Hexavalent Chromium Plating

The first hexavalent chromium plating bath used for decorative plating, sometimes referred to as the conventional bath, consisted of an aqueous solution of chromic anhydride (CrO3) and a small amount of soluble sulfate (SO4=), referred to as the catalyst. The latter was added as sulfuric acid or as a soluble sulfate salt, such as sodium sulfate. When dissolved in water, the chromic anhydride forms chromic acid, which is believed to exist in the following equilibrium:

The ratio of chromic acid to sulfate, generally given as the weight ratio of chromic anhydride to sulfate, governs the current efficiency for chromium metal deposition. The cathode current efficiency also is affected by solution variables, such as concentration of chromic acid, temperature, and content of metallic impurities. The latter variable is an important consideration for commercial operations, because an excessively high content of impurities such as copper, iron, zinc, and nickel seriously affects bath conductivity, cathode current efficiency, and both throwing and covering power, even if the ratio of chromic anhydride to sulfate is within optimum limits for the application.

Most decorative chromium is deposited within these operating limits:

• Chromic anhydride, 200 to 400 g/L (27 to 54 oz/gal)

• Chromic anhydride-to-sulfate ratio, 80:1 to 125:1

• Cathode current density, 810 to 1880 A/m2 (75 to 175 A/ft2)

These wide limits encompass a broad variety of decorative applications. As of 1994, the trend is toward using chromic-anhydride concentrations of 250 to 300 g/L (33 to 40 oz/gal) and avoiding more highly concentrated baths and their attendant environmental and recovery concerns. The ratio of chromic acid to sulfate ion is usually maintained at levels near 100:1.

With the development of duplex, microcracked, and crack-free applications, specialized bath compositions and operating conditions have come into use. However, many of these are either proprietary or are not subjects of general agreement. The compositions and operating conditions for a general, additive-free, decorative chromium plating bath and a bright, crack-free bath are given in Table 2.

Table 2 Compositions and operating conditions for two chromium plating baths

Constituent or condition

General decorative bath

Bright, crack-free bath

Chromic acid

250 g/L (33.0 oz/gal)

260-300 g/L (35-40 oz/gal)

Ratio of chromic acid to sulfate

100:1 to 125:1

150:1

Operating temperature

38-49 °C (100-120 °F)

52-54 °C (125-130 °F)

Cathode current density

810 - 1885 A/m2 (75 - 175 A/ft2)

2690 - 3230 A/m2 (250 - 300 A/ft2)

To meet specific requirements for plating speed, nickel activation, and crack pattern, the chromium anhydride and sulfate concentrations should be properly correlated with temperature and cathode current-density limits. In preparing a bath and establishing operating conditions, these relationships should be considered:

• An increase in the temperature of the bath, except for mixed-catalyst baths, will decrease the cathode efficiency, decrease the number of cracks per unit length, decrease coverage at low current density, increase the limiting current density at which burning occurs, and increase passivating action on nickel.

• An increase in the weight ratio of chromic anhydride to sulfate will decrease the crack density and increase the nickel passivity. The chromium will whitewash (have a milky appearance) when plated over passive nickel.

• An increase in sulfate, at a constant chromic-acid concentration, temperature, and cathode current density, will increase the cathode current efficiency to an upper limit; beyond this point, any further increase in sulfate concentration can cause a decrease in cathode current efficiency.

In dilute chromic-acid solutions containing as low as 150 g/L (20 oz/gal) of sulfate, any small carryover of soluble sulfates from earlier solutions can quickly upset the balance of the solution. However, dilute solutions do have a higher cathode efficiency and a slightly wider bright range, although they require higher tank voltages to maintain desired current density. Even when other plating conditions (such as temperature and current density) are held constant, the plating operation can be seriously disrupted by any change in the ratio of chromic acid to sulfate.

Because there are advantages and disadvantages to using either high or low chromic-acid contents, some compromise is necessary. The size and shape of the article to be plated and the available equipment and power often determine exactly which solution should be used. In decorative chromium plating, all variables must be kept in the proper relationship. Frequent bath analysis and prompt adjustments are essential to maintain balanced conditions.

