Guide
How do you specify black oxide on steel and stainless steel to MIL-DTL-13924?
Black oxide is a conversion finish that turns the surface of iron or steel into a thin, black iron oxide layer instead of adding a plated or painted film on top of it. On a drawing it is commonly called out to MIL-DTL-13924, and the two things buyers most often get wrong are the class, which has to match the substrate (carbon steel and black oxide stainless steel parts go through different baths), and the oil or wax topcoat, which the specification leaves to you and which sets most of the corrosion life.
By Saola Manufacturing engineering team · Published · Updated
This guide works from the text of the current revision, MIL-DTL-13924E, and from the military specification it points to for the supplementary preservative. It is written for design and quality engineers who need a callout that a finisher anywhere can quote without a clarification round, and for buyers checking whether a quote actually covers what the print asks for.
On this page
What does MIL-DTL-13924 actually control, and what does it leave open?
It controls the blackening process class, coverage, color, smut, surface attack, embrittlement relief and a small set of acceptance tests. It does not choose the preservative topcoat, the sample size or, for most classes, any corrosion test, so those belong on your drawing or purchase order.
The scope of MIL-DTL-13924E is blunt about performance: black oxide, with or without a supplementary preservative, gives only very limited corrosion protection under mildly corrosive conditions. Its notes say the finish suits moving parts that cannot tolerate the buildup of a more protective coating, and that it is not recommended for parts going into long-term storage unless a preservative or a desiccated package is added. Read that as a design statement: you choose black oxide for fit, appearance and low light reflection, and you buy corrosion resistance separately through the topcoat and the packaging.
Three requirements in the body of the specification matter on almost every machined part. The coating is applied after all machining, forming, welding, cold straightening and heat treatment unless you say otherwise. The process must not pit or intergranularly attack the surface. And the notes state that the finish should produce no appreciable dimensional change, so the dimensions on the drawing are the dimensions after coating. That last point is why a black oxide note rarely needs a pre-finish dimension table, unlike anodizing or plating.
Which class does black oxide stainless steel need?
Austenitic 300-series stainless normally takes Class 4, the only class with a salt-spray test, while hardened 400-series grades take Class 2 or Class 3 depending on the temperature they were tempered at, or Class 4 when no salt-spray result is needed. Carbon steel, low-alloy steel and irons take Class 1.
The classes are not grades of quality; each one is a different chemistry built for a different substrate. Per MIL-DTL-13924E, Class 1 is a boiling chrome-free alkaline bath for wrought iron, cast and malleable irons and plain carbon or low-alloy steels. Per MIL-DTL-13924E, Class 2 is an alkaline chromate bath for corrosion-resistant alloys tempered below 900°F (482°C), and Class 3 is a fused-salt bath for corrosion-resistant alloys tempered at 900°F (482°C) or higher. Class 4 is a proprietary alkaline bath for other corrosion-resistant steels, and it is the only class with a salt-spray requirement, which applies to 300-series parts only. The tempering rule exists because the bath must not soften the part: the specification forbids any process that reduces hardness or exposes the steel to its temper embrittlement range. A hardened 440C or 420 part tempered low and then sent through the hot fused-salt bath could come back softer than the print allows. The notes also say Class 3 is used on both 300- and 400-series grades, and that martensitic 400-series grades and cast irons can be blackened in a Class 4 bath but will not meet the 300-series salt-spray requirement.
| Class and process | Intended substrate | Approximate bath temperature (°F / °C) | Approximate immersion (min) |
|---|---|---|---|
| Class 1, alkaline oxidizing, chrome-free | Carbon and low-alloy steels, wrought, cast and malleable irons | 285 to 305 / 141 to 152, boiling | 5 to 60 |
| Class 2, alkaline chromate | 400-series corrosion-resistant steels tempered below 900°F (482°C) | 240 to 260 / 116 to 126 | 30 to 45 |
| Class 3, fused salt | Corrosion-resistant steels tempered at 900°F (482°C) or higher; the notes list 300- and 400-series use | 750 to 850 / 399 to 454, molten | 30 |
| Class 4, alkaline, proprietary | 300-series grades that must meet the salt-spray test (notes: at least 17Cr-7Ni); 400-series grades with no salt-spray criterion | 250 to 265 / 121 to 130 | 15 to 30 |
| Class 4 salt-spray requirement | 300-series only, tested before the preservative is applied, per ASTM B117 | No corrosion after the exposure set in section 3.9 | — |
Why does the supplementary oil or wax set the corrosion life?
