Toxic Heavy Metal Free Stabilized Electroless Nickel Processes
Abstract: The ELV/RoHS/WEEE directives have imposed restrictions on certain hazardous substances used in manufactured goods, which can pose harm to consumers and potentially cause a pollution risk to their environment after end-of-life. From the substances mentioned in these directives lead (Pb) and cadmium (Cd) which are used in electroless nickel fall under restriction. This paper will introduce an advanced alternative to Pb and Cd-free electroless nickel processes, which is completely free of toxic heavy metal (THM) stabilizers. The process and deposit characteristics will be discussed and compared to the traditionally Pb stabilized electroless nickel.
Electroless nickel plating is an important and well established process, which is used in a wide spectrum of applications e.g. the oil and gas, automotive, electronics, white goods, aerospace, construction and textile industries.
The electroless nickel deposit provides an outstanding uniform coating thickness, which is able to improve the functionality of the plated parts by providing wear and corrosion resistance, excellent hardness and also abrasion resistance.
Traditionally, Pb was the commonly used stabilizer in electroless plating baths as a most effective catalytic poison for electroless nickel deposition. The addition of the stabilizer in the ppm range to the electroless nickel plating solution prevents spontaneous decomposition of the metastable plating bath. Cd has been traditionally used as an additive in the medium phosphorus electroless nickel baths to enhance the brightness and promote leveling.
ELV, RoHS and WEEE directives restrict the content of Pb to <0.1% and of Cd to <0.01% by weight in the coating. In order to avoid the necessity of tight analytic control of plated deposits, efforts have been undertaken to eliminate the use of Pb and Cd from electroless nickel plating completely.
In this context, Pb and Cd-free electroless nickel processes were developed and introduced. Nevertheless, the state-of-the-art electroless nickel plating processes still contain other toxic heavy metals as a stabilizer system. Such metals are not restricted by legislations today, but might be so in the future as there is a general demand for minimizing environmental impact of industrial processes.
An advanced generation of electroless nickel processes, which is free of Pb, Cd and toxic heavy metal stabilizers was introduced recently to the market. A process designed for the deposition of high phosphorous electroless nickel coatings (> 10 % phosphorus incorporation) was introduced in 2012 and a process generating medium phosphorous electroless nickel coatings (6 to 8 % phosphorus incorporation) was introduced in 2013. The challenge for this new technology is to offer a sustainable answer to the market with equivalent or better deposit and process properties compared to the traditionally stabilized electroless nickel processes.
The THM stabilizer free electroless nickel processes have similar bath operating conditions as traditionally Pb stabilized electroless nickel processes. All plating tests were made according to internal technical data sheet operating conditions (1, 2).
The old generation electroless nickel processes using Pb and Cd often contain strong complexing agents like EDTA, in order to prevent the precipitation of nickel phosphite. Addition of EDTA results in very difficult waste water treatment. The new generation THM stabilizer free electroless nickel processes have been developed without use of EDTA.
The stress of the deposit was tested according to an internal standard operating procedure by using stress strips (3).
The nitric acid test of the deposit was performed according to an internal standard operating procedure (4).
3. Results and Discussion
3.1 Solution life and deposition rate
One of the important features of autocatalytic nickel solutions is the solution life, which is the time the solution can be used to produce deposits within acceptable quality requirements. The solution life is usually measured by the “metal turnover” (MTO). One metal turnover is achieved when the quantity of nickel contained in a new bath is converted into NiP deposit. An electroless nickel process operating at 6 g/l nickel has achieved one MTO when 6 g/l nickel has been deposited. High phosphorus electroless nickel processes suffer especially from very limited solution life. Production experience has shown that it is possible to extend the bath life from approx. 5 MTO to 7 MTO with the new generation high phosphorus electroless nickel process. Furthermore, the deposition rate of THM stabilizer free high phosphorus electroless nickel is higher compared to Pb stabilized high phosphorus electroless nickel processes (Fig. 1). The higher deposition rate as well as the longer operating life of the new generation electroless nickel process has major economic advantages for the applicator. The plating time is reduced, the make-up cost is reduced and the process bath is discarded less frequently which decreases the cost for waste treatment.