Electroless Nickel Technology Offers High Tolerance to Zinc Contamination

Atotech’s Nichem MP 1188 supports the mid-phosphorous EN process.

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Aluminum alloys are common substrates for electroless nickel (EN) plating. However, in order to form adherent EN coatings, they require specialized pretreatment because of their ability to form a passive oxide layer when exposed to oxygen.

Platers often use a “double zincate” pretreatment process based on cleaning, etching and desmutting the surface prior to a zincate step. The aluminum oxide is dissolved and a zinc coating is formed by immersion plating, keeping the aluminum surface from re-forming its oxide. The first zincate is stripped and the aluminum is zincated again for a more uniform and fine grain, producing a better adhesive bond between the substrate and coating.

The parts are then immersed in the EN plating bath and the zinc layer dissolves as the plating reaction initiates, resulting in zinc buildup in the EN bath. The zinc buildup acts as a contaminant and slows the plating rate, causes tensile stress and blistering of the deposit, and reduces the life cycle of aluminum-plating EN baths to about half that of EN baths used for plating steel.

When plating aluminum, zinc buildup can be avoided using an EN strike prior to the main EN process. The EN strike is highly tolerant to zinc and absorbs the contamination to extend the life of a conventional EN, but does require another step.

Atotech's new mid-phosphorous EN process, Nichem MP 1188, is said to suit the demands of high aluminum throughput. 

Caleb Morrison and Brad Staples from Atotech’s North American TechCenter collaborated with Svetlana Satinskaya and Roland Vogel from the company’s Germany R&D headquarters to help develop Nichem MP 1188. The result, they say, is a consistently bright deposit with a phosphorous content of 6 to 8 percent.

“The composition of the bath gives it a high tolerance to zinc contamination, so it can be used for direct plating of aluminum without an EN strike,” Morrison says. For a conventional EN bath, he adds, zinc contamination limits  are roughly 100 ppm, whereas the Nichem MP 1188 has a limit of around 250 ppm.

Staples says that Nichem MP 1188 supports at least twice the zinc contamination tolerance compared with a standard mid-phosphorous EN process, leading to a longer bath life, higher deposition rate and a consistent appearance.

“An additional benefit is the ability to operate at extremely low bath loading, which can be important for job shop platers when the workloads can vary in bath loading,” Staples says.

Fully bright deposits have been obtained at bath loading below 0.02 ft2/gal (0.05 dm2/l), and Staples says the process works well in multi-metal applications with enhanced initiation on copper and brass, no rectifier initiation needed.

“The process is pH self-regulating and the deposit stress ranges from neutral to compressive,” he says.
Nichem MP 1188 marks another step forward in the Atotech green technology roadmap. It is EDTA-, boric acid-, lead-, and cadmium-free, and it fulfills all ELV, RoHS and WEEE regulations. In addition, Morrison says the Nichem MP 1188 supports process stability from its mixed stabilizer system. 

 

For information on Nichem MP 1188, please visit atotech.com.

 

 


Originally published in the November 2016 issue. 

 

 

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