Chromium Plating By Nabil Zaki Vice President of Technology MacDermid, Inc. Kearny, New Jersey Visit PF's Plating Zone PF's Tools Despite the simplicity of their formulations, chromium plating baths are more complicated to operate than most plating baths, and they require rigorous controls. CHROMIC-ACID-BASED BATHS (Hexavalent type) In plating from hexavalent chromium baths, sulfate and fluoride ions act as catalysts. Temperature, current density and bath composition affect the film characteristics and current efficiency. These parameters are therefore carefully controlled in order to obtain specific deposit properties and plating rates. Thicknesses of deposits vary from 0.25 to 0.5 micron (0.001 inch), or more for hard chromium plating. Bath Composition Chromic acid and sulfate are the necessary ingredients. Chromic-to-sulfate ratios range from 75:1 to 250:1. The composition depends primarily on whether the bath is co-catalyzed, e.g. with fluorides, fluosili-cates or fluoborates, and on the application (decorative or hard chromium). Plumbing fixtures and fitting shave been attractively finished using a tribalent chromium plating solution. (Photo courtesy MacDermid Inc., Kearny, NJ) Cr+6 is the source of chromium depos-ited from these baths. Chromic acid, Cr03, is the main component in solution make-up. Cr+6 is reduced to Cr+3, which in turn is reduced to unstable Cr+2 and further to Cr0. Some Cr+3 is normally found in operating baths, and indeed in the absence of Cr+3, little or no deposit is obtained. The introduction of small amounts of reducing agents to a new solution helps in bath start-up. An amount of Cr+3 exceeding two-three pct of the chromic acid content, however, reduces cathode efficiency and causes a variety of plating problems. The presence of other oxides of metals, e.g. iron, copper and nickel, combined with Cr+3, hurts bath performance. While the simple chromic-sulfate baths produce about 12 pct efficiency, co-catalyzed baths may deposit at efficiencies up to 22 pct. For a given composition, there is an optimum range of current density and temperature that produces a bright deposit. When a specific composition is found to produce the most desirable results for a given application, it should then be tightly maintained by periodic analytical and control methods. Table I—Single Catalyst Plating Bath Parameters: Chromic Acid Chromic/Sulfate ratio Temperature Current density Cathode efficiency Co-Catalyzed and High-Speed Baths Chromic Acid Chromic/Sulfate ratio Temperature Currently Density Cathode efficiency Decorative 225-300 g/liter 100:1 to 150:1 35-46C 7.0-15 A/dm2 6-12 pct Decorative 210-270 g/liter 150:1 to 250:1 35-55C 7.0-15 A/dm2 10-16 pct Hard (heavy) plating 250-400 75:1 to 100:1 49-65C (120-150F) 22-100 A/dm2 10-15 pct Hard (heavy) plating 200-300 g/liter 120:1 to 230:1 43-55C 22-90 A/dm2 15-22 pct Typical single-catalyst and co-catalyst plating baths have the parameters shown in Table I. In single-catalyst baths, cathode efficiency increases proportionally with chromic acid concentration, up to 250 g/ liter, and thereafter decreases (Fig. 1). Solutions with higher concentrations of chromic acid tolerate a higher level of trivalent chromium and iron oxide contaminants. Additions of secondary catalyst improve cathode efficiency at high concentrations of chromic acid, up to 300 g/liter. Analyses of chromic acid (Cr+6), sulfate, secondary co-catalyst, e.g. fluoride, as well as trivalent chromium (Cr+3) are needed to maintain the bath composition within the required limits. Temperature Chromium plated from trivalent solutions now has color closer to that of chromium from hexavalent chromium solutions. (Photo courtesy Atotech USA, Rock Hill, SC) Temperature is closely related to current density in its effect on brightness and coverage of deposit. Generally, the higher the current density, the higher the temperature requirement (Fig. 2). An optimum temperature range exists for a given concentration of chromic acid. Below or above that range, dull deposits result. In general, for decorative baths, the range is 35C (95F) to 46C (115F). For hard chromium, the range is 49C (120F) to 65.5C (150F). Since the chromic acid solutions used are fairly concentrated and viscous, stratification may occur. This results in uneven temperature distribution within the solution. Agitation is therefore required to equalize the bath temperature, to produce uniform brightness, and in the case of hard chromium, to improve deposit hardness. Preheating of parts to optimum bath temperature may be needed before they are introduced to the plating tank, and in rare instances cooling of parts may be required, in order to insure uniformity of deposit. Using both heating and cooling coils in the same tank may be necessary to maintain a precise temperature. 