Mixed-Catalyst Baths. Since the mid-1950s, a number of mixed-catalyst proprietary chromium plating baths have been developed. The advantages of these baths are increased cathode current efficiencies, increased activating action on nickel and stainless steel, improved coverage at low current density, broader bright plating ranges, and improved decorative chromium applications, including dual, microcracked, and bright or dull crack-free.

Mixed-catalyst compositions contain chromic acid, sulfate, and fluorine compounds (frequently, fluosilicate ions) as active ingredients. Proprietary baths, formulated to regulate the concentration of catalyst ions, contain strontium, calcium, or potassium salts, to control the solubility of fluosilicate ions. Details on mixed-catalyst compositions are provided in several U.S. patents and at least one British patent (see "Selected References" at the end of this article). Most control requirements applicable to standard baths also apply to proprietary baths.

With the exception of ratio control, the problems associated with low, medium, or high chromic-acid concentrations in mixed-catalyst baths are the same as those for conventional chromium baths. After optimum conditions are found, the same close control must be maintained to prevent mischromes (absence of plate) in areas of low current density and blue, matte, or burnt deposits in areas of high current density. Because supplies for mixed-catalyst solutions are more expensive than chromic acid alone, using less-concentrated solutions can provide a cost advantage.

Baths for Microcracked Chromium. Typically, two chromium solutions are used successively to produce microcracked chromium plate. The first chromium solution can either be conventional or proprietary and is operated in a normal manner. A plating time of 8 min is preferred when recessed areas are involved, although plating times of 5 to 6 min are often used. Surging of the current can be used to increase coverage. Composition ranges and operating conditions for nonproprietary, first-plating solutions that are used to plate steel and zinc parts are given in Table 3.

Table 3 Bath compositions and conditions for plating microcracked chromium

Substrate material

Constituents

Chromic anhydride to sulfate ratio

Temperature

Current density

Chromic acid

Fluoride

g/L

oz/gal

g/L

oz/gal

°C

°F

A/m2

A/ft2

First plating bath

Steel

338-375

45-50

100:1

46-52

115-125

1075-1615

100-150

Zinc

375-413

50-55

140:1

46-52

115-125

1290-1720

120-160

Second plating bath

Steel

165-195

22-26

1.5-2.25

0.20-0.30

180:1

43-54

120-130

970-1345

90-125

The second chromium solution is similar to the first and can be either proprietary or nonproprietary. The chromic-acid concentration is lower and fluosilicate ions must be present in the bath to promote cracking. The plating time is approximately the same as in the first solution, 5 to 8 min, and current surging can be used, if desired. Composition ranges and operating conditions for the second chromium bath also are given in Table 3. The plating conditions are governed by the nature of the parts being plated. Solutions for parts having deep recesses should have a higher chromic acid and fluoride content and a lower sulfate content. However, thickness must be weighed against other influences on microcrack formation. For this reason, operating conditions can be established on a firm basis only by actual operation with the parts to be processed. On simple shapes, the second plating bath formulation can be used alone.

The use of a rinse or rinses between the two chromium solutions is not essential to the process, but it may help to avoid control problems, because of drag-out from the first chromium solution into the low-concentration second solution. When used, these rinses can be operated as reclaim tanks to minimize drag-out losses.

Solution Control. Regardless of which chromium bath is used, periodic analyses are required. Information on control procedures is provided in the article "Industrial (Hard) Chromium Plating" in this Volume.

Temperature. All chromium plating solutions require the control of temperature, current density, and solution composition. The exact temperature at which bright, milky, frosty, or burnt deposits occur depends on solution composition and current density. Chromium plating is usually performed within the range of 38 to 60 °C (100 to 140 °F), but the most common operating range is from 46 to 52 °C (115 to 125 °F). At room temperature, the bright plating range is impractically narrow. In a process set up to plate at 50 °C (120 °F) with all variables properly controlled, the temperature need vary only 1.5 or 2 °C (3 or 4 °F) to move the electrodeposit out of the clear, bright range. Consequently, an accurate temperature controller and facilities for rapid cooling and heating of the bath are essential. Temperature variation outside the bright operating range can either cause an unacceptably high rejection rate or necessitate costly stripping and replating operations. The preheating of heavy parts is necessary to avoid plating solution cooling and temperature fluctuation.

