Alkaline Non-Cyanide Zinc Plating With any plating process, deposition is the most interesting aspect. However, pretreatment is the most important... by Pavco, Inc. Cleveland, OH Visit PF's Plating Zone PF's Tools Concerns over environmental and occupational safety hazards forced the plating industry to develop a cyanide-free alkaline zinc plating system. 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, EDTA or triethanolamine. These baths worked fine for a short period of time, but as iron was dragged in and complexed, codeposition 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 tying up not only zinc but also copper, nickel and other metals. Improved organic addition agents used in alkaline zinc baths eliminated the need for chelating agents. These organic addition agents brought with them several problems. The most insidious of them being delayed blistering. Most suppliers have eliminated these problems with a new generation of organic reaction products. "Monday morning blues" became a significant problem with the elimination of cyanide. The alkaline non-cyanide baths existing at that time had very narrow operating parameters. High zinc concentrations (>1.2 oz/gal), high temperatures (>90F), and high caustic levels (>14 oz/gal) all produced severe problems.uppliers 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 three to five years. Today's systems are much more reliable and consistent. Platers have a choice of low-chemistry alkaline non-cyanide zinc (low-metal bath) and high-chemistry alkaline non-cyanide (high-metal bath). Table I compares the two types of baths. TABLE I—LC and HC Bath Comparison Low Chemistry Bath* High Chemistry Bath* Efficiency 50 to 65 pct 70 to 95 pct Conductivity Poor due to low concentration of caustic Good due to high concentration of caustic Operating Cost Moderate due to low concentration of chemicals Higher due to higher concentration of chemicals Rinsability Good due to low alkalinity Fair due to high alkalinity Operating Parameters Narrow Wide * Neither of the above plating baths are recommended for plating cast-iron, malleable, and high carbon steel. General operating instructions for alkaline non-cyanide zinc baths are as follows: Routinely analyze the plating bath for caustic and zinc levels for a successful, smooth plating operation. Hull cell tests or other plating tests should be done on a daily basis. Cleaners and acids must also be analyzed, maintained, and dumped on a regular basis. Preventive maintenance reduces production problems and minimizes costs. A checklist of problems and solutions can help reduce downtime and rejects, which in turn results in higher profits. Automatic feeders for liquid components can eliminate human error. Look for obvious symptoms of problems such as oil floating on the cleaner tank, dirty rinse prior to plating or precipitation in the plating solution. Immediate action can help prevent problems worsening. For troubleshooting, follow the suppliers' instructions carefully. Find the source of the problem and implement the proper corrective measures. AUXILIARY zinc tank. 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. The selection of a particular option varies depending upon cost, time factors, and/or manpower availability. 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. Protective gear including suitable gloves, aprons, boots, and safety glasses should be worn when making the plating solution. Carefully follow all supplier instructions when making up the plating bath. Bath Constituents Caustic (Sodium Hydroxide). Sodium hydroxide keeps the zinc in solution and provides conductivity. High chemistry baths use less power because of the greater caustic concentration. The use of Rayon grade flake or granular caustic is strongly recommended. For bath maintenance, 50 pct liquid Rayon grade caustic can be added to the plating bath manually or by metering pump. Zinc. Many grades of zinc are commercially available. Use only special high-grade (99.99 pct) zinc. Lower grades will introduce contaminants that can cause plating problems. 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. 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. Water. Water quality plays a major role in alkaline non-cyanide zinc performance. The use of soft or DI water is suggested, not only at the time of bath makeup, but also on the plating line and in rinses prior to plating. When hard water is used, water softening agents can be added in the rinse tank just prior to the plating bath, and, in some instances, directly to the plating bath. Water hardness causes a dull zinc deposit, which can lead to increased consumption of proprietaries and purifiers. This problem is more severe in rack operations as opposed to barrel operations. Water hardness can be evaluated by test strips available from several suppliers. Proprietaries. Normally, three to four additives are used in a plating bath: Refining agent Brightening agent Purifier Water conditioner (water-softening agent) Wetting agent All of these additives work