There is something wrong with your nickel deposit. Whatever it
is, heres a fix-it guide. After a certain point in the guide,
it may become necessary to know exactly what is wrong with the
deposit, but there is a good chance that as you adjust the solution
and make improvements in the physical plant, the problem will
disappear before you know what caused it.
Most bright nickel solutions contain a stress reliever and an
auxiliary brightener whose concentration can be determined by
simple analytical techniques. The leveling portion of the brightener
is best determined by the appearance of the deposit.
Now that the basics are in order, take another look at the plated
work coming off the line. Is the quality satisfactory . . . or
do you find yourself searching for a word that will describe the
appearance of the work in unflattering terms? The use of the vernacular
is inappropriate, satisfying though it may be. We need to describe
the problem in analytical terms. The deposit may be dark in the
recesses, rough, pitted, etc. Here is a partial list of possible
complaints and probable causes. The guide is useful only if the
aforementioned parameters are in range (i.e. neither too high
nor too low).
Topics presented in this article: General Troubleshooting Guide;
Brittle Deposits; Darkness in the Recesses; Poor Adhesion; Surface
Roughness; Pitting and/or Blue Haze; Black Spots on Barrel Work;
and Stardusting; Hull Cell Tests.
General Troubleshooting Guide
Check the following, adjusting as you go:
Brittle Deposits
The solution contains excessive amounts of the leveling component
or is contaminated with breakdown products. Brighteners function
by breaking down at the cathode surface and interfering with the
deposition of metal ions. The result is the formation of organic
compounds that are different from the original chemicals that
were present in the addition agent. The brighteners in modern
formulations do not form particularly harmful breakdown products,
as evidenced by the long life of plating solutions. The baths
will, however, eventually contain products that have to be removed
either by drag-out or adsorption on activated carbon.
Brittle deposits are also sometimes characterized by poor leveling.
Carbon treatment can be very effective in restoring the ductility
and leveling properties of a plating solution. Carbon treatments
can be effective at rates of 1 to 6 9 of carbon/L of solution
volume. The use of carbon filtration (i.e., merely filtering the
solution through much smaller amounts of carbon) is also worthwhile
if the contamination is minor. Most plants utilizing a preventative
maintenance scheme of control will filter continuously through
fresh carbon to prolong the useful life of a plating solution
between batch treatments. This procedure saves downtime and is
probably cheaper in the long run.
Bench testing of different concentrations of carbon is the most
satisfactory method for deciding if carbon treatment will improve
the deposit. A panel is plated at 2 A for 10 min and the high-current-density
corner of the panel is bent to get a good idea of deposit ductility
before and after the contemplated treatment. A few minutes of
laboratory work can save untold hours in the plant. There is no
point in treating the whole plating solution if improvements cannot
be detected in the Hull Cell. If an impurity does not respond
to carbon treatment, try activated clay, peroxide or potassium
permanganate treatment.
A lab test should always be performed to determine the efficacy
of the treatment. Allow the powdered carbon to react with the
solution for 30 min with agitation at 140 F.
A deposit that has excellent leveling and brightness but is brittle
may just be too high in organics due to addition of excessive
amounts of brightener.
To test this hypothesis, plate two or three consecutive Hull Cell
panels in the laboratory, adjusting pH before each test. If the
deposit becomes progressively more ductile, the same thing will
happen in the tank if you shut off the brightener feeder and continue
to plate.
Darkness in the Recesses
Darkness in the recesses can be caused by metallic impurities,
high concentrations of the leveling agent, or interactions between
the two. Analyzing the solution for copper, zinc, lead or cadmium
by atomic absorption (AA) spectroscopy may give some indication
of the type and extent of contamination. Compare the AA data to
the baseline concentration of these elements in your solution.
In other words, it is possible to have a certain level of metallic
contaminants in the bath and still be able to plate without difficulty.
A one-time analysis can be misleading due to the interacting effect
of brighteners and metallics.
Darkness can be eliminated most easily and completely by dummy
electrolysis. Both the metallic contaminants and organic brighteners
are plated at 2 to 4 A/ft2 of cathode area. Assuming the brightener
feeder is turned off, and the source of metallics is eliminated,
the levels of both bath constituents will drop to allow satisfactory
plating. Prior to electrolysis, the tank should be dragged to
remove dropped parts and equipment.
The harmful concentrations of metallic impurities and breakdown
products from brightening agents is usually in the range of tenths
of a g/L. Because they are preferentially plated at the low current
density being employed, these materials will be depleted rapidly
from the cathode film. Therefore, agitation of the solution is
imperative for efficient removal of the materials from the solution.
It is important to start out with smooth, clean, nickel-plated
steel dummy panels (preferably corrugated) and continue electrolysis
until the deposit has a light gray color at the higher current-density
edges.
Poor Adhesion
Poor adhesion is in most cases due to a cleaning or activation
problem of the basis metal or previous deposit. Some addition
agents of copper cyanide and acid copper plating baths will have
to be removed from the surface of the copper before nickel plating.
An acid dip will sometimes help. In general, the pH of the last
rinse prior to nickel plating should be 4 to 7.
High brightener concentration in the nickel bath can sometimes
cause poor adhesion. The use of live entry has often been successful
in preventing peelers. The reason may be because these addition
agents are surface active and can passivate the basis metal before
plating is initiated.
