| 摘要 |
An aluminium surface is chromated using a solution at a temperature from 12 to 160 DEG F., having a pH of between 1.1 and 2.3 and containing 0.5 to 10 g./l. hexavalent chromium, 1.6 to 27 g./l. of the fluoride ions and from 0.1 to 4.0 g./l. of tungsten in the form of tungstate, and in addition to the fluoride ions aluminium fluorides having the formula AlFx, where x is an integer from 1 to 6, in an amount equivalent to 2.2 to 32 g./l. A/F3. The fluoride ions may be introduced as hydrofluoric, fluosilicic, and fluoboric acid, and the sodium, potassium or ammonium salts thereof. The tungsten may be present as tungstic acid and its sodium, potassium or ammonium salts. The solution may also contain nitrates and sulphates. In addition to replenishing the solution, part of the solution is circulated through a strongly acidic cation exchange resin as described in Specification 846,363. The solution may be applied to the aluminium by brushing, spraying, dipping or by atomizing the solution on to the surface as described in Specification 863,098. |
