention date, July 1, 1947. [Class 56] Aluminium-rich alloy surfaces to be enamelled are placed in contact with an aqueous alkaline solution of a chromate, heat treated between 350C. and the heat tolerance limit of the alloy and then enamelled with a vitreous enamel which melts below about 580C. The alkalinity of the chromate solution is between pH 8.0 and pH 14.0 and is imparted by alkali metal hydroxides or salts such as alkali metal carbonates or bicarbonates. Any water soluble chromate or dichromate may be used though alkali metal chromates are preferred, the chromate content varying from 0.25-25 per cent expressed as potassium chromate. The alloy may be cleaned with grease solvents, e.g. trichlorethylene. The chromate solution is applied in any manner but preferably by immersing the alloy in the solution at a temperature (about 20C.) and boiling point (100C.) for 1-30 minutes, and then rinsing in water. The preferred enamels contain, in mols. per cent, PbO 10-18, SiO 2  38-65, Li 2 O 5-12, Na 2 O 0-22, K 2 O 0-20, TiO 2  0-11, the total alkali being between 25-36, and the ratio SiO 2 +2 (TiO 2 ) alkali oxide between 1.8 and 3.0. The enamel should be fired between 480- 580C. (in some cases 480-510C.) for 2-30 minutes. The heat treatment after immersion in the chromate is preferably at 400- 550C. for 2-10 minutes in atmospheric conditions. The various time factors vary with different alloys, thus: (1) Al alloy containing 0.25 per cent Cu, 0.6 per cent Li, 1.0 per cent Mg, and 0.25 per cent Cr is immersed for 5 minutes in a bath containing by weight NaOH 4 per cent, K 2 CrO 4  20 per cent, H 2 O 76 per cent at 50C., rinsed, dried, heated at 500C. for 5 minutes, cooled, dipped in a slurry of an enamel consisting of by weight, PbO 38.1 per cent, SiO 2  25.3 per cent, Li 2 O 2.2 per cent, Na 2 O 10.2 per cent, K 2 O 8.3 per cent, TiO 2  8.7 per cent. Sb 2 O 3  2.9 per cent, B 2 O 3  4.3 per cent with 3 per cent boric acid and 0.5 per cent K 2 CrO 4  added to give a coating of 15-30 gms. per square foot (several colouring or opacifying coatings may be applied and separately fired) and fired at 450C.-515C.; (2) Al alloy containing 5 per cent Li treated as in (1) above except that the chromate immersion is at 40C. for 5 minutes; (3) Al alloy containing 1.2 per cent Mn is immersed for 15 minutes at 40C. The firing at (1), (2) and (3) is at 480-560C. but an alloy containing 2.5 per cent Mg and 0.25 per cent Cr is fired at 480- 510C. Other baths contain, by weight (a) H 2 O 89 per cent, K 2 CrO 4  2 per cent, NaHCO 3  2 per cent, Na 2 CO 3  7 per cent, and (b) H 2 O 90 per cent, K 2 CrO 4  0.5 per cent, NaHCO 3  2.5 per cent, Na 2 CO 3  7 per cent, Too short immersion in chromate gives an incomplete protection against spalling, too long an immersion leads to etching of the alloy, at higher temperatures a shorter time of immersion is required, with higher alkalinity the treatment is more rapid, and with increased chromate more flexibility of the time of treatment is possible, but the speed of action is decreased. The enamelled alloys have improved resistance to weathering and moist atmospheres. Specifications 580,688 and 634,655 are referred to.