How to Write a Zinc-Plating Specification: A Checklist for Purchasing and QC

Example: SCM435 class 10.9 bolt, zinc electroplate to ISO 4042, Cr(VI)-free trivalent clear/blue passivation, 8–12 µm on significant surfaces, approved seal/topcoat, customer-defined white/red corrosion criteria, hydrogen-relief bake, GO-gauge thread acceptance, thickness report and lot traceability.

The numbers and coating system are not universal defaults. Select them from the customer drawing, service environment and dimensional allowance. The goal is one shared requirement before production.

Give the material grade—SWCH, S45C, SCM435—and property class or post-heat-treatment hardness. High-strength steel carries a different hydrogen-embrittlement risk from mild steel.

Attach the drawing, thread size/class, blind holes, welds, masking and no-plate zones. If grade or hardness is unknown, say so and plan verification rather than guessing.

ISO 4042 addresses electroplated fasteners, dimensions and hydrogen-risk recommendations, with Amendment 1 published in 2026. ASTM B633 covers electrodeposited zinc on general iron and steel articles and points purchasers of fasteners toward a fastener-specific specification.

ISO 19598 covers Cr(VI)-free zinc and zinc-alloy systems within its scope. “Per ISO” alone is ambiguous; include the standard number and applicable edition/customer drawing.

State a range or minimum on the significant surface and show the measurement point. Barrel-plated parts vary with geometry and contact, so “8 microns” without a location is incomplete.

Coating adds size on every surface, especially threads, holes and grooves. Check allowance before calling for a thick deposit and agree on a measurement method suitable for the substrate and geometry.

Clear/blue, yellow or black is not a corrosion specification by itself. State trivalent/Cr(VI)-free passivation and any sealer, topcoat or lubricant needed for corrosion or friction performance.

For controlled supply chains, attach the actual RoHS, REACH, ELV or customer restricted-substance requirement. Do not infer chemistry from appearance.

State the test method, hours, separate white- and red-corrosion limits, excluded contact/edge areas and sampling plan.

Salt-spray hours are comparative laboratory criteria, not a direct conversion to service years. Add a representative environmental test when field exposure is critical.

State part hardness/strength and any pre-treatment or post-coating bake required by the governing standard and customer drawing, including the maximum delay before baking. “Bake” without conditions is not auditable.

Define thread class, GO/NO-GO gauge, mating pair and any torque–tension or coefficient-of-friction requirement because coating changes both size and friction.

Agree samples per lot, locations and acceptance for appearance, adhesion, thickness, corrosion and thread fit, plus the data required on the certificate.

For repeat work, approve a first article and require notice before changing passivation chemistry, topcoat, material source or another critical process. That prevents disputes more effectively than comparing colour by eye at delivery.