By Nabil Zaki
SurTec International
Zwingenberg, Germany

Dana Keeps Zinc Plating Under Control While National Plating has created a new line using automated barrels, Dana has automated its entire zinc plating line through the use of some interesting software. To find out more on how centrifuges can be used in wastewater treatment, check ou the PF Online article, "Centrifuge Improves Waste Treatment."

Despite the simplicity of their formula- tions, chromium plating baths are more complicated to operate than most plating baths, and they require rigorous controls.

Chromic-Acid-Based Baths
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 0.25-0.5 micron, 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, for example, with fluorides, fluosilicates or fluoborates, and on the application (decorative or hard chromium).

Cr+6 is the source of chromium deposited from these baths. Chromic acid, Cr03, is the main component in solution makeup. 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 startup.

An amount of Cr+3 exceeding 2-3% of the chromic acid content, however, reduces cathode efficiency and causes a variety of plating problems.

The presence of other oxides of metals, such as iron, copper and nickel, combined with Cr+3 hurts bath performance.

While the simple chromic-sulfate baths produce about 12% efficiency, co-catalyzed baths may deposit at efficiencies up to 22%. 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.

Plumbing fixtures and fittings have been attractively finished using a trivalent chromium plating solution.

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 decreases thereafter (see Figure 1). Solutions with higher concentrations of chromic acid tolerate a higher level of trivalent chromium and iron oxide contaminants. Additions of a 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, for example, fluoride as well as trivalent chromium (Cr+3) are needed to maintain the bath composition within the required limits.

Temperature. 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 (see Figure 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 35-46C. For hard chromium, the range is 49-65.5C.

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, produce uniform brightness and, in the case of hard chromium, 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 to ensure uniformity of deposit. Using both heating and cooling coils in the same tank may be necessary to maintain a precise temperature.

Chromium platings from trivalent solutions now have acolor closer to that of chromium from hexavalent chromium solutions. (Photo courtesy Atotech USA, Rock Hill, SC).

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 a current density, burning or roughness of deposit occur. At too low a current density, 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 5% 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 amp/dm2 for decorative plating and 23.25-100 amp/dm2 for hard chromium plating.

Anodes. Insoluble lead or lead-antimony alloys 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.

In hard chromium plating, the closer the anode-to-cathode distance, the better the deposit distribution. Lead anodes ideally should be 90% as long as the cathodes. Tops and bottoms should be slightly below and above the tops and bottoms of the cathodes (parts) to prevent excessive build-up at the plated ends.

1. Effect of chromic acid concentration on current efficiency.

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 2 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.

2. At constant temperature, cathode efficiency increases with current density.

Microporous Chromium. 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.

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 ensures 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 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.

TRIVALENT CHROMIUM PLATING
Toxicity, 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.

3. Membrane separating anode box from cathode compartment in trivalent chromium plating

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.

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 “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 (see Figures 3 and 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, 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.

4. Trivalent chromium can be plated from a cell with shielded anode (left) or from a single cell.

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 amp-hr basis, using chemcial-feed pumps actuated by an amp-hr 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 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, trivalent chromium deposits are microporous. Typical density of the pores in a microdiscontinuous deposit is 20,000-60,000 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, 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/min.

5. Cathode selector for use in reoxidation of trivalent chromium

The elevated-temperature chloride process plates at about 0.25 micron/min.

The hardness of the deposit is similar to that of traditional hexavalent chromium—about 700-1,000 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 water 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.

Effluent treatment consists of acidification of the rinse water 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 + 2NaHSO3 + 2H2SO4
Cr2 (SO4)3 + Na2SO4 + 3H2

Finally the solution is neutralized, causing precipitation of chromium hydroxide.

The theoretical requirement is 3 kg of sodium bisulfite (60-62% SO2) plus 2-3 kg of sulfuric acid to reduce 1 kg of hexavalent chromium. A trivalent chromium electrolyte may contain only 5 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 water 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; and
• 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 with their probable causes and some remedial steps.

