Anodic Growth vs. Dissolution 1

[Fibergeek]

I am aware that anodizing involves two competing reactions; the growth of the the anodic layer, and the dissolving (dissolution) of that layer by the action of the acidic (sulfuric acid) electrolyte.

It's obvious that the rate of layer growth is proportional to current density, but I have been led to believe that the dissolution reaction is independent of current density. Is this true? I would have thought that dissolution would have been accelerated by the flow of current.

I hope I'm posting this in the right forum.

 

[kenvlach]

What you were told is correct. While the dissolution of a bare metal is normally accelerated by an anodic current (ionization: Me --> Me⁺ + e^-), this is not so for an oxide (Me⁺ = Me⁺).

The dissolution rate of the anodic layer is primarily dependent upon temperature and acid concentration; also, upon solution agitation, dissolved aluminum concentration, and anodizing additives. The Arrhenius-type temperature dependence is 1 of the 2 main reasons that hard anodizing is typically conducted at low temperatures, it gives a more dense oxide since the acid electrolyte causes less increase of the pore diameters. The other reason for low temperatures is to remove the large amount of heat generated without risk of ‘burning,’ which is a sort of electrochemical dissolution of hot points (especially a problem in intermetallic-containing alloys such as Al 2024).

The temperature effect is why a conventional sulfuric anodize coating (MIL-A-8625F, Type II) is softer and the thickness is 2/3 ‘in,’ 1/3 ‘out’ or ‘buildup.’ E.g., for 1 mil total anodize, penetration into original surface is 2/3 mil. Whereas, for hard anodize (Type III), penetration is ~ ½ of the total anodic thickness.

A consequence of the competing growth and dissolution processes is that a maximum anodic thickness will be reached for a given current density. Continued anodizing will maintain a constant anodic thickness, while the substrate metal becomes thinner. Sheasby and Pinner, in The Surface Treatment of Aluminum and Its Alloys, 6th Edn., p. 967, (2001) mention that for certain Al-Mg & Al-Mg-Zn alloys, the anodic coating can also become thinner.

Hope this helps,
Ken