By PAVCO, Inc.
Cleveland, OH

The first cyanide-free alkaline zinc plating system became commercially available in the early 1960’s. Cyanide was replaced by complexing or chelating agents such as sodium gluconate, triethanolamine, or polymeric amines. However, as iron was dragged in and complexed, co-deposition of iron resulted. This iron-zinc alloy created problems in chromating, causing discoloration, particularly in blue-bright and yellow chromate films. Chelating agents also caused problems in waste treatment by rendering some metals difficult to remove from the waste stream.

Improved organic addition agents in some baths have eliminated the need for chelating agents. Unfortunately, these organic addition agents had drawbacks as well, the most detrimental being delayed blistering. Most suppliers have eliminated these problems with a new generation of organic reaction products.

Suppliers learned well from these early growing pains. As a result, most of today’s alkaline non-cyanide zinc plating processes are chelate-free and produce deposits exhibiting good brightness, throwing power, ductility, and chromate receptivity.

Significant technological advancements have been made in the last 10 - 15 years. Today’s systems are much more reliable and consistent. Platers have a choice of low-chemistry alkaline non-cyanide zinc (low-metal bath) or high-chemistry alkaline non-cyanide (high-metal bath). Also, potassium ion based baths are providing platers with an alternative for faster plating speeds and higher efficiencies. Table I compares the two types of baths.

General operating instructions for alkaline non-cyanide zinc plating are as follows:

• Bath analysis, Hull cell testing, and other plating tests on a daily basis.
• Cleaners and acids analyzed, maintained, and dumped on a regular basis.
• Preventive maintenance to reduce production problems and minimize costs.
• Automatic feeders for liquid components can eliminate human error.
• For troubleshooting, follow the suppliers’ instructions carefully.

Bath Makeup

Three options are available for bath makeup:

A. With caustic and zinc oxide
B. With ready made zinc concentrate
C. With zinc anodes and caustic.

Option A is labor-intensive. The materials costs are moderate. Caution must be exercised with this option as the reaction is highly exothermic (more than 250F).

Option B has higher material costs, but is the least labor-intensive and the fastest.

Option C is the least expensive overall, but requires a delay for zinc dissolution, as well as possible low-current-density electrolysis to remove unwanted metallic impurities.

CAUTION: Carefully follow all supplier mixing and safety instructions when making up the plating bath.

Bath Constituents

1a. Caustic (Sodium Hydroxide). Sodium hydroxide keeps the zinc in solution and provides conductivity. The use of Rayon grade flake or granular caustic is strongly recommended or

1b. Caustic Potash (Potassium Hydroxide). Potassium hydroxide containing baths have been used more frequently over the last 5-10 years. Utilizing newer technology, potassium baths are more efficient, have faster plating speed, rinse more easily and accept chromates better, thus yielding higher corrosion protection, and eliminating the carbonate problems associated with sodium baths.

2. Zinc. Use only special high-grade (99.99 pct) zinc.

3. Zinc oxide. Zinc oxide is used only during a new bath makeup and rarely after startup of the plating bath. Zinc oxide (80.3 pct zinc metal by wt.) should contain no more than 0.002 pct lead (Pb) and no more than 0.005 pct cadmium (Cd). Request a Certificate of Analysis before purchase.

4. Zinc concentrate. Zincate is a concentrated solution of zinc oxide and sodium hydroxide. This mixture should not contain any additives and should be manufactured with pure ingredients.

5. Water. Water quality plays a major role in alkaline non-cyanide zinc performance. Water conditioning agents can be added to the rinse tank just prior to the plating bath, and, in some instances, directly to the plating bath. High water hardness causes a dull zinc deposit, which can lead to increased consumption of proprietaries and purifiers.

6. Proprietaries. Normally, two to three additives are used in a plating bath:

a. Refining agent—to give a semi-bright uniform zinc deposit. It can be controlled by calculating the amount dragged out.
b. Brightening agent—adds luster to the deposit and is consumed by electrolysis and added back on an amp-hour basis.
c. Purifier—treats heavy metals and impurities introduced from zinc anodes and caustic, and it also affects the low-current-density brightness.

