Canadian Finishing & Coatings Manufacturing - May/June '24 - 26

PLATING AND ANODIZING: ELECTROLESS NICKEL
regulations
Historically legislation
have
been
drivers
and
for
innovation in surface finishing
technology, and today more sustainable
chemistries are required to meet the
demands of global regulatory bodies.
Additionally, integrating sustainable
practices into the development of new
technologies is becoming an important
selection criterion for future-focused
companies. Since the adoption of
technologies to meet the EU's ELV
Directive 2000/53/EC Annex II in the
early 2000s, innovation in ENP has
been confined to niche technologies
such as poly-alloys, composite coatings,
and post-treatment colouring. In recent
years, nickel has been a substance of
interest to health officials around the
world, with nickel compounds classified
as human carcinogens, so the efficient
and responsible use of nickel is key
to its continued use. New approaches
to electroless nickel are now being
examined with this in mind, and the
focus on more sustainable approaches
is a prominent theme.
The vast majority of conventional
ENP processes operate at roughly 6
g/L (0.1 mol/L) Ni metal. A sustainable
alternative to this standard is one in
which the nickel metal concentration
is lowered, thus reducing its impact on
both the plating process itself and any
downstream operations. With a focus
on safety and sustainability, a complete
optimisation effort was undertaken to
create a new class of Reduced Ion (RI)
processes that reduce the concentration
of nickel metal to roughly 3-4 g/L (0.050.07
mol/L).
These efforts resulted in processes
that operate at lower Ni metal
concentrations while delivering lower
solution densities over the solution's
operating lifetime, when compared to
conventional chemistries.
As depicted in Figure 1, the average
solution density difference between the
RI ENP and the conventional system
is roughly 0.025 g/cm3
. A MTO (metal
turnover), a common measure of ENP
bath lifetime, is when the original or
make up concentration of nickel metal
is depleted from the solution and
subsequently replenished. Based on
conventional operation the industry
has come to define 1 MTO = 6 g/L Ni2+
deposited from solution, so therefore
8 MTO = 48 g/L Ni2+
deposited. It
is interesting to note that the
equivalent specific gravity of the RI
ENP occurs at roughly +1 MTO,
meaning the RI ENP gains an
additional MTO of solution life
compared to conventional EN (the
functional end of a bath's life is often
reached when the solution density starts
to approach saturation). In addition
to the extended operating solution
lifetime, other potential advantages
of this reduction in solution density
include but are not limited to, reduced
drag out of EN process solution to
post-plate rinses and reduced nickel in
emissions from the process solution. An
example of said reduction of Ni drag out
from the process solution is illustrated
in Figure 2.
Figure 2: Drag out of nickel metal into first
post-plate rinse.
Figure 2 depicts the result of an
experiment in which five consecutive
panels (0.015 m2 surface area) were
plated in two process solutions at the
same stage of solution life (MTO). After
plating for 30 minutes, the panels were
removed from the process solution and
allowed to idle over the solution for five
seconds before being transferred to a
beaker with clean DI water and rinsed
thoroughly. The collected rinse water
was brought to a standard volume and
the Ni concentration was analysed via
atomic absorption spectroscopy. The
analysis shows that the RI ENP results
in a 55 per cent reduction in Ni metal
concentration in the first post-plate
rinse, which is a consequence of both
the reduced viscosity and lower Ni
metal concentration. This effect can be
even more pronounced on articles with
rougher substrates and/or with more
complex geometries.
Figure 1: Solution density comparison for conventional ENP (6 g/L Ni2+) vs RI ENP (3 g/L Ni2+)
26
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EN processes operating at lower
metal concentrations require less Ni to
be stabilised in the solution, resulting
in improved plating and operational
efficiencies. Stability of an ENP solution
is critically important for economical
and consistent operation. Since ENP is
an autocatalytic reaction, if the process
is not sufficiently stabilised it can
deposit nickel on the tank walls and any
ancillary equipment that the solution
comes into contact with. If the solution
stability is very poor, the ENP bath could

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Canadian Finishing & Coatings Manufacturing - May/June '24

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