Guide
Yellow zinc plating vs clear, black and olive: what changes besides the color?
Yellow zinc plating is electroplated zinc on steel finished with a colored conversion coating, a thin film formed by reacting the fresh zinc in a chromate or chromium(VI)-free passivate bath. The color comes from that film, not from the zinc, and the same zinc layer can be finished clear (often faintly blue), yellow, black or olive drab. What really changes between them is the chemistry of the film, how long it holds off white corrosion, whether it contains hexavalent chromium, and how the drawing has to call it out.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers choosing a zinc finish and for buyers reading a supplier’s finish note. It works from ASTM B633, the specification for electrodeposited zinc on iron and steel, and quotes only what its published scope and type definitions say. The short version: the color word on a drawing is the least useful part of the callout. The B633 type number, the thickness class and the RoHS status carry the engineering meaning; the color is what the part happens to look like once those are chosen.
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What does the color of zinc plating actually tell you?
The color tells you which conversion film sits on top of the zinc and, roughly, how heavy that film is. It does not tell you how much zinc is on the part, and since the move to chromium(VI)-free chemistry it no longer reliably tells you whether the film contains hexavalent chromium.
Every zinc-plated part has two layers. The zinc itself is the sacrificial metal: it corrodes in place of the steel and is the reserve that keeps red rust away. On top sits the conversion film, a very thin layer grown out of the zinc surface rather than laid on it like paint. Its job is to slow the zinc’s own corrosion, the white, powdery zinc oxide and hydroxide that appears long before any red rust.
In the traditional hexavalent chemistry, the colors formed a ladder. Clear or blue was the lightest film, chosen when the part should look like bright zinc. Yellow, the familiar iridescent gold, carried more film and more protection. Olive drab was heavier still and became the military and heavy-equipment look. Black was its own family, used for appearance, low glare and optical parts, and its protection depended on the bath and on whether a sealer went over it.
Trivalent and other chromium(VI)-free passivates broke the link between color and chemistry. A trivalent bath can be dyed or formulated to look yellow, black or olive, and the result may be paler, more rainbow-like or more uniform than the hexavalent finish it replaces. Two parts of the same color can therefore come from different chemistries with different corrosion results, and a part that looks “like the old yellow” may or may not be RoHS compliant. That is why every serious callout names a type, not a color.
How do zinc colors map to the ASTM B633 types?
ASTM B633 does not name colors as types; it names the film. Clear or blue maps to Type III (hexavalent) or Type V (chromium(VI)-free), while yellow, olive and other colored films map to Type II (hexavalent) or Type VI (chromium(VI)-free).
The current edition, ASTM B633-23, defines six finish types: Type I is zinc as plated with no film, Types II to VI are its five supplementary finishes, and Type IV is a phosphate coating, normally a base for paint. The four that matter for the color question come in pairs: a colorless and a colored chromate (Types III and II), and a colorless and a colored chromium(VI)-free passivate (Types V and VI). There is no separate black or olive type, so a black or olive film is specified as a colored type plus a color note. The table below lists the minimum neutral salt spray hours B633 sets for each type, judged on white corrosion; the companion salt spray hours guide explains what those figures do and do not prove.
B633 also sets four thickness classes, Fe/Zn 5, 8, 12 and 25, where the number is the minimum zinc thickness in micrometers per ASTM B633. Class and type are chosen independently. A common error is to upgrade the color when the part actually needs more zinc: a heavier film delays white corrosion, but only a heavier zinc class adds time before red rust once that film is consumed.
| Color as usually ordered | B633 type | Film | Chromium(VI) | Minimum hours to white corrosion |
|---|---|---|---|---|
| Clear / blue, hexavalent | Type III | Colorless chromate | Present | 12 |
| Yellow, olive drab, bronze, black, hexavalent | Type II | Colored chromate | Present | 96 |
| Clear / blue, trivalent | Type V | Colorless passivate | None | 72 |
| Yellow, black, olive, trivalent | Type VI | Colored passivate | None | 120 |
| As plated, no film | Type I | None | None | No requirement |
Is yellow zinc plating RoHS compliant, and what replaced hexavalent yellow?
