Silver plating
Stainless steel and nickel-alloy threads tend to gall: under load and heat the thin oxide on the metal is scraped away, the bare surfaces weld, and a nut can seize and strip when it is forced off. A thin layer of silver on the nut is soft and ductile and works as a solid lubricant, so the nut runs on and comes off without seizing. It is plated by outside Houston platers.
Activate, strike, plate.
- STEP 01
Clean
The plater cleans the nuts of oil and machining residue.
- STEP 02
Activate with a nickel strike
Stainless steel and nickel alloys carry a passive oxide that silver will not bond to. A nickel strike bath removes it and leaves a thin, active nickel layer.
- STEP 03
Silver strike
A dilute silver bath lays down a thin, adherent first layer; the parts go in with the current already on.
- STEP 04
Silver plate
Silver is built up to the thickness on the order, matte, semi-bright or bright. Matte silver of 99.9% purity is preferred for high-temperature parts.
- STEP 05
Relieve and check
Stress and hydrogen relief are applied where ASTM B700 and the base metal call for them, and adhesion and thickness are checked.
- Joints opened often: aerospace guidance limits silver-plated fasteners to two refits.
- Titanium: silver-plated parts should not touch titanium, because of the corrosion risk.
- Oversize tapping reduces thread engagement. Agree it and name it on the order (ASTM A194 S7).
Finishes are applied by outside coaters and platers in Houston, to the specification on the order.
A soft lubricant, on a thread with no room to spare.
Standard nut threads have no allowance for plating, and on a 60° thread every 0.0001 in. of plating takes about 0.0004 in. of pitch-diameter clearance. A nut to be silver-plated is therefore tapped oversize first, by an amount agreed with the plater. Any change to an A194 nut's thread dimensions is named on the order, because it can reduce the nut's load capacity.
Silver works by smearing: it is soft and ductile, so it shears between the threads instead of letting the stud and nut surfaces weld. Some smearing after use is normal. Aerospace fastener guidance advises against silver on fasteners removed and refitted more than twice, and against silver touching titanium.
ASTM B700 covers stress relief before plating and hydrogen embrittlement relief after it. Give the base alloy and its hardness on the order so the plater can apply them.
Designations and classes.
How to call it out.
- Give the nut, specification, purity and thickness, for example ASTM A194 Grade 8M heavy hex nuts, silver-plated to ASTM B700 Type 1, matte, thickness per drawing, tapped oversize to suit.
- Say whether the whole nut or only its thread is to be plated, and whether a supplementary surface treatment is wanted.
- Ask for thickness readings on the certificate if you need them.
The tests behind it.
Coating thickness and salt spray
Coating thickness is measured with a magnetic or eddy-current gauge, or by X-ray fluorescence for thin platings, and checked against the class ordered. Salt-spray exposure checks corrosion protection on a sample from the lot when the specification calls for it.
Does every dimension and thread meet the drawing?Thread gauging and dimensional inspection
Threads are checked with Go and No-Go ring and plug gauges, and pitch diameter is measured by the three-wire method or on a thread comparator. Heads, flats, lengths and radii are measured with calibrated instruments and, for complex parts, on the CMM.
Silver plating: common questions.
What causes galling?
On stainless steel and nickel alloys, the thin oxide that keeps the metal surfaces apart can be scraped off under load and heat. The bare metals then weld and tear, and a nut can seize and strip its thread when it is forced off.
How thick is the silver?
It is set on the order or drawing. Because a nut thread has no allowance, the nut is tapped oversize to suit.
Can silver-plated nuts be reused?
Sparingly. Some smearing is normal after tightening, and aerospace guidance limits silver-plated fasteners to two refits.
Compare with.
Passivation of stainless steel
A nitric or citric acid treatment that dissolves free iron left on the surface by tooling and lets the natural chromium-oxide film reform. It is not a coating; it is how a machined stainless part gets its full corrosion resistance back.
Uniform, hard and corrosion resistant, even inside threadsElectroless nickel
A nickel-phosphorus deposit laid down by chemical reduction rather than current, so it builds evenly on every surface, including thread roots and bores. It is harder than zinc and resists chemicals and wear.
Low friction, easy break-out, the oil and gas standardPTFE / fluoropolymer coating
A thin fluoropolymer film, usually blue or green, applied over a phosphate base. It gives a low, consistent coefficient of friction, so less torque is needed for the same bolt tension and the torque is set for the coated parts; it resists the atmosphere and chemical splash, and helps nuts come off at the next turnaround.