1. Effect of chromic acid concentration on current efficiency. Current Density At given solution composition and temperature, current density affects cathode efficiency, brightness and hardness. Generally, the optimum current density is recommended by the manufacturer of the plating chemicals used. At too-high current densities, burning or roughness of deposit occur. At low current densities, lack of chromium coverage can be expected. Be sure to use the proper current source. Deposit hazing and/or peeling can be traced to electrical problems, especially in conventional chromic-sulfate (single catalyst) baths. Three-phase rectifiers with a maximum of five pct AC ripple should supply an uninterrupted flow of current throughout the plating cycle. Standard current densities are in the range of 7.75-15.5 A/dm2 (0.5 to 1.0 asi) for decorative plating, and 23.25 to 100 A/dm2 (1.5 to 6.5 asi) for hard chromium plating. Anodes Insoluble lead or lead-antimony alloy are used. Plating of inside diameters and recessed areas may require that conforming anodes be fixtured at close proximity (0.5-1 inch) to the surface to be plated. 2. At constant temperature, cathode efficiency increases with current density. at constant current density, cathode efficiency decreases with temperature. In hard chromium plating, the closer the anode-to-cathode distance, the better the deposit distribution. Lead anodes ideally should be 90 pct as long as the cathodes. Tops and bottoms should be slightly below and above the tops and bottoms of the cathodes (parts) respectively to prevent excessive build-up at the plated ends. Properly operating lead anodes form a layer of dark brown lead oxide. A yellow or light colored coating on the anodes indicates a build-up of lead chromate, which results from too little or no current flow, such as during idle periods. This light colored layer has poor electrical conductivity. Cleaning and reactivating lead anodes can be performed chemically, using cleaners specially formulated for that purpose. Once cleaned, the anodes should be properly rinsed and promptly returned to the tank. A satisfactory distribution of anodes consists of three anodes two inches in diameter or four 1.5-inch-diameter anodes per foot of anode bar. Self-Regulating High-Speed Baths This type of solution incorporates fluoride complexes such as silicofluorides, in addition to sulfates. Salts of low solubility are selected and used to release the desired anions on a controlled basis. Mixtures containing potassium or sodium silicofluoride, and dichromate, for example, regulate the release of fluorides by common-ion effect. Mixtures of strontium sulfate and chromate regulate the release of sulfate. Consequently, at higher temperatures, the cathode current efficiency increases as a result of the increased solubility of catalysts in this type of bath. Crack-Free Chromium This is deposited from baths with low catalyst ratio and high concentration of chromic acid. Since the deposit is hard and brittle, it cracks when subjected to strain, forming a macro-cracked layer. Micro-Cracked Chromium Microcracking is produced by proprietary dual-catalyst baths. The micro discontinuity of the chromium layer results in spreading the corrosion potential that exists between chromium and an underlying nickel plate. This reduces the anodic current flowing to any one location on the nickel, greatly retarding the rate of corrosion. Crack densities of 27-50 cracks per millimeter are specified for optimum corrosion resistance. Microporous Chromium 3. Membrane separating anode box from cathode compartment in trivalent chromium plating. Microporous chromium also improves the corrosion resistance of nickel-chromium deposits, in a manner similar to that just described for microcracked chromium. Regular