I. Introduction Alkaline non-cyanide zinc (NCZ) plating has gained commercial success for its low cost of operation, ability to produce cosmetically acceptable deposits, environmentally friendly electrolyte, and ability to plate with excellent distribution distribution over complex geometry. NCZ chemistry is used in manual rack and barrel plating lines as well as large programmed automatic machines. With increasing environmental restrictions and competitive pricing, alkaline non-cyanide zinc will continue to grow replacing cyanide processes and in some instances the more costly acid Zinc chloride process. II. Pre-treatment Cleaning The general rule of thumb for NCZ plating on steel is that the cleaning cycle shall be at least as good or preferably better than what is considered good for bright nickel and acid zinc plating. This typically will require the use of soak cleaning, electrocleaning, acid activation, electroclean, a mild activation, an alkaline pre-dip, and then NCZ plate. This extensive pre-treatment pre-treatm ent is required due to the absence of cyanide in the electrolyte. The work must be free of oils and smut prior to plating if uniform, bright, adherent deposits are to be obtained. Acid Activation The proper use of acid pickles is essential in NCZ plating. In order to ensure a bright, uniform, adherent deposit, all heat scales, smut, or other surface defects must be eliminated. Due to the high hydrogen overvoltage of the NCZ electrolyte, zinc plating will not initiate over heat scales or carbon that has been brought to the surface. Pre-Dip The use of the alkaline pre-dip is unique to NCZ plating. Iron contamination in the bath will result in poor chromating and with some brightener systems, blistered deposits. The pre-dip should contain 2 oz/gal of ENTHOBRITE® 4211 water conditioner to chelated soluble iron, and 4 oz/gal caustic soda to precipitate any iron dragged into the plating bath. To avoid drag-in of chelated iron, the last electrocleaner should be chelate free. Plating Zinc Die Cast The ENTHOBRITE NCZ 916 A, B & 966 C system has proven to be excellent for producing bright, adherent deposits On zinc die cast. The recommended cycle is as follows: 1. Soak Clean, ENPREP® 72 SE, 180, or 182. 2. Electroclean Anodically 15-20 ASF, ENPREP 72 SE, 285, or 288. 3. Counter-Flow Rinse. 4. Acid Activation, ACTANE 73 or ENPREP 340. 5. Counter-Flow Rinse. 6. NCZ Electroplate. III. Solution Control NCZ baths are composed of zinc (as sodium zincate, Na2Zn(OH)4), caustic soda, water conditioner, and organic additives. The economy economy of NCZ plating is derived from the low metal content of the bath, 0.7-1.4 oz/gal, and the use of a low priced conducting salt, caustic soda. The effects of these bath parameters on the plating characteristics will be discussed in detail. Zinc concentration The concentration of zinc in the bath influences the efficiency of the deposition process in all NCZ systems. in the case of ENTHOBRITE NCZ 916, the zinc concentration will also influence the throwing power and covering power of the bath. Throwing power is defined as the thickness ratio between high and low current density areas. Covering power is defined as the ability to plate a bright deposit in low current density areas.
