Guide
What does ASTM B633 specify, and how do you read a callout like Fe/Zn 8, SC 2, Type V?
ASTM B633 is the ASTM International specification for electrodeposited (electroplated) zinc coatings on iron and steel parts, and a callout written to it fixes two things: how much zinc goes on and what goes on top of it. The thickness is set by a class such as Fe/Zn 8, which is tied to a service condition from SC 1 (mild) to SC 4 (very severe); the topcoat is set by a supplementary finish type from Type I (as plated) to Type VI (a colored chromium(VI)-free passivate). Read Fe/Zn 8, SC 2, Type V as zinc on steel, at least 8 micrometers thick, for moderate indoor service, finished with a clear passivate that contains no hexavalent chromium.
By Saola Manufacturing engineering team · Published · Updated
This guide is a callout decoder for design engineers who write zinc finish notes and for buyers who receive them on a drawing. It takes each element of a B633 note in turn, gives the class, service condition and type values in tables you can check a drawing against, shows how much a class grows a thread, and explains when the note also needs a hydrogen embrittlement bake. It works from the published scope of ASTM B633-23 and the thread tolerance standards, and it covers specification only: the plating itself is a job for a zinc finisher, and the machining before it is where most callout mistakes get caught or missed.
On this page
What does each part of an ASTM B633 callout mean?
A complete ASTM B633 callout has four parts: the standard and its edition, a thickness class written Fe/Zn plus a number, the matching service condition SC 1 to SC 4, and a supplementary finish type from I to VI. The class sets the minimum zinc thickness and the type sets the conversion coating or passivate applied over it.
In the class designation, Fe is the chemical symbol for the iron or steel substrate, Zn is the zinc coating, and the number is the minimum zinc thickness in micrometers per ASTM B633. So, per ASTM B633, Fe/Zn 8 means at least 8 µm of zinc on the significant surfaces of the part. The service condition is not a separate requirement; each SC number points to exactly one class, so writing Fe/Zn 8, SC 2 says the same thing twice. Plenty of drawings carry both anyway, and the repetition does no harm as long as the pair matches.
The type is the part most drawings get wrong. Per ASTM B633, Type I is zinc with no supplementary finish, Types II and III are the traditional colored and colorless chromate conversion coatings, Type IV is a phosphate coating, and Types V and VI are the colorless and colored passivates that ASTM added so that non-chromate treatments could replace hexavalent chromium under RoHS-type restrictions. A note that says only “zinc plate, clear” leaves the finisher to guess between Type III and Type V, which differ in chemistry, in salt spray requirement and in whether the part can ship into equipment covered by the EU RoHS Directive. The edition matters too: B633 has been revised several times, and the current one is B633-23 (ASTM B633-23).
| Service condition | Typical exposure | Class | Minimum zinc thickness | Pitch diameter growth at minimum thickness |
|---|---|---|---|---|
| SC 1, mild | Dry indoor air, rare condensation, little wear | Fe/Zn 5 | 5 µm (0.0002 in) | 20 µm (0.0008 in) |
| SC 2, moderate | Mostly dry indoors, occasional condensation, some wear or abrasion | Fe/Zn 8 | 8 µm (0.0003 in) | 32 µm (0.0013 in) |
| SC 3, severe | Condensation, occasional rain, cleaners, perspiration | Fe/Zn 12 | 12 µm (0.0005 in) | 48 µm (0.0019 in) |
| SC 4, very severe | Frequent wetting, salt solutions, likely denting or abrasion | Fe/Zn 25 | 25 µm (0.0010 in) | 100 µm (0.0039 in) |
Which ASTM B633 thickness class and service condition should you choose?
Choose the lowest service condition that honestly describes where the part lives, because each step up adds zinc that has to fit on threads and fits. B633’s own examples for SC 2 include tools and machine parts; parts that see condensation or rain usually justify Fe/Zn 12, and Fe/Zn 25 suits plain parts without tight threads.
