Guide
When does zinc-plated steel need a hydrogen embrittlement bake, and how do you specify it?
A hydrogen embrittlement bake is a low-temperature heat treatment applied to hardened or high-strength steel parts soon after electroplating, so that hydrogen picked up in the cleaning and plating tanks can diffuse out of the steel before it causes cracking. It is not a finish and it does not change how the part looks; it is a safeguard against a brittle, delayed fracture that no visual or dimensional inspection can see.
By Saola Manufacturing engineering team · Published · Updated
This guide is for mechanical engineers and buyers who put “zinc plate” on a drawing for a turned, stamped or heat-treated steel part and need to decide whether the note also needs a bake. It works from the governing documents: ASTM B633 for electroplated zinc on steel, ASTM B849 and B850 for the treatments before and after plating, ASTM F1941/F1941M for plated fasteners, and the NASA fastener design manual for the history of how bake requirements were tightened. It ends with drawing and purchase-order wording you can adapt, and a short note on what changes when the parts are plated in one country and assembled in another.
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Why does zinc plating put hydrogen into steel?
Electroplating and the acid cleaning before it both release hydrogen at the steel surface, and some of that atomic hydrogen enters the metal instead of leaving as gas. In soft steel it does little harm, but in hardened steel under load it can collect at stress points and crack the part.
Zinc is deposited from solution by passing current through the part, and water in the bath is reduced to hydrogen at the same surface. Pickling in acid to remove scale does the same thing before plating starts. The NASA fastener design manual puts it plainly: most plating processes are electrolytic and generate hydrogen, which is why most of them call for baking afterward (NASA RP-1228).
The same manual separates three hydrogen problems, and only one of them concerns a plated part. Hydrogen that stays dissolved in the steel after processing is called internal hydrogen embrittlement, and it produces delayed failures: a bolt or clip passes its proof test, is installed, and fractures hours or days later with no external sign that anything was wrong. The other two cases, hydrogen reacting chemically with the steel and hydrogen supplied by a high-pressure gas environment in service, are different problems that a bake does not address.
Zinc adds one complication. The plated layer is a partial barrier to hydrogen, so once the part is coated the hydrogen already inside has a harder path out. That is why the bake is tied to the time of plating and done at an elevated temperature: it keeps the hydrogen moving while it can still escape or become trapped harmlessly.
Which parts need a bake after zinc plating?
Parts whose steel is hardened to high strength need a bake, while ordinary low-carbon turned or stamped parts generally do not. The exact trigger comes from the specification you cite, and it turns on how strong or hard the steel is, not on what kind of part it is.
ASTM B850 is a guide to heat treating steel after it has been coated, with the purpose of lowering its susceptibility to hydrogen damage (ASTM B850-98(2022), scope). Its published scope does not reproduce the conditions themselves, so read the full text before choosing one. Whatever the table, the practical consequence for a drawing is the same: the engineer who knows the heat treatment has to state it. A finisher looking at an unmarked part cannot tell a quenched and tempered spring clip from a low-carbon washer, and a bake requirement that depends on strength only works if strength is on the drawing.
At the top of the range, zinc electroplating stops being the right process at all. ASTM B633 leaves steel above 1700 MPa tensile strength (roughly 247 ksi or 46 HRC) outside what it covers for zinc electroplating (ASTM B633-23, scope). Parts in that range need a coating route chosen with the embrittlement risk in mind, not a longer bake.
Watch for the parts that are hard without looking like it. Case-carburized or induction-hardened shafts, through-hardened pins, spring-steel stampings, and thread-forming or self-drilling screws all carry a hard surface or a high core strength. ASTM F1941/F1941M, the standard B633 points fastener buyers toward, names high-strength and surface-hardened fasteners specifically when it sets out precautions for managing hydrogen risk and for hydrogen embrittlement relief (ASTM F1941/F1941M-16, scope). Soft, low-carbon parts turned from free-machining bar or stamped from mild strip sit at the other end, which is why most everyday zinc-plated brackets and spacers are plated without a bake.
