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Finishers' Think Tank Finishers’Finishers’ ThinkThink TankTank Stephen F. Rudy, CEF • Enequist Chemical Co. 100 Varick Avenue • Brooklyn, NY 11237 • 718/497-1200 • E-mail: [email protected] 2001: A Plating & Surface Finishing Odyssey The Journey Continues, Part 3 As the P&SF Express pulled out of Bath Constituents—Low-Cyanide Bath Orlando in February, we reviewed some of the features and applications Constituent Range Opt. Range Opt. of zinc plating in this column’s travel oz/gal oz/gal g/L g/L brochure. With great anticipation, the Zinc metal 1.0-2.0 1.3 7.0-15.0 10.0 Express sped along the orange groves * Zinc oxide 1.2-2.5 1.7 9.0-19.0 12.5 to our fi rst stop. * Zinc cyanide 1.8-3.6 2.3 13.5-27.0 20.2 Welcome to Zincmalia. This is per- Sodium cyanide 1.5-4.0 2.4 11.0-30.0 18.0 haps the most densely packed land of Sodium carbonate 8.5-12.0 10.2 63.8-90.0 76.5 zinc plating. Its geographical bound- Caustic soda 10.7-12.0 11.3 80.0-90.0 85.0 aries include these industrious states: * Optional to add either zinc oxide or zinc cyanide Acid, Alkaline, Cyanide, Alloy, and Ratio of sodium cyanide to zinc metal: Range (1.5-2.2) optimum = 1.8 Galvanize. Our initial border crossover into Zincmalia enlightened us as to the Bath Constituents—Medium-Cyanide Bath region’s continued strength and pros- perity: cheapest metal to electroplate Constituent Range Opt. Range Opt. and excellent sacrifi cial corrosion pro- oz/gal oz/gal g/L g/L tection. There is no indication that Zinc metal 2.0-3.35 2.7 15.0-25.0 20.0 zinc’s position in our industry will * Zinc oxide 2.5-4.0 3.4 19.0-31.0 25.0 change. Barrel, rack, continuous strip, * Zinc cyanide 3.6-6.0 4.9 27.0-45.0 36.0 and hot dip process lines all provide a Sodium cyanide 4.0-8.7 6.7 30.0-65.0 50.0 large variety of fi nished parts. World- Sodium carbonate 9.0-12.6 10.7 67.5-94.5 81.0 wide markets include: consumer, com- Caustic soda 8.0-10.7 9.3 60.0-80.0 70.0 mercial, engineering, functional, mil- * Optional to add either zinc oxide or zinc cyanide itary, aerospace, and automotive. No Ratio of sodium cyanide to zinc metal: Range (2.0-3.0) optimum = 2.5 wonder Zincmalia operates on a non- stop universal schedule. Let’s visit our Bath Constituents—High-Cyanide Bath fi rst state, updating its maintenance, Constituent Range Opt. Range Opt. control, and individual contributions to oz/gal oz/gal g/L g/L zinc fi nishing. Zinc metal 4.0-5.4 4.7 30.0-40.0 35.0 * Zinc oxide 5.0-6.7 5.9 37.0-50.0 44.0 Cyanide * Zinc cyanide 7.2-9.7 8.4 54.0-72.8 63.4 This state contributed much of the Sodium cyanide 10.0-14.7 12.3 75.0-100.0 87.5 technology development and experi- Sodium carbonate 8.0-12.6 11.0 60.0-94.5 77.2 ence related to zinc plating. For sev- Caustic soda 10.0-12.0 11.0 75.0-90.0 82.5 eral decades, cyanide zinc plating was * Optional to add either zinc oxide or zinc cyanide a major backbone of zinc fi nishing. Ratio of sodium cyanide to zinc metal: Range (2.5-3.5) optimum = 3.0 Ecological considerations, waste treat- ment upgrades, and newer process developments have increased the Three distinct cyanide level baths: low, and health hazards. Use the recom- applications of acid, alkaline, and medium, and high comprise the work- mended safety clothing and related alloy zincs. These states eagerly await ing electrolytes. equipment when handling these addi- the P&SF Express. Our present trip tives and the plating solutions. Consult itinerary has our train pulling into Safety Caution with responsible plant personnel, ven- Port Cyanide station, to review cya- Cyanide additives and the plating solu- dors, and safety engineers. nide zinc plating. We make a quick tion formulas listed are poisonous and Each of the plating solutions, when exit to Technology Blvd., confi rming corrosive. Always read the appropriate correctly mixed and prepared, chemi- the chemistry that makes it work. MSDS sheets to understand the risks cally form two distinct electrolytes: Sodium zinc cyanide .......... Zn(CN) + Operating Parameters 2NaCN = Na Zn(CN) 4 4 Low - Medium - High Cyanide Baths Sodium zincate .......... Zn + 4NaOH = Na2Zn(OH)4 Parameter Rack Barrel Cathode Current Density 1-20 ASF 3-6 In preparing the solution, about 75-90 Voltage 3-6 9-18 percent of the zinc metal forms the Temperature 65-120°F (18-49°C) 65-120°F (18-49°C) Sodium Zincate complex, with the Anodes *High purity grade only *High purity grade only remainder in the form of sodium zinc Current Effi ciency 60-90% 60-90% cyanide. Each bath constituent contrib- Anode/Cathode ratio 1.5:1.0 or 2.0:1.0 1.5:1.0 or 2.0:1.0 utes specifi cally to the plating opera- tion. * >99.99% pure zinc. This is very important to avoid cadmium contamination. Steel may also be used to help control desired zinc metal