Your print says “Zinc plate per ASTM B633, SC3, Type II”. Here is what the service condition buys you, what the type actually is, which types are hexavalent, and how to write the callout so the shop cannot get it wrong.
ASTM B633, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, is the commercial standard for barrel and rack zinc plating in North America. The current issue is B633-23. It covers material and process requirements for zinc electrodeposited on iron or steel to protect it from corrosion — nothing else.
Three limits before you cite it: it does not cover continuously coated wire or sheet, it points threaded mechanical fasteners at F1941/F1941M instead, and it says high strength steels above 1700 MPa tensile (247 ksi, 46 HRC) should not be zinc electroplated to this specification at all.
Prints outlive specifications. Federal Specification QQ-Z-325, Zinc Coating, Electrodeposited, was the government document for this work and it was canceled; the cancellation notice names ASTM B633 as the commercial standard covering the material. If your drawing still calls QQ-Z-325, process to B633 — but do not assume the numbering carries across. QQ-Z-325 had its own type and class structure. A QQ-Z-325 type number is not a B633 type number. Re-state the requirement in B633 terms on the purchase order and agree it in writing before parts run.
B633 also does not control bath chemistry. Whether the zinc came from an alkaline zinc bath or an acid chloride zinc bath is a process choice the specification is silent on. That silence is why two compliant shops hand you visibly different parts.
The service condition is the only part of the callout that sets how much zinc you get. B633 states thickness in micrometers; the inch figures below are conversions for reading a drawing, not values printed in the standard.
| Service condition | Classification No. | Min. thickness | Approx. | Intended environment |
|---|---|---|---|---|
| SC 4 — very severe | Fe/Zn 25 | 25 µm | 0.0010 in | Harsh exposure, or frequent wetting by moisture, cleaners and saline solutions. Standard’s examples: plumbing fixtures, pole line hardware. |
| SC 3 — severe | Fe/Zn 12 | 12 µm | 0.0005 in | Condensation, perspiration, infrequent wetting by rain, and cleaners. Tubular furniture, window fittings, builder’s hardware. |
| SC 2 — moderate | Fe/Zn 8 | 8 µm | 0.0003 in | Mostly dry indoor atmospheres, occasional condensation, wear or abrasion. Tools, machine parts. |
| SC 1 — mild | Fe/Zn 5 | 5 µm | 0.0002 in | Indoor atmospheres, rare condensation, minimum wear or abrasion. Buttons, fasteners. |
These are minima on significant surfaces, not averages. On a part with deep recesses or blind holes the thin point governs — a bath-selection problem, covered below.
The type is the supplementary treatment applied after the zinc. It sets corrosion life, color and paint adhesion. Watch the vocabulary: B633 uses chromate for the hexavalent films and passivate for the hexavalent-free ones, defining a passivate as a conversion coating that shall not contain hexavalent chromium. That one word is the whole RoHS decision.
| Type | B633 description | What it is | Usual appearance | Min. salt spray |
|---|---|---|---|---|
| I | As plated, without supplementary treatments | Bare zinc. No conversion coating. | Bright to matte gray, dulls quickly | None specified |
| II | With colored chromate coatings | Hexavalent chromate conversion coating | Conventionally yellow iridescent; olive drab and black are also colored chromates | 96 h |
| III | With colorless chromate conversion coatings | Hexavalent chromate, thin and clear | Clear to faint blue (“clear” or “blue-bright” zinc) | 12 h |
| IV | With phosphate conversion coatings | Phosphate, per Guide D2092. A paint base, not a standalone corrosion film. | Dull gray, absorbent | None specified |
| V | With colorless passivate | Hexavalent-chromium-free, in practice trivalent | Clear to blue | 72 h |
| VI | With colored passivate | Hexavalent-chromium-free, in practice trivalent | Iridescent, yellow-toned or black depending on chemistry | 120 h |
Those hours are what the specification requires, tested to ASTM B117 and measured to the appearance of corrosion products of the coating — white corrosion — with white products within 6 mm of an edge not counting as failure. They are a qualification floor, not a service-life prediction and not a shop performance claim. Nothing in that column is a red-rust number.