Current Density. The standard sulfate bath is usually operated at 1075 to 1720 A/m2 (100 to 160 A/ft2). A current density of about 1075 A/m2 (100 A/ft2) is used for solutions maintained at 38 °C (100 °F). A higher current density, sometimes as high as 3230 A/m2 (300 A/ft2), is required for solutions operated at 55 °C (130 °F). The choice for a specific use depends on such variables as the complexity of the articles being plated and the equipment available. After the current density has been established, close control must be maintained.

Changes in the ratio of chromic acid to sulfate require compensating adjustments in current density. If the sulfate content is increased (lower ratio), then current density must be increased to maintain full coverage in areas of low current density. If the sulfate content is decreased (higher ratio), then current density must be decreased to prevent burning in areas of high current density.

An increase in temperature may require an increase in current density to ensure full coverage in areas of low current density. A decrease in temperature may require a decrease in current density to prevent gray (burnt) deposits in areas of high current density.

As chromic-acid content increases, higher current densities can be used. The average cathode efficiency of most conventional chromium solutions is about 13% over a wide range of concentration, making it possible to plate for shorter times when using the most concentrated solution. Rectifiers with low ripple, not exceeding 5%, must be used to maintain trouble-free, uniform deposition.

Anodes. In chromium plating, insoluble lead or lead-alloy anodes are almost always used. Chromium metal is supplied by the chromic acid in the electrolyte.

Pure lead anodes are often attacked excessively by idle baths, which causes the formation of a heavy sludge of lead chromate on the bottom of the tank, making pure lead anodes impractical for all but continuous operations. During plating, a coating of lead peroxide forms on the anode. The coating favors oxidation of trivalent chromium at the anode. However, when the bath is idle, the coating dissolves to some extent in the solution, making attack on the anode possible.

To reduce the attack of the chromic-acid bath on the anode, several lead alloys are used. For conventional sulfate baths, 6 to 8% antimonial (Sb) lead is preferred, whereas for solutions containing fluoride, lead alloys with 4 to 7% Sn are used.

For an anode to provide optimum throwing power and coverage, it must be positioned properly in relation to the workpiece and have a continuous, uniform film of lead peroxide on the entire surface. Anodes with crusty surfaces have low conductivity and should be cleaned periodically by wire brushing or alkaline cleaning to ensure proper current distribution. The function of the anode is not only to conduct the plating current, but to oxidize trivalent chromium, which forms at the cathode, back to hexavalent chromium. To accomplish this, the anode area should be adjusted to provide the optimum anode current density for the oxidation necessary to keep the trivalent chromium at the desired level, usually 0.25 to 1.0 g/L (0.033 to 0.13 oz/gal). In decorative chromium plating, an anode-to-cathode area ratio of 2:1 is common for proper reoxidation and balance. If trivalent chromium continues to increase above the desired level, then the anode area should be increased to the point where the trivalent chromium concentration remains stable. Overheating of the bath can occur if the anode area is so small that resistive heating becomes a factor.

Anodes with round cross sections are most commonly used. When maximum anode area is desired, corrugated, ribbed, ridged, and multi-edged anodes are used. The round anode is preferred, because its surface is active on its entire circumference, enabling it to carry higher amperage at lower voltage. The absence of inactive areas on this anode minimizes the formation of lead chromate film, reducing maintenance requirements. If the weight of the anode presents a problem, then hollow, round anodes can be used. Although such anodes provide a 25 to 40% reduction in weight, their current-carrying capacity is less than that of solid anodes.

Anodes are manufactured by extrusion. Contact with the bus bar can be provided by a copper hook homogeneously burned to the extruded anode. Pure nickel, nickel-plated copper, and lead-coated copper are also used for hooks. Several hook styles are used, but the knife-edge hook is preferred. The hook and the top of the anode are covered with plastisol for protection against corrosion by fumes and drag-out drip. Bags typically used to cover the anodes in nickel- and copper-plating processes should not be used in chromium-plating processes. Roughness that is due to nonuniform anode corrosion is not a problem in chromium-plating operations. Therefore, the resulting particles do not have to be captured by the bags.