The thickness class buys time to red rust. Zinc protects steel sacrificially: it corrodes first, and the steel stays protected until the zinc on a surface is used up, so more zinc means a longer reserve. That is a different job from the supplementary finish, which mainly delays white corrosion of the zinc itself. An engineer who wants a longer life outdoors should therefore look at the class first, and treat the type as the second lever. Per ASTM B633, the classes are 5, 8, 12 and 25 µm minimum, so the jump from SC 3 to SC 4 roughly doubles the zinc.
The cost of a heavier class is dimensional more than financial. Electroplated zinc builds up unevenly: outside corners and edges collect more, while recesses, bores and the roots of internal threads collect less. A heavy class that looks reasonable on a flat bracket can close up a tapped hole or push a turned diameter out of tolerance. For that reason SC 4 is usually reserved for parts with open geometry, and threaded or close-fitting parts that need more protection than SC 2 often move to a different coating instead; the zinc-nickel comparison covers that route. If service life in a specific environment is what matters, say so in the RFQ rather than raising the class by reflex.
What do ASTM B633 Types I to VI mean, and how many salt spray hours does each need?
The type names the finish over the zinc, and per ASTM B633 only Types II, III, V and VI carry a neutral salt spray requirement, judged on white corrosion. Type VI (colored, Cr(VI)-free) carries the longest requirement and Type III (clear chromate) the shortest.
Types V and VI are the ones to specify on new drawings for most markets, because they do the job of the old Type III and Type II without hexavalent chromium. Type II is the classic iridescent yellow chromate, still common on legacy prints, and it is the usual answer to the search question “is zinc Type I or Type II”: a part that looks gold or iridescent is Type II or Type VI, while plain bright zinc with no conversion coating is Type I. Type IV, a phosphate coating, is a base for paint and has no salt spray requirement of its own.
The salt spray hours check that the conversion coating was applied and aged correctly; they are not a prediction of years in service. The test cabinet is run per ASTM B117, which itself states that it sets no exposure period and gives no interpretation of results (ASTM B117-26a). The salt spray hours guide explains how to write the test and acceptance clause; the table below is the short version for decoding a callout.
| Type | Finish over the zinc | Usual look | Hexavalent chromium | Minimum hours to white corrosion |
|---|---|---|---|---|
| Type I | None, as plated | Bright or dull zinc | No | No requirement |
| Type II | Colored chromate conversion coating | Iridescent yellow, also olive or bronze | Yes, in traditional chemistry | 96 |
| Type III | Colorless chromate conversion coating | Clear to faint blue | Yes, in traditional chemistry | 12 |
| Type IV | Phosphate coating, paint base | Gray, matte | No | No requirement |
| Type V | Colorless passivate, Cr(VI)-free | Clear to blue | No | 72 |
| Type VI | Colored passivate, Cr(VI)-free | Iridescent, often yellow | No | 120 |
How much does an ASTM B633 class change threads and fits?
On a 60-degree thread, an even coat of zinc grows the pitch diameter by four times the coating thickness, and because a B633 class is only a minimum, ASME B1.1 asks for six times that minimum to fit inside the allowance. On that basis no class above Fe/Zn 5 fits a standard class 2A or 6g thread up to 1/2 in or M12, so decide on the drawing whether thread dimensions apply before or after plating.
The four-to-one ratio is geometry: zinc is measured square to each flank, while pitch diameter is measured square to the thread axis, so a uniform layer on both flanks of a 60-degree thread moves the pitch diameter by four layer thicknesses. ASME B1.1 then adds a margin for plating that is specified only as a minimum: unless a thickness tolerance is given, it assumes the coating may run 50% over the stated figure, so the allowance is sufficient only if six times the specified thickness fits inside it (ASME B1.1, section 7). Per ASME B1.1, the class 2A allowance on a 1/4-20 UNC external thread is 0.0011 in (28 µm), which by that rule leaves room for under 5 µm of specified zinc. In ISO 965-1, the fundamental deviation of tolerance position g on a 1 mm pitch, as in M6 x 1-6g, is 26 µm, which gives a similar limit (ISO 965-1:2013). Class 3A and tolerance position h threads have no allowance at all, so any plating on them needs a note on when the dimensions apply and a thread made undersize to suit.
