84    PRECIPITATION TREATMENT OF SPENT ELECTROLESS
NICKEL PLATING BATHS
Wei-chi Ying, Associate Scientist
Robert R. Bonk, Associate Chemist
Michael E. Tucker, Senior Technician
Occidental Chemical Corporation
Grand Island Technology Center
Grand Island, New York 14072
ELECTROLESS NICKEL PLATING PROCESS
Nickel coating on an object via electroless nickel plating (EN) is produced by controlled chemical
reduction of nickel ions on a suitably treated surface. The nickel coating itself is catalytic to the
reduction reaction, and the deposition continues as long as the object remains in contact with the EN
bath. Many important physical properties such as uniformity, corrosion and wear resistances, hardness, lubricity, and ductility of EN deposit are better than those of electroplated nickel.' These
advantages have made EN an attractive process for product finishing. EN baths are blends of several
chemicals, each performing a specific function — source of nickel ions (nickel chloride or sulfate),
reducing agent to supply electrons for the reduction of free nickel ions, complexing agent to control
the amount of free nickel ions, buffering agent to resist the pH changes associated with the Ni-
reduction, accelerator to enhance the speed of the reaction, and inhibitor to moderate the deposition
process. The temperature range for EN is 87-93°C2
Sodium hypophosphite (NaH2P02-H20) is by far the most common reducing agent because of the
lower cost, greater ease of control, better overall plating quality relative to most other reducing
agents —sodium borohydride (NaBH4), aminoboranes such as dimethylamine borane (DMAB,
(CH3)2NHBH3), or diethylamine borane (DEAB, (C2H5)2NHBH3) and hydrazine (N2H4-H20).3
Many organic acids —citric, acetic, hydroxyacetic (glycolic), succinic, lactic, malic, propionic, and
aminoacetic acids —are employed as the complexing and/or buffering agents.4 For hypophosphite-
reduced EN baths, succinic acid also serves as the accelerator, and thiourea or lead is useful as the
inhibitor. Table I presents compositions of six typical hypophosphite EN plating baths. Compared to
the basic baths which are suited for plating on plastics, the acidic baths are more popular because of
higher plating rate, better stability, easier process control, and improved corrosion resistance.5
A typical EN process flow diagram for plating ferrous alloys is shown in Figure 1. Nitric acid
stripping is often used to remove nickel from improperly plated articles for re-plating and to remove
nickel deposits from the surfaces of plating equipment. The major types of wastewater resulting from
an EN process are spent EN baths, stripping solutions, and rinse waters. The spent baths are very high
in total soluble nickel species (Ni(II)), reducing and complexing agents; the stripping solutions are
acidic and usually contain Ni2+ but no complexing agents; the rinse waters are diluted plating
solutions. On-site treatment of spent EN baths for meeting the effluent discharge limits, often as low
as 1.2 mg/L,6 is sometimes difficult due to the presence of complexing agents in high concentrations,
while off-site disposal is very expensive, up to more than Sl.OO/gal, if it is feasible at all.7
WASTEWATER TREATMENT TECHNOLOGIES
Existing treatment methods for removing Ni(II) from industrial wastewaters fall into three general
categories: 1) reduction to elemental nickel (Ni<>); 2) precipitation as insoluble nickel compounds; and
3) separation by adsorption, electrostatic force, applied electrical potential and hydraulic or mechanical pressure. Table II summarizes methods which have been utilized for treating nickel-containing
wastewaters; a brief introduction of these treatment technologies is given below.'-9
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