Acid chloride zinc plating
has a long and varied history. The first zinc electro- plating baths were acidic
and based upon zinc sulfate. The deposits from these first baths were dull and
flat gray. Acid zinc plating technology has come a long way in its almost 200
years of existence.
Today, there are three
primary types of acid zinc plating baths: straight ammonium chloride; straight
potassium chloride; and mixed ammonium chloride/potassium chloride.
Advantages of Acid Zinc
Plating
Acid zinc plating systems have several advantages over alkaline cyanide
and alkaline non-cyanide zinc plating systems:
• Less waste treatment,
since no cyanide treatment is required and no chelating agents are used;
•
Deposits with outstanding brightness that rival nickel chromium in their luster;
•
High cathode efficiencies, 90-95%, at normal operating current densities;
•
Plates difficult substrates such as castings and carbonitrited pieces, which can
be plated directly without any special pretreatment;
• Excellent leveling;
•
Substantially less hydrogen embrittlement than cyanide and non-cyanide processes;
and
• Wetter systems typically employed in an acid process are relatively
free rinsing when compared to alkaline systems.
Disadvantages of Acid Zinc
Plating
In acid zinc plating, the electrolyte is extremely corrosive. Any
solution that becomes entrapped in crimped or spot-welded areas can eventually
bleed out and discolor or corrode the part. Because the plating solution is so
corrosive, special tank construction and equipment designed to withstand the corrosive
nature of the solution are required.
The surface preparation
of parts can cause problems as well. Improper cleaning and/or pickling can lead
to serious problems. Other disadvantages include the loss of ductility with thick
deposits and the need for continuous filtration to remove iron from the solution.
Ammonium Chloride Zinc
Plating
The ammonium chloride bath is the most forgiving of the three major
types of acid zinc plating because of its wide operating parameters. The primary
drawback of this system is the high level of ammonia, which can cause problems
in wastewater treatment. Ammonia acts as a chelator, and if the rinse waters are
not segregated from other waste streams, removal of metals to acceptable levels
using standard water treatment practices can be difficult and expensive. Ammonia
is also regulated in many communities.
Potassium Chloride Zinc
Plating
Potassium chloride zinc plating solutions are attractive because they
contain no ammonia. The disadvantages of this system are a greater tendency to
burn on extreme edges and higher operating costs. The potassium bath also requires
the use of relatively expensive boric acid to buffer the solution and prevent
burning in the high-current-density areas, functions performed by the ammonium
chloride in the other systems.
Mixed Ammonium Chloride/Potassium
Chloride Zinc Plating
This bath combines the best of the ammonia and ammonia-free
baths. Because potassium chloride is less expensive than ammonium chloride, the
maintenance costs of the mixed bath are lower than the ammonia bath, and it does
not require boric acid. The ammonia levels in the rinse waters are low enough
that it does not significantly interfere with wastewater treatment, even if plating
nickel and copper in the same plant with mixed waste streams. If local regulations
restrict the level of ammonia discharged, special waste treatment equipment will
be required, and the non-ammonia bath is most likely the best choice.
Typical Process Cycle
The
typical process cycle for plating a ferrous substrate, regardless of the type
of zinc being applied, is as follows:
1. Soak Clean
2. Electroclean
3.
Rinse
4. Acid Pickle
5. Rinse
6. Zinc Plate
7. Rinse
8. Nitric
Dip
9. Chromate
10. Rinse
11. Dry
The cleaning cycle is critical
when plating acid zinc. Using cyanide zinc, poor cleaning could be overcome by
the inherent cleaning properties of the plating solution. This is not the case
in acid plating. Poor cleaning will result in blisters, lack of adhesion and hazy
deposits.