Color is not a specification. B633 says “colored” and “colorless”. It never says yellow, gold, olive drab, blue or black. If shade matters, call color out separately and agree a sample — two shops can both run compliant Type II and hand you different-looking parts.
This is the most common misunderstanding on a zinc print. Nobody restricts zinc. RoHS restricts hexavalent chromium, and hexavalent chromium enters a zinc-plated part through the conversion coating, not through the plating.
EU RoHS (Directive 2011/65/EU as amended by 2015/863) caps hexavalent chromium at 0.1 % by weight in any homogeneous material in electrical and electronic equipment. A chromate film is its own homogeneous material, so a traditional yellow chromate does not squeak under the limit by being thin — it fails on concentration. The ELV Directive (2000/53/EC) applies the same logic to vehicles, and the Annex II allowance that once covered corrosion-preventive coatings ran out on 1 July 2007. Automotive supply chains have been trivalent since. That is why an automotive or heavy-truck part and a general industrial part can carry the same B633 line and mean different chemistry.
The mapping is straightforward. Types II and III are chromates — the hexavalent route. Types V and VI are passivates, which by B633’s own definition shall not contain hexavalent chromium; ASTM added them specifically to give the standard a compliant path. Type I has no conversion coating and Type IV phosphate is not a chromium film. In RoHS or ELV scope, Type V or Type VI is the unambiguous answer.
Type III is where engineers get burned. Many shops run a trivalent clear and still certify Type III, because the part looks identical and the customer asked for “clear zinc”. Read strictly, Type III is a chromate. Do not rely on a Type III callout to deliver a hexavalent-free part: call Type V, or write the prohibition into the callout in words.
What you give up is real. A hexavalent chromate contains soluble chromium that migrates to a scratch and re-passivates it — the self-healing behavior that made yellow chromate hard to beat. A trivalent passivate has no equivalent reservoir; on its own it is a thinner, harder, more brittle film. The industry answer is a sealer or topcoat over the passivate, silicate or organic, which fills microcracks and raises the barrier, and is how a trivalent system reaches the same salt spray requirement. It is part of the system, not an extra. So the callout has to say whether a sealer is permitted or required, and the RoHS declaration has to cover the sealer as well as the passivate.
Writing a RoHS-compliant zinc callout. Do not write “RoHS zinc” and hope. Write the type, then the prohibition, then the sealer position: Zinc plate per ASTM B633-23, SC 3, Type VI. Passivate and any sealer shall contain no intentionally added hexavalent chromium; hexavalent chromium ≤ 0.1 % by weight per homogeneous material. Supplier to provide RoHS declaration covering plating, passivate and sealer.
Zinc plating is cathodic. Hydrogen is generated at the part, some enters the steel, and in hardened steel it collects at stress concentrations and cracks the part days later under static load. B633 handles it with two heat treatments, before and after plating.
Before plating, steel above 1000 MPa ultimate tensile (about 31 HRC) that has been machined, ground, cold formed or cold straightened is stress relieved per ASTM B849. After plating, steel above 1000 MPa (31 HRC) and surface-hardened parts — carburised, induction-hardened — regardless of core strength are baked for embrittlement relief. B633 prints no bake schedule; it sends the plater to Table 1 of ASTM B850, Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, which selects temperature and duration from the steel’s strength and the part’s class. B850 is a guide; the acceptance test is that baked parts do not crack or fail by fracture.
Two sequencing rules do the real work. The bake starts within 4 hours of the part leaving the last process, because hydrogen keeps diffusing inward while the part sits and a late bake drives it deeper instead of out. And the bake happens before the supplementary treatment, because chromate and passivate films are dehydrated and damaged at bake temperatures. Bake first, then convert. If parts were chromated and then baked, the film on them is not what the specification describes.
Put substrate and hardness on the RFQ, not on a later phone call. “4140, 42 HRC” changes the routing; a hardness discovered at receiving inspection changes nothing. Our design guide covers what else belongs on a plating drawing.
