1 · Degrease and pickle
Hydrogen enters the steel here as well as in the plating bath. Acid strength, time and temperature all matter; over-pickling is a common cause of problems that only appear weeks later.
2 · Electroplate
Zinc or zinc-nickel. Thickness distribution across the part — edges, threads, recesses — sets the upper limit of achievable salt spray, no matter what is applied afterwards.
3 · Rinse
Static rinse water accumulates zinc, iron and passivation drag-in and deposits it back on the part. Counter-flow and conductivity-based dumping are the controls.
4 · Passivate
Trivalent blue-white, yellow or thick film. Film integrity and colour uniformity are set here; sealing cannot repair a damaged passivation layer.
5 · Seal
Barrier film, typically 1–3 µm. Film weight, coverage at thread roots and drying behaviour determine whether the barrier is continuous.
6 · Dehydrogenate (if required)
190–220 °C for 4–24 h, started within hours of plating. If the sealer was applied before this step, it must survive the temperature.
Four possible routes — and what each one costs
| Route | Advantage | Cost / risk |
|---|---|---|
| Seal → bake | One thermal step, shortest lead time, film applied to a clean passivated surface. | Sealer must be rated for the bake. If it is not, salt-spray performance drops after dehydrogenation. |
| Bake → seal | Sealer never sees the bake; wide choice of chemistry. | Bare parts are handled and stored between steps; surface oxidation can reduce film adhesion. |
| Bake → seal → cure | Best film integrity; sealer fully cured. | Two thermal steps: energy, handling, lead time. Cure temperature must not undo the bake. |
| High-temperature sealer, single step | Seal first, then dehydrogenate; the film retains corrosion resistance through the bake. | Requires a datasheet statement to that effect. Not every "high temperature" product is one. |
Where the process most often leaks
Racking and contact marks
Every contact point is a place with no coating and no film. Rotate rack positions and inspect the marks before blaming the chemistry.
Drip time and orientation
Thread roots, blind holes and overlapping faces are where the film is thinnest. Racking orientation is cheaper to change than concentration.
Oven uniformity
Set-point is not part temperature. Loading density can leave a 20–30 °C spread; the coldest part may never reach the specification.
Packing while warm
Condensation inside a closed carton produces white rust in transit — the most common "the sealer failed" claim that has nothing to do with the sealer.