Aluminium anodising: types and thickness

What anodising does to the surface

Anodising is an electrochemical process that grows an aluminium oxide layer on the surface of the part. The layer is not a coating sitting on top of the metal. About half of it grows into the original surface and about half sits above it, so a 20 micron layer adds roughly 10 microns to each dimension. That growth matters on a bore, a thread or a mating face.

The oxide is hard, electrically insulating and porous at the surface while it is still in the bath. The pores are what allow a dye to enter and what make sealing possible. Once sealed, the pores close and the coating becomes a barrier against moisture and handling marks.

The alloy matters as much as the process. Extruded 6063 takes a clean, even anodic layer and is the usual choice for housings. Cast alloys with high silicon or copper content anodise unevenly and often go dark or patchy. If a casting has to be anodised, the alloy and the casting process need to be agreed with the finisher before the die is cut.

Type II and Type III compared

The two common sulphuric acid anodising types differ mainly in thickness and hardness. Type II is the standard decorative and protective coating. Type III, often called hard anodising, uses a colder bath and a higher current to build a thicker, denser layer.

The choice is driven by wear and environment, not by appearance alone. A housing that is handled daily and wiped with a cloth needs Type II. A wear plate, a slide or a part exposed to abrasive dust may need Type III. The comparison below sets out the practical differences.

Thickness: what the number means

A thickness call of 20 microns is not a single measured value. The layer grows at a rate that depends on current density, and current density depends on how the part sits in the bath. Edges and corners draw more current and build faster. Deep recesses and shielded faces build more slowly.

That is why a drawing should state a minimum thickness on a defined surface, not a nominal figure with a wide tolerance. A common call is a minimum of 15 microns on visible external faces, with the measurement method named. The finisher can then position the part and run the bath to meet the minimum at the weak points.

Measurement is usually done with an eddy current gauge on a flat, clean area, or by a microscope on a cross-section for a dispute. Both methods have a place. The gauge is fast and non-destructive, but it needs a calibration block of the same alloy and a flat spot to sit on. A cross-section gives a direct reading but destroys the sample.

Colour, dye and batch variation

Colour is added after anodising, while the pores are still open. The part is dipped in a dye, then sealed. The dye sits in the pore and the depth of colour depends on the pore structure, the dye concentration, the dip time and the alloy.

This is why colour matching across batches is difficult. Two batches of the same alloy and the same dye can still differ if the bath is older or the part geometry changes the pore size. A drawing that says black is not a specification. A colour chip, a Pantone reference or a measured colour value with a tolerance gives the finisher something to aim at.

Sealing also shifts the appearance. Hot water sealing, cold sealing and nickel acetate sealing all close the pores in different ways, and some dyes shift colour slightly during sealing. If a customer-facing part has to match a previous shipment, keep a sealed sample as the reference and state that it is the master.

Dimensions, threads and masking

Because the oxide grows into the metal, an anodised thread is not the same thread as the one that was cut. A male thread grows and may not fit its nut. A female thread closes slightly and may not accept its screw. The usual fix is to mask threads, or to cut them undersize and let the coating bring them back to size.

The same logic applies to bores, dowel holes and mating faces. A bore that is already at the top of its tolerance before anodising will be out of tolerance after it. Either leave stock and finish after anodising, or mask the feature and keep it bare.

Masking is also how an earth path survives. An anodised layer insulates, so any point that must carry current needs to be masked or left unfinished. Mark those areas on the drawing with a clear note, because a masking instruction in a drawing comment is easy to miss.

What to put on the inspection record

An anodising certificate should record the type, the thickness result and the sealing method. For a visible part, add a colour reference and a sample. For a functional part, add the dimensions that the coating affects, measured after finishing.

Cost and lead time

Anodising cost is driven by surface area, the type, the colour and the amount of masking. Type III takes longer in the bath and needs more careful racking, so it costs more than Type II. A bright dye costs more than a natural or black finish. Masking is labour, and a drawing with many small masked areas is expensive to run.

Racking is the hidden variable. Parts are held on jigs that leave a contact mark where the current enters. The mark is usually on a face that will not be seen, but someone has to decide where. If the drawing does not say, the finisher chooses, and the mark may land on a visible face.

Lead time depends on the bath schedule. A finisher who runs a colour once a week will hold parts until the batch is full, so ask for the schedule before the order is placed.