The oxide layer is thin and porous, so on carbon steel it is the preservative held in that porosity that slows rust, not the oxide itself. MIL-DTL-13924 requires the preservative to follow the end-item specification or “as otherwise specified,” which means a drawing that names no topcoat has not really specified one.
The notes to the specification suggest a water-displacing preservative such as MIL-PRF-16173 Grade 3, or a comparable material giving equal or better protection. That reference is worth understanding before you copy it. MIL-PRF-16173E covers solvent-cutback, cold-applied corrosion preventive compounds in two VOC classes (Class I high VOC, Class II low VOC) and five grades: Grade 1 is a hard film, Grade 2 a soft film, Grade 3 a water-displacing soft film, Grade 4 a transparent non-tacky film and Grade 5 a film removable with low-pressure steam. Grade 3 is the usual pairing because it pushes rinse water out of the oxide pores, but it stays soft and slightly oily, which is not what you want on a part that is handled, packed against cardboard or bonded later. That specification also asks the buyer to state both the class and the grade, so a note that says only “MIL-PRF-16173, Grade 3” is still incomplete.
So decide the topcoat from what happens to the part next. A part that goes straight into a lubricated assembly can take a water-displacing oil. A part handled by assemblers or end users usually suits a dry-to-touch film or a wax; the notes to MIL-PRF-16173 point to Grade 4 where handling, stacking and counting need a tack-free coating. A part that will be painted, bonded or potted needs either no preservative or one that the next process can remove, and the drawing should say which. On 300-series stainless the preservative matters less for corrosion, since the Class 4 salt-spray requirement is judged on the bare oxide before any oil goes on.
What changes when the steel is hardened?
Per MIL-DTL-13924, at around 39 HRC and harder the specification calls for stress relief before blackening and hydrogen embrittlement relief after it, and warns about caustic embrittlement during blackening. If your part is hardened, the drawing should state the hardness so the finisher knows these steps apply.
Per MIL-DTL-13924E, parts harder than 39 HRC that carry residual stress from forming and hardening must be stress relieved before cleaning and coating, at a temperature that does not drop hardness below the specified minimum. Surface- or through-hardened parts at 39 HRC and above also get an embrittlement relief bake after the final rinse and before the preservative, using the baking table in SAE AMS2759/9, and springs or flexing parts must not be flexed before that bake. The notes add a separate caution that high-strength steel under internal or applied stress can crack spontaneously from caustic embrittlement during blackening. None of this is visible on a finished part, which is why it belongs in the callout rather than in a phone call. The same logic applies to plated hardware; the guide to hydrogen embrittlement baking after zinc plating covers the bake itself in more detail.
What should the drawing note and the RFQ actually say?
Name the specification and revision, the class, the supplementary preservative by specification and grade, and any test or sampling you expect to see reported. Section 6.2 of the specification lists exactly the items it expects the buyer to supply.
A callout for a carbon-steel shaft might read: “BLACK OXIDE PER MIL-DTL-13924E, CLASS 1. SUPPLEMENTARY PRESERVATIVE PER MIL-PRF-16173, CLASS II, GRADE 3. DIMENSIONS APPLY AFTER FINISH.” For a 304 bushing it becomes “BLACK OXIDE PER MIL-DTL-13924E, CLASS 4. SALT SPRAY PER SECTION 3.9 REQUIRED, RESULTS TO BE REPORTED. NO SUPPLEMENTARY PRESERVATIVE.” The notes say Class 4 blackens corrosion-resisting steels of at least 17Cr-7Ni, and 316 can sit just under 17 percent chromium, so confirm a 316 part with the finisher before relying on the salt-spray line. For a hardened 4140 part, add the hardness and a line such as “EMBRITTLEMENT RELIEF REQUIRED PER SECTION 3.10.”