B633 does not choose the bath, so you should — or at least tell the shop what the part needs. The two commercial zinc chemistries fail in opposite directions.
| Alkaline (non-cyanide) zinc | Acid chloride zinc | |
|---|---|---|
| Throwing power | Better. Even thickness into recesses, blind holes, ID features, complex geometry. | Poorer. Builds on edges and high-current areas, thin in recesses. |
| Cathode efficiency | Roughly half that of acid; slower, more energy per mil. | Near complete; fast deposition. |
| Brightness | Good, but generally less brilliant. | The brighter, more decorative deposit. |
| Ductility | Fine grain, lower stress; better for parts formed or crimped after plating. | More stressed deposit. |
| Castings and cast iron | Poor. Not the route for cast iron, malleable iron or powdered metal. | The practical route for castings, cast iron and heat-treated or case-hardened steel. |
| Operating cost | Higher bath cost and control burden. | Generally cheaper to run and maintain. |
Short version: if the SC minimum has to be met at the bottom of a blind hole or inside a weldment, alkaline zinc makes the print achievable. If the part is a casting, or hardened, or wants cosmetic brightness at production speed, acid chloride zinc is the bath. Where the drawing wants a phosphate paint base rather than a bright finish, that is zinc phosphate coating — Type IV territory.
Zinc protects sacrificially and it is cheap, and for most fasteners, brackets and enclosures that ends the discussion. It runs out in three places: sustained heat, marine or road-salt exposure, and galvanic contact with aluminum. Zinc-nickel — an alloy deposit, typically 10 to 15 percent nickel — stays sacrificial to steel but corrodes far more slowly, holds up better after thermal exposure, and sits much closer to aluminum galvanically, which is why aerospace and defense programs use it as a cadmium replacement. It costs more and it is a fussier bath. If you are converting a legacy cadmium callout, start with zinc-nickel versus cadmium plating; cadmium itself runs at our McAllen plant.
A complete callout names the standard with its revision, the service condition and the type, then anything the standard does not control that you care about: hexavalent chromium status, color, sealer, bake, and the substrate condition that drives the bake.
Zinc plate per ASTM B633-23, SC 1, Type V. No hexavalent chromium in passivate or sealer. Sealer permitted. Threads to gauge after plating.
SC 1 is 5 µm, right for a dry indoor part. Type V is the colorless hexavalent-free passivate, and the prohibition is written out rather than implied.
Zinc plate per ASTM B633-23, SC 3, Type VI. Passivate and sealer free of hexavalent chromium. Alkaline zinc required; SC 3 minimum to be met inside the two blind holes shown.
Type VI carries the highest salt spray requirement in the standard. Naming the bath and naming where the minimum applies stops a thin recess passing on an edge reading.
Zinc plate per ASTM B633-23, SC 3, Type II. Substrate 4140, 42 HRC. Bake for hydrogen embrittlement relief per ASTM B850 within 4 hours of plating, prior to chromate.
Hardness is on the drawing, so the bake is not a judgment call. Type II here is a deliberate hexavalent choice for a part with no RoHS or ELV obligation.
Zinc is our highest-volume line. Gleco Plating has been finishing metal in Texas since 1979, family-owned, with plating plants in Rowlett and McAllen.
Alkaline zinc and black zinc run at our Rowlett plant, 2220 Grisham Drive, alongside zinc phosphate and the rest of the Rowlett process set. That is the plant for zinc plating in the Dallas area — see Dallas and Rowlett metal finishing, and the zinc plating overview for the line itself.
Our clear passivate is trivalent. That makes it a Type V under B633, not a Type III — and we will certify it as Type V, because the point of the distinction is that your incoming inspection and your RoHS declaration can rely on it. Our yellow and black finishes are trivalent passivates as well, which puts them under Type VI. We run hexavalent chromate only where a controlling drawing or a legacy specification calls for it, and we will tell you which you are getting before we quote rather than after you receive the parts.
If your drawing currently says Type III and hexavalent chromium is prohibited on your program, that is worth a conversation before the next release. It is a two-character change on the print and it removes an argument that otherwise happens at receiving.