Control of Current Distribution. Chromium plating baths have poorer throwing and covering power than most other plating baths. To obtain thickness and coverage in areas of low current density, special auxiliary anodes are sometimes used.

Any workpiece of complex shape constitutes a problem of proper current distribution when nonconforming anodes are used. The current density and the thickness on a workpiece varies from highest on corners, edges, and areas closest to the anode to lowest on recesses and areas distant from the anode. Variations in current density result in differences of cathode efficiency, which accentuate the problems of uneven plate, burning, or complete absence of plate. These problems can be overcome, to some extent, by special racking and shielding techniques, such as:

• Wide spacing of concave parts on rack

• Increasing the distance between workpiece and anode

• Intentional shielding of a projection on one piece with a depression on an adjacent one

• Orienting areas of low current density toward the periphery of the fixture

• Moving the parts in the center of the rack closer to the anodes than those on the periphery of the rack

Improved coverage on areas of low current density can be achieved with striking, that is, plating at high current density for a short period of time. The striking time duration is kept to a minimum, usually 5 to 20 s, to avoid burning. Plating is continued at normal current density after the strike.

Current Shields. A nonconducting plate or panel (current shield) can be mounted on the plating rack to direct current away from areas of high current density or to direct additional current into areas of low current density. Figure 3 illustrates the use of a device to divert some of the current that would otherwise cause excessive current density and possible burning at the work areas closest to the anodes. The position and size of current shields are extremely important for their effective use and can be established best by trial and error. The use of shields, however, is always accompanied by some increase in drag-out.

Fig. 3 Current shield

Thieves or robbers made of metal conductors can be positioned near edges and points to shunt away current from these

areas. Rods with a diameter of 9.5 to 16 mm (- to -in.) are sometimes suspended vertically on both ends of a plating

rack to prevent burning or rough plate on the edges of the cathodes. Maintenance of robbers is of utmost importance, because they can be the source of large drag-out losses if metal is allowed to build up excessively.

Auxiliary Anodes. Special racks and auxiliary anodes are used only when conventional techniques fail to produce satisfactory coatings. Parts with deep recesses, such as coffeepots and small appliance housings, require auxiliary anodes. Auxiliary anodes are also used for parts with concave surfaces that are difficult to plate uniformly (Fig. 4). Auxiliary anodes also offer potential cost reductions by directing the plate into areas of minimum plate thickness without the penalty of overplating areas of high current density. The use of such devices should be considered even for some parts that do not present serious problems in meeting specifications for plate thickness. The shapes of many die castings make the use of auxiliary anodes particularly applicable. The current supply for auxiliary anodes can be the same as the major plating circuit with a separate current control, such as a rheostat. Greater flexibility is obtained if a separate current source is used for the auxiliary anodes.

Fig. 4 Use of auxiliary anode for a part difficult to plate to uniform thickness because of concave surface

Auxiliary anodes are mounted on the plating rack, insulated from cathode current-carrying members, and provided with means of direct connection to the anodic side of the electrical circuit (Fig. 5). In still tanks, the connection can simply be a flexible cable equipped with battery clamps. In fully automatic machines, cables are permanently mounted on the carriers, and contact brushes riding on an anode rail are provided to pick up the current. Connections must be positive. An interruption or drastic reduction of current could cause the auxiliary anode to function as a robber or shield, resulting in local interruption of plate, with consequent darkening and loss of adhesion.

Fig. 5 Rack assembly for decorative chromium plating

The auxiliary anode need not follow the contour of the part closely. An anode-to-work spacing of 13 mm (2 in.) or slightly more is usually effective. The auxiliary anode mounting must be designed carefully to prevent the anode from interfering with efficient racking and unracking of parts. The anode can be designed for removal while parts are being loaded on the rack, but good contact must be preserved. The auxiliary anode should be held rigidly to prevent it from short circuiting against the cathode.

