Zinc electroplating
Specifying electrodeposited zinc on steel, supplementary treatments, dimensional allowances and acceptance.
Tags: processes, finishing, electroplating, zinc
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An example 9.980 mm pin with 8–12 µm radial zinc finishes at 9.996–10.004 mm before supplementary layers. The high end exceeds an assumed 10.000 mm maximum by 0.004 mm. Include passivate, sealer and local buildup in the finished fit and qualify threads and tightening separately.
Process and protection mechanism
Electrodeposited zinc protects steel sacrificially in suitable exposure. AGA distinguishes electroplating from hot-dip, mechanical plating, metallizing and zinc-rich paint. Specify the complete finish stack. AGA zinc-coating overview, updated October 2026
Hot-dip galvanizing immerses the article in molten zinc and forms zinc–iron alloy layers. Electroplating uses a different deposition route and should retain its own preparation, dimensional and acceptance requirements. An approval for one process does not authorize substitution of the other simply because both finishes contain zinc. AGA zinc plating, AGA hot-dip coating metallurgy
Select the standard and finish system
ASTM B633-23 covers material and process requirements for electrodeposited zinc on iron or steel articles for corrosion protection. Its scope provides four thickness classes and supplementary-finish options, excludes continuous zinc-coated wire/sheet processes, and directs purchasers of mechanical fastener coatings to consider F1941/F1941M. It also says the document does not provide all the background needed to specify a coating. Supply the actual part, material and functional requirements. ASTM B633-23, DIN's B633-23 scope record
Do not automatically label every passivated zinc finish “ISO 2081.” ISO 2081:2025, edition 5, November 2025, specifically covers zinc electroplates on iron/steel with supplementary treatments using hexavalent chromium compounds. ISO 19598:2016, edition 1, November 2016, confirmed in 2022, covers zinc and zinc-alloy deposits with Cr(VI)-free passivation. The latter includes zinc–nickel and zinc–iron systems; these are not compositionally identical to unalloyed zinc. ISO 2081:2025, ISO 19598:2016
A useful order identifies the governing edition, deposit composition, thickness requirement, conversion treatment, any sealer/topcoat, significant surfaces and tests. Colour alone does not identify chemistry or demonstrate regulatory conformity. Obtain the actual material declaration where required rather than inferring composition from a blue, clear, yellow or black appearance.
Conversion coatings, sealers and a named example
The zinc layer and its supplementary treatments perform different jobs. MacDermid's ZinKlad 96 brochure describes an 8 µm zinc deposit with a TriPass trivalent-chromium passivate. Its alternatives include a thin-film passivate plus clear topcoat to change corrosion/friction performance, or a high-build passivate supporting subsequent organic-paint adhesion. The same brochure distinguishes zinc's sacrificial protection from passivate protection against white corrosion and topcoat modification of friction. MacDermid ZinKlad 96, version 2.0
That named system is a bounded example, not a generic minimum zinc thickness or universal coefficient of friction. Specify the approved stack and delivered condition. For fasteners, coating thickness and lubricant/topcoat selection must remain compatible with thread fit and the qualified tightening behavior. ASTM's current fastener catalog lists F1941/F1941M-16(2025) separately from hot-dip and zinc-flake specifications. ASTM fastener standards
Substrate, geometry and hydrogen embrittlement
Provide alloy, tensile strength or hardness, heat treatment, surface condition and manufacturing history before approving the route. Cleaning, activation, plating, stripping and rework should be addressed as one controlled sequence. A finish certificate cannot compensate for an unidentified high-strength substrate.
B633's scope specifically advises against zinc electroplating high-strength metals, including steels above 1700 MPa, in accordance with that specification. This is a specification-specific boundary, not a statement that every lower-strength part is immune to hydrogen embrittlement or that a universal bake makes any part acceptable. Obtain material-appropriate pretreatment, post-treatment and required testing from the ordered specification and design authority. B633-23 public scope
For current general guidance, ISO 24251-1:2025, edition 1, August 2025, addresses prevention of hydrogen-assisted brittle fracture in high-strength steel components, linking material selection, manufacturing, heat treatment and coating. It replaced the withdrawn ISO 9587:2007 and ISO 9588:2007. Do not quote a legacy post-plating heat-treatment recipe as a universal current requirement. Preserve any contractually specified historical edition while resolving applicability with the responsible authority. ISO 24251-1:2025, ISO 9588 withdrawal record
Identify significant surfaces, recesses, threads, masking and permissible electrical-contact marks on the drawing. Agree where thickness is required and measured; an accessible face or overall average should not silently substitute for a local minimum at a functional surface. Retain final dimensional limits and the inspection method for the completed stack.
Original bounded dimensional example
Assume a perfect cylindrical pin is 9.980 mm before coating. A hypothetical qualified route adds a uniform 8–12 µm radial zinc layer, with no substrate removal or other dimensional change:
minimum zinc-coated diameter = 9.980 + 2 × 0.008 = 9.996 mm.
maximum zinc-coated diameter = 9.980 + 2 × 0.012 = 10.004 mm.
Against an assumed final interval of 9.995–10.000 mm, the maximum result is 0.004 mm oversize before considering any supplementary-layer buildup or measurement uncertainty. These values are arithmetic assumptions, not a process-capability claim. Resolve incoming size, local deposit variation and the complete finish stack together. This simple cylinder calculation is not a thread allowance formula.
Corrosion acceptance and release
Distinguish white corrosion of the zinc-based finish from red corrosion of the steel substrate. The ZinKlad brochure reports these as separate endpoints in its ASTM B117 comparison; the report does not make either endpoint a field-life prediction. State test exposure, duration, specimen preparation, significant surfaces, post-treatment and acceptance endpoint for the actual ordered system. MacDermid ZinKlad 96 performance table
Thickness, adhesion, appearance, corrosion resistance, thread fit and embrittlement controls answer different questions. Retain substrate identity/condition, processing and any heat-treatment records, layer stack, measured results and approved rework. Recheck requirements after stripping or replating rather than treating another cycle as equivalent to the original route.
Source versions: B633 is the 2023 edition; the current ASTM fastener catalog identifies F1941/F1941M's 2025 reapproval. ISO editions are stated above. MacDermid's two-page brochure prints ©2017 and Version 2.0 – 04/12/17; the ambiguous numeric date is retained without assigning a month/day interpretation. Its historical product example does not establish current OEM approval. The original dimensional example is separate from supplier ratings.