Zinc Metal Solution Maintenance
and Control
The zinc content of the bath is normally maintained by the dissolution
of the anodes, as the bath operates at nearly 100% anode efficiency. The anode
levels should never be allowed to drop below half of the zinc in the basket. If
this happens, the titanium baskets will begin to dissolve and the zinc metal will
drop, as zinc is being plated out of solution faster than it can be replaced by
dissolution of the zinc anodes. As the anode area decreases, the voltage will
have to be increased to maintain the amperage required to achieve the desired
plating thickness. As the voltage is increased, oxygen is formed at the anode
that causes the oxidation of the organic components of the bath, increasing the
concentration of organic breakdown byproducts. These byproducts then become incorporated
into the deposit, resulting in a stressed and brittle plate.
If the zinc metal must
be adjusted by the addition of zinc chloride, it is important to deduct the added
chloride before determining the amount of ammonium chloride and/or potassium chloride
that is to be added. Zinc chloride contains 48% zinc and 52% chloride. To raise
the zinc metal content by 1 oz/gal requires 1.92 oz/gal of zinc chloride powder
or 1.7 fluid ounces of 9.4 lb/gal zinc chloride concentrate solution per gallon
of solution.
High zinc metal concentrations
can allow operation at higher current densities without burning but cause a decrease
in the throwing power and increase operational costs by increasing the zinc content
of the dragout and wastewater treatment costs due to higher zinc levels in rinse
waters.
Determination of Zinc Metal
Concentration
Equipment
5.0 ml pipette
250 ml Erlenmeyer flask
100
ml graduated cylinder
50 ml burette with stand
Reagents
Xylenol orange indicator—Grind
together 0.1 g xylenol orange tetrasodium salt with 100, of reagent-grade sodium
chloride until thoroughly mixed. (Do not use iodized salt.)
Buffer solution—Dissolve
90 g of anhydrous sodium acetate in about 500 ml of distilled or deionized water.
Add 15 ml of reagent-grade acetic acid and dilute to 1 liter.
0.0575 M EDTA solution—Dissolve
21.4 g of reagent-grade EDTA disodium salt and 6 g of reagent-grade sodium hydroxide
in about 500 ml of distilled or deionized water. Dilute to 1 liter.
Procedure
1. Pipette
a 5 ml sample of the plating solution into a 250 ml Erlenmeyer flask.
2. With
a graduated cylinder, add 50 ml of distilled or deionized water.
3. Add sufficient
buffer (about 25 ml) to produce a pH of approximately 5.15 and mix sample. (The
solution should be clear with no haze.)
4. Add a pinch of xylenol orange indicator
to produce a violet color.
5. Titrate immediately with 0.0575 M EDTA to a yellow
or gold endpoint.
Calculation
Total zinc metal (oz/gal) = ml 0.0575 M EDTA
titrated × 0.1
Chloride
Maintenance
of the chloride concentration is very important, since it is the primary contributor
to the conductivity of the solution. Chlorides are typically lost by dragout and
are added based upon analysis.
Ammonium chloride contains
66% chloride ion. Therefore, to raise the chloride content in the bath by 1 oz/gal
requires 1.52 oz/gal of ammonium chloride. Only untreated grades of ammonium chloride
should be used. Commonly marketed grades of ammonium chloride contain additives
used to promote its efficiency as a galvanizing flux. These often are detrimental
to its use in a plating electrolyte.
Potassium chloride is 48%
chloride. Therefore, to raise the chloride content in the bath by 1 oz/gal requires
about 2 oz/gal potassium chloride. A chemical-grade potassium chloride that is
free of clay should be used.
Low-chloride concentrations
reduce the bath conductivity, cause hazy or dull deposits in the low current densities,
reduce low-current-density coverage, reduce anode corrosion and reduce the plating
rate.
High-chloride concentrations
can lower the cloud point of the bath, increase burning in the high current densities,
increase the rate of zinc dissolution and cause some brightener systems to “salt
out.”
Additions of chloride to
the bath should never exceed 100 lb of ammonium or potassium chloride per 1,000
gal of plating solution at any one time. Large single additions of chloride can
shock the plating bath by lowering the temperature of the solution and possibly
breaking out some of the wetter and brightener components. Frequent small additions
are preferable to single large additions.