Some platers connect auxiliary anodes electrically only during chromium plating, a practice that is usually satisfactory for still tank operation, where an anode can be physically mounted immediately before chromium plating. In an automatic plating machine, however, the auxiliary anode should be connected in the acid-copper bath, the nickel bath (at least, in the last half of the tank), and the chromium bath to avoid low thicknesses and low current density effects that could detract from appearance and cause difficulty in chromium plating.

Unless they are made of insoluble material, auxiliary anodes are consumed in plating. Their design should therefore permit easy replacement. Plastisol-coated steel bushings with locking screw heads protected by stop-off lacquer are satisfactory. As anodes become thin, they must be carefully inspected for replacement to avoid shorting out. The diameter of rod used should be as large as is compatible with the size of the part and with construction requirements to minimize the need for frequent replacement. A diameter of 13 mm (2 in.) is suitable for a variety of parts, ranging from small brackets to instrument panels and moldings. On larger parts, diameters as large as 25 mm (1 in.) or specially cast sections can be useful.

Bags should not be used on auxiliary anodes, because of the resulting solution contamination from drag-in. Avoiding roughness from bare anodes requires serious consideration if the anodes are to be immersed in copper and nickel undercoating baths. Roughness is not a problem when the anode is to be immersed only in the chromium solution. Leadalloy or steel anode material has been used satisfactorily for this purpose. Graphite rods also have been used to a limited extent. Auxiliary anodes are most frequently made of platinized titanium.

Bipolar anodes are a special variation of auxiliary anodes, in which current is not supplied by external connection. In use, collector plates are mounted at the cathodic end (the end closest to the tank anodes) of the bipolar anode to draw current from a larger section of the bath. Bipolar anodes can be used on conveyorized systems when a special bus bar is unavailable. Although adequate for some purposes, bipolar anodes are usually less effective than other auxiliary anodes and must be carefully maintained to avoid the problems of roughness from loosely adherent deposits of nickel and chromium on the collector plates. The metal deposited on the collector plates is often not reusable.

Stop-offs are not widely used in decorative chromium plating. However, when selective plating is required, a number of materials have the necessary qualifications, including ease of application and removal, resistance to hot cleaners and plating solutions, and excellent adherence and electrical insulation characteristics during use.

Special racks are sometimes used to prevent plating solution from entering tapped holes and areas where plate is not wanted. Figure 6 shows a plated lever with a 7.92 mm (0.312 in.) diameter hole that had to be free of plate. If conventional racking had been used, then the hole would have had to be reamed to remove the plate.

Fig. 6 Racking arrangement to prevent plating of chromium in the hole of a shift lever

Tanks. Chromium plating tanks can be constructed of steel and lined with flexible plastic-type materials, such as fiberglass or polyvinyl chloride (sheet form or sprayed) or lead alloy (6% Sb). Lead-alloy linings should be approximately

3.2 mm (-in.) thick. Plastic liners should range in thickness from 2.4 to 4.8 mm (— to —in.). Plastic linings are

preferred, particularly for proprietary baths with fluoride-containing anions, which may have a greater rate of attack on lead linings. Rubber mats or plastisol-coated metal ribs are often used to protect the sides or bottoms of lead-lined tanks from shorts that are due to either accidental contact or being punctured by dropped anodes or workpieces. Lead linings can cause serious bipolarity problems, because of their electrical conductivity.

Heating. Chromium plating tanks can be heated internally or externally. Internal heating, by steam coils or electric immersion heaters, is usually used for small tanks. External heating by heat exchangers is used for large tanks. Coils for internal heating can be made of lead, a lead alloy such as 4% Sn or 6% Sb, or tantalum. Titanium can be used for baths that do not contain fluoride ions. Immersion heaters should be quartz-covered. Heat exchangers can be made of tantalum, lead alloy (4% Sn or 6% Sb), high-silicon cast iron, or heat-resistant glass. Tantalum is preferred for heating coils or heat exchangers when proprietary solutions containing fluoride ions are used, because titanium is attacked by the fluoride. Consultation with vendors on specific material/process compatibility is suggested.

