Trivalent Chromium Plating

The use of the trivalent chromium ion, instead of the hexavalent ion, in solution to deposit chromium has been of interest for many years. The first commercially successful decorative trivalent chromium process began in England in 1975 and in the United States in 1976. Environmental, safety, and productivity advantages have been the driving forces for the commercialization of trivalent chromium processes.

Hexavalent chromium ions are also considered carcinogenic and can cause skin ulcerations. The trivalent chromium ion is estimated to be about 100 times less toxic than hexavalent chromium ions.

Trivalent chromium processes have reduced misting to the extent that scrubbers, such as those used with hexavalent chromium processes, are presently not required to meet federal and state air-quality discharge standards. Waste-treatment costs are reduced by a factor of 10, because less than one-tenth of the chromium contained in hexavalent processes is used in the trivalent process (8 to 23 g/L, or 1 to 3 oz/gal, versus 115 to 300 g/L, or 15 to 40 oz/gal). In addition, the solution drains faster, so that less solution is removed with the parts. Finally, the chromium in the rinse water is already in the trivalent state, which eliminates the expensive and sludge-volume-building reduction step required with hexavalent chromium ions.

Some of the trivalent chromium processes plate up to three times faster than hexavalent chromium processes. This increases productivity in some shops. Increased throwing and covering powers, lack of burning, and tolerance to current interruptions and ripple also reduce rejects and can increase the allowable number of parts on a rack.

One of the main difficulties with the development of trivalent chromium baths was the formation of hexavalent chromium at the insoluble anodes during plating. Hexavalent chromium ions are a contaminant in trivalent chromium processes. They initially cause a poor deposit appearance and eventually result in the cessation of plating.

Two well-known proprietary approaches were developed to address the problem of hexavalent chromium formation during plating. The oldest and most frequently used technique incorporates several lines of defense against hexavalent chromium ions. Under normal operating conditions, hexavalent chromium cannot form. If it does manage to get into the plating solution, then it is reduced to the trivalent state, which eliminates it as a contaminant. This technique is referred to as the single-cell process, in contrast to the second technique, which isolates the insoluble graphite anodes from the trivalent-chromium-containing plating solution to restrict the formation of hexavalent chromium.

This second technique, commonly referred to as the double-cell, or shielded anode, method, uses an ion-selective membrane to create a barrier around the anode. Conventional lead anodes are used in a 10% sulfuric-acid electrolyte. The membrane keeps the trivalent chromium from contacting the anode, thereby preventing the formation of hexavalent chromium.

Solution Compositions. Depending on the process used and its operating conditions, the trivalent chromium ion content typically ranges from 5 to 20 g/L (0.67 to 2.67 oz/gal). It is introduced as a water-soluble salt and forms a stable specie upon combining with the stabilizing agents/catalysts. These agents permit the trivalent chromium ion to be stable in solution until it is plated out at the cathode. However, the stability process is not strong enough to interfere with the normal precipitation sequence used with chromium during waste treatment.

In comparison to hexavalent chromium solutions, which have good conductivity, the conductivity of the relatively high-pH and low-metal-content trivalent plating solution is increased by the addition of conductivity salts/buffers. Lower amperes but higher volts are required for trivalent chromium processes, compared with hexavalent chromium process requirements. Surfactants are added to reduce the surface tension of the solution for mist suppression, as well as to act as additives in the plating operation.

Solution Operation. The typical operating conditions for trivalent chromium processes are summarized in Table 9. High current density spiking at the onset of plating increases the already excellent covering and throwing powers of trivalent chromium processes, when compared with those of hexavalent processes. In general, wherever nickel can be plated, trivalent chromium can be plated. Hexavalent chromium processes fall short, particularly around holes and slots and in low current density areas. Process control, while plating at high current densities, is not a serious concern for trivalent processes, because they have less tendency to produce burnt deposits, compared with hexavalent processes. However, some earlier trivalent processes did produce thick deposits, over 1.3 pm (50 pin.). This thickness is sufficient to produce macrocracking. The cathode efficiency decreases with increasing current density. Therefore, the plating speed does not increase proportionally with an increase in current density.