On the RFQ itself, give the material grade and condition, the hardness if the part is heat treated, the number of samples to be tested from each lot (the specification leaves this to the procuring activity), whether test results must be accepted before the lot ships, and how the parts will be packed. If the program is aerospace, check whether your customer flows down SAE AMS2485 instead, since a finisher working to one is not automatically working to the other.
What are the most common black oxide callout mistakes?
The usual ones are a note that says only “black oxide,” a class that does not match the alloy, no preservative named, and an expectation of salt-spray performance the specification never promises.
“Black oxide” alone lets the finisher pick the chemistry, and a cold or room-temperature blackening product, which typically deposits a copper-selenium compound rather than converting the surface, will look similar on the first article and behave differently in service. Calling Class 1 on a stainless part names a bath built for carbon steel and iron, not for stainless, so the finisher has to come back with a question or pick another class without telling you. Leaving out the preservative can produce parts that arrive dry, which on carbon steel risks rust spots after a humid ocean crossing. Asking for a salt-spray duration on a Class 1 part with oil invents a requirement the specification does not contain, so it has to be written as your own acceptance criterion with its test method, as the guide to salt-spray hours on zinc plating explains for plated parts. Finally, stainless parts that need both passivation and blackening need the order of operations stated; see ASTM A967 passivation for machined stainless parts for what passivation does and does not do.
How is black oxide handled on a Saola order?
Black oxide is not on the Saola finishing menu, which lists the finishes our partner plants apply, so a black oxide callout has to be raised in the RFQ before anything is quoted.
The finishes listed on the surface finishing page are electroplating, anodizing on aluminum, gold plating, zinc plating, RoHS-compliant blue trivalent chromate, powder coating, pad printing and oil-dip rust protection, with salt-spray test standard and duration confirmed in the quote. If a turned or milled steel part from the CNC turning line needs a black MIL-DTL-13924 finish, say so in the RFQ so the gap is visible before anything is quoted, rather than discovered at first article. If the real requirement is corrosion protection rather than a black surface, zinc plating with trivalent chromate or an oil-dip finish may already cover it, and the comparison of anodizing, powder coating and zinc plating is a good place to check. Send the drawing through the quote page with the finish requirement written out, and the reply will say which part of it can be done here.
Frequently asked
Does black oxide change the size of a machined part?
Not appreciably. The notes to MIL-DTL-13924E say drawing dimensions are the dimensions after coating, which is why black oxide is often chosen for close-fitting moving parts, gauges and threaded features where plating buildup would be a problem.
Is black oxide the same as black zinc or black chromate?
No. Black zinc is a zinc plating with a black passivate or chromate on top, so it adds a sacrificial metal layer and real thickness. Black oxide converts the steel surface itself and adds essentially nothing, which also means it offers far less protection without its preservative.
Can I require a chrome-free process?
Class 1 is chrome-free by definition, and the current revision says chrome-free finishes should be used instead of hexavalent chromium ones wherever all performance criteria are met and documented. Classes 2 and 3 use chromium compounds and a chromic acid dip, so on 400-series stainless ask whether Class 4 can meet your need.
How is a black oxide coating checked at receiving inspection?
MIL-DTL-13924E uses a visual and white-cloth smut check for every class, an oxalic acid spot test for Classes 1 to 3 before the preservative goes on, a magnified look for pitting or intergranular attack, and the salt-spray test for 300-series Class 4. If you want any of these reported, put the requirement and sample size on the purchase order.
Is black oxide a good choice for outdoor parts?
Rarely on carbon steel. The specification itself describes only very limited protection under mildly corrosive conditions, so outdoor or wash-down parts are usually better served by plating, a painted system or a stainless grade chosen for the environment.
Sources
- MIL-DTL-13924E, Coating, Oxide, Black, for Ferrous Metals, U.S. Department of Defense, preparing activity U.S. Army, September 12, 2018
- MIL-PRF-16173E, Corrosion Preventive Compound, Solvent Cutback, Cold-Application, U.S. Department of Defense, January 6, 1993 (the Navy's 2017 interim amendment 2 keeps the same classes and grades)
- SAE AMS2485, Coating, Black Oxide, SAE International, accessed October 2026
- SAE AMS2759/9, Hydrogen Embrittlement Relief (Baking) of Steel Parts, SAE International, accessed October 2026
- ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2026