Aluminium chromate conversion coating: corrosion protection and electrical contact

What a conversion coating is

A chromate conversion coating is formed by a chemical reaction between the aluminium surface and a bath. The surface metal is converted into a thin mixed oxide layer. Unlike paint or plating, there is no meaningful build up, so the part does not change size and a thread does not close.

The layer is typically a fraction of a micrometre thick. It provides a barrier against moisture, improves the adhesion of a later paint or powder coat and leaves a colour that ranges from clear to iridescent yellow depending on the process and the alloy.

That thinness is the point and the limit. It holds a tight tolerance and keeps a joint closer to metal to metal contact than anodising would. It also wears through at a sharp edge or under a sliding contact, and the protection at that point is gone.

Electrical contact: what the coating allows

A chromate layer is not a good conductor in the way that silver or tin is. It is thin enough that a bolted joint with sufficient contact pressure can break through it at the asperities, so the joint resistance may stay close to a bare aluminium joint. That is why conversion coatings are used on busbars, connector bodies and earth points where a full plating is not wanted.

The result depends on the pressure, the surface roughness and the area. A small screw at low torque may sit on the coating without breaking through. A large bolted lap joint with a specified torque and a Belleville washer will break through at enough points to carry the current. The joint design is doing the work, not the coating.

This is why a drawing that says conversion coated and expects a low resistance joint is incomplete. State the contact resistance limit, the torque, the washer and the preparation of the mating faces. If the joint is critical, mask the contact area and leave it bare, or plate it.

Conversion coating versus anodising

The choice between a conversion coating and anodising is usually a choice between a thin, conductive barrier and a thick, hard insulator. The comparison below sets out the rows that matter for a part with a joint or an earth path.

The process and the controls that matter

The line is short but sensitive. A typical sequence is clean, rinse, deoxidise, rinse, de-smut, conversion dip, rinse and dry. Each step has a time and a temperature, and the result depends on all of them.

Cleaning is the step that most often fails. Oils, machining coolant and fingerprints stop the bath from reaching the metal, and the result is a patchy coat that looks like a flow mark. Deoxidising removes the natural oxide and any smut from machining. If the deoxidiser is weak, the coating forms over a contaminated surface and adhesion fails later under paint.

Bath chemistry, pH and immersion time set the coating weight. A short dip gives a thin, clear film. A longer dip gives a heavier, more coloured film. The supplier should control the bath by titration and record the coating weight on a coupon, not judge the result by eye alone.

Hexavalent and trivalent processes

Traditional chromate conversion uses hexavalent chromium. It gives a strong corrosion result and a self healing effect at scratches, but it is restricted in many markets because of its toxicity. Trivalent chromium processes are the common alternative and are widely used on aluminium.

The two are not identical. A trivalent process may give a different colour, a different coating weight and a different corrosion result in a salt spray test. If a drawing or a customer specification was written around a hexavalent process, changing to trivalent is a change that should be validated, not assumed to be equivalent.

The regulatory position also varies by market and by application. A buyer importing a coated part should know which process was used and be able to show it in the documentation, because a declaration that says conversion coated is not enough to prove compliance. RoHS and REACH obligations sit with the importer, and the coating is part of the product.

Where the coating fits and where it does not

The coating suits aluminium parts that need a corrosion barrier, a paint base or a low cost surface treatment without a dimensional change. Housings, brackets, busbar bodies, connector shells and earth plates are typical. It is also a common pretreatment before powder coating, because it improves adhesion and slows underfilm corrosion at a scratch.

It does not suit a part that sees abrasion, a sliding contact or a high current joint with no masking. The film wears through at an edge, and the joint resistance depends on the pressure and the area. For those parts, a tin or a silver plating is the better route, or a masked bare contact area with the coating on the rest of the part.

Specifying and inspecting the coating

A conversion coating is too thin for a magnetic or an eddy current paint gauge in the usual sense. Coating weight is measured by stripping the film from a defined area and weighing the loss, or by a test method agreed with the finisher. Colour and appearance are checked against a sample, not against a word on the drawing.

For a joint, the useful check is electrical. A contact resistance or a millivolt drop test on a sample joint, at the specified torque and with the specified hardware, proves the assembly rather than the coating. For a painted part, an adhesion test and a salt spray result give the corrosion evidence.

The drawing should state the process type, the colour or the appearance reference, the coating weight or the test method and any masked areas. If a customer specification names a standard, quote it. A note that says chromate conversion with no class or method leaves the supplier to choose, and the choice may not match the last shipment.