Guide
Zinc plating vs galvanizing: which should you specify on small steel parts?
Zinc plating is an electrodeposited layer of zinc, usually finished with a thin chromate or passivate topcoat, applied to steel parts in an aqueous plating bath under ASTM B633. Hot-dip galvanizing is a much thicker zinc coating formed by immersing cleaned steel in a bath of molten zinc under ASTM A123/A123M or ASTM A153/A153M. Both protect steel the same way, sacrificially, but they are sized for different parts: one for small, dimensionally sensitive components, the other for structural steel and larger hardware.
By Saola Manufacturing engineering team · Published · Updated
This guide is written for engineers who have to choose a corrosion finish for turned bushings and shafts, stamped brackets or other small steel parts, and for buyers who see both terms used loosely in a quote and want a straight zinc plated vs galvanized comparison. It works from the specifications that actually set the numbers: ASTM B633 for electroplated zinc on iron and steel, and ASTM A123/A123M and ASTM A153/A153M for hot-dip galvanized coatings on iron and steel products and hardware. Hot-dip galvanizing is not offered on Saola orders; this guide explains the difference so you can specify the right one and know where to send the other.
On this page
What is the difference between zinc plating and galvanizing?
Zinc plating deposits zinc from solution using electric current, in a process that runs at room temperature and adds a thin layer. Hot-dip galvanizing bonds zinc to steel by immersion in a molten zinc bath, in a process that runs hot and adds a much thicker, alloyed layer.
In zinc plating, the steel part is the cathode in an aqueous zinc salt solution, and current draws zinc ions onto its surface as the part is racked or barrel-plated. The coating is close to pure zinc, thin enough to preserve fine detail and tight tolerances, and it is almost always finished with a chromate or passivate conversion coating to slow white corrosion and set the color.
In hot-dip galvanizing, cleaned and fluxed steel is lowered into a kettle of molten zinc. The bath is at least 98 percent pure zinc held at 815 to 850°F (435 to 455°C), per ASTM B6, as the American Galvanizers Association describes the process (AGA, HDG process). The iron in the steel reacts with the zinc to form a series of zinc-iron alloy layers topped by a layer of free zinc, so the coating is metallurgically bonded rather than simply deposited, and it comes out far thicker than anything an electroplating line produces.
Both coatings work the same way once they are on the part: zinc is more reactive than steel, so it corrodes first at a scratch or edge and protects the exposed steel underneath. The difference that matters for a drawing is thickness, and thickness is what the rest of this guide is about.
How thick is zinc plating compared to hot-dip galvanizing?
ASTM B633 zinc plating classes run from 5 to 25 µm (about 0.2 to 1.0 mil) minimum, per ASTM B633. Even the lightest ASTM A123/A123M or ASTM A153/A153M galvanizing class starts above that range, and most run several times thicker.
Per ASTM B633, the four classes are named Fe/Zn 5, 8, 12 and 25, tied to service conditions SC1 (mild) through SC4 (very severe), where the number is the minimum zinc thickness in micrometers on significant surfaces. In inch units those minimums are about 0.20, 0.31, 0.47 and 0.98 mil (0.0002 to 0.001 in), converted from the micrometer values per ASTM B633. A Fe/Zn 25 class, the heaviest zinc-plating class in the standard, still tops out at 25 µm, per ASTM B633.
Hot-dip galvanizing starts heavier than that and does not come down to meet it. Per ASTM A123/A123M, the minimum coating grade for the lightest category, wire under 1/16 in (1.6 mm) thick, is 35 µm, and most structural shapes, bar and plate call for 45 to 100 µm depending on steel thickness (AGA, ASTM A123/A123M overview). ASTM A153/A153M, the companion standard for hardware and fasteners that are centrifuged to fling off excess zinc, sets lighter classes for small parts, but even its lightest class, Class D for fasteners under 3/8 in (9.5 mm) diameter, still requires a minimum average of 43 µm (1.7 mils) (AGA, ASTM A153/A153M overview).
The table below lines up the two families of standard side by side.
| Property | Zinc plating (ASTM B633) | Hot-dip galvanizing (ASTM A123/A123M, A153/A153M) |
|---|---|---|
| Process | Electrodeposited from an aqueous zinc solution | Immersion in a bath of molten zinc |
| Typical part size | Small turned, stamped and cast steel parts | Structural steel, plate, larger hardware and fasteners |
| Minimum coating thickness | Fe/Zn 5, 8, 12 or 25 (micrometers minimum) by service condition | 35 to 100 micrometers (A123/A123M) or 43 to 86 micrometers (A153/A153M) by category or class |
| Threaded features | Coating adds thickness on threads; mask or allow for it | Threaded areas excluded from the thickness requirement; nuts over-tapped after coating |
| Supplementary chromate or passivate | Standard, semi-permanent finish (Types II, III, V, VI) | Optional, temporary (about six weeks), mainly to prevent wet storage stain |
| Hydrogen embrittlement trigger | Above 1700 MPa (247 ksi, 46 HRC): not to be electroplated to this specification | Above roughly 150 ksi (1,100 MPa): precautions per ASTM A143/A143M |
| Run at Saola Manufacturing | Yes, through a partner finishing plant we manage | Not offered |
Does the extra thickness of galvanizing change thread fit more than zinc plating does?