The property of decreasing efficiency is responsible for the throwing power of the NCZ electrolyte. By efficiency decreasing at higher current densities, deposit thickness will not build as rapidly thereby producing more uniform thickness distribution. Generally speaking, the lower the zinc concentration, the more Inefficient plating becomes in the high current density area. This effect is quite clear with the ENTHOBRITE NCZ 916 system. Current Density (ASF ) With the ENTHOBRITE NCZ 8018 system, the effect of metal concentration is not as evident. The ENTHOBRITE NCZ 8018 system illustrates that the brightener system has very significant effects on efficiency. Therefore, it can be somewhat misleading to say that a system that operates at higher metal operates more efficiently than one at lower metal. In the ENTHOBRITE NCZ 8018 system, the combination of increased zinc concentration and increased temperature has greater impact on the efficiency of the system at higher current densities. Current Density (ASF) To obtain the best distribution with ENTHOBRITE NCZ 916, low zinc concentration and low temperature are the preferred settings. This is in agreement with the efficiency curves. To get the best distribution, conditions that lower the high current density efficiency should provide the best distribution. Under these conditions, ENTHOBRITE NCZ 916 is able to produce an optimum ratio of 0.64. The data did not produce any variables that were significant with ENTHOBRITE NCZ 8018. This is interpreted as meaning that this process is robust to variation of these parameters. The NCZ 8018 system produced a thickness ratio of 0.76 regardless of the parameter settings. NCZ 8018 is then capable of producing a better distribution than NCZ 916 at higher zinc concentrations and higher temperatures which means at faster plating rates. The ability to plate bright low current density areas is a characteristic of the individual brightener system. It is important to recognize the limitations of any system. With the ENTHOBRITE NCZ 916 system, controlling the zinc concentration and temperature are very important in order to plate bright low current densities. The zinc operating range for ENTHOBRITE NCZ 916 is 0.8 to 1.4 oz/gal and the best low current density plating is accomplished at low zinc concentration. The recommended temperature range is 70 to 850F, again with the best low current density plating is done at the lower operating temperature. Most NCZ baths should not be operated above 850F. The ENTHOBRITE NCZ 8018 system is very tolerant to Zinc concentration and temperature with regards to bright low current density plating. NCZ 8018 Is capable of producing bright low current density plate at 1.4 oz/gal zinc and 1100F. One characteristic that is common to both Systems is the influence of zinc concentration on brightener demand. Both systems require less grain refiner to produce a smooth deposit at low metal concentrations. As the zinc concentration increases, the demand for the grain refiner increases. For the best economy of the additives, 0.8 to 1.0 oz/gal of zinc is recommended. Caustic soda The caustic soda provides the supporting electrolyte for electroplating and provides the ligand for the dissolution of the zinc anode during the plating process. The concentration of caustic soda also affects the following: covering power, anode polarization, metal build-up, and blue chromating. Caustic soda concentration does not influence efficiency but does influence covering power. Covering power of ENTHOBRITE NCZ 916 increases when the caustic soda concentration is
raised from 10 to 14 oz/gal. This increased covering power reaches maximum benefit at about 14 oz/gal. increased caustic levels beyond 14 oz/gal will not increase covering power, but will have detrimental effects on the grain refiner. This is seen as high current density burning as a result of degradation of the grain refiner. Understanding the influence of the caustic soda concentration on anode dissolution is critical to controlling the zinc concentration in the bath. When the caustic soda concentration is at 10 oz/gal, the anodes will polarize at an anode current density of 30 ASF. At this current density, a heavy black film forms on the anode, which inhibits zinc dissolution. The oxygen which is generated at the anode will eventually break the anode film, releasing it as large particles which lead to roughness. The voltage must be increased in order to overcome the insulating effect of the film so that the amperage can be maintained. The efficiency curve at 14 oz/gal caustic soda is 100% to 60 ASF. While the anode does darken, the voltage does not drop nor is oxygen generation seen, therefore the anode is dissolving properly without creating roughness or electrical problems. A curve for 12 oz/gal caustic soda was attempted. The data points were very irregular. For example, the efficiency at 30 ASF was 77%, 102% at 40 ASF, 43% at 50 ASF, and 101% at 60 ASF. The conclusion is that 12 oz/ gal caustic soda is the borderline concentration between anode polarization and 100% anode efficiency. Anode Current Density (ASF) Metal build-up during idle periods has been a problem with NCZ Systems. The metal build-up is due to the galvanic interaction between the zinc anode and the steel baskets. The rate of the galvanic dissolution is related to the caustic soda