Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 364 of 707

Table III. High-Temperature, Alkaline Electroless Nickel-Phosphorus Bath Nickel sulfate Sodium citrate Ammonium chloride Sodium hypophosphite pH Temperature H2PO2— + H2O H+ + HPO3 2— 33 g/L 84 g/L 50 g/L 17 g/L 9.5 85° C (185° +H2 F) (4) Alkaline nickel-phosphorus deposits are generally reduced by sodium hypophosphite. These alkaline baths can be formulated at low temperatures for plating on plastics. Deposits provide good solderability for the electronics industry, and operating energy costs are reduced due to some solutions' low oper- ating temperatures; however, less corrosion protection, lower adhesion to steel, and difficulty in processing aluminum due to high pH values are drawbacks. One such bath consists of the components shown in Table II. An example of a high-temperature, alkaline, electroless nickel-phosphorus bath is given in Table III. Acid nickel-phosphorus deposits normally consist of 87-94% nickel and 6-13% phosphorus, operating at 77-93° C (171-200° C (176-180O F), with a pH of 4.4-5.2. Low phos- phorus electroless nickel baths contain 1-4% phosphorus and normally operate at 80-82O sodium hypophosphite. The resultant deposit melting point is 890° F), with a pH of 6.0-6.5. The reducing agent is commonly C (1,635° F) for 8-9% phosphorus baths and will vary dependent on the amount of phosphorus alloyed in the deposit. The pH of the solution is the controlling factor affecting the phospho- rus content of the deposit. The higher the pH, the lower the phosphorus content, resulting in deposit property changes. Lower phosphorus-containing deposits (i.e., 1-3%) typically have less corrosion resistance than 10% alloys. Low phosphorus deposits do have good corrosion protection against alkaline solutions such as sodi- um hydroxide. Also, deposits containing phosphorus in excess of 8.0% are typically nonmagnetic. When the pH drops below 4.0, subsequent nickel deposition virtually stops. As-deposited nickel-phosphorus hardness is 500-600 Vickers hardness num- ber (VHN), and maximum values of 1,000 VHN may be realized by post-heat-treat- ment of the coating at a temperature of 399° C (750° F) for 1 hour. The temperature is a dominant factor in determining the final deposit hardness. Careful consid- eration should be given to the choice of temperature in order not to affect Table IV. Acid Hypophosphite-Reduced Electroless Nickel Bath Nickel sulfate Sodium acetate Sodium hypophosphite Lead acetate pH Temperature 28 g/L 17 g/L 24 g/L 0.0015 g/L 4.4-4.6 82-88° C (180-190° F) 363

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