Spring plating: zinc, nickel, tin and passivation compared

What a spring finish has to do

A spring is a difficult shape to coat. It flexes, it has tight inside radii, and it is often made from high strength wire that is sensitive to hydrogen. The finish has to cover the whole surface, including the gaps between coils, without bridging the coils together or cracking at the bend when the spring moves.

The job of the finish is usually one of four things: slow corrosion of the base metal, keep contact resistance low and stable, improve solderability or wear resistance, or restore the corrosion resistance of stainless steel after forming.

Thickness is the other constraint. A heavy coating can close the clearance between coils and crack at the outside of a bend. A thin coating may not cover the wire evenly. The drawing should give a thickness range and say which surfaces matter.

Zinc: the corrosion barrier for steel

Zinc protects steel in two ways. It forms a barrier on the surface, and it acts as a sacrificial anode, so a scratch exposes steel that is still protected by the surrounding zinc. That is why zinc is common on steel springs.

The weakness is hydrogen. Electroplating zinc on a high strength spring wire can introduce hydrogen into the steel, and a spring under load is exactly the shape that will crack if hydrogen is present. The process needs a bake at a stated temperature and time, with a limit on the delay between plating and baking. Mechanical zinc avoids the electrolytic hydrogen but gives a different surface and thickness range.

Zinc is not a contact finish. Its corrosion products are insulating, and the contact resistance at a zinc surface is high and unstable. If the spring is also an electrical contact, zinc is the wrong choice.

Nickel: wear and a stable surface

Nickel plating gives a harder surface than zinc or tin, with good wear resistance and a reasonably stable contact resistance. It is used on springs that rub, on contacts that need a durable surface and on parts that must resist mild corrosion without a thick coating.

Nickel is not a sacrificial coating in the way zinc is. A scratch exposes the base metal and protection depends on the coating staying intact. On steel, a thin nickel layer can also be porous, so a copper or nickel underlayer is sometimes used to build a barrier before the final coat.

The process still carries hydrogen risk on high strength wire, and the bake controls apply. Nickel can also be magnetic, which matters near sensors. On stainless, nickel improves the surface but does not fix a chloride problem in the base metal.

Tin: the contact finish

Tin is soft, ductile and easy to solder. It gives a low contact resistance and it is the usual finish on terminals and many spring contacts. A thin tin layer also survives bending better than a hard, brittle coating because it deforms with the wire.

Tin is a poor corrosion barrier on its own. It is cathodic to steel, so a scratch can concentrate attack on the exposed steel, and the coating is thin enough that porosity matters. On a damp steel spring, tin alone is usually not enough unless the environment is mild and the coating is sound.

Tin whiskers are the other known risk. Thin tin coatings can grow conductive filaments over time, especially under stress, and a whisker can bridge a fine pitch. For critical electronics, a nickel underlayer or a thicker coating may be specified. Tin is also soft with a low melting point, so it is not a wear finish.

Passivation: a treatment, not a coating

Passivation is a chemical treatment that removes free iron and other contamination from the surface of stainless steel and helps the chromium oxide layer reform. It adds no measurable thickness, changes no dimensions and does not fill a scratch. It is used after forming, machining or handling that may have embedded iron particles.

Passivation only works on stainless steel. It does nothing for carbon steel or music wire, because those alloys do not have enough chromium to form a passive layer. On a stainless spring, it restores the corrosion resistance that cutting, coiling and grinding may have reduced.

The standard matters. Passivation to a named standard, with a stated test for free iron, is a real specification. A note that says passivate without a standard leaves the bath, the time and the acceptance open. It helps in chlorides but does not replace a molybdenum grade.

Comparing the four finishes

The table summarises the practical differences. Use it to narrow the choice, then confirm the thickness and the test with the supplier.

Selecting by environment and function

For a dry indoor steel spring that is not a contact, zinc is usually the lowest cost route, provided the bake controls are in place. For a damp environment or a part that rubs, nickel may be better. For an electrical contact, tin is usual, with a nickel underlayer where whiskers or porosity are a concern.

For a stainless spring, the first decision is the alloy, not the finish. Passivation is the normal final step. A coating is only needed when the environment is beyond what the alloy can handle.

If the spring is high strength and the finish is electrolytic, the hydrogen question comes before the corrosion question. A finish that protects against corrosion but embrittles the wire is not a finish, it is a failure waiting for a load.

What to inspect and what to put on the drawing

Inspect thickness on a representative surface, adhesion after a bend, and coverage in the coil gaps. For a contact finish, measure contact resistance before and after any ageing that represents service. For a corrosion finish, run the agreed test and record the acceptance criteria in advance.

• Finish type and the standard it is made to
• Thickness range and the surfaces that must be covered
• Bake temperature, time and maximum delay after plating
• Adhesion test and acceptance
• Contact resistance limit, if the part conducts
• Corrosion test method and duration