Guide
Zinc plating salt spray hours: how many should clear and yellow zinc pass?
Zinc plating salt spray hours are the minimum time a plated steel part must spend in a neutral salt fog cabinet before a stated kind of corrosion appears on its significant surfaces. The number is set by the finish specification, not by the test method, and for zinc on steel it depends mainly on which conversion coating sits on top of the zinc: clear, yellow, or one of their chromium(VI)-free successors.
By Saola Manufacturing engineering team · Published · Updated
This guide is written for engineers who have to put a corrosion requirement on a drawing, and for buyers who receive a salt spray report and need to know what it proves. It works from two documents: ASTM B633, which sets thickness classes and supplementary finish types for electroplated zinc on iron and steel, and ASTM B117, which only defines how the salt fog cabinet is run. Keeping those two roles apart is most of the job. The first explains what number to ask for; the second explains why that number is a quality check and not a forecast of service life.
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How many salt spray hours does ASTM B633 require for clear and yellow zinc?
ASTM B633 ties the hours to the supplementary finish type: yellow chromate (Type II) carries the longest requirement of the chromate finishes and clear chromate (Type III) the shortest. The chromium(VI)-free passivates added for RoHS, Types V and VI, carry their own figures, with the colored one again ahead of the clear one.
The requirement is judged on white corrosion, the powdery zinc corrosion product, not on red rust from the steel underneath. That surprises people who read a report showing hundreds of hours to red rust and assume the plating beat the specification by a wide margin. The B633 figures are a check that the conversion coating was applied and aged properly; they say little about how much zinc is on the part. Type I (as plated) and Type IV (phosphate, usually a paint base) have no salt spray hours of their own in the table.
| B633 type | Topcoat | Usual appearance | Chromium(VI) | Minimum hours to white corrosion |
|---|---|---|---|---|
| Type I | None (as plated) | Bright or dull zinc | None | No requirement |
| Type II | Colored chromate conversion coating | Iridescent yellow, also olive or bronze | Present in traditional chemistry | 96 |
| Type III | Colorless chromate conversion coating | Clear to faint blue | Present in traditional chemistry | 12 |
| Type IV | Phosphate conversion coating | Gray, matte | None | No requirement |
| Type V | Colorless passivate, Cr(VI)-free | Clear to blue | None | 72 |
| Type VI | Colored passivate, Cr(VI)-free | Iridescent, often yellow | None | 120 |
Why does yellow zinc last longer in salt spray than clear zinc?
Yellow zinc lasts longer because its conversion film is heavier, and the color is a side effect of that extra film. A clear or blue finish is a much thinner layer, so it delays white corrosion for a shorter time.
A chromate or passivate is not paint. It forms by reacting the fresh zinc surface in an acidic bath, so the film is part of the surface rather than a layer laid on top. With traditional hexavalent chemistry, the film holds soluble chromate that can migrate to a scratch and slow attack there, which is part of why the older yellow finishes earned their reputation. Clear chromate was always the cosmetic option: it keeps the part looking like bright zinc and gives the shortest protection before white corrosion.
The chromium(VI)-free passivates changed the chemistry but not the ranking. Trivalent and other Cr(VI)-free films do not self-heal the way hexavalent films did, so suppliers often add a sealer or topcoat to close the gap. In practice, that means “yellow” on a drawing no longer tells you which chemistry you will get. Two parts of the same color can come from different baths with different results, which is why the type number matters more than the color word.
Two practical cautions follow. First, color is a poor acceptance criterion; judge the finish by type, thickness class and test result. Second, conversion films are sensitive to heat and to handling while fresh, so a finisher who bakes, packs or tests too soon after passivating can lose hours that the bath chemistry would otherwise deliver.
Does a thicker ASTM B633 class add salt spray hours?
Not in the B633 requirement itself, because its hours depend on the finish type and are judged on white corrosion. A thicker zinc class buys time to red rust and longer service, which the white corrosion figure does not measure.
ASTM B633 offers four thickness classes, Fe/Zn 5, 8, 12 and 25, where the number is the minimum zinc thickness in micrometers per ASTM B633, matched to service conditions SC1 (mild) through SC4 (very severe). The conversion coating sits on the surface and is consumed first; once it is gone, the zinc itself corrodes, sacrificially protecting the steel until it is used up. More zinc means more of that sacrificial reserve. So the class you choose drives how long it takes for red rust to appear, while the type you choose drives how long the part stays free of white corrosion.
If your application genuinely depends on time to red rust, the drawing has to say so, because B633 does not set a red rust hour figure for general parts. For threaded fasteners, ASTM F1941/F1941M is the document to cite; its scope covers coating thickness, hexavalent and non-hexavalent conversion coatings and corrosion resistance for fasteners, and it deals with the thread fit that thick zinc affects (ASTM F1941/F1941M, scope). Remember that thickness has a dimensional cost too: on threads and press fits, heavier classes need room in the tolerance, as the tolerances guide discusses for coated features in general.
What does an ASTM B117 salt spray result not tell you?