concentration. • Zinc Complexes—A secondary source of metal, complementing the Under the Hood Troubleshooting Guide anodes. • Cyanide—Functions as a “carrier” Defect Probable Cause for metal deposition. Additional Poor adhesion Poor surface prep. Hex-chrome contamination. properties include: enhancing effec- Delayed blistering due to hydrogen absorption. tiveness of the organic brightener Bath out of balance. agents, promoting satisfactory Burnt deposit Low cyanide and or caustic levels. Excess current. deposit adhesion, promotes anode Hazy, deposit stains Organic/metallic contamination. corrosion & cathode effi ciency, and Solution/brightener out of balance. “last stop” cleaning and descaling. Dark deposit Organic/metallic contamination. • Caustic Soda—Helps prevent anode Solution/brightener out of balance. polarization, contributes to zinc Low conductivity Low temp. Low: zinc, caustic, cyanide or combination deposition rate & speed, and Poor throw High conc.: zinc, caustic, cyanide or combination. improves coverage of complex Shelf roughness Suspended solids in bath. shaped parts. Polarized anodes Improper anode area. • Sodium Carbonate—Promotes Low: cyanide, caustic or combination. deposit fi ne grain structure. Zinc conc. rises Excessive anode area. Cold bath crystallizes High carbonates. Analysis Contributes to anode polarization & low cond. The plating process depletes active concentrations of the bath salts. Solu- Cooling—Coil or chiller recom- rosion protection. Baths are easy to tion drag out also affects their con- mended to maintain below upper operate and maintain. By optimizing centrations. All of these bath com- temperature limit. the basic operating parameters (plating ponents can be readily analyzed by Ventilation—*Mechanical to maintain current density, time, temperature, simple, very accurate titration proce- levels below permissible exposure concentrations of salts & addition dures. Addition agents (brighteners) limits . agents, surface preparation), excep- such as: aldehydes, long chain alco- Anode Baskets—Steel tional throwing power and coverage hols, inner transition metals, and com- Agitation—Not essential. Non air-agi- are realized. Solution temperature and plexing agents in specifi c ratios, pro- tated solution movement is pre- bath chemical composition especially duce a modifi ed deposit grain struc- ferred . affect plating speed and current effi - ture. The resulting deposit exhibits Filtration—Not essential. Good for ciency. A minimum of 0.0002-in. zinc good brightness and leveling through- particulate removal. thickness is required for suffi cient out the operating current densities. chromating. Most commercial appli- Concentration of a polysulfi de addi- *Organic wetting agents do form a cations require at least 0.00025-in. tive (purifi er) is maintained to precipi- stable, thin foam blanket during plat- zinc, readily meeting most thickness tate metallic bath contaminants, such ing. This signifi cantly reduces corro- requirements. Steel parts such as: as copper. After confi rming concen- sive mist and spray. Care should be stampings, welded frames, fasteners, tration of bath salts with appropriate taken to prevent thick, stable foam washers, nuts, bolts, and nails are additions, the hull cell method is used blankets, that may result in hydrogen easily cyanide zinc plated. In this to monitor desired level of the bright- gas explosions. system, iron is not a contaminant. ener. Polysulfi de test papers are nor- mally suffi cient for semi-quantitative Owners Manual determination of the purifi er. The majority of commercial zinc plat- Troubleshooting Tips ers have some exposure to cyanide • High zinc metal concentration Equipment zinc plating. The deposit typically reduces throwing power and The bath doesn’t require lined tanks or meets the fi nish applications for a increases the burning limit. Low special corrosion-resistant equipment. wide variety of industry parts. Bright zinc metal contributes to deposit Related cost savings are realized. cyanide zinc deposits develop aesthet- burning. ically “chrome white” deposits and • High sodium cyanide level reduces Tank—Mild steel, reinforced polypro, with application of a chromate conver- brightness, contributing to or fi berglass. sion coating, provide excellent cor- increased additive brightener con- sumption. Low cyanide produces a its, and affects chromating. Addi- chemical maintenance of the bath. brittle zinc layer. Maintaining a suf- tions of sodium hydrosulfi te effec- • Toxicity of cyanide enhances the fi cient cyanide concentration reduces tively reduce hexavalent chromium switch to cyanide-free zinc plating bath sensitivity to impurities. (Cr +VI) to it’s insoluble trivalent systems.
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