What is hard anodising?

The process conditions differ from standard anodising: a chilled electrolyte, usually sulphuric-acid based, and a higher current density. The layer is formed from the aluminium itself. It is much thicker and harder than a decorative anodised layer, wear-resistant and electrically insulating, and usually grey to dark bronze or black depending on alloy and thickness. Typical uses include pistons, cylinders, guides and other sliding and wear surfaces. The aluminium beneath the layer does not get harder; the hardness is in the oxide layer only.

What does hard anodising depend on?

How the layer turns out depends on the alloy, the thickness and the process. Alloys high in copper, such as the 2xxx series, and casting alloys high in silicon give a less even, softer or more porous layer; the anodiser decides what is achievable. The layer grows perpendicular to the surface, partly inwards and partly outwards. A rule of thumb often quoted is about half outwards, but the split varies with alloy and process. At a sharp external edge the layers from the two faces do not join, leaving voids or cracks; a large enough radius gives a closed layer. The layer is brittle and can lower the fatigue strength of the part.

What does the standard say about hard anodising?

Hard anodising is often specified to ISO 10074, which sets requirements for hard anodic oxidation coatings on aluminium and its alloys; US drawings often call up Type III to MIL-PRF-8625, or on older drawings MIL-A-8625. A callout is only unambiguous with its conditions: the thickness, usually as a minimum local thickness on designated surfaces, whether the layer is sealed, which surfaces are masked and whether the drawing dimensions apply before or after anodising. Sealing improves corrosion resistance but can lower wear resistance, so wear surfaces are often left unsealed. Thickness is usually measured by eddy current.

How does hard anodising work in practice?

Hard anodising is usually subcontracted, with its own price and lead time, and the build-up needs preparation in your own route. Fits are machined to a pre-coat size, masked or machined after anodising. On a 60° thread the pitch diameter changes by about four times the build-up per face, so masking or an adjusted size is often the practical route. Edges are rounded or chamfered, and the rack contact points are agreed with the anodiser, because no layer forms there. Settle the alloy, thickness and masking before you confirm material and price.

What is hard anodising often confused with?

Hard anodising is not decorative anodising: the layer is far thicker and harder, the build-up larger and the colour choice limited. Nor is it a heat treatment: heat treatment changes the properties of the metal itself, hard anodising only the surface layer. And it is not a deposited coating such as electroless nickel, which is laid on top of the metal and grows entirely outwards.

How do you see hard anodising in Blake?

In Blake subcontracted hard anodising goes into the estimate as its own line, priced from your subcontract library. If that price is missing, the line stays open and does not count as zero. The finish from the drawing sits with the part as a proposal; you confirm or correct it. More on estimating.