Pour a box of aerospace fasteners onto a bench and the first thing you notice is color. Bright silver. Iridescent gold. A blue-grey sheen. Flat black. Those colors are not decoration and they are not a branding choice. They are the surface plating, and the plating is doing most of the work you never see.
A fastener is a structural part, but it is also a chemical part. It sits in a joint with dissimilar metals around it, it gets rained on, salted, heated, torqued, and untorqued. Bare steel in that environment loses. The plating is the layer that takes the abuse so the base metal does not.
The Trade Every Plating Makes
There is no best plating. There is only the right plating for the joint, and every choice is a trade across three things:
- Corrosion protection — how hard the finish fights back against moisture, salt, and dissimilar-metal contact.
- Dimensional change — plating adds material. On a threaded part, added material changes thread fit and changes the torque you feel going in. Some finishes add almost nothing. Some add enough to matter.
- Cost and compliance — what it costs to apply, how many shops are qualified to apply it, and what the regulatory picture looks like now and five years from now.
Push hard on any one of those and you give something up on the other two. That is the whole game.
The Finishes, One At A Time
- Cadmium plate — the long-running aerospace benchmark. It is a sacrificial coating, meaning it corrodes first and preferentially so the steel underneath stays intact. It has excellent salt and marine corrosion resistance, it is naturally lubricious so it resists galling and gives steady, repeatable torque-tension behavior, it conducts well for bonding and grounding, and it plays nicely against aluminum structure without setting off a galvanic fight. The catch is that cadmium is toxic, the regulatory pressure on it keeps tightening, and the list of shops still plating it keeps getting shorter. It remains specified on plenty of defense and legacy airframe work, and it will be around for a long time yet.
- Zinc-nickel — the leading modern answer to the cadmium problem. It keeps the sacrificial behavior and delivers strong corrosion protection without the toxicity baggage. If a program is being designed today and someone asks what replaces cad, this is the first name on the list.
- Yellow zinc chromate — the gold iridescent parts. Zinc plating with a chromate conversion layer over the top, and that top layer is what gives the color and a real step up in corrosion resistance over plain zinc. It is inexpensive and widely available. It earns its keep on commercial hardware, ground support equipment, tooling, and interior fittings. It is not the answer for a salt-spray marine environment or a flight-critical joint.
- Clear and blue zinc passivate — the bright, slightly blue-silver parts in the same family. Same zinc base, different conversion chemistry, cleaner cosmetic result, and generally a bit less corrosion protection than the yellow. Chosen when appearance matters or when the gold look is not wanted.
- Black oxide — a conversion coating rather than a plated-on layer, which is its whole advantage: it changes the surface instead of adding to it, so thread fit and torque are essentially untouched. It is dark and low-glare, which matters in a cockpit or anywhere reflection is a problem, and it is cheap. Corrosion protection is mild, and it needs a supplementary oil or wax to deliver even that. Good for non-critical, indoor, controlled-environment hardware. Not a substitute for real corrosion protection.
- Passivation — strictly speaking not a coating at all. It is a chemical treatment for stainless that strips free iron off the surface and lets the chromium oxide film rebuild itself. Nothing is added, so there is no measurable dimensional change and no effect on thread fit. It is environmentally friendly and inexpensive. The limits are real: it only works on stainless, and it adds no lubricity and no wear resistance. It restores the metal to being properly itself, nothing more.
- Anodizing — for aluminum and titanium hardware. Like passivation it converts the surface rather than plating onto it, but unlike passivation it grows an oxide layer outward, so it does affect tolerances and you have to account for it. Type II is the general-purpose, dyeable version. Type III hardcoat is the thicker, tougher one for wear surfaces. One thing to plan for: the anodic oxide is an insulator, so anodized hardware in a grounding or EMI path needs a deliberate answer.
- Electroless nickel — applied autocatalytically rather than by electric current, which means it deposits at the same thickness everywhere, including down in threads, into bores, and inside blind features where an electroplated finish goes thin or misses entirely. It is hard, wear resistant, and corrosion resistant. When geometry is complicated and coverage has to be uniform, this is the one.
- Dry film lubricant — a bonded PTFE or molybdenum disulfide film, and it is about friction rather than corrosion. It gives a predictable, repeatable coefficient of friction so torque actually correlates with clamp load, and it stops titanium and stainless from galling and seizing on assembly. It works across a wide temperature range and leaves nothing to migrate or contaminate the way a liquid lubricant would. Frequently applied over another finish rather than instead of one.
- Silver plate — the high-heat specialist. Anti-seize behavior at temperatures that would defeat almost anything else, plus excellent electrical conductivity. This is engine and exhaust-section hardware, and it pairs with the superalloys used out there.
How To Actually Choose
Start with the environment. Salt, moisture, and heat narrow the field fast, and they narrow it before cost ever enters the conversation.
Then look at what the fastener touches. Dissimilar metals in contact create a galvanic path, and the finish is your first line of defense against it. Aluminum structure in particular rewards careful thinking here.
Then check fit. On a fine thread or a close-tolerance hole, a finish that adds material can turn an easy assembly into a fight. If dimensional stability is the priority, the conversion treatments earn their place.
Then check whether torque matters. If the joint is torque-controlled, friction is not a detail, it is the specification. That is where lubricity stops being a nice-to-have.
Cost comes last, and it comes last for a reason. The cheapest finish that fails in service is the most expensive part on the aircraft.
The Short Version
The plating is not the finish on the part. On an aerospace fastener, the plating is a good share of the engineering. Two bolts can be the same alloy, the same grade, the same thread, and the same dimensions, and behave completely differently in a joint because of a layer you could measure in fractions of a hair.
Know what the colors mean and you know what you are holding.
Thomas E Aerospace & Defense supplies fasteners and hardware across the full range of aerospace surface finishes. If you are not sure which finish your application calls for, get in touch and we will work through it with you.