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 in the bath to exceed the recommended maximum of around 2%, 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:

1. Periodic reoxidation of bath by electrolysis.
2. Continuous reoxidation of bath by means of a reoxidation cell within the plating tank.
3. 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 to cathode area ratio has been maintained. Specific pointers on each of the three alternative procedures follow.

Periodic Reoxidation of Bath
Raise the plating-bath temperature to at least 145F, or to the highest temperature permissible 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 Figure 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 1% 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 5% to 4% than to reduce it from 2% to 1%.

It may be necessary to electrolyze the bath for several overnight periods or over a weekend to lower the Cr+3 con-centration to the desired level (1% of the chromic acid present). If the current/volume ratio is 5 amp/gal of solution, the anode current density 20 asf and the cathode current density 600 asf, it will take about 2 hr 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.

TABLE IV—Working Parameters of Single-Cell Trivalent Baths
Room Elevated Temperature Temperature
Concentration of Chromium (g/liter) 15-25 15-25
pH 2.8-3.5 2.3-2.9
Temperature (C) 20-22 27-44
Current Density (amp/dm2) 8-13 8-13
Maximum Thickness millionth 50 1,500+
Anodes Graphite Graphite
Line of Anodes Indefinite Indefinite
Throwing Power and Covering Power Better than hexavalent Better than hexavalent
Reaction to Current Interruption Tolerant Tolerant
Effluent Low levels of Cr3 Low levels of Cr3
Misting None None
Skin contact Mild effect, similar Mild effect, similar to nickel to nickel
Deposit Structure less than 25 millionth Microporous Microporous greater than 25 Microcracked Microcracked
Plating Rate 4 millionths/min 10 millionths/min
Filtration Only to remove solids Only to remove solids

TABLE V—Conversion of Excess Sulfate to Barium Carbonate Required
Bath Volume (gal)
100 200 300 400 500 600 700 800 900 1000
Barium Carbonate Required (oz)
0.01 2.2 4.4 6.6 8.8 11.0 13.2 15.4 17.6 19.8 22.0
0.02 4.4 8.8 13.2 17.6 22.0 26.0 30.8 35.2 39.6 44.0
0.03 6.6 13.2 19.8 26.4 33.0 39.6 46.2 52.8 59.4 66.0
0.04 8.8 17.6 26.4 35.2 44.0 52.8 61.6 70.4 79.2 88.0
0.05 11.0 22.0 33.0 44.0 55.0 66.0 77.0 88.0 99.0 110.0
0.06 13.2 26.4 39.6 52.8 66.0 79.2 92.4 105.6 118.8 132.0
0.07 15.4 30.8 46.2 61.6 77.0 92.4 107.8 123.2 138.6 154.0
0.08 17.6 35.2 52.8 70.4 88.0 105.6 123.2 140.8 158.4 176.0
0.09 19.8 39.6 59.4 79.2 99.0 118.8 138.6 158.4 178.2 198.0
0.10 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 VI—Conversion of Excess Sulfate to Barium Carbonate Required
Bath Volume (gal)
100 200 300 400 500 600 700 800 900 1000
66 Degrees Bé Sulfuric Acid Required (fl oz)
0.01 0.5 1.0 1.6 2.1 2.6 3.1 3.7 4.2 4.7 5.2
0.02 1.0 2.1 3.1 4.2 5.2 6.3 7.3 8.4 9.4 10.4
0.03 1.6 3.1 4.7 6.3 7.8 9.6 10.9 12.5 14.1 15.7
0.04 2.1 4.2 6.3 8.4 10.4 12.5 14.6 16.7 18.8 20.9
0.05 2.6 5.2 7.8 10.4 13.0 15.6 18.2 20.9 23.5 26.1
0.06 3.1 6.3 9.6 12.5 15.6 18.8 21.9 25.0 28.2 31.3
0.07 3.7 7.3 10.9 14.6 18.2 21.9 25.6 29.2 32.9 36.5
0.08 4.2 8.4 12.5 16.7 20.9 25.0 29.2 33.4 37.6 41.8
0.09 4.7 9.4 14.1 18.8 23.5 28.2 32.9 37.6 42.3 47.0
0.10 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