Guide
Zinc phosphate coating, manganese phosphate or black oxide: which dark finish should a steel part get?
Zinc phosphate coating is a crystalline conversion layer grown on steel by immersing it in an acid zinc phosphate bath, which turns the surface gray to black and leaves a porous film designed to soak up oil, wax or paint. Manganese phosphate is its heavier, more wear-oriented sibling, and black oxide on carbon steel is a much thinner magnetite film formed in a boiling alkaline bath. All three look like a dark, low-build finish on the print, and all three depend on what goes on top of them far more than buyers expect.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers choosing a finish for steel shafts, pins, fasteners, gears and sliding parts, and for buyers checking whether a finish note on a drawing will actually produce what the designer had in mind. It works from the two US military detail specifications that many commercial drawings still cite, MIL-DTL-16232 for heavy phosphate and MIL-DTL-13924 for black oxide, and every requirement and figure quoted here comes from those documents. The short version: phosphate is chosen for oil retention, break-in and paint adhesion; black oxide is chosen when nothing may grow; and neither one is a corrosion finish without the supplementary oil or preservative that the type and class number calls for.
On this page
What actually protects the steel: the conversion coating or the oil?
Mostly the oil. The phosphate specification puts the major part of the corrosion protection in the supplementary treatment, and the black oxide specification calls the protection from the oxide alone very limited.
MIL-DTL-16232H describes heavy phosphate as a base for holding supplemental coatings “which provide the major portion of the corrosion resistance”, and MIL-DTL-13924E says only very limited protection, under mildly corrosive conditions, comes from black oxide itself. The numbers in the table make the point. A bare manganese phosphate coat has to survive just 1.5 hours of neutral salt spray per MIL-DTL-16232H, Table I, while the same coat impregnated with preservative oil as Class 2 must reach 48 hours. Zinc phosphate goes from 2 hours bare to 72 hours with its Class 2 preservative, per MIL-DTL-16232H.
The two phosphates are steered toward different jobs. Zinc phosphate has the lower minimum coating weight, the specification says the heavy zinc coat may be used when paint and supplemental oil coatings are both required, and its Class 2 is the general-purpose choice where corrosion protection is the primary concern. Manganese phosphate carries the heavier minimum weight and is the one the specification associates with wear, break-in and sliding friction. Black oxide is a different animal: it is a thin oxide film without the open crystal structure of a phosphate, so it has far less capacity to act as an oil reservoir and a black-oxided part holds a thinner film of preservative than a phosphated one.
| Property | Zinc phosphate, MIL-DTL-16232 Type Z | Manganese phosphate, MIL-DTL-16232 Type M | Black oxide, MIL-DTL-13924 Class 1 |
|---|---|---|---|
| Minimum coating weight | 11 g/m² | 16 g/m² (11 g/m² when specified) | No weight requirement; oxalic acid spot test instead |
| Expected thickness (guideline, not a requirement) | 5 to 15 µm | 5 to 10 µm | No appreciable dimensional change |
| Salt spray, no supplementary treatment, Classes 1 to 3 | 2 h | 1.5 h | No requirement |
| Salt spray, Class 2 oil or preservative | 72 h | 48 h | Preservative per end-item specification |
| Salt spray, Class 4 chemically converted, no supplement | 24 h | 24 h | Required only for Class 4 on 300-series stainless |
| Service temperature limit (guidance) | 93 °C (200 °F) | 121 °C (250 °F) | Not stated |
| Steel at 39 HRC or harder, unless the contract says otherwise | Stress relief before if machined, ground or cold worked after hardening; embrittlement relief after | Stress relief before if machined, ground or cold worked after hardening; embrittlement relief after | Stress relief before; embrittlement relief after |
How much does zinc phosphate coating change the size of a part?
Phosphate adds a measurable layer, a few micrometers to roughly fifteen, and the drawing dimensions apply before coating; black oxide adds essentially nothing, and the drawing dimensions apply after coating.