hexavalent chromium deposits are typically made microporous by one of two different methods. The bright nickel layer can be topped with another thin layer of nickel plated from a bath containing a very fine suspension of inert particles, which codeposit with the nickel. Chromium plated over this layer deposits around these particles, creating a microporous structure. Another way to produce microporous chromium is to lightly spray the surface of the chromium deposit with a hard, fine abrasive material such as 60- or 80-mesh alumina, or sand. The brittle chromium deposit breaks at the point of impact, forming micropores and exposing the bright nickel layer beneath the chromium. For best corrosion resistance, the pore density in both cases must exceed 10,000 pores per square centimeter. Cleaning and Surface Preparation Inadequate cleaning of the basis metal will lead to poor plating results. In decorative plating, hazy, pitted or non-adherent nickel deposits will be obtained as a result of inadequate surface preparation. The thin chromium deposit will magnify and reflect these defects. Table II—Comparison of Working Parameters of Trivalent Sulfate Type Versus Hexavalent Baths Concentration of chromium g/liter pH Temperature (C) Current density A/dm2 Anode:Shielded Anode Anode:Insoluable Anode Throwing power Reaction to current interruption Effluent Skin contact Deposit structure Plating rate Trivalent 5-6 3.5-3.9 40-50 4-9 Lead (alloy) Composite metal oxides Good Tolerant Low level of Cr3 Mild effect, similar to nickel Microporous 0.1 micron/min at 7 A/dm2 Hexavalent 100-250 0 40-50 10-15 Lead (alloy) Poor Causes "white wash" High level of Cr3 Strong acid burn and ulceration Induces microdiscontinuities 0.1 micron/min at 10 A/dm2 Allowing a nickel plated surface to dry during transfer will passivate the nickel and produce milky, hazy or no deposit when chromium plated. In hard chromium plating, surfaces of the basis metal should be free of oil, grease or rust. Etching of steel and stainless steel insures proper adhesion. The extent of etch needed depends on the composition of the steel. Carbon steel should be etched for 15-30 sec and up to 45 sec if the steel has been heat treated. Etching is best performed in non-catalyzed solutions of 210-225 g/liter (28-30 oz/gal) chromic acid to prevent overetching by sulfate, and particularly by fluoride catalysts. Reverse etching in the same plating bath usually introduces an excessive amount of iron into the bath and should be avoided. 4. Trivalent chromium can be plated from cell with shielded anode (left) or from single cell. TRIVALENT CHROMIUM PLATING Toxicity of Cr+6, low current efficiency, poor metal distribution, burns in high-current-density areas, "white-wash" and lack of coverage around holes are some of the problems associated with Cr+6 plating baths. These factors led to the eventual development of a safer and more efficient system, based on trivalent chromium. Trivalent chromium plating baths for decorative applications enjoyed a steady, although initially slow, acceptance as a substitute for hexavalent chromium plating. Their main attraction lies in the fact that they eliminate many of the shortcomings of hexavalent chromium solutions. Bath Chemistry Several advances in technology have taken place, making the process more readily acceptable to platers. Single-Cell Trivalent Chromium A major distinction between trivalent chromium processes is the chemistry used to prevent the formation of hexavalent chromium at the anode during plating. This must be done, since hexavalent chromium is a poison to all trivalent chromium plating processes. Fig. 4 shows the anode configurations of the shielded anode type and single-cell trivalent chromium plating. The single-cell process has built into its chemistry mechanisms to prevent the presence of hexavalent chromium. No hexavalent can form at the anode, and if hexavalent chromium were added, the chemistry of the single-cell bath would immediately convert the hex to tri. This ability to convert hex to tri instantly permits continued deposition of chromium from the single-cell bath. The working parameters of the two available versions of the single-cell process are shown in Table A. The room-tempera-ture process produces deposits similar in color to that