Plating Cycles. Typical system cycles for the application of six decorative chromium plating systems to identical workloads are given in Table 4. Each system is identified by the specific combination of metals successively deposited, the total thickness of plate, and the total plating time. The plating times and power requirements listed in Table 4 are theoretical values for perfect coverage. In practice, these values would be considerably higher to ensure adequate plate thickness in all areas. Table 5 provides the requirements for the design of several installed machines for the continuous plating of zinc die castings of average complexity. The higher-than-normal designed current density is related to potential future needs that exceed present requirements.

Table 4 Typical system cycles

System

Cycles

• (a) mm

^m

^lin.

Cu + Ni + Cr

A, B, D, F

50

1970

48

Cu + Cr

A, B, F

20

790

14

Ni + Cr

D, F

30

1180

36

Ni + Cr + Cr

D, G

32

1260

41.5

Ni + Ni + Cr

C, E, F

30

1180

36

Ni + Ni + Cr + Cr

C, E, G

32

1260

41.5

Operating parameters

A: Copper strike

Current density

325 A/m2 (30 A/ft2)

Plating time

2 min

Heat(b)

49-65 °C (120-150 °F)

Filtration

Yes

Agitation

Optional

B: Acid copper plate, high speed, bright (20 ^m, or 790 ^in.)

Current density

430 A/m2 (40 A/ft2)

Plating time (100% efficiency)

10 min

Heat(b)

21-27 °C (70-80 °F)

Filtration and agitation

Yes

C: Nickel plate, semibright (23 ^m, or 900 ^in.)

Current density

430 A/m2 (40 A/ft2)

Plating time(100% efficiency)

26 min

Heat(b)

55-65 °C (130-150 °F)

Filtration

Yes

Agitation

Usually

D: Nickel plate, bright (30 ^m or 1180 ^in.)

Current density

430 A/m2 (40 A/ft2)

Plating time (100% efficiency)

34 min

Heat(b)

55-65 °C (130-150 °F)

Filtration

Yes

Agitation

Usually

E: Nickel plate, bright (8 ^m, or 315 ^in.)

Current density

430 A/m2 (40 A/ft2)

Plating time (100% efficiency)

34 min

Heat(b)

55-65 °C (130-150 °F)

Filtration

Yes

Agitation

Usually

F: Chromium plate (0.3 ^m, or ^in.)

Current density

1550 A/m2 (144 A/ft2)

Plating time

Conventional (10% efficiency)

2 min

High speed (25% efficiency)

54 s

Heat(b)(c)

46-65 °C (115-150 °F)

Filtration and agitation

No

Ventilation(d)

Yes

G: Chromium plate, microcracked (0.64 ^m, or 25 ^in.)

Current density

1550 A/m2 (144 A/ft2)

Plating time (25% efficiency)

2.5 min

Heat(b)(c)

45-65 °C (115-150 °F)

Filtration and agitation

No

Ventilation(d)

Yes

(a) Power requirements and plating times given are theoretical values for perfect coverage. In practice, these values would be approximately doubled to ensure adequate thickness of plate in all areas. Table 5 has data for practical conditions.

(b) For operating temperature indicated.

(c) Cooling as well as heating may be required.

(d) Chemical suppressant (mist or spray) may be used in addition to ventilation.

Table 5 Design basis of equipment for continuous chromium plating of zinc-base die castings

Metal deposited

Designed current density

Minimum thickness of plate

Nominal thickness of plate

Plating time, min

A/m2

A/ft2

^m

• (a) ^in.

^m

^in.

Copper cyanide strike

1075

100

0.25

10

3-4

Bright copper

320

30

15

590

20

790

25-30

Semibright nickel

810

75

15

590

20

790

30

Bright nickel

1075

100

5

197

7.5

295

17

First chromium

2150

200

0.3

12

0.5

20

6.5

Second chromium

1615

150

0.25

10

0.5

20

6.5

(a) On parts of moderate complexity

Maintenance. The importance of proper solution maintenance and electrical, mechanical, and other equipment used in plating processes cannot be overemphasized. Table 6 identifies the daily, weekly, monthly, and annual inspection and correction operations that should enable the setup of an adequate maintenance program for chromium plating.

Table 6 Chromium plating maintenance schedule

Frequency

Action

Daily

Fill plating tank with solution from save-rinse and boil-down tanks.