synergistically, as well as individually. The function of a refining agent is to give a semi-bright uniform zinc deposit. Used in conjunction with a purifier and a brightening agent, it makes the zinc deposit uniformly brighter. Refining agents, for all practical purposes, can be controlled by calculating the amount dragged out. Brightening agents add luster to the deposit. A high brightener level can lead to blistering and ductility problems. Brighteners are consumed by electrolysis and added back on an amp/hour basis. Purifiers treat heavy metals and impurities introduced from zinc anodes and caustic. They also affect the low-current-density brightness. Water conditioner treats water hardness. Wetting agents put a thin foam blanket over the surface of the plating bath to suppress any fumes or spray created during operation. It is recommended that suppliers' recommendations be followed carefully for optimum levels of proprietary agents when making up and/or maintaining the plating bath. 7. Carbonates (Sodium Carbonate). In general, sodium carbonate is not essential for the system, although some suppliers do recommend the addition of a small amount for initial startup. It is strongly recommended that there be no more than eight oz/gal sodium carbonate. An elevated level may cause high resistance in the plating bath, which in turn will result in increased energy costs. Also, in the high-chemistry alkaline non-cyanide zinc bath, excessive sodium carbonate levels will create problems with the solubility of the organics. Excess sodium carbonate can be frozen out by the use of a refrigeration unit. When the temperature is below freezing, the bath solution can be pumped to an outside storage tank to effectively remove the carbonates. Process Steps Alkaline Cleaning. Alkaline cleaning is a very important step in alkaline non-cyanide zinc plating. Cyanide baths provide some cleaning action but alkaline non-cyanide zinc plating baths do not. Soak cleaning followed by electrocleaning must remove all dirt and oils. Cleaners can either be powders or liquids; liquids have the advantages of easier handling and of adaptability to automatic feeders. The cleaner temperature and concentration must be maintained per supplier's recommendations. The use of an oil skimmer and/or filtration can be advantageous from the standpoints of cleaner longevity and the minimizing of the dragging of impurities down the line. Evaluate the cleaners used on the line for compatibility with the organics of the plating system, as additives from cleaners may play an active role in the plating bath, creating plating defects or precipitating proprietary organics. Pickle (Acid activation). Hydrochloric acid (20 to 30 pct) at room temperature or sulfuric acid (5-15 pct) at 105-120F should be used to activate steel parts prior to plating. If possible, strip reject parts off-line, which avoids the introduction of chromium contamination and extends the life of the acid. Chromium at 3-5 ppm in the plating bath can cause blistering problems in the zinc deposit. The use of proprietary acid additives and/or ammonium bifluoride may be beneficial in cleaning and activating parts, as well as increasing acid longevity. Acid additives may put a thin foam blanket over the surface of the pickle, which will prevent acid fumes from escaping into the atmosphere. 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 (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. ALKALINE ZINC produces columnar deposits shown schematically at the left. Acid zinc deposits are laminar (right). Equipment Considerations Plating Tank. The plating tank can be made of either low-carbon steel, polypropylene, PVC or rubber-lined steel. A good practice is to caustic leach all lined tanks prior to use. Low-carbon steel tanks should not be part of the electrical circuit. Rectification. For barrel operations, six-15 volts, five-10 asf is recommended. For rack operations, three-nine 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 is practical in most installations. Use 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. The use of a carbon packing filtration system is recommended. Agitation. Air or mechanical agitation is recommended for alkaline zinc rack operations. When air agitation is used, keep the air hoses under the anode. This will prevent parts from breaking contact or from being blown off the rack. 