The influence of the alloying elements on the Anodizing process

The post about the natural formed oxide layer shows how important it is to know about the alloying elements in the aluminum.

As for the natural formed oxide layer, it is obvious that the state of the aluminum surface is also of great importance to the result of the Anodizing process.

Anodic coatings formed on pure aluminum (0.05% impurities) are continuous and transparent transmitting light through the thin oxide coating. In coatings formed on aluminum containing 0.8% impurities dark-coloured areas appear. These areas contain particles of constituents and oxidation products that detract from the transparency of the coating. This indicates how important or rather detrimental impurities (= constituents of an alloy) are to the anodizing result.

Some of the most common phases found in alloyed aluminum behave differently, when anodized.

Si: does not oxidize or dissolve at all but will be found in the formed aluminum oxide layer
CuAl2: oxidizes or dissolves more rapidly than the aluminum
AlMg: oxidizes or dissolves more rapidly than the aluminum
FeAl3: partly dissolves and partly remains in the coating

This is due to the chemical composition and electrolectrochemical behavior of the alloying elements.

Wernick, Pinner and Sheasby´s book The Surface Treatment And Finishing of Aluminum And Its Alloys gives a summary of the effect of various alloying elements on the anodic oxide layer.

Iron - detracts the specular brightness of the high purity bright anodizing alloys even when it is found in small amounts

Silicon - in small amounts cloudiness is obtained, when the amount exceed 5 % dark gray to black coatings will be produced

Magnesium - less than 3 % gives almost clear coatings

Copper - less than 2% will give colorless coatings, higher levels will give a discoloring and anodizing becomes more difficult

Zinc - good and protective coatings are formed with contents up to 5%, colorless when the microstructure is homogeneous or else quite brown when second phase precipitates are present

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