Black Oxide Coating for Steel Parts
Alkaline black oxide per AMS 2485 with oil seal for carbon and alloy steel, about one micrometer growth so fits and threads hold, dimension verified after process.
Black Oxide Where Dimensions Must Hold
Black oxide is a conversion coating: the steel surface reacts into magnetite, growing a layer of roughly a micrometer, so holes, threads and fits survive untouched. The trade is honest: corrosion protection is modest until the oil seal goes on. For precision steel parts that live indoors or inside an assembly, that trade is usually exactly right.
When to Choose It
| Part | Route | Why |
|---|---|---|
| Precision steel parts with tight fits | Black oxide, oil sealed | ≈1 μm growth |
| Shafts, rotors, brackets in 20#, C45, 4140 | Alkaline black oxide | uniform dark finish |
| Tooling and fixture hardware | Black oxide | low cost, no size change |
| Outdoor corrosion service | Zinc plating instead | see electroplating |
Specifications We Control
| Item | Specification |
|---|---|
| Standard seen on drawings | AMS 2485 (black oxide, steel details) |
| Dimension change | About 1 μm growth; fits and threads unaffected in practice |
| Seal | Rust-preventive oil or wax dip after conversion, standard |
| Applicable alloys | Carbon and alloy steel (20#, C45 / 1.0503, 4140, tool steels); stainless gets a different route |
| Verification | Appearance uniformity, coating presence, dimensions after on critical features |
| Compliance | RoHS compatible; declarations available per shipment |
How We Run It and Check It
Route: clean and degrease, alkaline black oxide bath, rinse, oil or wax seal, dry, verify, pack. Post-process dimension checks focus on the features the drawing flags as critical, and the shipment carries the finish certificate alongside the CMM report. Pre-cleaning matters more than most buyers expect: residual cutting oil shows as blotch, so parts are washed twice on visual-critical families.
Material side: carbon and alloy steel machining. Aluminum counterpart: black anodizing.
Cross entrances: by material | CNC machining | inspection and QC.
Frequently Asked Questions
Black oxide or zinc plating on my steel part?
Black oxide when the part carries tight fits, because the conversion layer adds about a micrometer and the dimensions effectively hold. Zinc plating when corrosion in service is the priority and the coating thickness can be planned for. On one robot program both run in the same shipment: black anodize on the 6061-T6 housings, black oxide with oil seal on the 20# and C45 steel parts.
Does black oxide prevent rust by itself?
No, and we say so plainly. The conversion layer is decorative and mildly protective; the real corrosion barrier is the oil or wax seal applied after. Parts leave oil-sealed, and storage guidance goes on the packing note.
Can black oxide go on drawings that call anodize for steel?
We flag it at quote time. Steel cannot anodize; on one shipment review a C45 bracket had anodize on the drawing, and the corrected route was black oxide after engineering confirmation. Better to catch it in the quote than at your goods-in.
What standards apply?
Black oxide on steel details per AMS 2485 on the drawings we process, with the alkaline bath route and oil seal as the standard practice. RoHS compatible, declarations available.
Send Your Drawing
Quote with engineering feedback within 24 hours; prototypes in 7 to 15 days.
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