Anodising is an electrochemical treatment that turns the surface of aluminium into an oxide layer (alumina). The part, connected as the anode, is immersed in an acid bath through which a current passes: the oxide is not deposited on the metal, it grows from it. It is therefore part of the component and does not flake like paint. The layer is porous: it can absorb a dye, and is then sealed, that is, closed up, to lock in the colour and improve corrosion resistance.
There are two main families. Sulphuric anodising, from a few micrometres to around twenty micrometres thick, is used for protection and decoration: natural, black or coloured finishes. Hard anodising, much thicker, is aimed at wear and friction resistance (pistons, slides, tooling) and gives dark shades that cannot be freely chosen. In both cases the oxide is an electrical insulator: earthing or contact areas must be masked.
The result depends on the aluminium alloy. Alloys rich in copper or silicon, and cast parts, give less even colours; two parts in different alloys will not take colour identically. The layer also changes dimensions: part of its thickness grows outwards, the rest penetrates the metal, which matters for a tight fit. Prior blasting gives a uniform matt base.
In a quote request
Specify the type (sulphuric or hard), the colour, the target thickness or acceptable range, the reference standard (for example ISO 7599 for standard anodising, ISO 10074 for hard anodising) and the areas to mask: threads, bores, electrical contacts. State whether dimensions apply before or after treatment, the exact alloy of the part and the inspection required, for example a thickness report. To compare processes, read the guide on choosing a surface treatment and consult surface treatment workshops.