Optional Additives:

d. Water conditioner (water-softening agent)—which treats water hardness
e. Wetting agent—to suppress any fumes or spray created during the plating opera- tion.

All of these additives work synergistically, as well as individually.

Follow suppliers’ recommendations carefully, providing optimum levels of proprietary agents when making up and/or maintaining the plating bath.

7. Carbonates (Sodium Carbonate). Sodium carbonate is not essential for the system, although some suppliers recommend a small amount for initial startup. High levels of sodium carbonate (>8 oz/gal) may cause high resistance in the plating bath, and in high-chemistry alkaline non-cyanide zinc baths, they will reduce the solubility of the proprietaries. Excess sodium carbonate can be frozen out with a refrigeration unit or when the outside temperature is below freezing, the solution can be pumped to an outside storage tank to effectively remove the carbonates.

Figure 1. Zinc Generation Tank – Overhead View

Process Steps

1. Alkaline Cleaning. Soak cleaning followed by electro-cleaning must remove all dirt and oils. The cleaner temperature and concentration must be maintained per supplier’s recommendations. Cleaner compatibility with the organics of the plating system is critical.

2. Pickle (Acid activation). Hydrochloric acid (20–30%) at room temperature or sulfuric acid (5–15%) at 105–120ºF (40–50°C) should be used to activate steel parts prior to plating. Stripping rejected parts off-line will avoid the introduction of chromium contamination and extend the life of the acid. Proprietary acid additives and/or fluoride salts may be beneficial in cleaning and activating parts, as well as increasing acid longevity.

3. Post-Plate Treatments. Chromate conversion coatings and lacquers are the usual post plate treatments for a zinc deposit. Chromate conversion coatings that provide up to 1,000 hrs to white salt formation per ASTM B 117 when used over zinc deposited from an alkaline bath are now commercially available. This can provide a cost-effective alternative to alloy plating.

Equipment

Plating Tank. The plating tank can be made of either low-carbon steel, polypropylene, PVC or rubber-lined steel. Caustic leach all lined tanks prior to use. Low-carbon steel tanks should be insulated from the electrical circuit.

Rectification. Barrel operations, 6–15 volts, 5–10 asf is recommended. Rack operations, 3–9 volts, 10–40 asf.

Heating and Cooling. Most baths operate at a broad range of temperatures. However, cooling equipment is essential and heating equipment may be needed in colder climates. Steel is the material of choice for any equipment in contact with the plating solution.

Filtration. Filters are essential for an alkaline non-cyanide zinc process. One to two turnovers of plating solution per hour are practical in most installations. Polypropylene cartridges, filter screens of 10-15 microns. Avoid paper or cellulose-type filter screens, as they can be attacked by the alkalinity of the system. Use a carbon-packing filtration system recommended.

Figure 2: THROWING POWER of alkaline zinc is better than that of acid zinc as evidenced by a thicker deposit in the threads of this screw.

Agitation. Mechanical agitation is optional for alkaline zinc rack operations. Air agitation is not generally recommended.

Anodes. Preferably all anodes used in the plating tank should be made of low-carbon steel, perforated, and with a thickness of 0.125 to 0.375 inch. Thicker steel has a higher current carrying capacity than thinner steel.

Titanium baskets are not recommended due to their high resistivity. Make sure that low carbon steel baskets are filled appropriately as per supplier’s suggestion when zinc anodes are used. Knife-edge anode hooks make better contact than other designs. Polypropylene material is recommended for anode bags. Cotton bags will be attacked by high alkalinity and dissolve in the plating bath. Ensure that the tops of the bags remain above the plating solution to avoid roughness.

The anode-to-cathode ratio should be about 1:1.

Zinc metal consumption: 2.7 lbs/1,000 amps per one hr at 100 pct plating efficiency.

Figure 3: Comparative COVERING POWER results from five-sided box.

