Anodising converts the surface of aluminium into a hard, porous oxide by making the part the anode in an acid electrolyte. Because the oxide grows out of the metal rather than being deposited on it, it is integral and cannot flake, and because it is porous it can be dyed and then sealed, which is how coloured aluminium parts are made.
The common types
Chromic anodising gives a very thin coating used where fatigue life and tight dimensions matter. Sulphuric anodising is the general purpose process and covers clear and dyed decorative work. Hard anodising, a thicker and denser sulphuric variant run cold, produces a genuinely wear resistant surface used on pistons, valves and sliding parts. The type decides thickness, colour options and cost.
Dimensions grow, and hard anodising grows most
The coating grows both into and out of the surface, roughly half each way, so an external dimension increases and a hole decreases. On decorative sulphuric anodising the change is small; on hard anodising it is substantial enough to matter on any fitted feature. Allow for it in the machined dimension or mask the feature, and say which on the drawing.
Colour and its variability
Dye is absorbed into the pores and then sealed in. The result depends on alloy, temper, surface preparation, coating thickness and dye batch, so colour matching between lots and between different alloys is genuinely difficult. Where appearance matters, agree a physical reference sample with the finisher rather than a colour name, and keep parts from one batch together.
Masking, racking and electrical contact
The anodic layer is an insulator, so any surface that must conduct has to be masked or the coating removed afterwards. The part must also be racked, and the rack makes electrical contact somewhere, leaving a mark. Both facts belong on the drawing: say what is masked and where the rack mark may fall.
Questions people ask about anodizing service
Does anodising change dimensions?
Yes. The coating grows both into and out of the surface, so shafts get larger and holes get smaller. Hard anodising grows enough to matter on any fit.
Which anodising type should I specify?
Chromic where fatigue and thin coating matter, sulphuric for general and decorative work, hard anodising where wear resistance is the requirement.
Why do my anodised parts not match in colour?
Because colour depends on alloy, temper, preparation, thickness and dye batch. Agree a physical reference sample and keep production batches together.
Can anodised surfaces conduct electricity?
No, the oxide is an insulator. Any earthing or contact surface must be masked during anodising or have the coating removed afterwards.