MIL-DTL-16232H lists expected thicknesses of 5 to 10 µm for Type M and 5 to 15 µm for Type Z, and it is careful to call these guidelines; the enforceable requirement is coating weight per unit area (MIL-DTL-16232H, section 3.5 and Table V). The same specification says parts must meet the drawing before phosphating and then meet it again “with allowance for the phosphate coating buildup.” If your print does not say how much allowance, nobody knows what to inspect against. Write the allowance in, or give before-coat and after-coat limits on the fits that matter.
Phosphate is friable, and the specification notes that close fits may show apparent interference that disappears when the surplus crystal is wiped or forced off at assembly. That is acceptable on a press fit that will be oiled anyway, and it is not acceptable on a gauged bore or a thread that must accept a GO gauge at incoming inspection. Do not count on the class 2A allowance of an external thread to absorb heavy phosphate: the pitch diameter of a 60-degree thread grows by four times the film on each flank, so the upper end of the Table V range can use up the allowance on common sizes. The arithmetic is the same as for plating and is covered in the guides to thread class 2B vs 3B and threads that bind after plating.
Black oxide is the opposite case. Per MIL-DTL-13924E, section 6.3.1, it should produce no appreciable dimensional change, so the drawing dimensions are the dimensions after coating. That single fact is why black oxide survives on gauges, tooling components, precision pins and moving parts where a phosphate crystal or a plated layer would disturb the fit.
Why is manganese phosphate the usual choice for sliding and break-in surfaces?
Because the heavy manganese crystal holds oil at the contact, wears in sacrificially during the first hours of running and reduces scuffing between new steel surfaces. The specification names wear prevention and break-in of bearing surfaces as the purpose of Type M Class 2.
The notes to MIL-DTL-16232H describe Type M Class 2 as suitable where moderate corrosion resistance is needed in storage, “to prevent wear, and assist in the ‘breaking-in’ of bearing surfaces.” Class 4, the chemically converted manganese coat, goes further: the specification recommends it for reducing sliding friction in preference to Class 2, and says it reduces the variation in torque versus tension usually found with threaded fasteners. That last point matters on bolted joints torqued to a value rather than to an angle, because scatter in friction becomes scatter in clamp load.
Zinc phosphate has its own wear role, mostly in forming. The same notes say Type Z may be used to prevent galling in cold extrusion and deep drawing, where the crystal carries the forming lubricant. It is also the classic base under paint and powder coat, though light paint-base phosphate is a separate specification (TT-C-490), not the heavy coating discussed here.
Two limits point the choice. Type M is more resistant to alkaline environments than Type Z, and the temperature guidance in the specification notes is 121 °C for Type M against 93 °C for Type Z, per MIL-DTL-16232H, sections 6.1.1 and 6.1.2. A gear, cam follower, piston pin or slide way that runs warm and sees alkaline coolant or cleaners is a manganese job; a bracket that will be oiled or painted is a zinc job.
When is black oxide the better call than phosphate?
When the part cannot tolerate any buildup, must keep a clean, low-reflection black on precise surfaces and will live indoors with a preservative or in protective packaging. It is the wrong call for parts headed into long-term storage or humid, outdoor service without further protection.
MIL-DTL-13924E says black oxide is particularly suited to moving parts that cannot tolerate the dimensional buildup of a more corrosion-resistant finish, that it is used for appearance and to cut light reflection, and that it is not recommended on parts going into long-term storage unless a preservative or desiccated package is used. Its classes follow the base metal: Class 1 for carbon and low-alloy steel and irons, Classes 2 and 3 for certain corrosion-resistant steels depending on their tempering temperature, and Class 4 for other stainless grades. Only Class 4 on 300-series stainless carries a bare salt-spray requirement (MIL-DTL-13924E, section 3.9); Class 1 on carbon steel is judged by coverage, a smut test and an oxalic acid spot test, not by corrosion hours.
