Chemical Conversion Coating (Alodine / Chromate)

What it is and when to specify it

Chemical conversion coating is a chemical process that grows a thin protective film directly on an aluminium surface. It is known by several names on drawings: chromate conversion coating, chem film, and Alodine, a trade name that has become everyday shorthand for the whole family. Unlike anodizing, which builds an insulating oxide layer, the conversion film stays extremely thin and leaves the part electrically conductive. That pairing of corrosion protection with conductivity is the main reason engineers specify it, and it is why the finish appears so often on electronics and enclosure work.

Specify chromate conversion when a machined aluminium part must remain conductive for grounding or shielding, or when you want corrosion protection without the added thickness and insulation of anodizing. Typical parts are enclosures, chassis, front panels, brackets, and heat sinks. It is also a sound paint base, because the film improves adhesion for a later coat. We arrange this finish through our vetted partner network, managed under one quality process, and stay your single point of contact from drawing to shipment.

Specifications

Two classes dominate this finish, and the drawing should state which one you want. Class 1A is the heavier coating: it is specified for maximum corrosion protection and is usually gold or a darker iridescent colour. Class 3 is the thinner, clear coating: it is specified when low electrical contact resistance matters more than heavy corrosion protection, so it suits grounding and EMI-shielding parts. Both classes are commonly called out to MIL-DTL-5541, the standard most engineering drawings reference.

Coating thickness sits in fractions of a micrometre rather than the tens of micrometres you expect from anodizing; a common trade range is well under one micrometre. The exact class, colour, and thickness requirement is confirmed per drawing before we quote, because these choices change conductivity, colour, and corrosion behaviour together.

Masking matters on this finish. Threaded holes, press-fit bores, and mating faces are frequently masked so those surfaces stay bare and conductive, and so assembly torque and fit are not disturbed. We mask per the drawing and confirm every masked area before the parts go into the line.

Which materials and parts it suits

This finish is for aluminium, and in practice the grades we finish most are the same four we machine every day: aluminium 2024, 5052, 6061 and 7075. All four take a clean, uniform conversion film. 6061 is the general-purpose choice for enclosures and brackets, 5052 suits formed and welded sheet where the part has to bend before it is coated, and 2024 and 7075 are used where higher strength is needed and the part still has to stay conductive.

The parts that benefit most are the ones where conductivity and shielding drive the design. Think electronics and communications hardware: RF shields, chassis, housings, and grounding brackets. Aluminium machined parts such as heat sinks, front panels, and cover plates also suit it well, and any part headed for paint often gets a conversion film first as an adhesion layer.

The finishing itself is applied through our vetted partner network, managed under one quality process. We remain the single point of contact, so you do not need to screen finishers yourself, and every order ships with a material certificate and pre-shipment photos.

Corrosion, wear and dimensional impact

On corrosion, the film works twice: it is a barrier that slows moisture reaching the metal, and the chromate within it acts as a self-healing reservoir that limits corrosion if the surface is scratched. This is why it protects better than a bare machined surface, while still being a much lighter finish than anodizing.

It is not a wear or hard coating. It does not add meaningful surface hardness, so it is the wrong choice if the part will see sliding or abrasive contact. Its real strength is dimensional: the film is thin enough that threads, bores, and press fits stay essentially unchanged, which is why it is specified on close-fit conductive parts.

Because conductivity is the whole point, masking threaded and mating surfaces is part of the job. We mask per the drawing so grounding pads and contact faces keep their electrical path, and we confirm the class, colour, and masking before production. See the wider surface finishes overview for how this finish compares with plating and anodizing.