A B117 result does not tell you how many years a part will survive outdoors. The standard itself says that salt spray results used alone have seldom been correlated with performance in natural environments.
ASTM B117 describes the apparatus and the conditions for a continuous neutral salt fog, and nothing else. It states that it does not prescribe the specimen, the exposure period or how to interpret the results (ASTM B117-26a, section 1.2). The hours and the pass criterion always come from somewhere else, such as B633, a customer standard or the drawing. Its significance section is blunt: the practice gives relative corrosion resistance information for specimens in a given chamber, and correlation or extrapolation to real service should be considered only where corroborating long-term atmospheric exposures exist.
Three consequences matter to a buyer. A constant wet salt fog never dries, while real service cycles through wet and dry, so a finish can rank differently outdoors than in the cabinet. Results vary between chambers even when conditions are nominally the same, as B117 section 3.3 notes, so a pass in one lab and a fail in another is not proof that the parts changed. And the result depends heavily on the evaluation criteria, which is why the acceptance clause below spells them out. Treat salt spray hours as a consistency check on the plating line, lot to lot, rather than a service life figure.
How should a drawing state the salt spray requirement?
Name the standard and edition, the thickness class, the finish type, the hours, and whether the criterion is white corrosion or red rust. Then say which surfaces count and how many parts are tested.
A typical loose note that only says “zinc plate, yellow, salt spray tested” leaves every argument open. A tighter version reads: ZINC PLATE PER ASTM B633-23, FE/ZN 8 (SC2), TYPE VI. NO WHITE CORROSION ON SIGNIFICANT SURFACES AFTER THE TYPE VI HOURS OF ASTM B633, TESTED PER ASTM B117. SIGNIFICANT SURFACES AS SHADED. If you need red rust performance as well, add it as a separate line with its own hours, and expect it to push the finish toward a heavier class, a sealer, or both.
Beyond the note, agree these points before the first order:
- Significant surfaces. Edges, contact points with the rack and deep recesses plate thinner and corrode first. Mark which surfaces are judged.
- Sample and frequency. How many parts per lot, and whether every lot or a periodic check is tested.
- Aging before test. Fresh conversion films are soft; ask the finisher how long parts rest before they go in the cabinet, and keep it consistent.
- Who tests. The finisher’s own cabinet, an independent lab, or both, and which result governs if they disagree.
- Handling. Parts scratched in tumbling or packing are not a fair test of the finish, and the report should say how parts were handled.
The finish comparison guide covers the RoHS side of the passivate choice, and the RFQ preparation guide shows where a finish note sits in the full quote package.
What changes when zinc-plated parts are made overseas and tested at home?
The finish has to survive ocean freight before anyone tests it, so the salt spray result from the plating line and the condition of the parts on arrival are two separate questions. Agree how both are checked before the first shipment.
Ocean containers see warm, humid days and cool nights, and condensation inside the box can start white corrosion on zinc that would pass a fresh cabinet test. That is a packing and logistics issue, not a plating defect, but it looks identical at incoming inspection. Dry packing, desiccant and avoiding bare contact with wet cartons do more for arrival condition than a higher hour figure on the drawing.
For the test itself, ask for the salt spray report to name the B633 edition, class and type, the cabinet standard, the hours run, photographs at the end of the test and the criterion applied. If your own incoming inspection or a local lab repeats the test, send both sides the same acceptance clause, because chamber to chamber variation is real. Steel parts such as stamped brackets and turned shafts are the usual candidates for zinc, and the surface finishing page lists the finishes our partner plants apply, and the quote names where each step is done. To check a finish note against a real part, send the drawing for a quote or write to rfq@saolamfg.com.
Frequently asked
Is “clear zinc” the same as “blue zinc”?
Usually, yes. Both names describe a thin colorless conversion film that can look faintly blue. On a drawing, replace the color word with the B633 type, Type III for the hexavalent version or Type V for the Cr(VI)-free one.
Does a spot of white corrosion fail the part?
Under a white corrosion criterion, white corrosion on a significant surface before the required hours is a failure. The same spot on an unmarked surface, or after the required hours, is not. This is why the drawing should mark significant surfaces.
Is yellow zinc still allowed under RoHS?
Yellow made with hexavalent chromate is not acceptable in equipment covered by RoHS. Yellow made with a Cr(VI)-free passivate, Type VI, can be. The finish comparison guide covers the directive.
Can a zinc die casting be held to the salt spray hours in ASTM B633?
Not as a B633 requirement. ASTM B633 covers electroplated zinc on iron and steel only, so a finish on a zinc or aluminum casting needs its own specification and its own test requirement on the drawing.
Why did my parts pass at the plater but fail at incoming inspection?
Common causes are chamber to chamber variation, a different evaluation criterion, damage from handling or packing, and moisture in transit. Compare the two reports line by line before assuming the plating changed.
Sources
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023 (scope sections 1.3, 1.4, 1.6 and 1.9; supplementary finish types in section 4.2)
- ASTM B117-26a, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2026 (sections 1.2, 3.1 to 3.3)
- ASTM F1941/F1941M-16, Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric, ASTM International, 2016, reapproved 2025