Stir solution thoroughly, using low-pressure air agitation.

Check solution for chromic acid, sulfate, and anti-spray additives; make corrective additions.

Check temperature controls for satisfactory operation; adjust temperature to proper range.

Inspect plating racks; repair as necessary.

Check ground lights to see that plating circuits are clear; do not start plating until grounds are clear.

Put dummy cathodes in tanks and electrolyze solutions at maximum voltage for 15 to 30 min at start of each day.

Check hull cell.

Weekly

Boil down the save-rinse solution.

Check auxiliary catalyst; make additions as necessary.

Monthly

Check solution for metallic impurity content (iron, zinc, copper, nickel).

Clean and straighten anodes.

Check solution for trivalent chromium content.

Annually

Check all ammeters and ampere-hour meters.

Inspect and adjust all temperature controllers.

Clean and repair outside of all tanks; clean and repair all ventilation hoods and ducts.

Pump out solution, remove sludge. Clean and inspect tank and heating coil; repair as needed. Disconnect all bus bar connections; clean, draw-file and reconnect, including all anode and cathode joints. Inspect anodes; clean, straighten or replace as required.

Troubleshooting. Plating problems can still develop, even when proper maintenance is used. Some typical plating problems and solutions are given in Table 7. Examples of actual problems and solutions used with a variety of chromium-plated parts are given in Table 8.

Table 7 Chromium plating problems and corrections

Defect

Possible cause

Possible remedy

Poor covering power or low deposit thickness

Temperature too high

Adjust temperature to standard range.

Current density too low

Increase current density.

Low chromic acid

Adjust chromic acid to standard range.

Fluoride catalyst too high

Reduce concentration (by dilution).

Low chromic acid to sulfate ratio

Adjust ratio.

Poor electrical contact

Correct electrical contact.

Bath contamination

Remove impurities.

Burning in high current density areas

Temperature too low

Adjust temperature to standard range.

Current density too high

Reduce current density.

Chromic acid low

Raise chromic acid.

High chromic acid/sulfate ratio

Adjust ratio.

Fluoride catalyst too low

Adjust concentration of fluoride catalyst.

Deposit color nonuniform

Underlying surface not clean or active

Remove any interfering films and provide active surface.

Bipolarity during entrance to chromium

Enter bath with precontact (live entry).

Bath contamination by metallic

Analyze bath, remove impurities.

impurities

Table 8 Case histories of plating problems

Condition

Cause and correction

Passivation

Nickel-plated business machine parts were stored submerged in cold water to await barrel chromium plating. Although these parts were acid activated before chromium plating, chromium coverage was poor to nil on parts that had been stored for only a few hours. To remedy the problem, parts were stored submerged in a 10% solution of potassium bitartrate (cream of tartar). After several days of storage, the parts could be electrolytically reactivated and barrel chromium plated satisfactorily.

Burning

Although bath composition and temperature were carefully controlled, burnt chromium deposits occurred on die castings plated at moderate amperage. This resulted from nonuniform distribution of current caused by the corrosion of mounting blocks attaching the bus bar to the anode bar. The situation was corrected by welding the bus bar to the anode rail, eliminating mounting blocks.

Poor electrical contact

L-shaped die-cast frames approximately 0.09 m2 (1 ft2) in area, although plated in identical racks in an automatic plating machine,exhibited nonuniformity of plate and, in some racks, burnt deposits. This was found to be caused by variations in current from rack to rack in the plating machine. To correct this, mechanical joints were eliminated from the electrical circuit. The mechanical joints were replaced by welding cables from the carrier contact brushes to the rack mounting bar.

Mischromes

Mischromes (absence of chromium on certain areas) occurred on die-cast window frames that occupied the lower portions of racks during plating in a full-return-type automatic plating machine. These defects were caused by the presence of short anodes at the exit end of the nickel plating tank. Replacement with anodes of the correct length solved the problem.

Inadequate rinsing

Inadequate rinsing after chromium plating, which failed to remove small amounts of bath impurities,resulted in nonuniform appearance of the plate on zinc die castings. The parts were given a hot rinse at 93 °C (200 °F) before customary room-temperature rinses to remove bath impurities.

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