Electroless nickel
Electroless nickel is a nickel-phosphorus alloy deposited by a chemical reaction instead of an electric current. Because no current is involved, it builds to the same thickness on every surface: edges, thread roots and blind holes alike. It can be heat treated afterwards for hardness. It is plated by outside Houston platers.
Clean, immerse, relieve, heat treat.
- STEP 01
Stress relieve if ordered
Steel over 1000 MPa (about 31 HRC) that was machined, ground or cold formed after hardening can be stress relieved before plating (ASTM B849), when the order calls for it.
- STEP 02
Clean
The parts are cleaned and prepared so the deposit bonds to the base metal.
- STEP 03
Plate without current
The parts are immersed in a nickel-phosphorus bath. The deposit grows by chemical reduction, with no electric current, at the same rate on every wetted surface, until it reaches the service-condition thickness.
- STEP 04
Relieve hydrogen
Where the order calls for it, high-strength steel is baked soon after plating to relieve hydrogen (ASTM B850).
- STEP 05
Heat treat to class
Class 1 stays as deposited. Class 2 is heated to 500 to 750 °F (260 to 400 °C) to harden the deposit to at least 850 HK. The other classes are lower-temperature bakes for adhesion on particular base metals.
- Thread fit: even SC2 fills the class 2A allowance of a 1 in. thread, so standard threads need a pre-plate size.
- Class 2 hardening runs at 500 to 750 °F (260 to 400 °C). Alloy socket screws are tempered at 700 °F minimum (ASTM A574), so the class has to suit the part's tempering temperature.
- High-strength steel: the process can bring in hydrogen, so stress relief and baking are planned from the part's strength.
Finishes are applied by outside coaters and platers in Houston, to the specification on the order.
Even on the thread, so it takes all the allowance.
On a 60° thread the pitch diameter grows by about four times the coating thickness. Service condition SC2 calls for at least 13 µm (0.0005 in.), which adds about 0.002 in.: the whole class 2A allowance of a 1-8 thread, and more than the allowance of anything smaller. Threads for electroless nickel are therefore made to a pre-plate size, or a lighter service condition is chosen.
Because the deposit is uniform, it coats thread roots and the inside of nuts as thickly as the crests, where electroplating leaves the least. A nut's standard thread has no allowance, so it has to be tapped to suit.
For steel above 1000 MPa, ASTM B733 provides stress relief before plating and hydrogen embrittlement relief after it, when the purchaser specifies them. Giving the base metal's strength on the order lets the plater plan them.
Designations and classes.
How to call it out.
- Give the Type, service condition and Class, for example electroless nickel to ASTM B733, Type V, SC2, Class 1.
- State the base metal and its tensile strength or hardness, so stress relief and the hydrogen bake can be planned.
- Say whether the threads have been made to a pre-plate size, and whether nuts are to be tapped to suit.
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.
Will a plated high-strength bolt crack in service?Hydrogen embrittlement relief and testing
High-strength steel can pick up hydrogen during pickling and plating and fail days later under load. Parts above 39 HRC are baked after plating to drive the hydrogen out, and a sustained-load test on fasteners from the lot proves the relief worked.
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.
Electroless nickel: common questions.
What do Type, SC and Class mean?
Type is the phosphorus content, SC the service condition that sets the minimum thickness, and Class the heat treatment after plating. All three belong on the order.
Why does it fit threads differently from zinc plating?
Electroplating builds up at tips and edges and is thinnest in the roots. Electroless nickel is the same thickness everywhere, so the whole thread grows by the same amount.
Can it be hardened?
Yes. Class 2 is heat treated at 500 to 750 °F (260 to 400 °C) to at least 850 HK, provided the base metal can take that temperature.
Compare with.
Silver plating
A thin silver layer plated onto the nut thread that acts as a solid lubricant. It stops austenitic and nickel-alloy nuts galling on their studs, including in high-temperature service.
Economical sacrificial protection for indoor and mild serviceZinc electroplating
A thin electroplated zinc layer with a clear, yellow or black chromate, or a chromium-free passivate. Zinc corrodes in preference to the steel underneath, so a scratch does not rust. It is the general-purpose finish for hardware that lives indoors or under cover.
Restoring the stainless surface after machiningPassivation 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.