What we ask for at RFQ, because it changes routing: service condition and type as written, substrate and hardness, RoHS or ELV scope, whether a significant surface includes a blind or recessed feature, and whether color is a requirement or a preference. If a print carries a canceled specification or a type number that is not in the current revision, we raise it before parts run.
Gleco is AS9100D and ISO 9001:2015 certified, ITAR Registered, DFARS compliant and RoHS compliant; the AS9100D certificate covers three sites. Detail on certifications and approvals.
QQ-Z-325 is canceled and the cancellation notice names ASTM B633 as the commercial standard covering the material, so B633 is where you go. What you cannot do is copy the type number across — QQ-Z-325 used its own type and class structure and it does not map one-for-one onto B633’s six types and four service conditions. Decide what the part needs, write it as a B633 service condition and type on the purchase order, and get it onto the next drawing revision.
Thickness and intended environment, nothing else. SC 2 (moderate) is 8 µm minimum, for mostly dry indoor atmospheres with occasional condensation, wear or abrasion. SC 3 (severe) is 12 µm minimum, for condensation, perspiration, infrequent rain and cleaners. Salt spray is set by the type, not the SC, so SC 2 Type VI and SC 3 Type VI carry the same 120 h requirement on different zinc thicknesses.
None of them, strictly. B633 reserves the word chromate for the hexavalent films — Types II and III — and passivate for the hexavalent-chromium-free films, Types V and VI. RoHS caps hexavalent chromium at 0.1 % by weight in any homogeneous material, and a conversion film is its own homogeneous material, so a hexavalent chromate fails on concentration however thin it is. For a RoHS or ELV part call Type V (colorless) or Type VI (colored), and write the prohibition into the callout in words as well.
The part might be; the callout is not. Type III as written in B633 is a colorless chromate — the hexavalent family. Plenty of shops run a trivalent clear and still certify Type III because the parts look the same, but you are then relying on their chemistry rather than your drawing. If hexavalent chromium is prohibited, say Type V or state the prohibition explicitly. Ambiguity in a callout gets resolved by whoever is holding the part.
Under B633, yes if the steel is above 1000 MPa ultimate tensile — roughly 31 HRC — and yes for surface-hardened parts such as carburised or induction-hardened components regardless of core strength. The bake starts within 4 hours of the part leaving the last process, and before the chromate or passivate is applied. B633 prints no temperature or time; it sends the plater to Table 1 of ASTM B850. Put substrate and hardness on the drawing so nobody is guessing at receiving.
Because color is not a specification requirement. B633 says “colored” and “colorless” and never names a shade, so shade drifts with chemistry, bath age, dwell, rinse and whether the film is hexavalent or trivalent — trivalent iridescent films sit in a different color range from traditional hexavalent yellow. If the part is cosmetic or color-coded, call color out separately from the type, agree a physical sample, and keep the lot on one line.
Not really. B633 points mechanical fasteners at Specification F1941/F1941M, which handles what fasteners bring with them — thread fit and gauging after plating, embrittlement, coating on the flanks. B633 also excludes continuously coated wire and sheet, and states that high strength steels above 1700 MPa tensile (247 ksi, 46 HRC) should not be zinc electroplated to it at all.
Zinc, unless one of three things is true: the part sees sustained heat, it sees marine or road-salt exposure, or it is bolted to aluminum. Zinc-nickel stays sacrificial to steel, corrodes much more slowly, holds up better after thermal exposure and sits far closer to aluminum galvanically. It costs more. If you are converting a legacy cadmium print, read the zinc-nickel versus cadmium comparison first — cadmium plating runs at our McAllen plant, so it is a routing question as well as a chemistry one.
Service condition, type, substrate and hardness. If the callout is a canceled specification or the type does not match the chemistry you need, we will tell you before the parts run.
Precision metal finishing since 1979. Plating, anodizing, coatings and passivation from three Texas facilities.
Our paint, powder & Cerakote division: Gleco Paint & Powder Coating
Gleco Rowlett
2220 Grisham Drive
Rowlett, TX 75088
972-475-4300
Gleco McAllen
3800 W. Ursula Ave
McAllen, TX 78503
956-800-4069
