Effects of alloying elements on the anodising appearance (Topic: 14007)

The effects of various alloying elements in aluminium alloys can be summarised as follows:

Iron - is the principal impurity which detracts from the specular brightness of the high purity bright anodising alloys, even in quite small amounts (0,08%). Iron additions to super-purity aluminium (99,99%) lead to dark grey or black streaks. When iron is at 0,3%, a typical level for alloy 1050, the coating becomes quite grey in comparison. It may be a uniform light grey colour, or striated or mottled according to the distribution of the iron-containing particles. The max. iron content of the alloys in this Design support manual is 0,7% (3003,3103, and 6061), but only one of them is a "real" extrusion alloy (6061).

Silicon - leads to cloudiness when out of solution (precipitation of AlFeSi or Mg2Si intermetallic particles), but about 0,8% can remain dispersed. It is beneficial with the iron impurity. In Mg alloys silicon goes more readily into solution. At the 5% level, alloys anodising to dark grey or black are obtainable. Alloy 6082 has the highest value with 1,3% of the alloys listed in this Design support manual.

Titanium - detracts from brightness in the same way as iron, but is added to produce fine grain. Along with boron it also promotes nucleation helping to get a fine uniform structure.

Magnesium alloys - can give clear colourless coatings for Mg up to 3%, probably because magnesium oxide has a refractive index very near to that of aluminium oxide. None of the alloys mentioned in the manual has Mg-content above 1,5%. Alloy 7108 has a max. value of 1,5%.

Copper - up to 2% can give alloys with a clear protective coating in the solution treated condition. This amount of copper is much higher than in the actual alloys in our manual where alloy 6061 has the highest content with max. 0,4%. Alloy 6463 with max. 0,20% Cu is suitable for bright anodising.

Manganese alloys - up to 1% Mn, may anodise to clear silver, grey, brown or mottled according to production history and particle size of the constituent. With the thicker anodic films Mn contents of 0,3ñ0,5% give quite brown anodic oxide coatings on all of the alloys where the metal is present. Four of the alloys described in the manual have high Mn content (above 0,4%) , but only two of them are "really" extrusion alloys. These are 6082 (max. 1,0%) and 6005 (max. 0,5%). They will turn brown in the anodising process. Alloy 3003 and 3103, both with max. 1,5% Mn, will obtain a colour after anodising according to the production history.

Zinc alloys - form good protective coatings with zinc up to 5%. These may be colourless when homogeneous or quite brown or marbled in appearance when second phase precipitates are present. Alloy 7108 has a zinc content of max. 5,8%. Too much zinc in the alloy or in the etching bath (pre-treatment before anodising) will cause spangle effect

Chromium - at 0,3% leads to coatings which are yellow in colour. There are only 2 alloys in this manual with a chromium content up to 0,3%. That is 6005 (max. 0,3%) and 6061 (max. 0,35%).