The common surface treatments for steel parts are zinc plating, zinc flake coating, hot-dip galvanising, phosphating, powder coating and electrocoating. Each gives a different level of corrosion protection, appearance and cost.
The right choice depends on where the part will be used, how long it must last, and whether it has threads or tight fits that a thick coating would spoil.
Zinc coatings
- Zinc electroplating puts a thin layer of zinc on the part, then a passivation layer. It suits indoor and mild outdoor use. ISO 4042 covers electroplated fasteners.
- Zinc-nickel plating gives better protection than plain zinc for the same thickness. It is common on auto parts.
- Zinc flake coating is applied as a dip-spin layer and cured. It suits high-strength bolts because it avoids hydrogen embrittlement. ISO 10683 covers it for fasteners.
- Hot-dip galvanising dips the part in molten zinc. It gives thick, long-lasting protection for structural and outdoor parts. ISO 1461 covers it for general articles.
Passivation colour and chromium
Older yellow passivation used hexavalent chromium. Many auto and industrial buyers now ban it, partly because EU rules on end-of-life vehicles and chemicals restrict it. Ask for trivalent passivation and state it in your specification.
Clear, blue, black and iridescent trivalent finishes are all available. A sealer or top coat on top improves salt spray life.
Phosphating
Phosphating converts the steel surface into a fine crystal layer of zinc, iron or manganese phosphate. On its own it gives little protection. With oil it gives a dark, lightly protected finish for gears and fasteners. Under paint it gives good adhesion and slows rust creeping under scratches.
Powder coating and electrocoating
Powder coating sprays dry paint powder onto the part, then cures it in an oven. It gives a thick, even film in almost any colour. Polyester powders suit outdoor use, and epoxy powders suit indoor use and chemical resistance.
Electrocoating, or e-coat, dips the part in a paint bath and uses an electric current to build an even film, even inside cavities. Many auto parts get e-coat as a primer, sometimes with a powder top coat.
Both depend on good pretreatment. Parts must be degreased and usually phosphated or given a modern conversion coat first.
How to specify and check
- Name the coating, its standard, the minimum thickness and the passivation or colour.
- State the salt spray hours required to white and red rust, as explained in the salt spray test.
- Mask threads and fits that must stay clean, or allow for coating thickness in the tolerance.
- Ask for thickness readings and adhesion tests on each lot.
Next step
We check whether coating is done in-house or sent out, and have lots tested for thickness and salt spray. Book a free sourcing review to set the right finish for your parts.