Traditional yellow zinc is a hexavalent chromate, and it is not acceptable in electrical and electronic equipment covered by the EU RoHS Directive. A trivalent yellow passivate, ASTM B633 Type VI, can be compliant and is the usual replacement.
The EU RoHS Directive 2011/65/EU lists hexavalent chromium among its restricted substances, with a limit of 0.1% by weight in each homogeneous material. The Directive defines a homogeneous material as one that cannot be separated by mechanical actions, abrasion included, so a conversion film is generally assessed as a homogeneous material in its own right. Hexavalent chromium is also a workplace hazard: the US OSHA hexavalent chromium standards govern worker exposure, including in the electroplating shops that still use it. Between regulation and customer policy, many OEMs now ban Cr(VI) finishes outright, even on parts that sit outside RoHS scope.
The engineering trade-off is real but smaller than its reputation. Hexavalent films hold soluble chromate that can migrate into a scratch and slow attack there, the so-called self-healing effect; trivalent films do not do this to the same degree. Against that, B633 sets higher white corrosion hours for the chromium(VI)-free types than for their hexavalent counterparts, as the table shows, and suppliers commonly close the remaining gap with a sealer or topcoat. Industry guidance also holds that hexavalent films lose much of their protection when heated, while trivalent films tolerate heat better, which matters for parts that see a bake, a paint cure or a hot engine bay.
If an older drawing says “yellow zinc” or “Type II” and the product now has to meet RoHS, the drawing has to change; the supplier cannot fix it quietly. Revise the note to Type VI (or Type V if the color no longer matters), state RoHS compliance, and accept that the yellow will look different from the old parts. Approve that appearance once, with a sample, rather than rejecting the first lot because it does not match a ten-year-old part in the drawer.
When does black or olive drab zinc make sense?
Choose black when appearance, low reflectance or a match to other dark hardware matters, and choose olive drab mainly to match an existing military or equipment specification. Neither is a default upgrade in corrosion resistance, so state the hours you need rather than assuming the darker film delivers them.
Black is the finish most often misunderstood. A black film looks robust, but black films commonly give less white corrosion protection than yellow on their own, and typical industry guidance is that trivalent black needs a sealer or topcoat to approach the performance of a colored yellow film. Black also shows white corrosion, fingerprints and handling scuffs more readily than yellow, so cosmetic rejects rise even when the corrosion requirement is met. If black matters for function, such as stray light in an optical housing, say so in the note, because that turns a cosmetic preference into a requirement the supplier must control.
Olive drab was historically the heaviest hexavalent film, which is why it was associated with long outdoor life. Chromium(VI)-free olive finishes exist, but they are less common, and the color varies more between suppliers than yellow or black. On a new design with no legacy requirement, a yellow or clear trivalent film on an adequate zinc class usually does the same job with fewer appearance disputes.
Two cautions apply to every colored film. Color uniformity depends on part geometry, racking and bath age, so recesses and edges often look different from flat faces. And a colored film over a rough or pitted substrate exaggerates the surface: machining marks and tumbling scratches show more under yellow or black than under clear.
How do you choose between clear, yellow and black zinc for a steel part?
Start from the environment and the regulatory market, then pick the zinc class, then the type, and leave color last. In most indoor and sheltered applications a clear or blue trivalent film on a moderate zinc class is enough, and yellow or black earns its place only when a longer white corrosion requirement or a specific appearance is needed.
- Where does the part live? A dry cabinet, a sheltered outdoor frame and a wet, salted chassis call for different zinc classes before any color decision. B633 links its classes to service conditions from mild to very severe.
- Which markets will it ship into? If any version of the product may fall under RoHS or an OEM Cr(VI) ban, rule out Types II and III at the start, not after the first audit.
- Does white corrosion matter? On a hidden bracket a little white bloom is harmless; on a visible part or a sliding fit it is a reject. Only the second case justifies paying for a heavier film or a sealer.
- Is the part heat treated, baked or painted after plating? High-strength steels need an embrittlement relief bake, and that bake is done before the conversion film is applied, because heat damages the film. The hydrogen embrittlement guide covers when the bake is needed.
- Are there threads or fits? The film adds almost nothing, but the zinc does. Allow for the zinc class on threads and press fits, as the threads after plating guide explains.
