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This volume is concerned with autocatalytic nickel plating, commonly
referred to as electroless nickel plating. In contrast with electroplating,
electroless nickel (EN) plating does not require rectifiers, electrical current
or anodes. Deposition occurs in an aqueous solution containing metal ions
a reducing agent, chelates, complexing agents and stabilizers. Chemical
reactions on the surface of the part being plated cause deposition
of a nickel alloy.

Since all surfaces wetted by the electroless nickel solution are plated,
the deposit thickness is quite uniform. This unique property of EN makes
it possible to coat internal surfaces of pipes, valves and other parts.
Such uniformity of deposit thickness is difficult, if not impossible,
to achieve by any other method.

The discovery of electroless plating is credited to Brenner & Riddell in the
1940's. Today EN has grown into a very substantial segment of the metal
products finishing industry.

Compared with plating of other metals, electroless nickel (EN) plating
is relatively young being commercially available for less than 30 years;
however, in the past decade the usage of the coating has grown to such
proportions that electroless nickel plated parts are found underground,
in outer space, and in a myriad of areas in between.

This guide seeks to provide the reader with more thorough understanding
of the process. The volume includes descriptions of deposit properties,
equipment required, process applicability and test procedures to the end
that a high quality EN deposit can be achieved and maintained.

The chemical reactions that occur when using sodium hypophosphite
as the reducing agent in electroless nickel plating are as follows:

    H2PO~2 + H2O ---------› H2PO~3 + H2

    Ni++ + H2PO20 + H2O Catalyst › Ni0 + H2PO~3 + 2H+

    H2PO~2+ H+ ---------› P + OH~ + H2O

An electroless nickel coating is a dense alloy of nickel and phosphorus.
The amount of phosphorus codeposited can range from less than 1%
to 12%, depending upon bath formulation, operating pH and bath age.
The deposition process is auto-catalytic; i.e., once a primary layer of nickel has formed on the substrate, that layer and each subsequent layer
become the catalyst that causes the above reaction to continue. Thus,
very thick coatings can be applied, provided that the ingredients in the plating bath are replenished in an orderly manner. In general commercial
practice, thicknesses range from 0.1 mil to 5 mils but in some salvage
operations 30 mil deposits are not uncommon.

Electroless nickel deposits are functional coatings and are rarely used
for decorative purposes only. The primary criteria for using electroless
nickel generally falls within the following categories:

    1) Corrosion resistance.
    2) Wear resistance.
    3) Hardness.
    4) Lubricity.
    5) Solderability and bondability.
    6) Uniformity of deposit regardless of geometries.
    7) Nonmagnetic properties of high-phosphorus nickel alloy.

In the early years, platers encountered many problems with electroless
nickel because of poor formulations, inferior equipment, misapplications
and a general misunderstanding of the process and the deposit. In the first
decade and a half of its existence, electroless nickel plating had an aura
of "black magic" attached to it. Modern bath formulations, however,
use only the purest grades of chemicals, delicately balanced and blended
to give the processor plating baths with long life, exceptional stability,
consistent plating rates, self-maintaining pH and most importantly,
reproducible quality. In addition, advancements in tank design, filtration
systems, heating and agitation have virtually eliminated the problems
that plagued the user years ago.

Furthermore, in the past decade, advancements have been made in auto-
catalytic nickel plating solutions. Reducing agents other than sodium
hypophosphite are used for special applications; composites of nickel with
diamonds, silicon carbide and PTFE are available; and ternary alloys may
be applied. Also, baths have been formulated to yield specific results; i.e.,
high corrosion resistance, brightness, high plating rate, improved ductility
and low or high levels of magnetic response.

It has taken many years of hard work and cooperative effort by the suppliers and users to arrive at the present state of the art.
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