Black oxide also wins on machined detail. Sharp edges, small holes, fine threads and knurls keep their geometry, and the finish does not hide a ground surface. On free-machining grades such as 12L14 and 1215, which rust readily, black oxide with oil is a common low-cost cosmetic finish; the guide to 12L14 vs 1215 free-machining steel covers why those grades need some coating at all.
Phosphate is the better call when the part will see moisture between assembly and use, when oil retention over months matters more than appearance, or when the surface will be painted. If the part needs real outdoor corrosion life and the fit allows some buildup, neither finish is the right family; electroplated zinc with a passivate is, and the guide to zinc plating salt spray hours explains what that buys.
How should a drawing or purchase order call out phosphate or black oxide?
Name the specification, the type and the class, and say what supplementary treatment goes on top. “Black finish” or “phosphate and oil” alone leaves the coating weight, the oil and the corrosion test open to the finisher.
Workable callouts look like this:
PHOSPHATE COAT PER MIL-DTL-16232H, TYPE M, CLASS 2. DIMENSIONS APPLY BEFORE COATING.For a sliding or break-in surface that will run oiled.PHOSPHATE COAT PER MIL-DTL-16232H, TYPE Z, CLASS 2.For general corrosion protection under oil on brackets, fasteners and hardware.PHOSPHATE COAT PER MIL-DTL-16232H, TYPE Z, CLASS 1, SUPPLEMENTARY TREATMENT: PRIME AND PAINT PER [paint spec].When the phosphate is a paint base.BLACK OXIDE PER MIL-DTL-13924E, CLASS 1, FOLLOWED BY WATER-DISPLACING PRESERVATIVE PER MIL-PRF-16173, GRADE 3. DIMENSIONS APPLY AFTER COATING.For carbon or low-alloy steel parts with tight fits.
Some drawings, aerospace ones in particular, cite SAE AMS2485 for black oxide instead; the MIL specification itself references AMS2485 for its oxalic acid spot test. Whichever document you cite, add the edition letter so the finisher works to the revision you reviewed. In the current editions, MIL-DTL-16232H says hexavalent chromium should not be permitted on coated items unless the drawing or contract says otherwise, and MIL-DTL-13924E calls for a chrome-free bath and a non-chromated sealer on Class 1.
Hardened parts need two extra lines. Unless the contract says otherwise, MIL-DTL-16232H requires a stress relief before coating for steel at 39 HRC or harder that has been machined, ground, cold formed or cold straightened after heat treatment, and a hydrogen embrittlement relief after coating for all steel at that hardness, at temperatures and times given in sections 3.3 and 3.4; MIL-DTL-13924E likewise calls for stress relief before and embrittlement relief after on hardened steel, and warns that high-strength steel can suffer caustic embrittlement during the blackening treatment. If the drawing cites a commercial or in-house finish standard rather than the MIL document, state the bake yourself. The logic is the same as for plated fasteners and is laid out in the guide to hydrogen embrittlement baking.
Finally, say whether salt-spray results are required on the lot, and whether the test is run with or without the supplementary oil. A result quoted for an oiled panel cannot be compared with a requirement written for a bare coating.
What are the most common mistakes with these finishes?
Expecting black oxide or bare phosphate to stop rust on its own, and then stripping the oil that was doing the work. Most of the other mistakes are sequencing errors.
- Specifying black oxide for outdoor or humid service. The specification itself calls the protection very limited. If the part will see weather, choose a plated or painted system.
- Degreasing at incoming inspection. Vapor degreasing or solvent wiping a Class 2 phosphate part removes the preservative the salt-spray requirement was written around. The specification notes that supplemental oil can also be removed by contact with absorbent material.
- Phosphating before the last machining step. MIL-DTL-16232H wants coating after all machining, forming, welding and stress relief. A coated part that is then turned or drilled has bare steel where the tool cut.
- Leaving out the type and class. “Phosphate per MIL-DTL-16232” with no type or class lets the finisher choose zinc or manganese, oiled or dry.
