Nickel plating for busbars: corrosion and temperature

What nickel plating does on a busbar

Nickel plating puts a hard, metallic barrier between the busbar and the air. On copper it slows tarnish and oxidation. On aluminium it covers a surface that would otherwise form an oxide within minutes of cleaning. In both cases the coating is thin, so the barrier is only as good as its continuity and its bond to the substrate.

The coating is not a cosmetic finish. It changes the contact resistance, the friction in a bolted joint and the way the bar behaves at temperature. Nickel is harder than tin or silver, which helps on threads and on faces that are assembled more than once. It is also less conductive than copper or silver, so a thick nickel layer adds resistance to the joint.

That trade-off is the whole decision. Nickel buys durability and temperature range at the cost of some contact resistance, which is small on a generous joint and significant on one already close to its limit.

Nickel, tin and silver compared

Tin, silver and nickel are the three common busbar finishes, and they are not interchangeable. Tin is cheap, easy to solder and gives a low contact resistance, but it is soft and has a low melting point. Silver gives the lowest contact resistance and the best high-current performance, but it tarnishes and costs more. Nickel sits between them on cost and resistance, with the best temperature and wear behaviour of the three.

The table below gives a practical comparison. The thickness ranges are starting points, not universal values, and the service temperature is limited by the substrate and the joint design as much as by the coating.

Temperature and oxidation

Tin melts at 232 degrees C, which is well above the normal operating temperature of a busbar but not far above the temperature of a poor joint or a fault. As tin approaches its melting range it softens, and a soft coating under a bolt loses clamp force. Tin also forms intermetallic layers with copper over time, and those layers grow faster at temperature.

Nickel melts at 1455 degrees C and oxidises slowly. Its surface remains stable at temperatures where tin would flow and silver would tarnish quickly. That makes nickel attractive for busbars near engines, furnaces, braking resistors or enclosed equipment with poor ventilation. The nickel is not immune to oxidation, but the oxide that forms is thin and does not grow at the same rate as copper oxide.

The limiting factor is often the substrate, not the coating. Aluminium loses strength at a lower temperature than copper, and a laminated busbar may be limited by its insulation. Nickel raises the surface limit, but the designer still has to check the bar, the joint and the insulation against the actual temperature.

Nickel on aluminium busbars

Aluminium forms an oxide layer immediately after cleaning, and that layer is an insulator. A bolted aluminium joint depends on breaking the oxide at assembly and keeping it broken. Nickel plating removes that dependency by giving the joint a stable metallic surface that does not reform an oxide in the same way.

Nickel does not bond directly to aluminium in a plating line. The usual route is a zincate treatment, which displaces the surface oxide and deposits a thin zinc layer, followed by nickel. The process needs tight control, because a poor zincate layer shows up as poor adhesion or as blistering after a bend. Electroless nickel can give a more uniform layer on a complex shape, but the bath and the pretreatment still decide the result.

Nickel plated aluminium is common where an aluminium bar meets a copper or nickel plated component. The coating reduces the galvanic couple at the joint face and gives both sides a similar surface. It does not remove the need for a barrier at the edge of the joint, where water can still sit between dissimilar metals.

Corrosion, porosity and salt spray

A nickel coating protects by covering the copper. If the coating has pores, pinholes or thin patches, the copper at the base of a pore becomes the anode in a small corrosion cell. The visible result is a dark spot or a green stain, and the electrical result is a rising contact resistance. Thickness and adhesion control the pore population more than the bath chemistry alone.

Salt spray testing to ISO 9227 is a common way to compare finishes, but the result is only as useful as the test report. The number of hours, the concentration, the temperature and the inspection criteria all matter. A nickel coating that shows no red rust after a stated number of hours on a steel coupon says little about a nickel layer on a copper busbar in a ventilated enclosure.

For a busbar, a more useful check is a bend test and a thermal shock test on a production sample. Ask for the adhesion method and the acceptance criteria, not only the hours.

Thickness, adhesion and the bend

Nickel thickness is usually measured by X-ray fluorescence on a flat witness area or by a coulometric method. The measurement should be taken on the part, not only on a coupon, because current density varies across a rack. A joint face and a thread can receive different thicknesses in the same load.

Adhesion is checked by a bend, a tape test or a thermal cycle. A bend test is simple and relevant for a busbar, because the coating sees the same strain as the bar. If the coating cracks at the outside of a bend, the crack becomes a corrosion site and a possible initiation point for a fatigue crack in the bar.

Thicker is not always better. A thick nickel layer is harder to bend without cracking, adds contact resistance and can change the fit of a thread. The drawing should call a range, not a minimum alone, and the range should match the process capability.

• Measure thickness on the part, not only on a flat coupon.
• Check adhesion with a bend or a thermal cycle that matches the route.
• Call a thickness range that the process can hold on a thread and a flat face.
• Keep nickel out of resistance weld zones unless the weld schedule is qualified for it.

When nickel is the right call

Specify nickel when the joint runs hot, when the bar is aluminium, when the surface sees wear from repeated assembly, or when the environment attacks tin. It is also a sensible choice for a thread that must survive several torque cycles without galling. In those cases the extra contact resistance is a fair price for a surface that stays put.

Do not specify nickel as a default replacement for silver on a high-current joint with a tight temperature rise. Silver still wins on contact resistance, and a nickel layer can add enough resistance to change the thermal result. If the joint needs both corrosion resistance and low resistance, a silver finish over a nickel strike is a common compromise, with the silver thickness called separately.