Plating deposits a thin metallic layer onto a part to change what its surface does: resist corrosion, conduct better, wear longer, solder more easily, or simply look right. It is a chemical process run by specialist shops, and the two things buyers most often get wrong are specifying thickness and forgetting that the coating grows on fitted features.
What each common coating is for
Zinc and zinc alloys are the cheap sacrificial corrosion protection on steel fasteners and brackets. Nickel gives corrosion and wear resistance and is often an undercoat. Chrome adds hardness and appearance. Tin gives solderability. Gold gives contact reliability and does not tarnish, which is why it survives on connectors despite the price. Silver and copper appear where conductivity dominates.
Thickness is the specification, not the colour
Plating is specified as a thickness range at defined points on the part, usually against a published standard, because that is what determines the corrosion or wear performance. A drawing that says only the metal name has not specified anything a plater can be held to. Say the standard, the class and where the thickness is measured, especially in recesses where deposition is thinner.
Dimensions, masking and threads
The coating adds to every surface, so a shaft grows and a hole shrinks, and on a threaded feature the effect is multiplied across the flanks. Either allow for the build up in the machined dimension, mask the feature, or chase the thread afterwards. Blind holes, deep recesses and internal surfaces plate unevenly or not at all, which is a design constraint rather than a plater's failing.
Hydrogen embrittlement and other traps
High strength steels can absorb hydrogen during acid cleaning and electroplating and crack later under load, which is why a bake after plating is specified for them. Springs and fasteners are the classic cases. Also plan for racking marks, since the part has to be held somewhere, and agree with the plater where those marks are acceptable.
Questions people ask about gold plating services
How do I specify plating on a drawing?
Name the coating, the applicable standard and class, the thickness range, where it is measured, which areas are masked, and any post plating bake. A metal name alone is not a specification.
Does plating change the size of a part?
Yes. It grows every exposed surface, so shafts get larger and holes get smaller. Allow for it or mask the fitted features.
What is hydrogen embrittlement?
Hydrogen absorbed during cleaning and plating can make high strength steel crack under sustained load. A post plating bake within a specified time is the standard mitigation.
Why is gold plating so expensive?
The metal itself, and the fact that it is usually plated over a nickel barrier layer, so it is a two stage process. It survives because contact reliability in connectors is worth more than the metal cost.