Yes, because a hot-dip coating is several times thicker than a plated one. That difference is large enough that galvanizing specifications handle threads differently from a plating specification.
Any coating on a diameter adds roughly its own thickness on each side, so a coating twice as thick removes about twice the clearance from a close-fitting hole, shaft or thread. The threads after anodizing or plating guide covers how this plays out for masked and coated features generally, and the same math applies to galvanizing at a larger scale.
ASTM A153/A153M solves the thread problem by taking threaded areas out of the coating-thickness requirement entirely and requiring the finisher to over-tap the internal threads of a nut after galvanizing, so the nut still turns onto a coated bolt (AGA, ASTM A153/A153M overview). That works for standard fastener threads with a known allowance built into the nut, but it does not help a custom thread, a press fit or a bore machined to a specific size.
Zinc plating handles the same problem the other way: the coating is thin enough that a thread class or a small machining allowance usually absorbs it, or the thread is masked and plated bare. On a precision turned part, that difference is the practical reason to choose plating over galvanizing even before service life or cost come up.
What supplementary chromate or passivate finishes apply to each coating?
Zinc plating almost always gets a chromate or passivate conversion coating as a standard part of the finish. Hot-dip galvanizing is normally left bare, with a passivation quench used only sometimes and only for a matter of weeks.
On electroplated zinc, the conversion coating is not optional in practice: ASTM B633 Types II, III, V and VI cover the colored and clear, hexavalent and trivalent chromate and passivate finishes that set both the color and most of the corrosion performance, and RoHS drives most buyers toward the trivalent options today. The anodizing vs powder coating vs zinc plating guide covers the RoHS question and the type-by-type detail in full; it is not repeated here.
Hot-dip galvanized parts are usually shipped with no topcoat at all, because the zinc coating itself, and the zinc carbonate patina it forms outdoors, is the corrosion protection. A chromate quench passivation is sometimes applied right after galvanizing, but its purpose is different: it delays wet storage stain, the white or gray film that forms when freshly galvanized parts are stacked wet in transit, and it is consumed in about six weeks before the part weathers on its own (AGA, passivation). It is not a decorative or long-term finish the way a plating passivate is.
So the two processes answer different questions with a similar-sounding step: on plated parts, the chromate or passivate is most of the finish note; on galvanized parts, passivation is a shipping precaution, not a specification line.
Does zinc plating or galvanizing carry a higher hydrogen embrittlement risk for high-strength steel?
Both processes carry a hydrogen embrittlement risk on high-strength steel, but from different steps and at different strength levels. Zinc plating's risk comes from the electroplating and pickling steps; galvanizing's risk comes mainly from the acid cleaning step before the dip.
ASTM B633 excludes high-strength steel outright: parts with a tensile strength above 1700 MPa (247 ksi, 46 HRC) should not be zinc electroplated to that specification at all (ASTM B633-23, scope). Below that limit, a post-plating bake is the usual relief step, timed and run before the chromate or passivate goes on; ASTM B850 is the general guide finishers use to set a bake schedule from the steel's actual tensile strength, and for threaded fasteners specifically, ASTM F1941/F1941M sets its own hydrogen-embrittlement relief requirement for high-strength and surface-hardened parts (ASTM F1941/F1941M, scope).
Hot-dip galvanizing usually avoids the problem a different way: the zinc bath runs hot enough that absorbed hydrogen tends to diffuse back out during the dip, so embrittlement is uncommon on ordinary structural steel. The American Galvanizers Association puts the risk threshold at roughly 150 ksi (1,100 MPa) tensile strength, well above most structural steel, with acid pickling before the dip as the main hydrogen source (AGA, hydrogen embrittlement). Above that threshold, the same source gives the precautions drawn from ASTM A143/A143M: a thermal treatment at about 300°F (150°C) after cleaning, or abrasive blast cleaning in place of an acid pickle, to keep hydrogen out of the steel in the first place rather than baking it out afterward (AGA, hydrogen embrittlement).
Neither process makes high-strength steel risk-free, and neither substitutes for the other's precautions. If a part is both high-strength and small enough to plate, follow the plating relief bake; if it is high-strength structural steel bound for galvanizing, that is a question for ASTM A143/A143M before it goes in the kettle.
Why isn't hot-dip galvanizing used on small precision steel parts?
Its lightest coating class is still much thicker than a small part's tolerance can absorb, and the batch process is built around structural fabrications, not turned or stamped precision. That is also why Saola does not offer it.
Turned bushings and shafts on CNC lathes and brackets from metal stamping are usually held to bore, shaft and hole tolerances measured in thousandths of an inch, and a coating has to leave room inside that band or be masked off the feature that needs it.
