Zinc Plating: When to Spec It

Zinc is anodic to steel, so in the presence of moisture the zinc corrodes and the steel does not. This is the whole point and it is why zinc outperforms a simple barrier coating like paint at a scratch. Paint exposes bare steel where it is damaged; zinc keeps protecting the exposed area from beside it.

That protection is consumed as it works. A thin coating in a wet environment simply runs out sooner, which is why thickness is a service life decision rather than a cosmetic one.

Where zinc stops working is chlorides and heat. In marine or salted environments zinc is consumed quickly, and above roughly 120 C the coating degrades. Those are the conditions that push you to zinc nickel, or off zinc entirely.

The other limit is aesthetic: zinc is a functional finish. If the part is visible and needs to look designed rather than industrial, zinc is usually the wrong answer regardless of its corrosion performance.

A drawing that says zinc plate has specified almost nothing. The two variables that decide performance are the thickness and the passivate applied over it, and both are covered by ASTM B633, which is the specification to cite.

Thickness classes run from a few microns for indoor parts up to 25 microns for genuinely severe service. The passivate, or chromate conversion, sits on top of the zinc and multiplies its life substantially while also setting the color.

The passivate is also where compliance lives. Traditional yellow and olive drab passivates used hexavalent chromium, which RoHS restricts. Trivalent alternatives are standard now, but a drawing that names a color rather than a chemistry can still get you the wrong one.

The reference chart carries the full class tables; what matters here is that you name a class and a passivate type rather than a color.

This is the failure that shows up at assembly, and it is entirely predictable.

Plating deposits on every surface, including both flanks of a thread. Because a thread engages on its flanks, the effective pitch diameter closes up by roughly four times the plating thickness, since the coating is added on both sides of both mating flanks. A coating that sounds trivially thin at 8 microns removes something like 32 microns from the fit.

On a nominal thread that is often enough to prevent a plated screw entering a plated hole at all. The result is stripped threads, chased holes and a lot of avoidable frustration.

There are three normal solutions. Tap after plating, which is clean but leaves bare threads. Specify an oversize tap class so the plated result lands in tolerance, which is the usual production answer. Or mask the threads, which costs manual labor per part.

Whichever you pick, decide it on the drawing. A shop that is not told will make the choice for you, and the choice they make may not be the one your assembly needs.

Electroplating puts hydrogen into the steel. In most parts it diffuses out harmlessly. In high strength steel it collects at grain boundaries and can cause sudden brittle failure under sustained load, sometimes days after assembly.

The threshold usually cited is a hardness around HRC 40 or a tensile strength above roughly 1000 MPa. Above that, the part needs baking after plating, typically several hours at around 190 to 220 C, and the bake has to happen soon after plating to be effective.

This catches people on fasteners, springs, retaining rings and heat treated shafts, which are exactly the parts most likely to be both high strength and plated.

If your part is heat treated and plated, put the bake requirement on the drawing and cite the specification. It is the kind of omission that produces a failure nobody can reproduce, because the part passed every inspection before it broke.

Zinc is the default for a reason: it is cheap, widely available and adequate for indoor and mild outdoor service. The alternatives earn their cost in specific conditions.

Zinc nickel is the usual step up. It gives several times the salt spray life of plain zinc and holds up better at temperature, which is why automotive under-hood parts use it. It costs more and not every shop runs it.

Electroless nickel gives a hard, uniform, corrosion resistant deposit that coats internal passages evenly, which zinc does not. It is a barrier coating rather than a sacrificial one, so a scratch exposes bare steel.

Hot dip galvanizing puts on far more zinc than plating does and lasts far longer outdoors, but the coating is thick and uneven, so it is unsuitable for anything with a fit.

And for many parts the honest alternative is a different material. If the part is small and the environment is wet, stainless with no coating at all removes a process step, a failure mode and a compliance question.

Send the drawing citing ASTM B633 with a thickness class and a passivate type, stated as chemistry rather than color, and say trivalent explicitly if compliance matters.

Say how threads are handled: plated after tapping, oversize tap class, or masked. Say whether the part is heat treated, and if it is, call out the post plating bake with its specification.

Mark any surface that must stay bare for electrical grounding or for a press fit, because plating goes everywhere it is not masked.

Then ask three questions. What thickness will actually be deposited, and where do you measure it? Is the passivate trivalent? And given my thread callout, will a plated fastener assemble into this plated hole without chasing?