The plating of nickel over nickel is difficult. It may be necessary
to activate the base nickel with several steps such as cathodic
cleaning, rinsing, cathodic acid salt treatment, and hydrochloric
acid dip. A Woods nickel strike may be required between nickel
layers in order to obtain perfect adhesion. Sometimes, it is better
to use a very low voltage in the strikeójust enough to
get a very mild gassing at the nickel surface for 15 to 30 sec.
Experiment with different voltages and concentrations to see what
works best for you.
Surface Roughness
Every plated item will have an upper and lower surface in relation
to its placement on the rack and in the plating tank. Roughness
of the deposit is often due to particulate matter that settles
on this upper surface during plating. The term "shelf roughness"
was coined to describe this condition. Roughness is usually caused
by precipitated iron, carbon or shop dirt. These precipitants
can be detected either by passing a finger over the surface to
feel for roughness or rubbing a cloth over the surface to see
if the cloth threads become caught on it. Filtration is the cure,
with additions of peroxide needed in the case of iron.
To remove iron, very cautiously use 1 to 2 pints of 30 percent
hydrogen peroxide/1000 gal of solution. Premix the peroxide with
5 parts of water. If iron is a constant problem, run the bath
at a pH of 4.5 to help precipitate the iron. You can also check
for ripped anode bags or other obvious paths of entry of particulate
matter.
Pitting and/or Blue Haze
While the two topics may not seem related, they often are caused
by the same factors. Even though the surface tension may be between
40 and 45 dynes/cm, the use of additional wetting agent can sometimes
eliminate pitting and hazes caused by high levels of brightener
or impurities dragged into the bath.
There are three types of wetting agents. Low-foaming wetting agents
are required for air-agitated baths. Wetting agents for still
or cathode rocker baths can be high foamers, and, indeed, these
are the only types that work in solutions that are not air agitated.
The third type does not lower the surface tension of the bath
to a great extent but is specially formulated to emulsify and
negate the effects of dragged-in shop oils. The wetting agents
should be added in increments of approximately 1/4 of a full charge
or less.
Compressed air should not be used for agitating the plating tank
because introduction of finely divided oil droplets or air into
the tank can cause pitting of the deposits. A low-pressure blower
should be used instead.
Black Spots on Barrel Work
Black spots are caused by ferrous iron dissolved in the bath.
The spots correspond to the holes in the barrel. Add 1 pint of
hydrogen peroxide/1000 gal of solution and run the filter to remove
precipitated iron. Note that only dissolved iron, not the iron
that is being picked up on the filter, will cause the spots.
While on the subject of barrel plating, it is useful to note that
particulate matter really does no harm in barrel plating, and,
for this reason, it is usually not necessary to bag your anodes.
Removing the bags will allow improved corrosion of the anode and
decrease the voltage requirements of the tank. The only reason
to filter precipitated iron out of the solution is to prevent
the iron from being dissolved when acid is added to the tank to
adjust the pH.
Stardusting
This phenomenon is usually caused by the precipitation of aluminum,
calcium or iron in the cathode film. The pH of the film is higher
than that of the solution, and iron may precipitate and become
incorporated into the deposit.
Aluminum and calcium may precipitate as an extremely fine material
in the body of the solution and cause what may be described as
a micro-roughness showing up at the areas of high current density.
The best way to remove these impurities is to filter the solution
with an addition of peroxide in the case of iron. Calcium sulfate
should be filtered at 155 F because it is less soluble at this
high temperature. The filter should be cleaned after this treatment
to prevent the calcium from re-dissolving at normal operating
temperature.
The source of the impurities should be eliminated. Calcium is
present in hard water and accumulates when tap water is added
to make up evaporation losses. A sign that calcium is a problem
is the clogging of air lines, which must be cleared by running
water through them every morning.
A troubleshooting guide would not be complete without mentioning
that many rejects encountered in nickel plating have very little
to do with the bath itself. It is hoped that the information in
this article can be used to help eliminate the nickel bath as
the source of the problem, so the plater can concentrate his efforts
on finding the actual source. The problems associated with preplate
operations and defects due to the basis metal are subjects for
another guide.
Hull Cell Tests
The Hull Cell can give misleading results but is an invaluable
tool when used in combination with other laboratory equipment.
While no one would suggest that a fully equipped laboratory brimming
with technicians is all that is necessary to assure satisfactory
plating, it is part of a successful plating operation.
A statesman of the plating industry, Dr. Don Swalheim, in an introduction
to a lecture on the Hull Cell (part of the AESF Intensive Training
Course series) said, "any plating foreman who does not know
how to plate a Hull Cell panel should be fired on the spot."
While many people in charge of plating operations have no business
actually plating Hull Cell panels, they should be able to intelligently
discuss and interpret laboratory results because thousands of
dollars for man hours and materials can be conserved by testing
theories in the laboratory rather than on the production floor.
An extension of this idea is that many plating solutions can be
saved from the waste-treatment pit if laboratory personnel can
prove, via Hull Cell testing and other types of analyses, that
there is nothing wrong with the solution or that an economical
treatment is available for the problem.
Environmental concerns will make it imperative that all of our plating solutions last as long as possible. Laboratory control will become ever more important as the industry is forced to approach zero discharge of hazardous chemicals.