Internal threads get no coating allowance either: ASME B1.1 treats standard 2B threads as having none, so a tapped hole that must gauge after plating has to be cut oversize. In practice a tapped hole, and a blind one most of all, collects much less zinc than the outside of the part, which reduces the effect on fit but also the protection inside the hole. The rule for the drawing follows from the table below. Fe/Zn 5 can be plated to size on standard 2A and 6g threads from about 3/8 in and M10 up. For anything heavier, for smaller threads, or for any 3A or h-position thread, either cap the maximum thickness in the note or state that thread dimensions apply after plating and let the machinist cut the external thread undersize. The threads after plating guide shows how to write that note.
| External thread | Allowance on pitch diameter | Largest minimum thickness (allowance ÷ 6) | Largest capped maximum (allowance ÷ 4) | B633 classes that fit as a minimum |
|---|---|---|---|---|
| 1/4-20 UNC-2A | 0.0011 in (28 µm) | 4.7 µm | 7 µm | None; Fe/Zn 5 only with a 7 µm maximum |
| 3/8-16 UNC-2A | 0.0013 in (33 µm) | 5.5 µm | 8 µm | Fe/Zn 5 |
| 1/2-13 UNC-2A | 0.0015 in (38 µm) | 6.3 µm | 9 µm | Fe/Zn 5 |
| M6 x 1-6g | 26 µm | 4.3 µm | 6 µm | None; Fe/Zn 5 only with a 6 µm maximum |
| M10 x 1.5-6g | 32 µm | 5.3 µm | 8 µm | Fe/Zn 5, with little margin |
| M12 x 1.75-6g | 34 µm | 5.7 µm | 8 µm | Fe/Zn 5 |
When does an ASTM B633 callout need a hydrogen embrittlement bake?
A bake is needed when the steel is hardened or high strength, because plating and the acid cleaning before it put hydrogen into the steel that can cause delayed cracking. The drawing has to state the steel’s hardness or strength, since the finisher cannot see it and the bake trigger depends on it.
B633 includes embrittlement-relief requirements of its own, and the companion guide for the bake after plating is ASTM B850, which bases the choice of bake on the actual tensile strength of the steel (ASTM B850). At the top of the strength range, B633 advises against zinc electroplating to the specification at all: steels with a tensile strength above 1700 MPa (247 ksi, 46 HRC) should not be zinc electroplated in accordance with it (ASTM B633-23, section 1.4). Typical candidates for a bake are through-hardened pins, case-hardened shafts, spring-steel clips and heat-treated alloy-steel fasteners, while soft low-carbon turned and milled parts are usually plated without one.
Sequence matters as much as the bake itself. B850 places the bake after plating and before any conversion coating, and in practice that order also protects the chromate or passivate, which loses much of its salt spray performance if it is heated after application. A callout for a hardened part should therefore name the bake, the sequence and the steel condition, for example: ZINC PLATE PER ASTM B633-23, FE/ZN 8, SC 2, TYPE V. MATERIAL HARDENED TO [HARDNESS]. EMBRITTLEMENT RELIEF BAKE PER ASTM B850 AFTER PLATING AND BEFORE PASSIVATE. The hydrogen embrittlement baking guide covers bake verification and purchase-order wording in detail.
Does ASTM B633 apply to fasteners, castings and other metals?
ASTM B633 covers electroplated zinc on iron and steel only. For mechanical fasteners it encourages the purchaser to consider ASTM F1941/F1941M instead, and zinc or aluminum die castings, stainless steel and other base metals need a different plating specification.