concentration. Zinc concentration is capable of doubling during a weekend shutdown. Methods to control the zinc in the plating bath and the galvanic dissolution have included pulling anodes during idle periods, operating with reduced anode area, operating at low caustic soda levels, pumping solution into holding tanks during idle periods, and use of slab anodes. Slab anodes will not dissolve because of the absence of iron in the galvanic cell. In general, most of these alternatives are not practical in large automatic operations. The preferred method for controlling zinc in a NCZ bath utilizes an external zinc generator tank. This method takes advantage of the galvanic dissolution between zinc anodes and steel sheets in an alkaline NCZ solution. The zinc balls are dissolved in this tank and then the concentrated zinc solution is pumped through a filter into the main plating tank. By controlling the feed rate of the zinc concentrate and the surface area of anodes in the generator tank, the zinc concentration in the plating tank is keep constant. In high amperage cases, such as barrel plating, it may be necessary to supplement the generator tank with additional zinc anodes in the plating tank. Generally, zinc generators are 10 to 20% of the total volume of the plating tank. The area of zinc anodes in the generator tank can be estimated by the following formula: Ft2 of Zinc Anodes = Total Amps x 0.02 This assumes a zinc dissolution rate of 350 grams per square meter per hour at 780 degrees F, and a plating efficiency of 60%. Water Conditioning In all NCZ solutions, hard water will diminish the quality of the zinc deposit. Hard water interferes with the action of the organic additives and produces hazes or blemishes in the deposit. These interferences are removed by the use of conditioning agents. Agents such as tartrates, sugars, or EDTA have been used in the past. These conditioners were either marginally effective or produced effluent problems because of their chelating properties. The recommended water conditioner for ENTHOBRITE NCZ systems is ENTHOBRITE NCZ 4211. This product is nonchelating and extremely effective in removing hard water interferences. ENTHOBRITE NCZ 4211
is typically used at 1 oz/gal on new make-ups. Extreme hard water may require 2 to 3 oz/gal NCZ 4211 to effectively remove the interference. By incorporating 2 oz/gal NCZ 4211 in the alkaline pre-dip, water that will be dragged into the bath will be treated and NCZ 4211 that is lost on dragout can be replenished. Chromating ENTHOBRITE NCZ systems produce deposits that readily accept blue, yellow, black, or olive drab chromate conversion coatings. The use of a 0.25% nitric acid dip prior to chromating will give more uniformity and extend the life of the chromate. The history of NCZ systems has been that they will not blue chromate as well as cyanide or chloride zinc deposits. NCZ deposits tend to have yellow highlights and lack the depth of color. The ability of an NCZ deposit to accept a blue chromate has been related to the grain size of the deposit. The smaller the grain, the better the blue chromate appearance. The two factors which will reduce grain size are caustic soda content and brightener concentration. To obtain the best blue chromate, the caustic soda should be maintained at 14 oz/gal and the deposit should be bright and uniform. Trivalent blue chromates have been used in placed of hexavalent blue chromates to improve the blue color of the finish with some NCZ systems. Presumably, the lack of hexavalent chrome leaching from porous zinc deposits maintains a more consistent color. NCZ deposits serve as an excellent base for the darker chromates. Green, olive drab and black chromate finishes typically are less iridescent and deeper in color that finishes obtained from a chloride electrodeposit. Yellow chromating of NCZ deposits is easily accomplished and the chromate films are very adherent and normally free of adhesion problems as seen in chloride systems. IV. Hull Cell Techniques for Troubleshooting Alkaline Non-Cyanide Zinc The following techniques are used in troubleshooting ENTHOBRITE NCZ plating solutions. While the examples are primarily for ENTHOBRITE NCZ 916 and 966, the observations are generally the same for any non-cyanide brightener system. These procedures are useful for troubleshooting existing problems as well as screening solutions as a preventative maintenance practice. High Levels of ENTHOBRITE NCZ 916 A Baths with high EAU content will generally experience some of the following characteristics: 1. 2. 3.
High current density strip out on barrel plated parts. Low plating efficiency at elevated temperatures and zinc concentration. Excess "B" consumption.
Two Hull cell tests that will indicate excess "A" are measuring thickness over current density and strip out in nitric acid. 1.
2.
Thickness over Current Density a. Plate a 2 amp 15 min panel. b. Determine thickness over the current density range. Measure at selected current densities based on Hull cell ruler or at selected distances from an edge. A bath with high "A" will show decreasing thickness as current density increases. Nitric Strip Out Test a. Plate a 2 amp 5 mm panel. b. Immerse in 0.25% nitric acid solution. c. Allow the panel to strip out from the low current density area to about the 24 ASF area. d. Observe high current density area.
