Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 144 of 707

electroplating solutions Determination of Phosphorus in Electroless Nickel Deposits BY DR. V. PERSITS, ISRAEL AEROSPACE INDUSTRIES LTD, PETAH, TIKVA The term "electroless process" is used to describe methods of plating by means of chemical reduction. Electroless nickel is produced by reducing nickel ions from solutions which contain phosphite, boron or hydrazine compounds. Electroless nickel deposits are not pure nickel but contain substantial amounts of phos- phorus. The phosphorus content can be varied, normally between 2–13%, to con- trol strength, ductility, stress, magnetic properties and structure. The phos- phorus content of deposits is strongly affected by the bath pH; phosphorus content increases with decrease in pH. Acid baths in common use produce deposits containing about 7– 13% phosphorus. An increase in hypophosphite con- centration increases the phosphorus content of deposits from a bath of given pH. The phosphorus content of deposit is affected by bath temperature. Heat treat- ment at 400°C results in precipitation of an intermetallic Ni3P phase and grain growth. Deposits containing about 7% (wt) phosphorus consist of 50% (vol) Ni3P and 50% nickel. Lower phosphorus deposits consist of Ni3P in nickel matrix and the reverse for higher phosphorus deposits. High phosphorus contents obtained in solutions with a low pH and a high ratio of phosphate to nickel ions favor high strength and low stress. The alloys are supersaturated solid solu- tions of nickel phosphide in nickel.[1] With the manufacturing necessity and the inability of the exact determination of phosphorus on the level 7%– 13% in nickel electroless deposits by EDS method (Energy Dispersive Spectrometry) were examined the methods of the determi- nation of phosphorus in nickel electroless deposits, using chemical methods. Phosphorus analyses embody two general procedural steps: conversion of the phosphorus form interest to dissolved orthophosphate and the determination of dissolved orthophosphate, using suitable chemical method. For the deter- mination of phosphorus in electroless nickel deposits were tested gravimetric and photometric methods. In gravimetric method, the determination of phosphorus- heteropolyphosphor- molybdate (NH4)3H4[P(Mo2O7)6] is precipitated washed, dried and weighed. The gravimetric method for the determination of phosphorus in different sam- ples, for example–steels, cast iron is accurate and often used for the check work, where a substance is determined more than one method [2,3]. SUMMARY OF THE METHOD The deposit is dissolved in nitric acid and the phosphorus is oxidized by means of potassium permanganate in acidic conditions. Ammonium molybdate is added and reacts with phosphorus (V) to form yellow precipitate of the het- eropolyphosphor molybdate (NH4)3H4[P(Mo2O7)6]. 143

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