Anodizing

Anodizing (anodising in British English, technically anodic oxidation) is an electrochemical process that forms a continuous layer of aluminum oxide on the surface of aluminum and its alloys. The part is connected as the anode in an electrolyte, most often a sulfuric acid solution, and direct current converts the metal surface into oxide. Unlike paint or electroplating, it is not a deposited coating but a layer grown out of the part's own material, so it does not peel off.

The layer grows partly into the base material and partly above the original surface, roughly half and half for hard anodizing. Every surface therefore grows by part of the layer thickness, so a shaft diameter increases, and a hole diameter decreases, by twice that growth. Decorative and protective anodizing (Type II under the US specification MIL-A-8625) is usually 5 to 25 µm thick. Hard anodizing (Type III) is produced at a low electrolyte temperature, is typically 25 to 100 µm thick and has much higher wear resistance.

A fresh layer is porous. The pores can be dyed with organic dyes (black, blue, red, gold) or colored electrolytically, which gives shades from bronze to black with better lightfastness. Finally, the pores are sealed, most often in hot deionized water, which improves corrosion resistance and locks in the color. Appearance also depends on the pretreatment: caustic etching gives a matte surface, bead blasting a fine, uniform matte finish and polishing a bright one.

The anodized layer is hard and electrically insulating. Results vary widely among aluminum alloys: the 6000 series (for example 6060, 6061 and 6082) and the 5000 series anodize very well, while copper-bearing alloys (2000 series) and casting alloys with a high silicon content produce a thinner, gray and uneven layer.

Anodizing is the standard finish for machined aluminum parts: instrument enclosures, brackets, flanges, frames, panels and consumer products that need corrosion protection and a clean technical look without a layer of paint. Hard anodizing is used on sliding and wear surfaces, such as pneumatic cylinder pistons, guides or rollers.

For outdoor parts in a specific RAL shade, for weldments and for high-silicon castings, powder coating is often the better choice. For prototypes, anodize the same alloy that will be used in production, because the shade differs from alloy to alloy.

The most trouble comes from overlooking the dimensional growth. For fitted holes, precision threads and bearing seats, specify whether they are to be masked or whether the dimension applies after anodizing, and with hard anodizing, account for the growth on the drawing from the start. The part hangs in the bath from a contact point that remains uncoated: state where it may be, and if the part has to conduct current (grounding), mark the surfaces to be left bare.

The layer is thinner on sharp edges and cracks there with hard anodizing, so round or chamfer the edges. Anodizing does not hide machining marks or scratches; it tends to highlight them. The shade varies between alloys, stock and batches, so for cosmetic parts agree on a reference sample and anodize the parts of one set together. In the RFQ, state the alloy, layer thickness, color, pretreatment (such as bead blasting) and whether sealing is required.