Tips for specifying electroless-nickel coatings

Nov. 2, 2000
Electroless-nickel plating is specified in a wide variety of applications to provide corrosion and wear resistance. Grooves, slots, blind holes, threads, and even the inside of tubing will have the same coating thickness.

Electroless-nickel plating is specified in a wide variety of applications to provide corrosion and wear resistance. Grooves, slots, blind holes, threads, and even the inside of tubing will have the same coating thickness.

The coating uniformity of the electroless-nickel process helps maintain tight tolerances on threaded fasteners. Such coatings also help fight corrosion and eliminate galling of the threads during use as well as extend service life.

Applications for electroless-nickel plating in the aerospace industry are not limited to engine components. The coatings help protect airframe assemblies such as landinggear components, ramp-locking systems, and flap and actuator components from corrosion and wear.

Electroless-nickel (EN) plating is also known as chemical or autocatalytic nickel plating. In contrast to the electroplating (galvanic) technique, EN plating baths work without an externally applied electric current. The plating operation is based on the catalytic reduction of nickel ions onto suitable substrates. The EN process deposits uniformly hard coatings on any section of a part exposed to fresh plating solution. Grooves, slots, blind holes, threads, and even the inside of tubing will have the same thickness of coating.

The EN plating process is, however, more than just dunking parts into a plating bath. In fact, the plating tank is only one component in a sequence of processing steps. Bath chemistry and composition play an important role during the EN plating process. But there are other considerations that contribute equally to coating success.

The best plating process is one that prioritizes all requirements such as the primary and secondary functions of the coating and the environment it must withstand. These parameters help the coating house establish a well-thought-out sequence of preplating, plating, postplating, and/or testing processes.

Preplating considerations — Always consider the necessity for preplate stress relief on hardened-steel components. Preplate stress relief reduces and/or redistributes localized residual stresses. These stresses come from manufacturing processes such as machining, forming, welding, and heat treatment.

In addition, the type of alloy, its hardness or ultimate tensile strength, and final end use may also dictate preplate stress relief. In fact, all major EN specifications make reference to stress-relief treatments before plating. It is usually prudent to incorporate stress relief on components made from hardened steel even if the plating does not require a particular specification. Time and temperature guidelines can come from industry specifications such as AMS 2404, AMS 2405, MIL-C-26074, and ASTM B-733.

Parts undergo mechanical finishing operations to improve surface finish or to remove gross surface contamination, such as mill scale or weld slag. Vibratory deburring, blasting, and tumble finishing improve the surface condition and let the EN plating perform better. Shot-peening not only improves finish, but may also serve to redistribute localized stresses arising from machining and fabrication processes.

Components made from unusual or difficult-to-plate alloys require chemical pretreatment to ensure the EN coating has adequate initiation, adhesion, and overall deposit quality. Often necessary are special activation processes, electrolytic strikes, or immersion preplate deposits.

EN plating chemistry — There are entire volumes of information and related data that detail properties and applications of EN. But most suppliers of EN chemistry have a few all-purpose workhorse plating chemistries. They may also have proprietary hybrid chemistry formulations that produce properties geared for highly specific applications. Generally it's best to look for a plating house that carries several different plating processes. Platers should also be able to handle a select number of hybrid EN formulations along with their workhorse chemistries. Important for them as well is good understanding of pretreatment processes as previously discussed.

Postplating requirements — As with stress relief, all major specifications make reference to hydrogen embrittlement relief after EN plating. This is especially true for highly hardened-steel components. Such components need proper baking to remove potentially detrimental effects resulting from the absorption of hydrogen during pretreatment and EN plating. However, there are some possible exceptions. These include parts not subject to extreme service conditions or those that would degrade from elevated temperatures needed for hydrogen embrittlement relief.

A postplate baking process can improve the adhesion of the EN deposit for some applications. Certain aluminum alloys, high carbon steels, and other materials may exhibit much better adhesion when baked after plating. Baking is said to reduce EN deposit stress and help eliminate localized blistering or other failures. Time and temperature guidelines for baking are included in the previously referenced EN specifications.

EN deposits are age-hardenable when heat treated at elevated temperatures — >49°F. Hardness values in excess of 68 RC are possible with carefully specified heat treatments. The downside to heat treatment, however, is that the plating has less corrosion resistance, especially on high phosphorous deposits. Heat treatment of EN deposits is said to help convert the nickel-phosphorous alloy from an amorphous to a crystalline structure. It's therefore prudent to exercise caution when specifying heat treatment. The substrate may also soften while the EN deposit is being produced. Many platers have converted to low or low midphosphorous chemistries, which provide similar hardness values straight from the bath, eliminating the need for heat treatment.

Information provided by Dan Englebert, Technical Director, Imagineering Enterprises Inc., South Bend, Ind., (219) 287-2941.

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