Which alloys are suitable for anodising? (Topic: 14005)

Pure aluminium has a better anodic response than its alloys, and an appreciable difference is to be detected between the appearance of coatings produced on the various grades of aluminium. The difference between commercial-quality material (99%) and super-purity aluminium (99,99%) is particularly noticeable. The table below gives a general guidance on alloy behaviour with respect to those suitable for protective anodising, decorative and colour anodising, bright anodising and hard anodising.

Guide to selection of finishes for wrought alloys [27].

Most aluminium alloys can be provided with an anodic oxide coating which is moderate to good, provided that the appearance is of no great concern and that protection is the objective. Also, for general decorative work, the majority of these will be satisfactory provided that they do not become excessively dark or patchy. However, where brightness is the primary requirement there are few alloys to choose from, as the table indicates. Only metal of high purity is classed as excellent. In the case of hard anodising characteristics the comments are based on the ability to produce a satisfactory 50 µm anodic coating.

Among the wrought alloys the Al-Mg-Si alloys (6000-alloys) with or without small additions of Mn, produce the best decorative and protective coatings, and Al-Zn-Mg alloys (7000-alloys) are also good. Alloys containing appreciable amounts of Cu and other heavy metals, usually produce coatings of inferior colour.