of stainless steel or pewter. The elevated-temperature produces blue-white chromium almost identical in color to that from hexavalent baths. These solutions are more sensitive to metallic impurities than hexavalent baths. The trivalent baths are easily purified. Metallic impurities can be removed from single-cell baths by three methods. They can be plated out on dummies. They can be removed quickly by adding a chemical purifier that precipitates large quantities of metallic impurities at one time. Or they can be removed by ion exchange of a continuous basis. The elevated-temperature single-cell process can produce deposits over 0.25 mil thick and maintain the physical properties of hexavalent chromium deposits of similar thickness. —Dr. Donald L. Snyder World Wide Technical Marketing Manager Atotech USA Cleveland, Ohio There are currently at least three basic types of trivalent chromium baths available. A single electrolyte bath, chloride or sulfate based, using graphite or composite anodes, and special additives to prevent oxidation of trivalent chrome at the anodes (see accompanying "Single-Cell Trivalent Chrome" sidebar). Another type, a sulfate-based system, uses shielded anodes. Conventional lead anodes are surrounded by boxes sealed on one side by a selective ion membrane (Fig. 3, 4) and filled with dilute sulfuric acid. The membrane used is a perfluorinated sulfonic acid, reinforced with an inert Teflon® fabric. The membrane prevents the migrating trivalent chromium ions in the solution from reaching the anode, thus preventing their oxidation to the hexavalent state. The mechanism provides for excellent pH stability during plating. The bath is capable of producing light-colored deposits very close to the appearance of deposits from hexavalent baths. Table II summarizes the differences between Cr+6 and Cr+3 plating-bath parameters. The sulfate type trivalent chromium solution is maintained as one would maintain a conventional bright nickel bath. It utilizes a primary additive containing the trivalent chromium ion and a secondary additive that contains grain refiners and brighteners. These materials are added on an ampere-hour basis, using chemcial-feed pumps actuated by an ampere-hour meter. Metallic impurities affect results as shown in Table III. These impurities can be plated out on dummies as can be done with nickel baths, or through an external purification cell on a continuous basis. Ion exchange columns can also be used, as well as precipitating agents followed by filtration. A new insoluble catalytic anode has been developed for use in direct contact with the electrolyte of the above sulfate-based system. The new anode is designed to maintain an electrode potential level that will prevent oxidation of trivalent chrome at its surface. No selective oxidation additives are used in the electrolyte. Consequently, the same high tolerance to metallic impurities, light colored deposits and pH stability of this sulfate-based process are maintained. The users have a choice in selecting the shielded anode or the insoluble direct contact type anodes with the same bath. This new development will make it possible to use conforming and auxiliary anodes for plating of complex shaped configurations. Deposit Characteristics Trivalent chromium baths produce deposits that are inherently microdiscontinuous. Under about 0.65 micron (25 millionths of an inch), trivalent chromium deposits are microporous. Typical density of the pores in a microdiscontinuous deposit is 20,000 to 60,000 pores/sq cm. At higher thicknesses deposits are microcracked. The effect of microdiscontinuous chromium plating is to dissipate electrochemical corrosion current over a wide surface area, thus improving corrosion resistance. Room-temperature and sulfate-type, trivalent chromium processes plate at rates similar to those of hexavalent chromium baths—about 0.1 micron or four millionths of an inch per minute. The elevated-temperature chloride process plates at about 0.25 microns or 10 millionths of an inch per minute. The hardness of the deposit is similar to that of traditional hexavalent chromium—about 700 to 1,000 Vickers. Another advantage of the trivalent chromium bath is its ability to tolerate current interruptions without passivating or production of white "clouds" and