Determination of Total
Chloride Concentration
Equipment
5.0 ml pipette
10.0 ml pipette
100
ml volumetric flask
250 ml Erlenmeyer flask
100 ml graduated cylinder
50
ml burette with stand
Reagents
4% chromate
solution—Dissolve 4 g of either reagent-grade potassium or sodium chromate
in 100 ml of distilled or deionized water.
0.1 N silver nitrate solution—Dissolve
16.99 g of reagent-grade silver nitrate in about 500 ml of distilled or deionized
water. Dilute to 1 liter. Store in a brown bottle to protect from exposure to
light.
Procedure
1. Pipette
5 ml of plating solution into a 100 ml volumetric flask.
2. Dilute to mark
with distilled or deionized water.
3. Invert the stoppered flask several times
to ensure that the solution is well mixed.
4. Pipette 10 ml of this solution
into a 250 ml Erlenmeyer flask.
5. Add approximately 50 ml of distilled or
deionized water.
6. Add 1-2 ml of chromate indicator solu- tion to give the
sample a yellow color.
7. Titrate with 0.1 N silver nitrate solution to a brick-red
endpoint.
Calculation
Total Chloride (oz/gal) = ml 0.1 M silver nitrate
titrated × 1.0
Boric Acid
Boric acid
is a critical component of the non-ammonia bath. Boric acid helps to control burning
in the high current densi-ties and buffers the solution against drastic pH changes.
High boric acid concentra-tions are not typically harmful but may contribute to
rough plating if the solubility limit is exceeded, which is about 4.5-5.5 oz/gal.
Determination of Boric
Acid Concentration
Equipment
5.0 ml pipette
250 ml Erlenmeyer flask
100
ml graduated cylinder
50 ml burette with stand
Reagents
0.1 N sodium
hydroxide solution—Dissolve 4 g of reagent-grade sodium hydroxide in about
500 ml of distilled or deionized water. Dilute to 1 liter.
Mannitol—Reagent
grade, purchase from chemical supply house.
Bromocresol purple indicator solution—Dissolve
0.1 g of bromocresol purple indicator powder in 1.85 ml of 0.1 N sodium hydroxide.
Dilute to 250 ml with distilled or deionized water.
Procedure
1. Pipette
5 ml of plating solution into a 250 ml Erlenmeyer flask.
2. Add approximately
50 ml of distilled or deionized water.
3. Add 5 g of mannitol.
4. Add 3-5
drops of bromocresol purple indicator.
5. Titrate with 0.1 N sodium hydroxide
to a blue-violet endpoint.
Calculation
Boric acid
(oz/gal) = ml of 0.1 N silver nitrate titrated × 0.165
Ammonium Chloride
The
ammonium chloride in the ammonia bath serves as the source of chloride for conductivity.
In the mixed bath, the ammonium chloride not only serves as a source of chloride,
but it also acts to buffer the solution against drastic pH changes, a function
served by boric acid in the non-ammonia bath. The ammonia in solution acts as
a complexor for the organic components, allowing operation at a higher current
density than the non-ammonia bath.
pH
Hydrochloric acid
is used to lower and dilute ammonium hydroxide is used to raise the pH of the
ammonia bath. Dilute potassium hydroxide is used to raise the pH of the potassium
and mixed baths. The pH will have a natural tendency to rise with operation. Therefore,
regular additions of hydrochloric acid will be required. The pH of the solution
should be checked and adjusted once or twice daily.
Care should be taken during
pH adjustments, since the pH can change quite rapidly with small additions. Although
the pH can be checked with pH papers, a pH meter is much more accurate. Do not,
however, place a pH probe directly into the plating tank with current applied.
This will instantly ruin the probe. Test the pH in a sample collected in a beaker,
or, if using in-line pH control, place the probe in the line from the filter to
the tank, not directly in the tank.