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 should not be used 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. The anode-to-cathode ratio should be about 1:1. Polypropylene material is recommended for anode bags. Cotton bags will be attacked by high alkalinity and dissolve in the plating bath. When anode bags are used, ensure that the tops of the bags remain outside of the plating solution in order to avoid roughness. Zinc metal consumption is 2.7 lbs/1,000 amps per one hr at 100 pct plating efficiency. Generally, zinc slab anodes cost less than zinc balls, but zinc balls provide a higher anode surface area. Zinc Generator. Maintaining the zinc metal level can be difficult in some plating installations, especially in the higher-efficiency baths. An off-line zinc generation tank which is 10-20 pct 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 becoming available that may reduce the size of or eliminate auxiliary tanks. Control of a Plating Bath Analytical Method. Suppliers provide analytical procedures to control the plating bath. Make use of the suppliers' laboratories if this service is offered. Zinc is titrated with EDTA using an acetate buffer and xylenol orange as the indicator. This method eliminates formaldehyde (a known carcinogen) from the lab. A method for the analysis of caustic is available which does not use cyanide. Hull Cell Plating Tests. Follow supplier's recommendations for control of the bath with the Hull cell, which can indicate the overall condition of the bath. A panel plated at one amp for 10-15 min with agitation is suggested for a barrel operation; without agitation for a rack operation. If steel anodes are used in the plating tank, use them in the Hull cell tests as well. Check the distribution by measuring the thicknesses on the panel at high-, mid-, and low-current-densities. After plating, immerse the bottom half of the panel in a fresh blue-bright chromate. This procedure detects metallic contamination in the bath, which results in discoloration of the zinc deposit during chromating. THROWING POWER of alkaline zinc is better than that of acid zinc as evidenced by a thicker deposit in the threads of this screw. Contamination. Plating bath contamination can consist of metallics or organics, or a combination of both. Introduction of contaminants to the plating bath may be caused by human error or by dragin during the plating cycle from the cleaner or the acid. Try to detect the root cause of the problem. Organic contamination affects the zinc deposit in many ways, such as a step plate, poor distribution, blistering, a dark low-current-density area, burning in the high-current-density areas, discoloration in the bright dip, and others. The source of the problem can be cleaner-related (such as oil dragin from the cleaner), a hydraulic oil drip from overhead, or other causes. Treat organic contamination with: 0.25-0.5 oz/gal activated powdered carbon One lb/1000 gals of potassium permanganate pre-dissolved in hot water and spread around the tank. Metallic contamination can also affect the zinc deposit in many ways, resulting in discoloration in the bright dip, blistering, high-current-density burning, a dark low current density, for example. The source of the problem may be the zinc anodes, the parts (leaded or copper-brazed) to be plated, copper anode bars, chromium contamination from the acid used to strip chromated reject parts, or other causes. Common contaminants are copper, cadmium, lead, tin, chromium, and iron. A contaminated bath can be treated by using various procedures, depending upon the type of contaminant present. Sodium bisulfite or sodium hydrosulfite (0.1 lb/1,000 gals) pre-dissolved in water and spread around the tank should be used for chromium contamination. Precipitated trivalent chromium can be filtered out of the tank. Activated zinc dust may help take copper, cadmium, lead, and tin out of the system. Zinc dust is not as effective in the treatment of these contaminants in alkaline non-cyanide baths as it is in chloride zinc baths. Low-current-density electrolysis may still be needed after zinc dust treatment. Follow-up atomic absorption tests are desirable. COMPARATIVE THROWING POWER results from five-sided box. 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 (Fig. 1). 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. PFD AUXILIARY zinc tank. ALKALINE ZINC produces columnar deposits shown schematically at the left. Acid zinc deposits are laminar (right). THROWING POWER of alkaline zinc is better than that of acid zinc as evidenced by a thicker deposit in the threads of this screw. COMPARATIVE THROWING POWER results from five-sided box. EFFICIENCY of alkaline non-cyanide zinc decreases as current density increases. Higher efficiency can be achieved with higher zinc concentrations (HC) and higher temperatures. ANALYTICAL INFORMATION The following analytical procedures are important in the elimination of pollution, health and occupational hazards from the laboratory. These safer procedures eliminate the use of 10 pct sodium cyanide (poison) in caustic analyses, as well as the use of 4 pct formaldehyde, which is regarded as a carcinogen (cancer-causing material). The illustrated methods also have sharper end points that are easier to read. Analysis Pipette (sample size) Additive Indicator Burette Color Change Calculation =*oz/gal Zinc (Erlenmeyer Flask) 5ml 50ml Acetate Buffer Xylenol Orange EDTA .1M Wine red to yellow ml tit. x 0.176 Caustic (Erlenmeyer Flask) 5 ml 10 ml D.I. Water **Indigo Carmine 0.95 N Sulfuric Acid Orange/brown to yellow ml tit. + Zn (oz/gal) *oz./gal. x 7.5=g/L ** Should be refrigerated when not in use. Visit or return to PAVCO.