| Document | What it controls | What it says that matters here |
|---|---|---|
| ASTM B633-23 | Electroplated zinc on iron and steel: classes, finish types, thickness, hydrogen embrittlement | Excludes steel stronger than 1700 MPa (about 247 ksi, 46 HRC) from zinc electroplating under this spec; points fastener buyers to F1941/F1941M |
| ASTM B849-02 (2019) | Treatments before plating | Procedures that reduce susceptibility to hydrogen embrittlement arising in electroplating, conversion coating and phosphating |
| ASTM B850-98 (2022) | Heat treatment after plating | Post-coating heat treatment to lower susceptibility; done after plating and before any secondary conversion coating; does not guarantee freedom from hydrogen damage; technically equivalent to ISO 9588 |
| ASTM F1941/F1941M-16 | Electroplated coatings on inch and metric mechanical fasteners | Precautions and embrittlement relief for high-strength and surface-hardened fasteners |
| ASTM F1940-07a (2019) | Process control for plated fasteners | Monitors the plating process with notched test specimens and trend analysis, instead of testing each fastener lot |
| ASTM F519-23 | Mechanical embrittlement testing of plating processes | Test methods and acceptance criteria for coating and plating processes that can embrittle steel |
| NASA RP-1228 (1990) | Fastener design guidance | Reports that most plating specs moved from a 3-hour bake started up to 4 hours after plating to at least 23 hours in a 350 to 400 °F band started no later than 2 hours after plating, for plated carbon steel fasteners, citing MIL-N-25027D |
What bake time and temperature do the standards call for?
The time and temperature come from the full text of the standard you cite, not from a shop habit. The published record also shows that a short bake once written into plating specifications was later judged inadequate and replaced with a much longer one.
For commercial zinc plating, ASTM B850 is the usual reference: it is written as a guide to heat treating steel after coating to lower the risk of hydrogen damage, and ASTM describes it as technically equivalent to ISO 9588 (ASTM B850-98(2022)). Citing either one on a drawing ties the finisher to a published table instead of a house habit. For fasteners, cite F1941/F1941M, which carries its own embrittlement provisions.
The NASA record is a useful warning about treating the bake as a formality. The NASA manual reports that plating specifications once required baking in a 350 to 400 °F band for only 3 hours, started up to 4 hours after plating, that this was found inadequate, and that most specifications were revised in 1981–82 to call for at least 23 hours at the same temperature, started no later than 2 hours after plating (NASA RP-1228). NASA frames the revised rule as what most plating specifications require for a plated carbon steel fastener, and points to MIL-N-25027D. It is not a default for every zinc-plated part, but it shows that a specification can fix both how long the bake lasts and how soon it must start.
Two other points follow from the same sources. Per ASTM B850, a bake reduces risk without guaranteeing freedom from hydrogen damage, so it is one control among several, not a certificate of safety. And the coating limits the temperature: the NASA manual notes that heating a plating toward its decomposition temperature can release hydrogen again, which is why bake temperatures are kept well below it.
Why must the bake come before the passivate?
Because ASTM B850 places the bake after plating but before any secondary conversion coating, and zinc parts almost always get one. The passivate is the heat-sensitive layer, so the part is baked bare and then passivated.
Almost every zinc-plated part leaves the line with a clear, yellow or black passivate over the zinc, and ASTM B633 treats these as supplementary finishes on top of the plating class. The order is set by ASTM B850: heat treatment happens after plating and before the secondary conversion coating (ASTM B850-98(2022)). The reason finishers give is practical: a conversion film baked after it is applied can lose much of its corrosion performance, so baking last trades one problem for another.
The sequence also has a clock. The bake loses value the longer the part sits between plating and oven, which is why the specifications that set a start window do so in hours rather than days. On a plating line, that means the bake has to be planned into the same run as the plating, with the passivate done after the parts come out of the oven and are reactivated. It is not something that can be added later to parts already passivated and packed.
For a buyer, the sequence is the easiest thing to check on paper. A process record that shows plate, bake, passivate in that order, with times, tells you the requirement was understood. A record that shows only a plating class and a passivate color tells you it may not have been.
How should a purchase order or drawing call out the bake?
Name the plating standard and class, state the steel’s strength or hardness, name the bake standard, and state the order of plate, bake and passivate. Then ask for a record of each bake with the lot it covers.
A drawing note that leaves nothing to interpretation might read: ZINC PLATE PER ASTM B633, Fe/Zn 8, TYPE V (CLEAR TRIVALENT PASSIVATE). BASE MATERIAL HARDENED PER NOTE 4. BAKE FOR HYDROGEN EMBRITTLEMENT RELIEF PER ASTM B850 AFTER PLATING AND BEFORE PASSIVATION. SUPPLY BAKE RECORD WITH EACH LOT. Check the type designation against the edition you cite: in B633-23 the passivates that replace hexavalent chromate are Types V and VI, and the older Type II is a chromate finish (ASTM B633-23). For fasteners, replace the first sentence with a call to ASTM F1941/F1941M and its designation for coating, thickness and embrittlement relief.
