MIL-DTL-13924
Coating, Inorganic, Black, for Ferrous Metals
Covers inorganic black conversion coatings on carbon steel, low-alloy steel, corrosion-resistant steel, wrought iron, and specified cast irons.
What MIL-DTL-13924 specifies
MIL-DTL-13924 defines four black-oxide process classes selected by substrate and heat-treatment condition. Class 1 is the conventional boiling alkaline process for wrought iron, cast and malleable iron, carbon steel, and low-alloy steel. Classes 2 through 4 address corrosion-resistant steels using alkaline chromate, fused salt, or other alkaline oxidizing processes. The coating is thin and provides only limited corrosion protection; oil, wax, or another supplementary preservative is normally critical. Requirements include complete uniform black coverage, smut and spot tests, and substrate-specific salt spray. A drawing should state the class, basis alloy and temper, supplementary preservative, appearance limits, and any hydrogen-embrittlement precautions.
MIL-DTL-13924 types and classes
Every designation the document defines, with the thickness it requires and the tests it imposes.
| Designation | Meaning | Requirement / test |
|---|---|---|
Class 1 | For wrought iron, cast and malleable irons, plain-carbon steel, and low-alloy steel. | Uniform black coverage, smut test, oxalic-acid spot test, and supplementary-finish requirements. |
Class 2 | For specified corrosion-resistant steels tempered below 900 °F (482 °C). | Uniform black coverage, smut test, oxalic-acid spot test, and applicable corrosion test. |
Class 3 | For corrosion-resistant steels tempered at 900 °F (482 °C) or above. | Uniform black coverage, smut test, and applicable corrosion test. |
Class 4 | For other corrosion-resistant steel alloys. | Uniform black coverage and smut test; 300-series specimens require 96 h salt spray without corrosion before preservative. |
Tests and the drawing call-out
Tests the document imposes
- Visual coverage and uniform black color
- Smut test
- Oxalic-acid spot test where applicable
- ASTM B117 salt spray for 96 h on applicable 300-series corrosion-resistant steel
- Hydrogen-embrittlement control for susceptible high-strength steel
- Supplementary preservative verification
How to cite it
Drawing call-out
BLACK OXIDE PER MIL-DTL-13924 CLASS <1|2|3|4>, [SUPPLEMENTARY PRESERVATIVE], [MASKING]Angle brackets mark the choices the drawing must make; every finish page shows a completed example.
Finishes specified to MIL-DTL-13924
MIL-DTL-13924 — frequently asked
What class of MIL-DTL-13924 is used for carbon steel?
Class 1 is the conventional selection for plain-carbon and low-alloy steels, as well as the listed iron substrates. The alloy and hardness still need review for processing and embrittlement controls.
Does black oxide provide strong corrosion protection?
No. The specification describes only limited protection under mildly corrosive conditions, so the supplementary oil, wax, or preservative is an important part of the finish system.
What are Classes 2, 3, and 4 for?
They are processes for corrosion-resistant steels, selected according to alloy and tempering temperature. They are not increasing thickness or quality grades.
MIL-DTL-13924 is a work of the US Government and not subject to copyright. The authoritative copy is the DLA ASSIST database; check it for amendments issued after this page was updated. Official source ↗