Addition Agents
There
are typically two additives used in contemporary acid zinc plating. These are
a brightener and a wetter, sometimes referred to as make-up, starter or carrier.
The wetter is lost only to drag-out and is typically added back to the bath based
on chloride additions. The brightener component is lost through electrolysis and
is best maintained by adding back to the solution using an amp-hr feeder. Brighteners
are typically added at a rate of 1 gal per 15,000-30,000 amp-hr of plating.
Hull Cell Testing
The
addition agents should be checked by daily or weekly Hull cell tests. For the
purpose of routine monitoring, panels should be run for 5 min at 2 amps for a
rack bath and 10 min at 1 amp for a barrel bath. The solution should be agitated
during the test, using air for rack baths and mechanical agitation for barrel
baths. The panel should be bright and clear across the entire current density
range. If it is not, refer to the troubleshooting guide.
Additions and corrections
should be made in the Hull cell before making them in the tank. For salt additions,
the addition of 2 g to a standard 267 ml Hull cell is the equivalent of a 1 oz/gal
addition to the tank. After determining the additions required, only add half
of these quantities to the tank. It is easier to add more material to the tank
than to take it out. Most of all, bear in mind that the Hull cell is only a tool,
albeit, a valuable one. The goal, however, is to produce good parts, not good
panels. Decisions regarding bath chemistry should be based on the appearance of
the work itself.
Iron
Iron is the most
common metallic contaminant found in an acid chloride-plating bath. The amount
of iron that a bath can tolerate varies from as little as 40 ppm to as much as
1,000 ppm, depending on the additive system. Symptoms of iron contamination are
black spots after nitric dipping or chromating. Iron can be controlled by additions
of 35% hydrogen peroxide or potassium permanganate with good filtration. It is
important to routinely drag the plating tanks for parts that may have fallen off
plating racks to keep as much iron out of the solution as possible.
Hexavalent Chromium
Hexavalent
chromium is another commonly found metallic contaminant, usually by dragin of
the chromating solution. As little as 50 ppm of hexavalent chromium can cause
adverse effects. The symptoms of chromium contamination are a lack of plating
in the low-current-density areas, dull deposits and blistering of the deposit.
Chromium contamination can be treated by additions of sodium hydrosulfite that
converts hexavalent chromium to trivalent chromium. This is only a temporary cure,
though, as trivalent chromium is converted back to hexavalent chromium at the
anodes. Preventive measures are best and include good rinsing, redesigning process
flow to prevent drag-over of chromate solutions and treatment of chromium using
sodium hydrosulfite in the cleaners or a static rinse tank before the plating
tank.
Lead and Cadmium
These
metals are introduced as impurities in the zinc anodes and zinc chloride, with
lead being the most critical. As little as 2 ppm of lead can cause dark gray/black
deposits in the low-current-density areas that quickly spread to cover the entire
current density range. Cadmium contamination has a similar effect but is not usually
evident until the concentration reaches 50-150 ppm. In either case, treat with
zinc dust or low-current-density dummying.
Copper
Copper is typically
introduced into the bath from racks, bus bars and anode bars. Tolerance to copper
can be as little as 10 ppm. Copper contamination is evident by a bright deposit
that turns dark only after bright dipping. Methods of removal are zinc dust treatment
or low-current-density dummying.
Solution Make-Up
1.
To a clean storage tank, add approxi- mately two-thirds of the solution volume
of water.
2. Heat the water to 100F and turn off the heat.
3. Dissolve all
of the ammonium chloride, potassium chloride and boric acid as required and add
the required amount of zinc chloride concentrate. Mix well. The dissolution of
the ammonium chlo- ride and potassium chloride is endother- mic and will cool
the solution.
4. Filter this solution into the clean plating tank and dilute
to approximately 90% of the final volume.
