Anodising, powder coating, zinc plating: choosing a surface treatment

A surface treatment modifies the skin of a part without changing its overall geometry: it protects against corrosion, improves wear resistance, adds colour, insulation or conductivity, or prepares for bonding. Four questions guide the choice:

• Which material? Anodising applies to aluminium, galvanising to steel, passivation to stainless steel: the material already rules out several options.
• Which environment? Dry indoor, outdoor, marine atmosphere, food or chemical contact do not call for the same protection.
• Which function? Appearance, corrosion protection, friction, hardness, electrical insulation.
• What dimensional constraints? A deposit of a few micrometres or of a hundred micrometres cannot be handled the same way on a tight fit.

Quantity matters too: many treatments are done in batches, with a minimum charge. They are generally entrusted to a workshop specialising in surface treatment.

Anodising electrochemically converts the surface of aluminium into an oxide layer integrated with the metal. It is not a paint: it does not flake, but it applies only to aluminium (and a few similar metals).

Sulphuric anodising, protective and decorative

The layer, from a few micrometres to about twenty micrometres, protects against corrosion and takes colours: natural, black, and many colours by dyeing in a bath. The result depends on the alloy: alloys rich in copper or silicon and cast parts give less even shades, and an assembly of different grades will not colour uniformly (see the materials guide).

Hard anodising

Thicker (several tens of micrometres) and harder, it markedly improves wear and abrasion resistance: slides, pistons, friction parts. Its dark shade cannot be freely chosen. In both cases, the oxide is an electrical insulator: mask contact or earthing areas.

A pigmented resin powder is sprayed onto the part electrostatically, then baked in an oven at around 180 to 200 °C, where it melts and polymerises. The result is a continuous film, resistant to impact and corrosion, in a very wide choice of RAL colours and finishes (matt, satin, gloss, textured).

It can be applied to steel, galvanised steel and aluminium, after careful pre-treatment (degreasing, conversion coating or shot blasting), but not to parts that cannot withstand baking. Epoxy powders suit indoor use; for outdoor use, prefer polyester powders, which are more UV-stable. The film, often around 60 to 120 µm, is thick: threads, bores and contact points must be masked, and it thins out on sharp edges. Hollow parts and internal corners are harder to coat uniformly.

Hot-dip galvanising

The steel part is dipped in a bath of molten zinc at around 450 °C. The coating is thick (often about 45 to 100 µm depending on the steel thickness; standard NF EN ISO 1461), also covers the inside of hollow sections and gives lasting outdoor protection. The appearance is grey and sometimes uneven. Limits: bath size, risk of distortion of thin sheet and large welded parts, vent and drain holes needed on hollow bodies, and threads and bearing surfaces to be designed with clearance. It suits structures, brackets and guard rails, not precision parts. It can then be painted (known as a duplex system).

Electro-galvanising

Zinc is deposited by electrolysis in a thin layer, from a few micrometres to about twenty micrometres. The appearance is bright, completed by a passivation (clear or bluish, iridescent yellow, black). Protection is moderate: indoor use, mildly aggressive atmospheres. It is the usual choice for fasteners, mechanical parts and thin sheet metalwork, because dimensions change little. Zinc-nickel performs better in more demanding environments.

Point to watch: on high-strength steels, electrolytic deposits expose parts to hydrogen embrittlement, which baking after plating limits. Tell the treatment shop the steel grade and hardness.

Stainless steel passivation

An acid bath (citric or nitric) removes free iron and the contamination left by machining, and helps rebuild the chromium oxide layer that gives stainless steel its resistance. There is no deposit, and no visible change in dimensions or appearance. It is recommended for food processing, medical, chemical and marine applications; the ASTM A967 standard is a common reference. Not to be confused with electropolishing, which smooths the surface.

Electroless nickel plating

This nickel-phosphorus deposit, produced without electric current, has a very even thickness, even in bores and complex shapes. It provides corrosion and wear resistance, and its hardness can be increased by heat treatment. It applies to steel, aluminium (with preparation) and copper alloys.

Chrome plating

Hard chrome plating gives high hardness and low friction (cylinder rods, moulds, rollers). Traditional processes rely on hexavalent chromium, a substance regulated under European law: chromium trioxide is listed in Annex XIV of the REACH Regulation (Regulation (EC) No 1907/2006), which makes its use subject to authorisation (granted for specific uses), and other sector-specific texts, such as the RoHS Directive for electrical and electronic equipment, limit its presence.

Thicknesses are orders of magnitude, to be confirmed with the treatment shop according to the alloy, geometry and specification.

A treatment changes dimensions. Allow for it before final machining, not afterwards:

• Anodising: about half of the layer thickness grows outwards, the rest penetrates the metal (order of magnitude, varying with the process). The diameter of a shaft therefore increases by roughly the layer thickness, and that of a bore decreases by the same amount.
• Powder coating, hot-dip galvanising: thick, imprecise films, incompatible with a tight fit. Mask threads, bearing surfaces and bores, or allow for clearance.
• Electro-galvanising, electroless nickel plating: thin, even deposits, to be factored into thread and bearing-surface dimensions.
• Passivation: no measurable effect.

State on the drawing whether dimensions apply before or after treatment. Machine and weld before treating, and rework only critical dimensions after treatment. Finally, indicate the areas where racking marks are tolerated. Break sharp edges before treatment: paint and deposits thin out there. For the tolerances themselves, see the guide to the ISO 2768 standard.

An incomplete treatment request produces quotes that cannot be compared. Specify:

• the type of treatment and, if your sector requires one, the reference standard or specification;
• the target thickness or the accepted range;
• for paint and anodising, the colour (RAL reference) and finish;
• the areas to mask or leave untreated, such as threads, bores and electrical contacts;
• the durability requirement, for example in neutral salt spray (NF EN ISO 9227) for a given duration;
• the inspections expected: thickness report, certificate of conformity.

Think about lead time too: treatment adds a step, often transport to the treatment shop and a pass through a batch before the parts come back. Flag this in the request and group the parts that need the same treatment.

Example wording: “Polyester powder coating RAL 7016 satin, 60 to 80 µm, M8 threads masked”. To send it all, post your request on Meaflow: posting is free and commission-free, and you compare quotes from several suppliers. For a sheet metal part, see also the guide to laser cutting and bending.