- Ignoring the temperature note. A zinc phosphate part that runs hot, for example on an engine or near a heater, is outside the specification guidance; manganese is the safer base.
- Writing a roughness callout with no sequence. Phosphate changes surface texture. MIL-DTL-16232H applies the drawing roughness before coating unless the drawing says otherwise, so say which you mean. The same specification calls for abrasive blasting before phosphating unless the drawing or contract waives it (section 3.2.2.1) and advises against phosphate where a highly polished surface is required after coating (section 6.5), so a ground or lapped face needs a note. The guide to surface finish before or after coating covers how to write it.
How are phosphate and black oxide handled on a Saola order?
Neither phosphate nor black oxide is on the finish menu Saola offers through its partner plants, and the surface finishing page is explicit that a finish outside that menu is not offered today.
The surface finishing page lists a fixed menu that our partner plants apply, with the quote naming where each step is done: electroplating, gold plating, zinc plating, RoHS-compliant blue trivalent chromate, anodizing on aluminum, powder coating, pad printing and oil-dip rust protection. Specialty conversion coatings are listed among the things not offered. For a steel part that only needs a dark, low-build look, that means saying so early rather than discovering it after samples. For a steel part whose real need is corrosion protection, zinc plating with the trivalent chromate topcoat or oil-dip rust protection may meet the requirement, and salt-spray results can be supplied on request, with the test standard and duration confirmed in the quote.
Steel bar parts such as bushings, spacers and shafts are turned at our lead CNC plant from grades including 12L14, C1117 and 52100, as listed on the CNC turning page. If your drawing calls for phosphate or black oxide, put it on the RFQ as written. You will get a plain answer on whether a finish on that menu fits the function, and no improvised coating.
Whatever supplier you use, an RFQ for a dark steel finish should carry: the specification with edition, type and class; the supplementary oil, preservative or paint; whether dimensions apply before or after coating; the hardness of the part and the required bakes; the corrosion test and whether it is run oiled or bare; and any surfaces that must stay uncoated, such as bores that are reamed after finishing.
Frequently asked
Is “parkerizing” the same as phosphate coating?
Yes, it is an old trade name for heavy phosphate coating, usually manganese or zinc phosphate on firearms and military hardware. On a drawing, replace the trade name with MIL-DTL-16232 and a type and class so the requirement can be inspected.
Can black oxide be applied to stainless steel?
Yes. MIL-DTL-13924E has separate classes for corrosion-resistant steels, chosen by alloy and tempering temperature. The process differs from the one used on carbon steel, so name the class rather than writing a generic black oxide note.
Is black zinc plating a substitute for black oxide?
Functionally it is closer to a corrosion finish than black oxide, because it adds a sacrificial zinc layer under the black film. It also adds thickness, so it is not a drop-in replacement on a part chosen for black oxide because the fit could not take buildup.
Can phosphated parts be painted or powder coated?
Yes, and that is one of the main reasons zinc phosphate is used. Do not oil a part that is going to be painted; specify Class 1 with the paint system as the supplementary treatment, or use a light paint-base phosphate specification.
Why do phosphated parts sometimes leave gray residue on hands and packaging?
The coating is crystalline and slightly friable, and loose surface crystal rubs off, especially on Class 3 parts with no oil. Good rinsing and a supplementary oil reduce it. MIL-DTL-16232H already checks for it: each lot is wiped with a dry white cloth after phosphating, and black or gray residue on the cloth halts processing.
Sources
- MIL-DTL-16232H, Detail Specification: Phosphate Coating, Heavy, Manganese or Zinc Base, US Department of Defense, September 15, 2020
- MIL-DTL-13924E, Detail Specification: Coating, Oxide, Black, for Ferrous Metals, US Department of Defense, September 12, 2018
- SAE AMS2485, Coating, Black Oxide, SAE International, accessed October 2026
- ASTM B117-26a, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2026