Due to the inefficiency at high current densities, a bath with high "A" will show strip out in the high current density area. The area that is likely to strip out will generally be a white, hazy deposit. This pattern can be misinterpreted as high "A" on occasion, therefore it is best to diagnose with one of these two tests. Three tests can be used to confirm the high "A" diagnosis. 1. 2.
3.
Cut the bath 50% with a stock solution of the same composition as the plating bath (minus wetters, brighteners, purifiers) Adjust zinc and caustic concentrations. The amount of "A" needed in the bath is proportional to the zinc concentration. If the bath is at 0.8 oz/gal, increase the zinc to 1.2 oz/gal. Dummy the solution at 2 amp 30 mm, adjust metal concentration and bath temperature.
If the bath is high in "A", any of the following should be seen: a. No need to add "A" as the HCD is burn free. b. Thickness no longer decreases as current density increases. c. No strip out seen in nitric acid.The dummy process may require repeated cycles to observe improvement. This amount of electrolysis could be used to approximate the amount of dummying needed on the process tank. High Levels of ENTHOBRITE NCZ 916 or 966 B High levels of ENTHOBRITE NCZ 916 or 966 B are indicated by skip plate around contact points, poor thickness distribution/efficiency, poor response to additional "B" adds. The best way to evaluate excess "B" is to run a 50% dilution cell. Add "A" as required to eliminate HCD burn or "C" to eliminate LCD bands. An overloaded bath will still be bright after the dilution. The amount of added "A" can be used as a rough guide to the "A" level in the bath. The bath may also be dummied to eliminate the excess "B", however due to the high mileage of the "B" component too much time would be required to plate out the brightener. Once a bath has been diagnosed as having excess "B", the best remedy is to discontinue additions until the bath shows a need for brightener. A bath that needs brightener will show good response to a three drop addition of "B" to the Hull cell. Often a bath that is high in "B" will generally be high in "A" and vise-versa. The "A" and "B" act together to produce brightness. If adds had been discontinued and the bath indicates a need for an addition, be sure to evaluate an "A" addition before adding more "B". Carbon treatment and peroxide treatment generally are not effective in NCZ systems, but they should be evaluated if the bath needs an immediate course of action. Dull Deposits Dull deposits are generally from poorly prepared substrates, too low cathode current density, hard water interference, or organic contamination. In the case of hard water interference, a 2 amp 5 mm panel generally will have a blotchy, hazy appearance that does not respond to brightener additions. In this case, add two grams ENTHOBRITE NCZ 4211 to the bath. If hard water is the problem, the 4211 generally will effervesce when added and the panel will be bright and free of haze. If ENTHOBRITE NCZ 4211 is not available, EDTA or tartaric acid may be used to demonstrate the point. If organic contamination is the problem, either a permanganate treatment will remedy the problem or a cut will be needed. Severe overloads of the brightener system will likely require extensive high current density dummying to overcome the problem. Flaking or Blistered Deposits
Blistered deposits have traditionally been the biggest concern with NCZ plating. The most important step in troubleshooting a blistering problem is to determine if the problem is bath related. Typically, the best test to determine if blistering is bath related is to plate a 2 amp 30 mm panel and follow b y baking the panel at 350°F for 2 0-30 minutes. In most cases, a panel that does not blister with this test indicates that the problem is not related to bath chemistry. The most likely causes are cleaning, substrate related, etc.. However this assumes that the bath is operating under recommended parameters. For example, ENTHOBRITE NCZ 916 has a long history of being blister free. It has been shown however, that baths that operate at low temperatures, 60°F, will show blisters, typically i n the high current density area. It is important t o control the Hull cell conditions to match those of the plating tank to be certain about your conclusions. Delayed blistering is difficult to demonstrate in a short time by Hull cell. The blistering that results from this effect is not always brought out by baking. This is seen with some of our competitor's baths. To effectively demonstrate delayed blistering, plate either a 2 amp or a 1 amp 15 mm panel and keep the panel up to two weeks. If delayed blistering is present, small blisters will generally appear in the mid to low