Even the lightest hot-dip class clears that room several times over before any masking allowance is considered: ASTM A153/A153M's Class D, its lightest, still requires roughly 43 µm (1.7 mils) minimum on hardware, and ASTM A123/A123M's lightest wire grade is 35 µm, both per the American Galvanizers Association's summaries of those standards (A153/A153M, A123/A123M). Zinc plating's lightest class, Fe/Zn 5, sits at a fifth of that, which is why it is the finish that reaches small turned and stamped parts without eating the tolerance.
The batch hot-dip process itself is set up for racks of structural shapes, bar, plate and fastener lots dropped into a kettle, not for the fixturing, masking and rack contact points a small precision part needs to plate cleanly. That equipment and process gap is the practical reason galvanizing is not offered here: it is the right answer for structural steel and larger hardware, and zinc plating, along with the other finishes on the surface finishing page, covers the corrosion need on parts this size.
Which finish should you specify: zinc plating or galvanizing?
Specify zinc plating per ASTM B633 for small turned, stamped or cast steel parts, and reserve galvanizing per ASTM A123/A123M or A153/A153M for structural steel and larger hardware. Send the part size and any threaded or press-fit features with the drawing so the right one gets written down.
A workable zinc plating note names the standard, service condition and type, along with what is masked: ZINC PLATE PER ASTM B633, SC2 (FE/ZN 8), TYPE III, CLEAR TRIVALENT PASSIVATE. MASK THREAD A AND BORE B. The anodizing vs powder coating vs zinc plating guide has more callout examples and the RoHS detail behind the type.
If a part genuinely needs galvanizing, the note should point to the right document for its shape and size: structural shapes, bar and plate cite ASTM A123/A123M; hardware, fasteners and small castings that will be centrifuged cite ASTM A153/A153M. Neither belongs on a drawing for a part that also has a tight thread, a press fit or a bore held to a few thousandths, because the coating class that meets those standards will not leave room for it.
Either way, the RFQ preparation guide covers where the finish note sits in the rest of the package, and a quote request with the finish note as written comes back with the class, type and any masking confirmed or questioned before production starts.
Frequently asked
Is galvanized steel the same as zinc-plated steel?
No. Both put zinc on steel, but hot-dip galvanizing bonds a much thicker, alloyed coating by immersion, and zinc plating electrodeposits a thin, mostly pure zinc layer. The coating thickness difference is usually the reason one is specified over the other.
Can small stamped brackets be hot-dip galvanized instead of zinc plated?
ASTM A153/A153M does cover small hardware, but even its lightest class adds more thickness than most stamped brackets' hole and bend tolerances can absorb. Small brackets are usually zinc plated for that reason, and galvanizing is kept for larger or structural parts.
Does hot-dip galvanizing avoid the hydrogen embrittlement risk that affects zinc plating?
Not entirely; it shifts where the risk comes from. Hot-dip galvanizing's risk comes mainly from acid pickling before the dip and appears above roughly 150 ksi (1,100 MPa), per the American Galvanizers Association's hydrogen-embrittlement guidance (AGA). Zinc electroplating's risk is addressed by a post-plating bake, and ASTM B633 excludes electroplating altogether above 1700 MPa (247 ksi), per ASTM B633 (ASTM B633-23, scope).
Is a chromate quench on galvanized steel the same as the chromate on plated zinc?
No. The chromate or passivate on plated zinc is a standard, semi-permanent part of the finish that sets color and adds corrosion resistance. A chromate quench on galvanized steel is an optional treatment that mainly delays wet storage stain for about six weeks after galvanizing.
Which lasts longer outdoors, zinc plating or galvanizing?
Galvanizing, because its coating is much thicker and zinc's protection life scales with coating thickness; the American Galvanizers Association notes that in atmospheric environments the time to first maintenance of hot-dip galvanized steel is directly proportional to coating thickness (AGA, ASTM A123/A123M overview). Zinc plating suits indoor or mild exposure and small, dimensionally sensitive parts, where its thin layer leaves room for tolerances that a galvanized coating would consume.
Why doesn't Saola offer hot-dip galvanizing?
Hot-dip galvanizing is a batch process built around structural shapes, plate and fastener lots, not the turned, stamped and cast steel parts Saola supplies. Zinc plating and the other finishes on the surface finishing page cover the corrosion need at that part size instead.
Sources
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023
- ASTM A123/A123M-24, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, ASTM International, 2024
- ASTM A153/A153M-23, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, ASTM International, 2023
- ASTM B850-98(2022), Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, ASTM International, 2022
- ASTM F1941/F1941M-16, Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric, ASTM International, 2016, reapproved 2025
- Overview of ASTM A123/A123M, American Galvanizers Association, June 3, 2021
- Overview of ASTM A153/A153M, American Galvanizers Association, February 25, 2011
- Hydrogen Embrittlement, American Galvanizers Association, March 27, 2012, updated 2026
- What is the HDG Process?, American Galvanizers Association, last updated February 2026
- Passivation, American Galvanizers Association, last updated December 2017