5. Check the pH and adjust if necessary.
6. Add the required amount of wetter and then the brightener. Mix well. Recheck
the pH and temperature before plating.
Tanks
Tanks should be
mild steel lined with Koroseal, hard rubber, polypropylene or another acid-resistant
lining. Any newly lined tank should be leached before use.
Anodes
SHG zinc (99.9%
pure) should be used. Slab zinc anodes should have titanium or Monel hooks above
the solution level. Zinc balls can be used in a titanium anode basket. The baskets
should be kept at least half full at all times. The anode to cathode ratio should
be maintained at about 1:1.
Anode Bags
Anode bags
are not required but highly recommended to prevent roughness. Bags should be constructed
of Dynel or polypropylene. Anode bags should be leached in 5-10% hydrochloric
acid prior to use.
Filtration
Continuous
filtration at a rate of one to two turnovers per hour is required for the removal
of iron that will build up in the bath. The filter media should be 20 microns.
Cooling
Cooling is recommended
for tanks in continuous operation. The coils should be constructed of titanium
or plastic.
Agitation
Air agitation
is recommended for rack plating. This aids in solution movement and the oxidation
of soluble ferrous iron to insoluble ferric iron, which can then be removed by
the filtration system. Air should be supplied by an oil-free low-pressure blower.
PFD
TABLE I—Comparison
of Operating Parameters
Ammonia Potassium Mixed
Zinc Metal 1.5-5 oz/gal
3-5 oz/gal 1.5-5 oz/gal
Ammonium Chloride 16-24 oz/gal 0 oz/gal 4-8 oz/gal
Potassium
Chloride 0 oz/gal 25-30 oz/gal 16-24 oz/gal
Boric Acid 0 oz/gal 3-5 oz/gal
0 oz/gal
pH 5-6 4.5-5.5 5-6
Temperature 65-105F 65-115F 65-120F
Acid Zinc Troubleshooting
Guide
Problem Possible Cause
Corrective Action
Black spots on deposit Iron contamination Check the bottom
of the tank for parts
before and/or after (More than 100 ppm). that have fallen
off racks or out of
chromating. High- barrels.
current-density areas
darker
after chromating. Treat with hydrogen peroxide.
Add 0.25-0.5 pint of 30-35%
hydrogen
peroxide per 1,000 gal of
solution volume. The hydrogen
peroxide should
be diluted at least 3:1
with water before addition to the tank.
The precipitated
ferric hydroxide is
removed by filtration. If filtering is
insufficient,
the precipitated iron will
again dissolve back into solution and
the spots
will reappear.
Deposit staining or Copper contamination Electrolyze solution
at 2-5 asf
black after chromating. (5-10 ppm) and/or for 8-12 hr.
cadmium
contamination
(10-20 ppm). Zinc dust treat at 1 lb per 1,000 gal of solution
and filter out zinc particles so as to redissolve the materials back into the
bath.
White staining. Poor rinsing
and/or Improve rinsing.
high brightener.
Add 0.5 fl/oz of hydrochloric
acid to the first rinse after the plating tank.
Reduce brightener additions.
No deposit in Chromium
Add 1 oz/gal sodium hydrosulfite low-current-density contamination per 100 gal
of solution per 100 ppm of
areas. (200 ppm). chromium to be removed.
Zinc dust treat at 1 lb
per 1,000 gal
of solution and filter out zinc particles
so as not to redissolve
the metals back
into the bath.
Electrolyze solution at
2-5 asf for 8-12 hr.
High brightener. Reduce
brightener additions.
Problem Possible Cause
Corrective Action
Poor adhesion and/or Chromium
Add 1 oz/gal sodium hydrosulfite blisters. contamination per 100 gal of solution
per 100 ppm of
(10-20 ppm). chromium to be removed.
Poor cleaning and/or Improve
cleaning, pickling and/or
rinsing. rinsing.
Organic contamination.
Filter the solution through a carbon
filter pack.
Low chloride. Analyze and
adjust.