current density region of the panel. Metallic Contamination Contamination by iron, cadmium, and copper will generally appear on plated work as dark areas after chromating, particularly blue chromating. This is easy to reproduce in Hull cells by plating 0.5 amp 20 minute panels. The panel is then immersed in 0.25 % nitric acid or blue chromate solution. If metallic contamination is present, the panel will blacken. It has been seen that agitation can be critical in reproducing the blackening. Panels should be run both still and agitated to be sure of your conclusion. The remedy for metallic contamination is addition of ENTHOBRITE 1969 (EDTA) or low current density dummy. After treatment, a panel should no longer blacken when dipped in nitric acid or blue chromated. Yellow chromating can also be influenced by metallic contamination. The final finish will vary from cloudy and hazy to blue-black. Treatment of the solution is the same. With all chromate appearance problems, the Hull cell panels must be chromated in freshly prepared chromate rather than on-line solutions in order to distinguish between a bath problem from a chromate problem. Anode Polarization Anode polarization problems that are related to contamination or additive overload may be seen in Hull cell tests. A freshly activated zinc anode is used to plate a 2 amp 5 minute panel. With a constant voltage rectifier, most NCZ baths will polarize the anode within 2-3 minutes. The time will vary with the actual anode area and the caustic soda concentration. This polarization is normal for NCZ baths. The color of the film that forms is the indicator of possible contamination. A normal NCZ 916 bath will produce a brown to golden film on the anode. The film is not completely adherent as it will fall apart as the anode current density increases. A bath that forms a very adherent black film is likely contaminated with sulfides. The sulfides may come from purifiers used in cyanide zinc plating, use of thiourea based purifiers for NCZ baths, or drag-in of sulfides from outside sources. The confirmations for sulfide contamination are: 1. Dilution of the bath with stock solution. 2. Zinc dust treatment. 3. Use of ST-2 service tool. The remedies will eliminate the anode filming problem and may also increase the conductivity of the solution by requiring less voltage to produce the amperage.
V. Procedure For Conversion of Non Enthone-OMI NCZ Bath to ENTHOBRITE NCZ Systems The focus on conversion of non-Enthone-OMI NCZ baths to ENTHOBRITE NCZ systems is compatibility of the systems with regards to blistering , efficiency, and response to ENTHOBRITE NCZ additives. Great care must be taken to evaluate the bath being converted for adhesion and efficiency. Potential profits from the new business can be readily consumed by starting a conversion on a process tank that has not been properly evaluated. Adhesion is evaluated by baking 2 amp 30 mm panels at 350 degrees F for 20 minutes. The effect on efficiency is evaluated by determining the thickness at 30, 20, 10, and 3 ASF on a 1 amp 15 min panel. A template should be prepared to mark the areas of the panel that are to be measured for thickness. The following plating cycles should be run three to five times to determine if a conversion can be recommended. The number of runs will be dependent on the ease of conversion. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
The first step is to analyze the bath for zinc metal, caustic soda, and dissolved iron. Plate a 1 Amp 15 mm "as is" panel and determine thickness at the specified areas. Plate a 2 amp 5 mm "as is" panel. Plate a 2 amp 30 mm panel. Bake at 350 degrees F, 20 minutes. Check for adhesion. If adhesion is a problem, electrolyze the bath until a 2 amp 30 min panel no longer blisters when baked. If adhesion is good, plate a 2 amp 5 min panel. Add ENTHOBRITE NCZ additives as needed to give desired response. Repeat step 6. Plate a 1 amp 15 min panel and determine thickness. Evaluate effect on thickness. Plate 2 amp 30 min panel, repeat bake test. Repeat steps 5 through 9, 3-4 times. Response to ENTHOBRITE NCZ additives with no detrimental effect on appearance, adhesion, or efficiency indicates conversion can be recommended.
ALKALINE ZINC AND ZINC ALLOY PLATING ZINC GENERATOR SYSTEMS Zinc generator systems are commonly used to supply zinc to solutions which operate with steel or mixed steel and zinc anodes. A general dissolution rate of zinc in alkaline zinc or zinc alloy solutions (in contact with steel) at 78°F is 350 gr ams per square meter per hour. This figure can be used to estimate the required surface area of zinc anodes in the generator tank. FT2 OF ZINC ANODE AREA = (TOTAL AMPS) X 0.02 This assumes an average cathode current efficiency of 60%.