Table 9 Typical operating conditions for trivalent chromium processes

Parameter

Value

pH

2.3-4.0

Temperature

27-50 °C (80-122 °F)

Current density

Cathode

430-1400 A/m2 (40-130 A/ft2)

Anode

540 A/m2 (50 A/ft2)

Agitation

Mild air

Rectifier voltage

6-15 V

Deposition rate

Single-cell process

0.20-0.25 |m/min (8-10 |in./min)

Double-cell process

0.08-0.10|m/min (3-4 |in./min)

Once the operating range has been established for a particular plating installation, the pH and temperature must be controlled well, because they influence plating speed, covering power, and color. The buffering ability of the solution is strong enough that large pH fluctuations do not occur. As the pH increases, the plating rate decreases, but the covering power increases. In general, trivalent chromium deposits do not have the blue-white color of hexavalent chromium deposits. Generally, they have a deeper, slightly darker appearance. However, the newer trivalent processes can produce deposits very close in appearance to hexavalent chromium deposits. In most cases, the color difference is noticeable only when the part is placed next to a hexavalent chromium-plated part.

Temperature. Depending on the process selected, either cooling or heating might be required for temperature control to maintain a bath at desired operating parameters. When lower operating temperatures are desired, some degree of cooling might be required to offset the power used for deposition. Cooler operating temperatures increase the covering power of the process, but slightly darken the color.

Anodes and Agitation. Anode current density should be maintained below 540 A/m2 (50 A/ft2) to promote anode life and consistent bath operation. The insoluble graphite anode used in the single-cell process should last indefinitely, if it is not physically damaged. Lead anodes will form protective insoluble films as long as the anodic current density is properly maintained in the double-cell process, resulting in a limited production of lead salts. Mild and uniform air agitation is used around the parts to assist in obtaining metal distribution and appearance.

Contamination Control. The major contributor to a change in appearance of the trivalent chromium deposits is solution contamination. Trivalent chromium solutions are much more sensitive to bath contamination, but are much more easily purified than are hexavalent chromium solutions. Organic contaminants, a minor problem, are typically removed by filtration through carbon. Organic contamination appears in the chromium deposit as white smears that resemble a pattern typically associated with poor cleaning.

Inorganic contaminants, such as iron, nickel, copper, and zinc, cause the deposit to have dark streaks and/or to lose covering power. The newest and easiest method for removing inorganic impurities is to continuously purify the plating solution by passing it through a specially designed resin. Using this technique, the inorganic impurity levels can be maintained much below the level that will cause any operational or appearance problems.

Three other general methods can be used to remove these contaminants. The slowest approach is to plate them out whenever the bath is not being used for production. An alternative method is to set a small plating unit, connected by a recirculating pump, to the main plating tank. Dummy sheets are used in the small unit to continuously plate out impurities without interrupting production.

A third method that is available for some processes is to use chemical purifiers that can remove large quantities of inorganic contaminants during one or two hours of downtime. Although this method is very fast, it has two disadvantages. The chemical precipitates the impurities within the plating bath. The precipitation itself does not cause any plating problems, but the precipitates could adhere to the parts as they leave the tank, causing them to have an objectionable white film. If chemical purifiers are improperly used, then the solution chemistry can be affected, resulting in a darker deposit and poorcoverage.

Plating Problems and Corrections. Some of the plating problems experienced with trivalent chromium baths can be ascribed to common operational problems. Poor coverage is typically due to low pH, high temperature, low current density, or lead or zinc contamination. Dark clouds or smudges on the work can arise from metallic contamination or low complexant or surfactant concentrations. White patches on the work can be caused by high concentrations of surfactants or other organics, lead contamination, or high wetting agent concentration in the nickel bath used prior to chromium plating.

Trivalent and Hexavalent Deposit Comparisons. The choice of chromium plating solution, whether hexavalent or trivalent, depends on the individual application under consideration. The characteristics of these processes are compared in Table 10.

Table 10 Trivalent and hexavalent chromium comparison

Parameter

Trivalent chromium

Hexavalent chromium

Throwing power

Good

Poor

Covering power

Good

Poor

Current interruptions

Completely tolerant

Intolerant

Rectifier ripple

Completely tolerant

Intolerant

Deposit structure (microdiscontinuous):

Single cell

Microporous and microcracked

Special processes required

Double cell

Microporous

Ease of burning

Very difficult

Easy

Ease of rinsing

Easy

Moderate

Color buffing requirement

Never

Occasional

Filtering requirement:

Single cell

Only after purification

Never

Double cell

Daily with carbon

Conditioning/dummying:

Single cell

Never

Start up to each day

Double cell

Start up and routinely

Passivity of nonplated surfaces

Needs post dip

"Chromate" surfaces

Color of deposit:

Single cell:

Ambient temperature

Pewter or stainless steel

Blue-white

Elevated temperature

Metallic white

Double cell:

Elevated temperature

Metallic white

Waste treatment

Easy

Moderate

Relative safety

Similar to nickel

Similar to cyanide

Misting

Almost eliminated

Heavy

Odor

Almost eliminated

Strong and dangerous

Nickel Plating

Revised by George A. Di Bari, International Nickel Inc.

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