Mil-Spec Coatings and Finishes for Fasteners

A drawing calls out a finish, and that finish tells you more about how the part will hold up than the base material does half the time. Two identical steel bolts — one cadmium plated, one black oxide — go into completely different environments. Get the finish wrong and you either fail a corrosion requirement or throw money at protection the application never needed. This guide covers the finishes you'll see most often on mil-spec and aerospace fastener drawings, what each one is actually for, and the one risk — hydrogen embrittlement — that matters on every high-strength plated part.

Engineering Data Disclaimer. The performance values shown are sourced from published industry and manufacturer literature. Actual performance in your application depends on base material, installation conditions, and environmental factors. For application-specific qualification, contact us; do not use these values as design loads without independent validation in your installation conditions. Eugene Fastener & Supply provides this data as a reference; we do not warrant performance for any specific application beyond the conditions described in the source specification.

Black Oxide (MIL-DTL-13924)

Black oxide is a chemical conversion coating, not a plated layer — it converts the surface of the steel into a thin layer of black iron oxide (magnetite) rather than depositing another metal on top. MIL-DTL-13924 is the governing spec. It's thin enough that it doesn't add measurable dimension to threads, which is one reason it's popular on precision hardware where a plated buildup would affect fit.

Black oxide gives minimal corrosion protection on its own. It's almost always paired with an oil or wax topcoat to do the real work of keeping moisture off the surface, and that oil needs to be maintained or reapplied in service. Think of black oxide as a cosmetic and light anti-galling finish more than a corrosion-resistant one. It's a good fit for indoor hardware, tooling, and fasteners that get an assembly-line coating of oil anyway, and a poor fit for anything exposed to weather or salt air without additional protection.

Cadmium Plating (QQ-P-416)

Cadmium plating, governed by QQ-P-416, has been the default aerospace fastener finish for decades. It gives excellent corrosion protection, it's naturally lubricious (good for consistent torque-tension performance), and it's compatible with aluminum without the galvanic corrosion issues some other finishes cause. That's why so much legacy aerospace hardware, and a large share of current drawings, still call it out.

The catch is cadmium's toxicity and environmental regulation. It's a restricted material under RoHS and increasingly restricted or banned outright in commercial manufacturing in many regions, and plating shops that still run cadmium lines have gotten harder to find and more expensive to use. On military and legacy aerospace work, cadmium is often still the specified finish because the drawing hasn't been revised, and re-qualifying a different finish on an existing design is its own cost. On new designs, zinc-nickel has become the practical substitute where the corrosion and galvanic performance of cadmium is needed without the same regulatory baggage.

Zinc Plating (ASTM B633)

Zinc plating under ASTM B633 is the general-industrial workhorse finish — the same family of finish you'll see on everyday grade 5 and grade 8 hardware, just with tighter process control when it's called out on a mil-spec drawing. Zinc offers decent corrosion protection through sacrificial action (the zinc corrodes before the base steel does) at a fraction of cadmium's cost and without cadmium's toxicity concerns.

On aerospace work, plain zinc plating shows up more on ground support equipment, brackets, and non-flight hardware than on flight-critical fasteners, where cadmium, zinc-nickel, or passivated stainless are more common callouts. Zinc's corrosion resistance and galvanic compatibility with aluminum are both a step below cadmium's, so it's not a drop-in substitute on a drawing that specifically calls for cadmium without an engineering disposition.

Zinc-Nickel — the Modern Cadmium Alternative

Zinc-nickel plating has become the go-to replacement where a design would have specified cadmium a generation ago. It delivers corrosion protection and galvanic compatibility with aluminum that's comparable to or better than cadmium, without the toxicity and disposal issues that have made cadmium plating lines increasingly rare. Zinc-nickel typically shows up in the finish suffix on updated drawings and is widely accepted across both military and commercial aerospace primes for new designs and for re-qualified legacy parts.

The tradeoff is cost and lead time — fewer plating houses run true aerospace-grade zinc-nickel lines compared to zinc or black oxide, and the process itself runs more expensive per part. If your drawing already calls for cadmium and there's no engineering change in progress, don't substitute zinc-nickel on your own judgment; it requires a documented material or process substitution approved by the customer's engineering authority.

Dry Film Lubricant (MIL-PRF-46010)

Dry film lubricant, specified under MIL-PRF-46010, is a thin bonded coating — usually a resin binder loaded with molybdenum disulfide or similar solid lubricant — applied over a base finish to control friction during installation. It's not a corrosion-protection finish by itself; it's typically layered on top of a plated or passivated part to get consistent, repeatable torque-tension results, especially on high-strength fasteners where friction variation between installations can throw off preload significantly.

You'll see dry film lube called out on fasteners going into critical joints where torque control matters more than usual — structural bolts, fittings with tight preload windows, and hardware that gets removed and reinstalled during maintenance. Because it's a thin, dry, non-oily coating, it doesn't attract dirt and grit the way a wet lubricant does, which matters for parts stored or handled in the field.

