Acid chloride zinc plating has a long and varied history. The first zinc electroplating 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 to 95 pct, 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.
| TABLE I—Comparison of operating parameters |
| Ammonia | Potassium | Mixed |
| Zinc Metal | 1.5 to 5 oz/gal | 3 to 5 oz/gal | 1.5 to 5 oz/gal |
| Ammonium Chloride | 16 to 24 oz/gal | 0 oz/gal | 4 to 8 oz/gal |
| Potassium Chloride | 0 oz/gal | 25 to 30 oz/gal | 16 to 24 oz/gal |
| Boric Acid | 0 oz/gal | 3 to 5 oz/gal | 0 oz/gal |
| pH | 5 to 6 | 4.5 to 5.5 | 5 to 6 |
| Temperature | 65 to 105F | 65 to 115F | 65 to 120F |
|
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 pct 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 by-products. These by-products 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 pct zinc and 52 pct chloride. To raise the zinc metal content by one 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 one liter.
0.0575 M EDTA solution:
Dissolve 21.4 g of reagent-grade EDTA disodium salt and 6g of reagent-grade sodium hydroxide in about 500 ml of distilled or deionized water. Dilute to one liter.
Procedure
1. Pipette a five 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 X 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 pct chloride ion. Therefore, to raise the chloride content in the bath by one 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 pct chloride. Therefore, to raise the chloride content in the bath by one oz/gal requires about two 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 lbs 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
Four pct chromate solution: Dissolve four 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 one liter. Store in a brown bottle to
protect from exposure to light.
Procedure
1. Pipette five 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 insure that the solution is thoroughly 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 one to two ml of chromate indicator solution 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 X 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 densities and buffers the solution against drastic
pH changes. High boric acid concentrations are not typically harmful, but may contribute to rough plating if the solubility limit is exceeded, which is about 4.5 to 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 four g of reagent-grade sodium hydroxide in about 500 ml of distilled or deionized water. Dilute to one 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 five ml of plating solution into a 250 ml Erlenmeyer flask.
2. Add approximately 50 ml of distilled or deionized water.
3. Add five g of mannitol.
4. Add three to five 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 X 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. The pH is controlled using dilute hydrochloric acid to lower and dilute ammonium hydroxide 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-hour feeder. Brighteners are typically added at a rate of one gal per15,000 to 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 five minutes at
two amps for a rack bath and 10 minutes at one 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 two g to a standard 267 ml Hull cell is the equivalent of a one 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 pct 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 that 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 two 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 to 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 approximately 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 chloride and potassium chloride is endothermic and will cool the solution.
4. Filter this solution into the clean plating tank and dilute to approximately 90 pct 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 pct 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 five to 10 pct 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
| Acid Zinc Troubleshooting Guide |
Problem
Black spots on deposit before and/or after chromating. High-current-density areas darker after chromating. | Possible Cause
Iron contamination
(More than 100 ppm) | Corrective Action
Check the bottom of the tank for parts that have fallen off racks or out of barrels.
Treat with hydrogen peroxide. Add 0.25 to 0.5 pint of 30 to 35 pct 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. |
| Problem | Possible Cause | Corrective Action |
|
Deposit staining or black after chromating | Copper contamination (5 to 10 ppm) and/or Cadmium contamination (10 to 20 ppm) | Electrolyze solution at two to five asf for eight to 12 hr.