The fastener carve-out matters because F1941/F1941M is written around threaded parts: it sets dimensional requirements for coated threads alongside coating thickness, conversion coatings, corrosion resistance and embrittlement relief for high-strength and surface-hardened fasteners (ASTM F1941/F1941M-16(2025)). A drawing for a standard bolt or screw is better written to F1941/F1941M; a machined steel part that happens to carry a thread, such as a turned stud or a fitting body, is usually still a B633 part with a thread note added.
Because B633 is limited to electrodeposited zinc, it does not apply to hot-dip galvanizing, mechanical plating or zinc flake coatings, which are specified to other standards and build a different coating; it also excludes continuously electrogalvanized wire and sheet. If a drawing says “galvanize” on a small machined part, the RFQ should confirm whether hot-dip galvanizing or electroplated zinc to B633 is meant; the zinc plating vs galvanizing guide sets out the difference.
How should you write an ASTM B633 callout on a drawing or RFQ?
Write the standard with its edition, the class with its service condition, the type, and any bake, masking and before-or-after-plating rule in one note. A callout that names all of these leaves the machinist, the finisher and the inspector far less to guess.
Three sample callouts cover most machined steel parts. For an indoor equipment part that must meet RoHS: ZINC PLATE PER ASTM B633-23, FE/ZN 8, SC 2, TYPE V. For a bracket or housing in sheltered outdoor service with a colored finish: ZINC PLATE PER ASTM B633-23, FE/ZN 12, SC 3, TYPE VI. THREAD DIMENSIONS APPLY AFTER PLATING. For a part that will be painted: ZINC PLATE PER ASTM B633-23, FE/ZN 8, SC 2, TYPE IV, PAINT PER NOTE 6. Add a masking note for bores, bearing seats or electrical contact faces that must stay bare, and shade the significant surfaces where thickness and corrosion are judged.
When we review a drawing for CNC turning or CNC milling with a B633 note, we check that the class and service condition match, that the type is stated and fits the market, that hardened material carries a bake note, and that every thread and close fit says whether it is measured before or after plating. Plating is done by an outside finisher, so the RFQ should send the full callout and say which party measures thickness and runs any salt spray test. To have a finish note checked against a real part, send the drawing for a quote.
Frequently asked
Is zinc plating Type I or Type II?
It depends on the finish over the zinc. Type I is plain zinc with no conversion coating; Type II is zinc with a colored chromate, usually iridescent yellow. For new designs, the chromium(VI)-free equivalents are Type V (clear) and Type VI (colored).
What is the difference between zinc plating and zinc phosphating?
Zinc plating applies a metallic zinc layer that protects steel sacrificially. Zinc phosphating is a crystalline conversion coating that mainly holds paint or oil. In ASTM B633, a phosphate over zinc plating is Type IV and is intended as a paint base.
Will zinc-plated steel rust?
Yes, eventually. The zinc corrodes first, showing white corrosion, and red rust appears once the zinc on a surface is used up. A heavier class delays red rust; the type mainly delays white corrosion.
Do I need to write both Fe/Zn 8 and SC 2?
No, because each service condition maps to one class. Writing both is common and harmless if they match; a mismatched pair is a contradiction the finisher has to query before plating, so check the pair before release.
What is the latest revision of ASTM B633?
The current edition is ASTM B633-23, which replaced B633-19. Cite the edition on the drawing so the classes, types and test requirements are fixed for the life of the part number.
Sources
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023 (scope sections 1.3, 1.4 and 1.6; classes in section 4.1, types in section 4.2)
- ASTM B117-26a, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2026
- ASTM B850-98 (reapproved 2022), Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, ASTM International, 2022
- ASTM F1941/F1941M-16 (reapproved 2025), Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric, ASTM International, 2025 (scope section 1.1)
- ASME B1.1-2019, Unified Inch Screw Threads (UN, UNR and UNJ Thread Forms), ASME, 2019 (class 2A allowances; coating accommodation in section 7). ASME has since issued B1.1-2024.
- ISO 965-1:2013, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data, ISO, 2013 (fundamental deviations). Withdrawn in 2026 and replaced by ISO 965-1:2026.