Passivation of Stainless Steel (AMS 2700)

Passivation, governed by AMS 2700, isn't a coating at all — it's a chemical treatment (typically a nitric or citric acid bath) that strips free iron and surface contamination from stainless steel parts left behind by machining, so the chromium oxide layer that gives stainless its corrosion resistance can form properly across the whole surface. Without passivation, machined stainless parts can show localized rust spots where embedded free iron corrodes even though the bulk material is stainless.

Passivation is standard practice on stainless fasteners for aerospace and defense work, and it's often called out as a required process step even when the print doesn't spell out every detail, because it's assumed as good manufacturing practice for corrosion- resistant steel hardware. If your drawing calls for CRES (corrosion-resistant steel) fasteners, expect passivation per AMS 2700 to be part of the certification package unless explicitly waived.

Finish Comparison

Hydrogen Embrittlement and Post-Plate Baking

Electroplating processes like cadmium and zinc plating use an electrochemical bath, and that process can drive atomic hydrogen into the surface of the steel. In low-strength steel this usually isn't a problem. In high-strength steel — generally parts heat treated above roughly 32 HRC hardness, which covers most aerospace-grade structural bolts and high-strength fasteners — that absorbed hydrogen can cause the part to crack under load days or weeks after plating, with no warning and no visible defect beforehand. That failure mode is hydrogen embrittlement, and it's one of the more dangerous things that can happen to a plated high-strength fastener.

The fix is baking. After plating, high-strength parts go into an oven at a controlled temperature (commonly cited in industry practice as roughly 375°F, though the acceptable range and hold time vary by spec and part hardness) for an extended hold, often many hours, to drive the absorbed hydrogen back out before it can cause a delayed crack. This bake has to happen within a defined window after plating — typically within an hour or so — because hydrogen migrates and the damage risk builds the longer the part sits unbaked.

What this means in plain terms for buyers:

• If you're ordering high-strength plated fasteners for a structural or flight-critical application, hydrogen embrittlement relief baking should be part of the plating process, documented on the certification paperwork — not an optional add-on.
• Low-strength hardware (soft washers, low-grade brackets) generally isn't at meaningful risk, which is why you won't see baking called out on every plated part.
• Don't accept plated high-strength fasteners without a bake certification if the spec or drawing requires it. This is exactly the kind of detail that shows up on an incoming inspection reject if it's missing.
• Baking adds cost and lead time to plated high-strength parts. Build that into your schedule rather than treating it as a surprise delay.

How Eugene Fastener Manages Coatings and Finishes

We don't run our own plating lines — nobody in this industry stocks every finish in-house and does it well. What we do is manage the process through certified processing houses that specialize in each finish, verify the process paperwork against your spec callout, and supply one consolidated certification package that covers both the base hardware and the applied finish. That means one point of contact and one cert package, instead of you chasing two separate suppliers for two separate sets of paperwork.

Frequently Asked Questions

Can I substitute zinc-nickel for cadmium if my drawing calls out QQ-P-416?

Not on your own authority. Zinc-nickel is widely accepted as a cadmium replacement on new designs, but if an existing drawing specifically calls out cadmium, substituting requires a documented material or process change approved by the customer's engineering authority, not a supplier-level swap.

Does black oxide provide any corrosion resistance by itself?

Very little. Black oxide is mainly cosmetic and mildly anti-galling on its own. The corrosion resistance people associate with it comes from the oil or wax topcoat applied afterward, and that protection degrades as the oil wears off or gets cleaned away in service.

Why does passivation matter if the part is already stainless steel?

Machining smears free iron across the surface of stainless parts, and that iron can rust even though the bulk material won't. Passivation under AMS 2700 removes that surface contamination so the stainless can form a clean, continuous chromium oxide layer — without it, you can see localized rust spots on an otherwise corrosion-resistant part.

Do I need to worry about hydrogen embrittlement on stainless fasteners?

It's primarily a concern for high-strength carbon and alloy steel fasteners, not stainless. Stainless fasteners are typically passivated rather than electroplated, so the classic hydrogen-charging risk from an electroplating bath doesn't apply the same way. Always check your specific alloy and process against the governing spec rather than assuming.

How do I know if a fastener needs hydrogen embrittlement relief baking?

It generally comes down to hardness and process. High-strength steel fasteners (commonly those heat treated above roughly 32 HRC) that go through an electroplating process are the ones at risk. The governing spec or drawing note should call out the bake requirement directly — if you're not sure, ask us to confirm against the applicable spec before you plate or accept plated parts.

Related Resources

• MS, NAS, and AN Fastener Numbering Systems Explained — decoding part numbers and dash codes
• Self-Locking Aerospace Fasteners — jet nuts, nut plates, and locking elements
• Aerospace & Mil-Spec Fasteners — capability overview
• Mil-Spec Certification & Paperwork Guide — what certs and traceability documents to expect
• DFARS-Compliant Fasteners Guide — domestic sourcing and specialty metals compliance