Zinc dust treat at one lb per 1,000 gal of solution and filter out zinc particles so as to redissolve the materials back into the bath |
| Problem | Possible Cause | Corrective Action |
| White staining | Poor rinsing and/or high brightener | Improve rinsing
Add 0.5 fl/oz of hydrochloric acid to the first rinse after the plating tank
Reduce brightener additions |
| Problem | Possible Cause | Corrective Action |
|
No deposit in low-current-density areas | Chromium contamination
(200 ppm) | Add one oz/gal sodium hydrosulfite per 100 gal of solution per 100 ppm of chromium to be removed
Zinc dust treat at one 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 two to five asf for eight to 12 hr |
| High brightener | Reduce brightener additions |
| Problem | Possible Cause | Corrective Action |
| Poor adhesion and/or blisters | Chromium contamination
(10 to 20 ppm) | add one oz/gal sodium hydrosulfite per 100 gal of solution per 100 ppm of chromium to be removed |
| Poor cleaning and/or rinsing | Improve cleaning, pickling and/or 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 pct by volume increments until optimum deposit is obtained. |
| Dull deposit in low-current-density areas (one to 20 asf) | High pH | Check the pH with a calibrated meter (do not rely on pH test strips) and lower with dilute hydrochloric acid. |
| Low ammonia and/or chloride | Analyze and adjust to range |
| Low brightener | Add in 0.5 pct by volume increments until optimum deposit is obtained. |
| High temperature | Lower to recommended range. |
| Iron contamination | Treat with hydrogen peroxide. Add 0.25 to 0.5 pint of 30 to 35 pct 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 current density range(20 to 30 asf) | High pH | Check the pH with a calibrated meter (do not rely on pH test strips) and lower with dilute hydrochloric acid. |
| Low ammonia and/or chloride | Analyze and adjust to range |
Dull or poor coverage
medium-current-density to low-current-density areas | Low ammonia and/or chloride | Analyze and adjust to range |
| 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 pct by volume increments until optimum deposit is obtained. |
| Problem | Possible Cause | Corrective Action |
| Low brightener | Add in brightener 0.05 pct by volume increments until optimum deposit is obtained. |
| Dull deposit across the entire current density range | High temperature | Lower to recommended range |
| Low brightener | Add in brightener 0.05 pct by volume increments until optimum deposit is obtained. |
| Poor surface preparation | Improve cleaning, pickling and/or rinsing |
| Excessive addition of hydrogen peroxide | Leave air agitation on during shutdowns to help dissipate excess peroxide. |
| | Add up to 100 ml of brightener per 100 gal of plating solution. |
| Bright, brittle deposit over 40 asf | High pH | Check the pH with a calibrated meter (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 pct by volume increments until optimum desposit is obtained. |
| Problem | Possible Cause | Corrective Action |
| Pitted deposit in medium-current-density to low-current-density areas | High ammonia and/or | Analyze and adjust to range |
| High brightener | Reduce brightener additions |
| Low wetter | Add in wetter 0.5 pct by volume increments until optimum deposit is obtained. |
| Trivalent chromium
(150 - 200 ppm) | Remove with filtration |
| Streaky deposit | Poor cleaning and/or rinsing | Improve cleaning, pickling and/or rinsing |
Soft, spongy or burnt
deposit in high-current-density areas | Low zinc | Analyze and adjust to range |
| Low ammonia and/or chloride | Analyze and adjust to range |
| 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 pct by volume increments until optimum deposit is achieved. |
| Problem | Possible Cause | Corrective Action |
| Iron contamination | Treat with hydrogen peroxide. Add 0.25 to 0.5 pint of 30 to 35 pct 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 solution | Filter the solution |
| | Check anode bags for tears and/or holes |
| Poor cleaning and/or rinsing | Improve cleaning, pickling, and/or rinsing |
| Low wetter | Add in wetter 0.5 pct by volume increments until optimum deposit is achieved. |
| Trivalent chromium | Remove with filtration |
| Advantages and Disadvantages of Typical Zinc Plating Systems |
| Advantages | Disadvantages |
| Cyanide zinc | Excellent covering power | Toxic materials |
| Good throwing power | Safety requirements high |
| Easy to control | Expensive waste disposal |
| Good corrosion protection | Lower cathode efficiencies at high current densities |
| Tolerant to purifiers | Low conductivity of plating solution |
| Chromates well | Unable to plate cast iron, some nitrided steels, malleable iron |
| Low equipment costs | |
| Alkaline non-cyanide zinc | Low dragout | Higher brightener costs than cyanide zinc |
| Lowest chemical costs | Blue-bright chromates tend to yellow with age |
| Lowest waste treatment costs | Pre-plate cleaning is more critical |
| Good coverage | Zinc metal concentrations more critical |
| Good plate distribution | Less tolerance to dragin of complexing agents from cleaners |
| Bright deposits | |
| Uses same equipment as cyanide zinc | |
| Acid zinc | Brilliant deposits obtainable | High equipment costs (requires corrosion resistant equipment) |
| Good leveling and hiding properties | Requires filtration |
| Faster plating rate | Requires bagged anodes |
| Ability to plate on cast iron and similar hard-to-plate bases | Requires superior pre-plate cleaning |
| Easy to control compared to other non-cyanide zincs | Subject to "bleed-out" problems |
| Low waste treatment costs | Blisters or flaking at higher thicknesses |
| Requires less voltage | Deposit tends to be brittle |
| | Poor throwing power |
|
