Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 221 of 707

electroplating solutions NICKEL PLATING BY GEORGE A. DIBARI INTERNATIONAL NICKEL INC., SADDLE BROOK, N.J.; Nickel plating is the electrolytic deposition of a layer of nickel on a substrate. The process involves the dissolution of one electrode (the anode) and the deposition of metallic nickel on the other electrode (the cathode). Direct current is applied between the anode (positive) and the cathode (negative). Conductivity between the electrodes is provided by an aqueous solution of nickel salts. When nickel salts are dissolved in water, the nickel is present in solution as diva- lent, positively charged ions (Ni2+ with two electrons (2e— ) and are converted to metallic nickel (Ni0 The electrochemical reaction in its simplest form is: Ni2+ +2e—=Ni0 Because the nickel ions discharged at the cathode are replenished by the nick- el ions formed at the anode, the nickel plating process can be operated for long peri- ods of time without interruption. Estimating Nickel Thickness The amount of nickel that is deposited at the cathode is determined by the product of the current (in amperes) and the time (in hours). Under ideal conditions, 26.8 A flowing for 1 hr will deposit 29.4 g of nickel (1.095 g/A-hr). If the area being plated is known, the average thickness of the nickel coating can be estimated. For example, if 29.4 g of nickel are deposited on 1 ft2 , the thickness of the deposit is 0.0014 in (Thickness equals the weight of nickel divided by the product of the area and the den- sity of nickel. It is important to use consistent units. The density of nickel is 0.322 lb/in3 .) Because a small percentage of the current is consumed at the cathode in dis- charging hydrogen ions, the efficiency of nickel deposition is less than 100%. This fact must be taken into account in estimating the weight and the thickness of nick- el that will be deposited under practical plating conditions. Table I is a data sheet on depositing nickel based on 96.5% cathode efficiency. The table relates coat- ing thickness, weight per unit area, current density, and time of plating. Some fac- tors useful in making nickel-plating calculations are given in Table II. Anode efficiency is normally 100%. Because anode efficiency exceeds cathode efficiency by a small percentage, nickel-ion concentration and pH will rise as the bath is used. Drag-out of nickel-plating solution may compensate for nickel metal buildup in solution to some extent, but at some point if may be necessary to remove a portion of solution from the plating tank and replace the solution removed with water and other constituents. The pH of the solution is normally maintained by adding acid. Metal Distribution It is desirable to apply uniform thicknesses of nickel on all significant surfaces to achieve predictable service life and to meet plating specifications that require min- 220 ). When current flows, divalent nickel ions react ) at the cathode. The reverse occurs at the anode where metallic nickel dissolves to form divalent ions.

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