Anodising, powder coating or galvanising: which should you choose?

These three treatments protect the metal, but by different mechanisms, which explains their fields of use.

• Anodising turns the surface of aluminium into an oxide layer by an electrochemical route. The layer is part of the metal: it does not flake.
• Powder coating applies a resin powder electrostatically, cured in an oven at around 180 to 200 °C, forming a thick, continuous paint film.
• Hot-dip galvanising dips steel into a bath of molten zinc, at around 450 °C: the zinc forms with the iron a layer of alloys that protects the steel, including on small scratches, through a sacrificial effect.

Each has its material, thickness, appearance and shape constraints: they cannot be freely substituted for one another. This guide helps you choose. For the detail of all treatments (zinc plating, passivation, nickel plating, etc.), see the guide surface treatment of metals and the surface treatment page to find workshops.

The material already rules out some options. The table compares the three treatments; thicknesses are orders of magnitude, to be confirmed with the treatment shop.

Aluminium

Choose anodising to keep the metallic appearance, the dimensions and, with hard anodising, wear resistance. Prefer powder coating for a precise RAL colour, large surfaces or good impact resistance, and when the alloy anodises poorly: castings and aluminium alloys rich in silicon or copper give uneven shades.

Steel, cast iron and stainless steel

A bare steel part rusts: hot-dip galvanising is the durable solution outdoors (structures, guardrails, supports), powder coating the one for indoors or for appearance, and combining the two (duplex system) adds up service life and colour. For fasteners and precision parts, electrolytic zinc plating is better suited (see the surface treatment guide). Stainless steel generally needs none of the three: passivation is often enough (see the guide stainless steel 304 or 316L).

• Dry indoors. All three are suitable: the decision comes down to appearance and cost. Hot-dip galvanising is rarely useful.
• Ordinary outdoors. Hot-dip galvanising gives steel protection that often lasts several decades in a mildly aggressive atmosphere. A polyester powder coating withstands UV well; anodised and sealed aluminium is also suitable outdoors.
• Coastal, industrial atmosphere. The attack is stronger: aim for a reinforced system (duplex, thick anodised aluminium, qualified paint) and set the requirement, for example resistance to neutral salt spray according to ISO 9227 for a given duration. For painted steel structures, ISO 12944 classifies environments from C1 (very low) to C5 (very high) and CX (extreme).
• Impacts and scratches. Powder coating withstands impacts well, but a chip leaves bare metal, and corrosion can progress under the film. Galvanising also protects small scratches through a sacrificial effect.
• Wear and friction. Hard anodising is made for this (slides, pistons). The oxide layer is hard but not very ductile: bend and machine before anodising, because deformation after treatment cracks it.
• Heat. Powder coating curing and the zinc bath heat the part: seals, plastics and bearings do not withstand them, and thin sheets or large welded parts can distort.
• Electricity. Anodic oxide and paint insulate: mask contact or earthing points.

Anodising lets the metal show: natural, black or coloured in the bath, matt or satin depending on preparation (brushing, blasting). The result depends on the alloy, and the shade can vary from one batch to another: have the parts of a visible assembly treated in the same batch. A weld seam also appears in a different shade after anodising.

Powder coating offers the widest choice: RAL colours, gloss, textured or metallic effects. A slight orange-peel appearance is normal with some powders. To validate a colour or texture, ask for a sample before the run: the look of a shade also depends on gloss and substrate.

Hot-dip galvanising gives a metallic grey, bright at first then matt with time, sometimes uneven depending on the steel composition (its silicon content influences thickness and appearance). A decorative appearance is not guaranteed: for a visible part, a powder coating is applied on top.

These typical cases show the reasoning; your part may justify a different answer, to be validated with the treatment shop.

A treatment changes dimensions, and the shape of the part determines its success. Allow for it before final machining.

• Dimensions. In anodising, about half of the layer thickness grows outwards (order of magnitude): a shaft gets bigger, a bore gets smaller. Powder coating and galvanising films, thick and imprecise, are incompatible with a tight fit: mask threads, bores and bearing surfaces. See also the guide on machining tolerances.
• Galvanising. Hollow bodies require vent and drain holes: without them, trapped air and moisture can cause zinc to spit. Sheet overlaps, where liquid seeps in, are to be avoided. Slag and weld spatter must be removed before treatment.
• Powder coating. Sharp internal corners, where powder penetrates poorly (Faraday cage effect), and sharp edges, where the film thins, are to be avoided: break the edges. Castings and porous welds can outgas during curing and cause blisters.
• Anodising. Assemblies with gaps (spot-welded, riveted) retain the bath acid and then weep. Provide an electrical contact point, whose mark will remain visible.

• The duplex system. Powder coating on galvanised steel offers a longer service life than each coating taken separately, provided the zinc is suitably prepared (degreasing, light mechanical abrasion) and the powder is compatible.
• The order of operations. Machine, bend, weld and deburr before treating, then rework after treatment only the critical dimensions. For welded parts, see the guide TIG, MIG, MAG or stick welding.
• Mixed assemblies. Aluminium and steel in contact, in the presence of moisture, form a galvanic couple: treat each material before assembly and insulate them if necessary.
• Over-specification. Galvanising then painting an indoor bracket costs time and money for no benefit. Conversely, powder coating alone on an outdoor steel structure ages quickly once the film is nicked.
• Lead time. These treatments are almost always subcontracted to a specialist workshop: allow for transport, batch scheduling and the curing or bath before the parts come back.

• the material and grade of each part, because they determine the possible treatment;
• the use and environment: indoor, outdoor, coastal, impact, wear, food contact;
• the appearance expected: RAL reference, gloss and texture, or anodising shade;
• the durability requirement and the reference standard, for example neutral salt spray for a given duration;
• the target thickness or the accepted range, and whether dimensions apply before or after treatment;
• the areas to mask (threads, bores, contacts) and the hanging points tolerated;
• the inspections and documents: thickness report, certificate of conformity;
• the quantity and lead time, grouping the parts that receive the same treatment.

An example of wording: “Polyester powder coating RAL 7016 satin, 60 to 80 µm, M8 threads masked”. On Meaflow, posting a request is free and commission-free: you compare quotes from several workshops and talk by messaging before choosing.