Influence of the anionic part of the stabilizer on electroless nickel-boron plating

Selectivity and smooth operation of electroless nickel plating require that a stabilizing agent is used. It operates by blocking catalytic activity on unwanted germination sites and regulating the activity of the substrate. In the case of alkaline electroless nickel-boron plating systems, which use sodium (or potassium) borohydride as reducing agent, lead and thallium salts are the most popular stabilizers. However, there is little knowledge about the way the stabilizer acts. In this study, 4 different lead-based stabilizers (tungstate, sulphate, nitrate and chloride) have been used, all other things left constant, in electroless nickel-boron plating baths. The thickness, composition, roughness, morphology, hardness and structure of all the obtained coatings have been investigated. Chloride led to thinner deposits and the boron content varied between 5.5 wt.% for lead tungstate and 6.5 wt.% for lead nitrate and lead sulphate, with a lead content between 0.2 and 0.25 wt.%. Coatings obtained with a lead tungstate stabilized bath were thicker, harder and contained less boron and lead than the others. This shows the influence of the anionic part of the stabilizing agent on the plating process.