Zinc Generator. An off-line zinc generation tank that is 10-20 % of the volume of the plating tank makes control of the zinc concentration easy. The zinc generation tank is a low-carbon steel tank with steel and zinc in contact. The zinc anodes are galvanically dissolved in the steel tank (low-carbon steel anodes are recommended in the plating tank). New technology is available that may reduce the size of or eliminate auxiliary tanks. See Figure 1 for an overview of the galvanic generator set-up.

Contamination. Plating bath contamination can consist of metallics or organics, or a combination of both.

Organic contamination affects the zinc deposit in many ways: step plate, poor distribution, blistering, dark low-current-density areas, burning in the high-current-density areas, discoloration in the bright dip, and others. Treat organic contamination with:

• 0.25-0.5 oz/gal activated powdered carbon
• One lb/1000 gal of potassium permanganate pre-dissolved in hot water and spread around the tank.

Metallic contamination can also affect the zinc deposit in many ways: discoloration in the bright dip, blistering, high current density burning, a dark low current density area. Treat metallic contamination using the following procedures: sodium bisulfite or sodium hydrosulfite (0.1 lb/1,000 gals) should be used for chromium contamination. Activated zinc dust may help remove copper, cadmium, lead, and tin from the system, although zinc dust is not as effective in treating metallic contaminants in alkaline non-cyanide baths as it is in chloride zinc baths. Low-current-density electrolysis may still be needed.

Figure 4. Efficiency of alkaline non-cyanide zinc decreases as current density increases. Higher efficiency can be achieved with higher zinc concentrations (HC) and higher temperatures.

Zinc Deposit Properties
Zinc deposit ductility, uniformity, and chromate receptivity in an alkaline non-cyanide bath is better than that achieved by chloride zinc baths. Unlike chloride zinc, the alkaline bath does not exhibit chipping or star-dusting when operated properly. The brighter the zinc deposit, the higher the occlusion of organics in the deposit. This makes the deposit less ductile and highly stressed. For these reasons, brightness, while beautiful in appearance, sacrifices quality. This remains true for all zinc plating systems. Zinc deposits from alkaline baths are columnar in structure. Chloride zinc deposits are laminar.

Throwing and Covering Power
Throwing power is the ability of a plating bath to deposit a uniform thickness of metal from high-current-density areas to low-current-density areas. Throwing power of an alkaline bath is significantly better than that of a chloride zinc bath. Covering power is the ability of a plating bath to deposit metal in a recessed area. Covering power from an alkaline bath is equal to or better than can be attained from a chloride bath.

The alkaline bath’s throwing power is approximately 40-65 pct (Haring cell), depending on the bath chemistry and/or type of additives used.

Any part, from large computer chassis to fasteners, plated in an alkaline plating bath will have thicker deposits in the low-current-density areas when compared to chloride zinc. As shown in Figure 2, the example of the screw plated in alkaline non-cyanide zinc will provide more protection to red rust because of the thicker zinc in the threaded area. Figure 3 shows comparative throwing power of alkaline non-cyanide zinc and chloride zinc in a five-sided box.

Efficiency
The bath efficiency of alkaline non-cyanide zinc decreases as the current density increases (see Figure 4). A higher efficiency can be achieved with higher zinc concentrations and higher temperatures.

Stress
Stress in alkaline non-cyanide zinc deposits depends upon the type of organics used. The type of stress in the zinc deposit obtained from the bath should be compressive, not tensile. Tensile stress is a suspected cause of immediate or delayed blistering. Check with your supplier about the type of stress obtained from their additives. Stress can be evaluated by using a contractometer or commercially available “Stress tabs.”

Current Trends
The plating industry is rapidly adopting alkaline non-cyanide zinc process technology over cyanide and chloride zinc processes. Alkaline non-cyanide zinc bath solutions are not as corrosive to equipment as are chloride zinc bath solutions. In some areas, restrictions on chloride levels in effluent may make the choice of an alkaline non-cyanide bath a more practical one. The majority of alkaline alloy baths (zinc-cobalt, zinc-nickel, and zinc-iron) are basically minor modifications of alkaline non-cyanide zinc baths. Alkaline non-cyanide zinc process usage is increasing and could claim the lion’s share of the market for zinc plating in the future.