DIN 50938 — black oxide (Brünieren) on iron and steel, and what the black finish really is

Scope — and what it does not cover

DIN 50938 is the German standard for Brünieren — black oxide finishing of components made of iron materials. It deals with the process itself and with how the result is examined: what the treatment does to the surface, how the finish is judged, and the after-treatment the finish depends on. It does NOT assign a corrosion life or a salt-spray duration, does not make the film an alternative to zinc plating, does not cover austenitic stainless steel (which blackens, if at all, in a different chemistry), and says nothing about the mechanical properties, thread tolerance or dimensional standard of the fastener underneath.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1The black is not something added to the part — it is the part. The bath converts the outermost iron into magnetite, Fe3O4. There is no anode, no cathode, no deposited metal. That is what makes it a conversion coating rather than a plated layer, and it is why the finish has nothing to peel or flake off.
• 2Because the film grows out of the substrate and consumes a little of it, thickness is counted in fractions of a micron up to a couple of microns, and the net change to a diameter is negligible. A thread that gauged before blackening gauges after it. This, not appearance, is why class 12.9 socket head cap screws are black and not electroplated.
• 3Bare black oxide is not a corrosion protection system. The film is thin and it behaves as an oil carrier rather than a barrier. Left dry it rusts; sealed, it performs modestly and honestly. Any salt-spray figure quoted for a black finish is a figure for the seal, not for the oxide.
• 4The seal is therefore part of the specification, not a courtesy at the end. Oil, wax and dry-to-touch preservatives behave differently downstream: oil fights thread adhesive and paint, wax changes the friction the assembly torque was calculated against, and an unsealed part can rust in its own box before it is ever fitted.
• 5No plating current means no cathodic hydrogen charging, which removes the mechanism that makes electroplating a decision on high-strength fasteners. It does not remove acid pickling from the cleaning line ahead of the bath, so an aggressive descale on a very hard part is still worth a conversation before the lot is booked.
• 6The finish is only as even as the steel beneath it. Colour and depth follow alloy content, carbon level, surface roughness and prior scale, so a lot that mixes forged and machined surfaces — or two heats of the same grade — can come out visibly two-tone while being entirely conforming. Sintered and other porous parts hold bath chemistry in the pores and bleed rust weeks later.

What to write on the drawing or PO

• The process and the governing document on one line — "black oxide (Brünieren) to DIN 50938", or the customer equivalent — plus the edition in force if the drawing is under change control.
• The after-treatment, named: oil, wax or dry-to-touch preservative, with the type where the assembly is sensitive to it. "Black oxide" on its own leaves the corrosion behaviour of the delivered part undefined.
• What happens to the part next — painting, bonding, thread locker, or a torque-controlled joint. Each of those rules out at least one seal, and it is far cheaper to know that before the parts go into the basket than after.
• The base material and its condition: grade, whether the parts are already quenched and tempered or case hardened, and whether anything in the lot is stainless, cast or sintered. Austenitic stainless does not blacken in an iron blackening bath.
• How appearance will be accepted — a boundary sample, or a written acceptance that shade variation between surfaces and between heats is not a reject. Colour is the argument black oxide lots actually end up in, and it is settled at quoting time or not at all.

What goes wrong when it is cited loosely

• Ordering black oxide where zinc plating was the real requirement. The two are not on the same scale of corrosion protection, and no amount of oil closes that gap in an outdoor or washdown environment.
• Writing a salt-spray duration against black oxide as if the standard assigned one. It does not. The hours, if any are agreed, belong to the oil or wax and must appear as their own line with their own test reference.
• Blackening parts that will later be bonded, painted or fitted with thread locker, without saying so. The oil that makes the finish work is precisely what stops the adhesive or the primer from working.
• Treating "black oxide", "black phosphate" and "black zinc" as one item on a purchase order. Three different surfaces, three thicknesses, three failure modes and three prices — and only one of them is Fe3O4.