The purchase order should repeat three things the drawing may not carry: the actual specified strength or hardness range of the steel, whether the supplier or its finisher is expected to stress relieve before plating under ASTM B849, and what record you expect back. A useful bake record names the lot, the plating completion time, the bake start and end times, the oven temperature chart, and the passivate step that followed.
If the part is soft steel and does not need a bake, say that too. A note such as NO HYDROGEN EMBRITTLEMENT BAKE REQUIRED; BASE MATERIAL NOT HEAT TREATED stops a cautious finisher from adding a step, and stops a later reviewer from wondering whether one was forgotten. For how the finish note fits with the rest of the finish choice, see the anodizing vs powder coating vs zinc plating guide.
How do you verify that the bake worked?
You cannot verify it by inspecting the parts, so verification rests on process records and on periodic embrittlement testing of the plating process. Dimensional checks, salt spray and appearance say nothing about hydrogen.
Two ASTM methods address the process rather than the part. ASTM F519 defines mechanical tests and acceptance criteria for plating and coating processes that can embrittle steel (ASTM F519-23), and ASTM F1940 monitors a fastener plating process with notched specimens and trend analysis instead of testing every lot (ASTM F1940-07a(2019)). For a critical part, ask whether the finisher runs one of these and how recent the last result is.
The distance between plating and assembly makes this sharper for offshore supply. When steel parts are turned or stamped and plated in Vietnam and assembled in the US or Europe, an embrittled part is not found at incoming inspection; it is found, if at all, as a cracked part after installation. The controls that work are the ones set before the first lot: strength on the drawing, the bake standard and sequence in the finish note, the bake record in the shipping documents, and a first-lot review of that record. The RFQ preparation guide covers the rest of the package.
Saola runs CNC turning at its lead CNC plant and manages stamping and surface finishing through partner plants, with the quote naming where each step is done. The strength note, the plating note and the bake sequence can therefore be reviewed together on one drawing. If you have a plated steel part with an unclear bake requirement, you can send the drawing for a quote and the question will be raised at review rather than after shipment.
Frequently asked
Can a bake be done days after plating if the parts were missed?
It is not equivalent. The specifications that govern baking can set a start window measured in hours after plating. In the revised rule reported in NASA RP-1228, the bake must start no later than 2 hours after plating, where the earlier wording allowed 4. Parts that missed the window should be treated as a nonconformance and dispositioned by the engineer, not quietly baked late.
Does a bake help if the part is already passivated?
ASTM B850 puts the bake before the secondary conversion coating. Baking a passivated part risks the passivate, and if it was also done late it may not deliver the relief the standard intends. The fix is to plan the sequence into the plating run.
Do zinc or aluminum die castings need a hydrogen bake after plating?
No. ASTM B849 and B850 are written for iron and steel, and hydrogen embrittlement in the sense they address is a problem of high-strength steel. Plating a die casting raises different questions, about the casting and the plating process, that fall outside the scope of those steel standards.
Is mechanical zinc plating a way around the bake?
It is a common alternative for hardened parts because it is not electrolytic, but cleaning and acid activation before it can still introduce some hydrogen. Treat it as a different specification to be chosen deliberately, not as a way to drop the question.
Who is responsible for deciding whether a bake is needed?
The design owner. Only the drawing owner knows the steel grade and heat treatment, and that is what decides whether a bake applies. A supplier can flag a missing requirement at review, but the drawing or purchase order should state it.
Sources
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023 (scope, paragraph 1.4)
- ASTM B849-02(2019), Standard Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement, ASTM International, reapproved 2019
- ASTM B850-98(2022), Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, ASTM International, reapproved 2022
- ASTM F1941/F1941M-16, Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric, ASTM International, 2016
- ASTM F1940-07a(2019), Standard Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners, ASTM International, reapproved 2019
- ASTM F519-23, Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments, ASTM International, 2023
- NASA Reference Publication 1228, Fastener Design Manual, R. T. Barrett, NASA Lewis Research Center, March 1990 (sections on platings and on hydrogen embrittlement)