hazes in the deposit. Stripping and replating over nickel can be carried out easily, with no adverse effects. Comparison of Effluent Treatment for Cr+6 and Cr+3 As trivalent chromium plating baths contain no hexavalent chromium, effluent treatment of the subsequent rinse waters is both simpler and cheaper. Most hexavalent chromium baths contain about 250 g/liter of chromic acid, equivalent to 130 g/liter of chromium metal. Even if a dragout rinse is used, the running-water rinses usually contain a high concentration of Cr+6. Table III—Metallic Impurities and Their Effects on Trivalent Baths Metallic Impurity Ni Cr (VI) Cu Zn Maximum Tolerance, mg/liter 500 30 30 70 Effect Produced If Exceeded Darkening of deposit Defects at low cd White blooms at high cd Darkness at low cd Effluent treatment consists of acidification of the rinse waters to obtain the required pH of 2.5, followed by reduction of the Cr+6 to Cr+3, using sulfur dioxide or sodium bisulfite, according to the following equation: CrO3 + 2 NaHSO3 + 2H2SO4 Cr2(SO4)3 + Na2SO4 + 3H2O Finally the solution is neutralized and this causes precipitation of chromium hydroxide. The theoretical requirement is three kilograms of sodium bisulfite (60-62 pct SO2) plus two-three kilograms of sulfuric acid to reduce one kg of hexavalent chromium. A trivalent chromium electrolyte may contain only five g/liter of chromium and require only neutralization to achieve precipitation. On precipitation, the volume of sludge generated by a hexavalent chromium electrolyte is approximately 30 times greater than that from a trivalent bath. A dilution of about 100 times with rinse waters might produce an effluent from the plating process of about 50 ppm (mg/liter), of Cr+3 before any further dilution occurs from other sources in the factory. Hexavalent Chromium Plating Troubleshooting Guide Common Plating Problems. Identifying the origin of a plating deficiency is the necessary first step in solving the problem. The basic causes of poor plating usually fall into three categories: Faulty bath chemistry. Improper temperature and/or current density. Poorly finished and/or inadequately cleaned basis-metal surface. Skill and experience will often permit a fairly precise identification of the source of any specific fault, or at least suggest the likely category in which it will be found. However, chemical analysis, Hull Cell tests, and reliable service recommendations give the best foundation for successful troubleshooting. The most common defects are listed here, together with their probable causes and some remedial steps. Table A—Working Parameters of Single-Cell Trivalent Baths Concentration of chromium, g/liter pH Temperature, C Current density. A/dm2 Maximum thickness millionths Anodes Line of anodes Throwing power and Covering power Reaction to current interruption Effluent Misting Skin contact Deposit structure less than 25 millionth greater than 25 Plating rate Filtration Room Temperature 15-25 2.8-3.5 20-22 8-13 50 Graphite Indefinite Better than hexavalent Tolerant Low levels of Cr3 None Mild effect, similar to nickel Microporous Microcracked 4 millionth per minute Only to remove solids Elevated Temperature 15-25 2.3-2.9 27-44 8-13 1500+ Graphite Indefinite Better than hexavalent Tolerant Low levels of Cr3 None Mild effect, similar to nickel Microporous Microcracked 10 millionth per minute Only to remove solids Milky Deposit Possible Cause: Corrective Step: High chromic acid/sulfuric ratio Chloride contamination Iron contamination Excess trivalent chromium Poor nickel surface High Temperature Increase sulfate additions Remove chloride with silver carbonate Dilute bath Clean anodes and reoxidize trivalent chromium Carbon treat nickel to remove organic contamination, improve rinsing of nickel and use a nickel activator Reduce to normal, check control Hazy Deposit Possible Cause: Corrective Step: High chromic acid/sulfate ratio High chloride contamination Iron contamination Excess trivalent chromium Low temperature Current density too high Intermittent current flow Poor nickel surface Increase sulfate concentration Remove chloride with silver carbonate Dilute bath Clean anodes and reoxidize trivalent chromium Increase temperature to normal Adjust anode-to-cathode ratio Check electrical contacts Improve