Low temperature. Check
and adjust to recommended range.
High brightener. Reduce
brightener additions.
Low wetter. Add in 0.5%
by volume increments
until optimum deposit is obtained.
Dull deposit in low- High
pH. Check the pH with a calibrated meter
current-density areas (do not rely
on pH test strips) and lower
(1-20 asf). with dilute hydrochloric acid.
Low ammonia and/or Analyze
and adjust to range.
chloride.
Low brightener. Add in
0.5% by volume increments
until optimum deposit is obtained.
High temperature. Lower
to recommended range.
Iron contamination. Treat
with hydrogen peroxide. Add 0.25-0.5 pint of 30-35% hydrogen
peroxide per 1,000
gal of solution
volume. The hydrogen peroxide should
be diluted at least
3:1 with water before
addition to the tank. The precipitated
ferric hydroxide
is removed by filtration.
Dark band in medium High
pH. Check the pH with a calibrated meter
current density range (do not rely
on pH test strips) and lower
(20-30 asf). with dilute hydrochloric acid.
Low ammonia and/or Analyze
and adjust to range.
chloride.
Dull or poor coverage Low
ammonia and/or Analyze and adjust to range.
medium-current-density chloride.
to
low-current-density
areas.
Problem Possible Cause Corrective Action
High pH. Check the pH with
a calibrated meter
(do not rely on pH test strips) and lower
with dilute
hydrochloric acid.
Low wetter. Add in wetter
0.5% by volume
increments until optimum deposit is obtained.
Low brightener. Add in
brightener 0.05% by volume
increments until optimum deposit until
optimum
deposit is obtained.
Dull deposit across the
High temperature. Lower to recommended range.
entire current density
range.
Low brightener. Add in
brightener 0.05% by volume
increments until optimum deposit is
obtained.
Poor surface Improve cleaning,
pickling and/or
preparation. rinsing.
Excessive addition of Leave
air agitation on during shutdowns
hydrogen peroxide. to help dissipate excess
peroxide.
Add up to 100 ml of brightener
per
100 gal of plating solution.
Bright, brittle deposit
High pH. Check the pH with a calibrated meter over 40 asf. (do not rely on pH
test strips) and lower with dilute hydrochloric acid.
High brightener. Reduce
brightener additions.
Lower wetter. Add in wetter
0.5% by volume
increments until optimum desposit is
obtained.
Pitted deposit in High
ammonia. Analyze and adjust to range.
medium-current-density
to low-current-density
High brightener. Reduce brightener additions.
areas.
Low wetter. Add in
wetter 0.5% by volume
increments until optimum deposit
is obtained.
Trivalent chromium Remove
with filtration.
(150-200 ppm).
Problem Possible Cause Corrective Action
Streaky deposit. Poor cleaning
and/or Improve cleaning, pickling and/or
rinsing. rinsing.
Soft, spongy or burnt Low
zinc. Analyze and adjust to range.
deposit in high-current-
density areas.
Low ammonia and/or Analyze and adjust to range.
chloride.
High pH. Check the pH with
a calibrated meter
(do not rely on pH test strips) and lower
with dilute
hydrochloric acid.
Low wetter. Add in wetter
0.5% by volume
increments until optimum deposit is
achieved.
Iron contamination. Treat
with hydrogen peroxide. Add
0.25-0.5 pint of 30-35% hydrogen
peroxide per
1,000 gal of solution
volume. The hydrogen peroxide should
be diluted at
least 3:1 with water before
addition to the tank. The precipitated
ferric
hydroxide is removed by filtration.
Rough deposit. Anode particles
in Filter the solution.
solution.
Check anode bags for tears and/or holes.
Poor cleaning and/or Improve
cleaning, pickling and/or
rinsing. rinsing.
Low wetter. Add in wetter
0.5% by volume
increments until optimum deposit is
achieved.
Trivalent chromium. Remove
with filtration.
