Finishing

§1Why finish a surface

Surface finishing adds a thin layer or altered skin to a component after it is made, to do what the base metal alone cannot — resist corrosion, resist wear, look right, or come back to size.

Most engineering metals corrode, wear or simply look unfinished, so a treatment is applied to the surface to serve one or more of four purposes: corrosion protection (a barrier or a sacrificial metal that keeps rust off steel), wear resistance (a hard skin on a soft or hardenable part), appearance (colour, gloss or a decorative metal), and dimension (building a worn or undersize surface back up with plating). The treatments fall into a few families — depositing a metal electrically (§2–3), growing a protective oxide (§4), coating with a sacrificial metal (§5), or applying an organic layer (§6). Which to use is decided by the purpose and the base metal, and the last section draws that choice together. Finishing is the final, and often decisive, step in making a part fit for service.

§2Electroplating

Electroplating deposits a thin layer of one metal onto another using electricity: the part is made the cathode in a solution of the plating metal's ions, which the current reduces onto its surface.

The part to be plated is hung as the cathode (negative) in an electrolyte containing dissolved ions of the plating metal, with an anode (often of the same metal) completing the circuit. Passing a direct current drives the metal ions onto the part, building an even layer atom by atom, while the anode dissolves to replenish the solution. Common plates each serve a purpose: chromium for a hard, bright, wear- and corrosion-resistant finish (and decoration); nickel for corrosion resistance and as an undercoat; zinc for cheap sacrificial rust protection of steel; copper as an undercoat and for electrical work; gold and silver for contacts and appearance. Because the layer grows with the current and the time, its thickness can be controlled precisely and calculated exactly (§3) — which is what makes plating suitable both for protection and for restoring a worn surface to size.

§3Faraday's law and thickness

How much metal plates onto a part is governed exactly by Faraday's law: the mass deposited is proportional to the electric charge passed — the current multiplied by the time.

§4Anodising

Anodising is not a coating added on but a controlled oxide grown out of the metal itself — chiefly aluminium — making its natural oxide skin far thicker, harder and more protective.

Aluminium already forms a thin, tenacious oxide that protects it (the materials pages), and anodising deliberately thickens that oxide by making the part the anode (positive — the reverse of plating) in an acid electrolyte and passing a current, which grows a hard, porous aluminium-oxide layer integral with the metal. The layer is hard and wear-resistant, improves corrosion resistance, and — because it is porous before sealing — can be dyed a range of colours, then sealed to lock the colour and close the pores. This is the origin of the coloured, durable finish on aluminium consumer goods, architectural sections and cookware. Unlike plating, anodising adds no foreign metal: it converts the surface of the aluminium into a thicker version of its own protective oxide. It suits aluminium (and a few other metals like titanium); it does not apply to steel, which is protected by other means (§5).

§5Galvanising and sacrificial protection

Steel is most cheaply protected from rust by coating it with zinc — galvanising — which guards the steel even where the coating is scratched, because the zinc corrodes in the steel's place.

Galvanising coats steel with zinc, usually by dipping it in a bath of molten zinc (hot-dip), leaving a metallurgically bonded zinc layer. Its power is that the protection is sacrificial, and this follows directly from the galvanic series of the materials pages: zinc is less noble than iron, so when the coating is scratched and both metals are exposed, the zinc — not the steel — corrodes preferentially, protecting the exposed steel until the surrounding zinc is consumed. A barrier coating like paint fails the moment it is scratched, but a galvanised coating keeps protecting a small bare spot because the zinc sacrifices itself to it — the same principle as the sacrificial anodes bolted to ships and tanks. This is why galvanised steel is the standard for fences, structures, roofing and outdoor hardware: cheap, tough, and self-protecting at scratches. The trade is appearance (a matt grey) and that the protection lasts only as long as the zinc.

§6Coatings and choosing a finish

Beyond the electro-chemical treatments lie the organic coatings — paint and powder — and the whole choice comes down to matching the finish to its purpose and the base metal.

Paint and powder coating apply an organic layer for corrosion protection and colour: powder coating sprays a dry polymer powder, electrostatically charged to cling, then bakes it to a tough, even film — more durable and cleaner than wet paint, the standard for appliances, furniture and architectural metalwork. These are barrier coatings — they protect only while intact. Choosing among all the finishes is a matter of purpose: for rust protection of steel, galvanising (self-protecting, structural) or zinc plating (cheaper, decorative), or paint/powder for a coloured barrier; for a hard, wearing, bright surface, hard chrome plate or, on aluminium, anodising; for appearance, decorative chrome, anodising in colour, or powder coat; for restoring size, plating built to thickness (§3). And the base metal narrows it — anodising for aluminium, galvanising for steel. The rule is to name the purpose first, then pick the finish that serves it on that metal.

§7Quick reference

The working core of the page on one card rack.