rinsing, use nickel activator Gray and Dull But Smooth Deposits Possible Cause: Corrective Step: High chromic acid/sulfate ratio Current surge when contact is made High temperature Increase sulfate concentration Reduce current Check temperature controllers Dull Spots in High-Current-Density Areas Possible Cause: Corrective Step: High chromic acid/sulfate ratio Passive nickel Bipolar condition Increase sulfate concentration Improving rinsing, use nickel activator Use live entry to chromium bath Blue Deposits Possible Cause: Corrective Step: High temperature Reduce temperature to normal Rough Deposits Possible Cause: Corrective Step: Low sulfate Low temperature Surface preparation Surface preparation Suspended particles in bath Add sulfuric acid to increase Adjust temperature to normal Improve cleaning and rinsing Filter bath and eliminate source Burned Deposits Possible Cause: Corrective Step: High chromic acid/sulfate ratio Low chromic acid Excess trivalent chromium Too-high current density Low temperature Increase sulfate concentration Add chromium salts Clean anodes and reoxidize trivalent chromium Reduce current density or increase temperature Increase temperature to normal; preheat large, cold parts Brown Spots or Rainbows Possible Cause: Corrective Step: Low sulfate or catalyst Inefficient contacts Increase sulfate concentration; submit sample for analysis Check racking, build-up on hooks and rack tips, contact on bus bars. Poor Adhesion Possible Cause: Corrective Step: Insufficient etch Surface contamination Intermittent contact Poor nickel deposit Increase etch time; check etch bath Improve rinsing and/or cleaning cycle Clean and check contacts; work should enter chromium bath live Check surface preparation before nickel plating and condition of nickel bath Poor Coverage Possible Cause: Corrective Step: Low chromic acid content Low chromic acid/sulfate ratio Plating current too low Oxidized contacts Scaled anodes High temperature Passive nickel Add chromium salts Precipitate excess sulfate with barium carbonate (See Table V) Raise current density Clean contacts Clean anodes Reduce temperature to normal Activate nickle surface in nickel activator Slow Deposition Rates Possible Cause: Corrective Step: High chromic acid/sulfate ratio acid Too-low current density Scaled anodes Oxidized contacts Insufficient power supply Iron contamination Excess trivalent chromium Too high temperature Add the proper amount of sulfuric acid Increase voltage; check parts distribution; check for current leakage Clean anodes Clean contacts Increase rectifier size Dilute bath Follow procedure for the reoxidation of trivalent chromium Reduce to normal temperature Partial Deposition Rates Possible Cause: Corrective Step: Too-low current density Uneven current density Passive nickel Gas pockets Increase voltage, clean rack contacts, clean anodes Improve arrangement of parts on rack Activate cathodically or immerse in hydrochloric acid 50 pct Suspend parts so gas escapes freely No Deposit Possible Cause: Corrective Step: Reverse polarity Defective contacts Excess sulfate Chloride contamination Make proper connections Clean contacts Check ratio and correct Remove chloride with silver carbonate Pitted Deposits Possible Cause: Corrective Step: Pitted nickel deposit Pitting in basis metal Solution contamination from magnetic particles on ground surfaces Gas pitting Filter nickel bath Improve basis metal preparation Improve grinding and cleaning procedure Reposition work on racks to avoid gas formation Table IV—Conversion of Excess Sulfate to Barium Carbonate Required Excess sulfate to be removed (oz/gal) Bath Volume (gal) 100 200 300 400 500 600 700 800 900 1000 Barium Carbonate Required (oz) 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 2.2 4.4 6.6 8.8 11.0 13.2 15.4 17.6 19.8 22.0 4.4 8.8 13.2 17.6 22.0 26.4 30.8 35.2 39.6 44.0 6.6 13.2 19.8 26.4 33.0 39.6 46.2 52.8 59.4 66.0 8.8 17.6 26.4 35.2 44.0 52.8 61.6 70.4 79.2 88.0 11.0 22.0 33.0 44.0 55.0 66.0 77.0 88.0 99.0 110.0 13.2 26.0 39.6 52.8 66.0 79.2 92.4 105.6 118.8 132.0 15.4 30.8 46.2 61.6 77.0 92.4 107.8 123.2 138.6 154.0 17.6 35.2 52.8 70.4 88.0 105.6 123.2 140.8 158.4 176.0 19.8 39.6 59.4 79.2 99.0 118.8 138.6 158.4 178.2 198.0 22.0 44.0 66.0 88.0 110.0 132.0 154.0 176.0 198.0 220.0 Note: 1 oz/gal = 7.5 