Electroless Nickel Bath Life Extension

What is the electroless nickel plating process?

Electroless nickel coatings are extensively used in the metal plating industry as the physical properties of the coatings (uniformity, corrosion resistance and lubricity) are better than electroplated nickel and the process can be used deposit metal coatings in situations which electrodeposition is not possible e.g. on plastics. Electroless nickel plating is a process whereby a nickel coating is deposited on a surface in a controlled chemical reduction; the process is termed "electroless" because the electrons are supplied by a chemical reducing agent and not electrically. The overall equation for the deposition of nickel by the reducing agent sodium hypophosphite can be written as:

NiSO₄ +2NaH₂PO₂+2H₂O --------> Ni+2NaH₂PO₃^- +H₂+ H₂SO₄

The nickel reduction only takes place on specific catalytic surfaces including nickel itself, which makes the reduction process autocatalytic. The reducing agent used for electroless plating not only supplies the electrons for the reduction, but some elements in the reducing agent can be incorporated into the nickel deposit improving its properties e.g. when sodium hypophosphite is used as a reducing agent the resultant deposit is a nickel phosphorus alloy. In addition to the nickel salts (generally nickel sulfate) and reducing agents in an electroless nickel bath, there are a variety of complexing agents, buffering agents and stabilizers/inhibitors present to control the amount of free nickel ions and the deposition process.

The limited lifetime of electroless nickel baths - a waste dilemma

Due to side reactions in the electroless nickel plating process, such as the evolution of hydrogen gas, for each gram of plated nickel-phosphorus alloy about 5 g of sodium orthophosphite is generated in the plating bath. After 6-8 metal turnovers (one metal turnover is when all the nickel originally in the bath has been plated) the orthophosphite level can compose up to 20 % of the bath and the plating solution may turn cloudy due to nickel orthophosphite precipitation producing rough deposits, poor corrosion resistance and occasionally causing spontaneous decomposition. During the plating process, the nickel sulfate and sodium hypophosphite raw materials are continuously depleted and must be replenished in order to maintain the chemical balance of the bath and addition of either ammonia or sodium hydroxide is necessary to keep the pH in the preferred range.This results in an accumulation of sodium and sulfate ions, along with orthophosphite, as the electroless nickel bath ages and consequently the plating rate can decrease from 18 mm hr^-1 to less then 10 mm hr-¹.Without an effective means to remove these components, the EN bath eventually becomes ineffective and must be disposed. Effluent treatment is one of the major weak points in electroless nickel technology and much of the recent efforts have been focused on extending electroless nickel bath life and regenerating electroless nickel baths.It is generally accepted that the recycling of electroless nickel solutions represents the one of the most important challenges facing the electroless nickel plating industry today.

Schematic of a single membrane pair electrodialysis cell for the regeneration of a spent electoless nickel bath

Schematic of a single membrane pair electrodialysis cell for the regeneration of a spent electroless nickel bath

Cleaning up the electroless nickel process

The long-term goal of a plating shop should be to prolong, for as much as possible, the life of the electroless nickel bath.By extending the number of metal turnovers available from a bath before disposal one can greatly reduce the cost per operating cycle of the electroless nickel process and minimize the amount of effluent requiring treatment.

Our laboratory is attempting to address the unique challenges in pollution prevention that the electroless nickel process presents.Some of the areas in which we are currently carrying out research are detailed below.

Extending electroless nickel bath life using electrodialysis

We have identified electrodialysis as a leading pollution prevention technology in the treatment and regeneration of spent electroless nickel baths (see schematic).This technology has become an increasingly popular technique for extending electroless nickel bath life in metal plating job shops but the losses of nickel, hypophosphite and complexing agents are high due to poor selectivity of the ion exchange membranes. Our initial investigations are focused on establishing baseline data for the removal rates of the various ions in electroless nickel baths as pH, voltage input, solution flow rates and temperature are varied. Once this initial data has been collected we intend to investigate the use of different electrodialysis cell configurations and to modify the membrane properties to induce selectivity for one ion over another.

Developing a closed-loop electroless nickel process

The ultimate goal of a zero waste electroless nickel plating process can only be achieved if the orthophosphite produced during the plating process can be reduced back to hypophosphite. Investigations on the redox properties of orthophosphite at a number of electrode surfaces and the possible reduction of the ion to hypophosphite are presently being conducted in our laboratories.

The electrodialysis unit for regenerating electroless nickel baths

A rapid analysis method for electroless nickel bath anions by ion chromatography

As part of the research on the extension of electroless nickel bath life we have developed a method for analyzing anions in electroless nickel baths by ion chromatography. The value of this method over the usual titration techniques is that it can rapidly and accurately quantify all the anions in the bath in a single run.

Contact Information:

Contact:
Michael Gonzalez
(513) 569-7180
gonzalez.michael@epa.gov
Fax: (513) 569-7677
Postal Address:
26 West Martin Luther King Drive
Mail Stop 443
Cincinnati, Ohio 45268