Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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plating procedures AUTOMATIC SYSTEM FOR ENDLESS OPERATION OF ELECTROLESS NICKEL BY HELMUT HORSTHEMKE, ENTHONE GMBH, ELISABETH-SELBERT-STRASSE 4, LANGENFELD, D-40764 GERMANY Wear- and corrosion-resistant nickel phosphorus alloy deposits from electroless processes have been used for various large-scale applications since their wider introduction to the industry in the 1980's. The phosphorus content (P-content) typically can be adjusted between 1 and 14%; high P-contents are chosen for high- est chemical and corrosion resistance, while low P-contents provide higher hardness and excellent wear properties. The coating thickness is uniform and almost entirely independent from substrate geometry, as chemical reducing agents are used and current density variation do not exist as with electrolytic processes. Nickel is replenished as a dissolved salt, traditionally nickel sulfate. In this way, sulfate and breakdown products of the chemical reducer build up in the process solution over time. Unless specific techniques are used to remove these sub- stances, the bath life will be limited to 4 to 10 metal turnovers (MTO). With typ- ical high-phosphorus applications, 4 to 5 MTO is the maximum bath life, as unwanted tensile deposit stress occurs above this age. As tensile stress is usually tolerated for mid- and low-phosphorus deposits, a bath life of 8 to 10 MTO can be expected, depending on the amount of drag out. The limiting factor for mid- and low-phosphorus is the buildup of specific gravity, and the related slowdown of the plating speed with increased solution viscosity. Typically, 1.3 g/cm³ specific gravity is the maximum for any electroless nickel (EN) process. From solution make-up to end of bath life, the process passes through different phases. It takes until 0.5 MTO to develop the desired compressive stress deposits, particular for high-phosphorus EN. By upward adjustments of temperature and pH with increasing bath life, the plating speed can be kept almost consistent. During this main operation phase, brightness, phosphorus content, hard- ness, and structure are subject to some degree of change and variation. Even at iden- tical speed over bath life, the brightness and hardness will decline, while the phosphorus content increases. This ongoing change is either tolerated by the end user, or different plating jobs are specified to specific bath ages only. In produc- tion, this pattern reduces flexibility significantly. Since 2004, patented sulfate-free processes have established on the interna- tional EN market. These processes do not entirely overcome the described problem of discontinuous operation, but significantly extend solution life and reduce overall production cost. ENfinity® processes provide a much slow- 370

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