g/liter Table V—Conversion of Excess Sulfate to Barium Carbonate Required Excess sulfate to be removed (oz/gal) Bath Volume (gal) 100 200 300 400 500 600 700 800 900 1000 66° Bé Sulfuric Acid Required (fl oz) 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.05 1.0 1.6 2.1 2.6 3.1 3.7 4.2 4.7 5.2 1.0 2.1 3.1 4.2 5.2 6.3 7.3 8.4 9.4 10.4 1.6 3.1 4.7 6.3 7.8 9.6 10.9 12.5 14.1 15.7 2.1 4.2 6.3 8.4 10.4 12.5 14.6 16.7 18.8 20.9 2.6 5.2 7.8 10.4 13.0 15.6 18.2 20.9 23.5 26.1 3.1 6.3 9.6 12.5 15.6 18.8 21.9 25.0 28.2 31.3 3.7 7.3 10.9 14.6 18.2 21.9 25.6 29.2 32.9 36.5 4.2 8.4 12.5 16.7 20.9 25.0 29.2 33.4 37.6 41.8 4.7 9.4 14.1 18.8 23.5 28.2 32.9 37.6 42.3 47.0 5.2 10.4 15.7 20.9 26.1 31.3 36.5 41.8 47.0 52.2 Note: Fluid ounces × 29.5737 = cubic centimeters Trivalent Chromium Oxidation. If anode deficiencies and bath contaminants (iron, copper, or other metal ions, unstable mist suppressants, or stray parts corroding on the floor of the tank) allow the trivalent chromium concentration (Cr+3) in the bath to exceed the recommended maximum of around two pct, reoxidation is necessary. Reoxidation of the Cr+3 requires that the bath be electrolyzed with clean (active) anodes, using an anode/cathode area ratio of 30:1 and the highest bath temperature permissible. Occasionally the electroplater can process loads of work using a high ratio of anode-to-cathode area to salvage his bath. More often he will adopt one the three alternatives: Periodic reoxidation of bath by electrolysis. Continuous reoxidation of bath by means of a reoxidation cell within the plating tank. Continuous reoxidation of the bath in a separate reoxidation tank. Regardless of the method used, the reoxidation requires an anode current density of approximately 20 asf. The cathode current density will then be about 600 asf, if the proper anode:cathode area ratio has been maintained. Specific pointers on each of the three alternative procedures follow. 5. Cathode selector for use in reoxidation of trivalent chromium. When using tank anodes for the intermittent procedure, determine effective anode area for a pair of anodes. For example, two anodes with 1.5 inches OD have an immersed length of 30 inches. The surface area is 2 x 1.5 x 3.14 x 30 or 282.6 in. To find the cathode length to use between the two tanks anodes, draw a line across from 282.6 in2 to the line showing the diameter of rod to be used. A 24-inch length of 1/8 inch OD rod should be used. Periodic Reoxidation of Bath. Raise the plating-bath temperature to at least 145F, or to the highest temperature permis-sible for the tank-lining material, and space the tank anodes evenly along the anode bar. Position a small-diameter cathode rod (copper) between each pair of anodes. The diameter/length relationship of the cathode rods should relate to the effective anode area as indicated in Fig. 5. In the event the cathode is not of the same length as the anodes, the indicated length to be plated should be centrally positioned. If necessary, mask or tape the top section to permit proper positioning. Electrolyze the chromium plating bath until the Cr+3 concentration is down to about one pct of the chromic acid concentration. The duration of the bath electrolysis will depend on the initial Cr+3 concentration, the total anode area available for the particular bath volume, and the temperature of the bath. The efficiency of oxidation will decline as the Cr+3 concentration drops. For example, less electrolysis will be needed to lower the trivalent chromium level from five pct to four pct than to reduce it from two pct to one pct. It may be necessary to electrolyze the bath for several overnight periods or over a weekend to lower the Cr+3 concentration to the desired level (one pct of the chromic acid present). If the current/volume ratio is five A/gal of solution, the anode current density 20 asf, and the cathode current density 600 asf, it will take about two hours of electrolysis (on the average) to reduce the Cr+3 concentration 0.1 oz/gal. PFD REFERENCES Dennis, J.K. and Such, T.E., Nickel and Chromium Plating, Butterworth Publishers, 1986. Canning, Handbook on Electroplating, 22nd edition, 1978. Salauze, J., Traite'De Galvanoplastic, 2nd edition, 1950. Zaki, N., "Advances in Trivalent Chrome Plating," AESF Chromium Colloquium, 1987.