Surface treatments: seven finishes, chosen by environment

Zinc plating is the highest-volume and cheapest finish. Parts go in as the cathode in a zinc-salt electrolyte and zinc deposits atom by atom, with thickness set by current density and time — commonly 5 to 15 μm. Zinc is less noble than iron, so where the coating is scratched the zinc corrodes first and protects the steel; that is sacrificial protection. The same mechanism is its limit: the zinc is consumed, and with thin deposits or prolonged damp and saline exposure the white-rust-to-red-rust cycle shortens sharply. Hydrogen embrittlement also has to be named — pickling and plating drive hydrogen into high-strength steel, so parts of class 10.9 and above, or cold-worked parts, must be baked after plating or they can fail in service days later. Blue-white, yellow and black variants are available; where salt spray matters, specify the coating class rather than just 'zinc plated'.

Hot-dip galvanizing dips pickled parts into molten zinc at about 450°C; zinc and iron react at the interface to form a zinc-iron alloy layer under a pure zinc skin. At 45 to 85 μm it is several times thicker than plating, which is why it lasts longest outdoors and is the default for heavy-corrosion projects. The price is heat: thin sections distort and high-strength bolts lose mechanical properties, so bolts of class 10.9 and above are usually zinc-flake coated instead. Threads must be cleaned or cut oversize after coating or the nut will not run on — the most common site complaint. The finish is dull grey with runs and zinc lumps, so it is wrong for cosmetic parts. Design in vent and drain holes, and add process holes to closed cavities or the zinc cannot reach the surface.

Black oxide, also called blackening, grows a film of magnetite a few tens of nanometres thick in a hot alkaline oxidising bath. Strictly it is not a coating at all — under 1 μm, so thread fit and precision dimensions are essentially untouched, which is its main value. The downside is almost no corrosion resistance: the oxide is porous and only works after oil, wax or a sealer fills it, which is enough indoors and fails as soon as it gets rained on. It conducts, so it is common on socket and set screws that must ground. To judge quality, look for even colour without blotches or smut, and a dry, non-tacky feel after oiling.

Zinc flake coating disperses flake zinc and aluminium in a water- or solvent-borne binder that cures into a thin layered scale structure. Corrosion has to travel around many overlapping flakes to reach the steel, so 8 to 12 μm can approach hot-dip performance — and because there is no heat and no pickling or electroplating, hydrogen embrittlement is designed out. That is why class 10.9 and 12.9 bolts and spring parts use it first. It sells as Dacromet, Geomet and similar. Three cautions: conductivity is poor, so it is wrong for grounding; the coating is soft and scratches through in assembly; and the look is matte silver-grey or black-grey, less refined than nickel. Specify thickness and class on the drawing — the number of coats changes performance a lot.

Nickel plating comes in decorative and engineering grades. Decorative nickel is about appearance — bright white and even — often built as a multilayer system with a microporous chrome topcoat for corrosion and tarnish resistance. Engineering nickel is about function: harder, wear resistant and corrosion resistant, used where parts are adjusted repeatedly or rub. On steel it is a cathodic coating: once scratched through to the substrate, corrosion concentrates at the breach and eats inward instead of sacrificing itself as zinc does, so thickness and sealing matter. It costs more than zinc systems and is normally reserved for parts with a real cosmetic or functional requirement. Hydrogen embrittlement applies here too — high-strength parts need baking.

Brass plating deposits a copper-zinc alloy, tunable from pale gold to antique bronze. It is mostly decorative, but also functional: the brass layer bonds chemically to rubber during vulcanisation, which is why tyre valves and rubber-to-metal vibration mounts use it, and its low contact resistance suits electrical terminals. Corrosion resistance is moderate — damp, sulphur-bearing air dulls it — so a clear sealer or chromate passivation is normally added. At 2 to 8 μm it is not for wear or heavy corrosion duty. Inspect colour evenness and tarnish after salt spray, not just the shine straight out of the bath.

Phosphate coating forms an insoluble crystalline phosphate layer on steel in a phosphate bath, graded light, medium or heavy by coating weight. It barely protects on its own — the porous structure actually holds moisture — so it is never used alone: it is a base for paint, where the film keys into the pores and adhesion jumps; an oil or wax carrier for short-term protection; or a lubricant carrier for cold heading and drawing. Its engineering value is adhesion and friction: a phosphated thread has a more stable friction coefficient and is far less likely to gall when tightened. Acceptance is by coating weight in g/m² and crystal uniformity — colour from light to dark grey means nothing on its own.