Color is a legitimate requirement when it does real work: color-coding fastener grades on an assembly line, matching existing hardware, reducing glare. Write it down in those cases. When it is only habit, the clear trivalent film is usually the simplest and most consistent finish to buy.
How should the drawing call out a zinc plating color?
Cite ASTM B633 with its edition, the Fe/Zn class, the type, and then the color as a separate appearance note. Add RoHS status, the significant surfaces and any salt spray requirement, and the note is complete.
Weak notes that cause trouble at the quote stage:
- “ZINC PLATE YELLOW”: no class, no type, no chemistry, no test.
- “YELLOW ZINC, RoHS”: workable, but it relies on the supplier to infer Type VI.
- “ZINC PER B633 TYPE II” on a RoHS product: a contradiction the supplier must stop and query.
- “BLACK ZINC” with a salt spray figure copied from a yellow part: may not be reachable without a sealer, and the note does not allow one.
Stronger versions, adapted to your own class and requirement:
- ZINC PLATE PER ASTM B633-23, FE/ZN 8, TYPE V (CLEAR/BLUE TRIVALENT). RoHS COMPLIANT. NO HEXAVALENT CHROMIUM.
- ZINC PLATE PER ASTM B633-23, FE/ZN 12, TYPE VI, YELLOW. RoHS COMPLIANT. COLOR TO APPROVED SAMPLE. SIGNIFICANT SURFACES AS SHADED.
- ZINC PLATE PER ASTM B633-23, FE/ZN 8, TYPE VI, BLACK, WITH SEALER. NO WHITE CORROSION ON SIGNIFICANT SURFACES AFTER THE HOURS STATED IN THE PO, TESTED PER ASTM B117.
For threaded fasteners, ASTM F1941/F1941M is the more specific document: its published scope covers dimensional requirements for coated inch and metric threads, coating thickness, hexavalent chromate and non-hexavalent conversion coatings, corrosion resistance and hydrogen embrittlement management. Use B633 for general steel parts and F1941 for screws, bolts and nuts, and do not mix the two on one note.
On the RFQ, send the finish note as it stands and flag anything you are unsure of: whether RoHS applies, whether the color is a real requirement, what salt spray result you need and who will test it. The RFQ preparation guide shows where the finish note sits in the rest of the package.
Frequently asked
Is blue zinc the same as clear zinc?
In practice, yes. Both names describe a thin colorless film that can look faintly blue depending on the bath and the light. On a drawing, replace the color word with B633 Type V for the chromium(VI)-free version or Type III for the hexavalent one.
Why do new yellow zinc parts look different from the old ones?
Most likely the supplier has moved from a hexavalent yellow chromate to a trivalent yellow passivate. The trivalent film is often paler, greener or more iridescent. Check the type on the certificate of conformance; if it is Type VI the change is expected, and the fix is to approve a new color sample rather than to reject the lot.
Can you tell hexavalent from trivalent by looking at the part?
Not reliably. Color and sheen overlap between the two chemistries. A chemical spot test or a laboratory analysis is needed to confirm the absence of hexavalent chromium, which is why a RoHS declaration from the finisher matters more than the appearance of the part.
Does a darker zinc color mean more corrosion protection?
Not by itself. In the old hexavalent ladder heavier films were darker, but black is a special case and trivalent films are formulated for color separately from protection. The B633 type, the zinc class and any sealer decide the result; specify those and test them.
Can a zinc die casting be ordered with a yellow or clear chromate to ASTM B633?
Not as a B633 requirement, because B633 covers zinc electroplated on iron and steel only. A conversion film on a zinc casting needs its own specification and its own test requirement written on the drawing.
Sources
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel (scope; supplementary finish types), ASTM International, 2023
- ASTM B117-26a, Standard Practice for Operating Salt Spray (Fog) Apparatus (scope), ASTM International, 2026
- ASTM F1941/F1941M-16(2025), Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric (scope), ASTM International, reapproved 2025
- Directive 2011/65/EU on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS), Official Journal of the European Union, 2011, as amended
- Hexavalent Chromium, safety and health topic page, US Occupational Safety and Health Administration, accessed October 2026