Equipment

The discussion of equipment that follows is confined largely to considerations that are specific to chromic acid plating processes. Mixed-catalyst and fluoride-free solutions have essentially the same equipment requirements as conventional sulfate solutions, except that all parts of the electrical system may need to be heavier to accommodate the increased current used. Equipment requirements for plating three specific parts are given in Table 9.

Table 9 Process and equipment requirements for hard chromium plating using conventional solutions

Item

Area of part

Area of load

No. of pieces/

Thickness of plate

Current density

Plating time, min

Temperature of bath

No. of work

Tank dimensions

mm

. 2 in.

mm

. 2 in.

8 h

fini

mil

A/dm2

A/in.2

°C

°F

rods

mm

in.

Small cutting tools

4,800

7.5

967,000

1500

10,000

1.3

0.05

30

2

5

50

120

1

1500 x 760 x 910

36

Shafts

20,000

30

600,000

930

200

25

1

30

2

63

50

x 910 x 910

36

Gun barrels(a)

15,000

23

543,000

828

180

25

1

45

3

40

54

x 610

x

(a) Plating of inside diameter 30-caliber gun barrels

Tanks and Linings. Figure 1 illustrates a hard chromium plating tank arrangement. Most tanks for chromium plating are made of steel and lined with an acid-resisting material. Because of their excellent resistance to corrosion by chromic acid, lead alloys containing antimony or tin may be used as tank linings.

Fig. 1 Tank and accessory equipment used for hard chromium plating. A, anode rods; B, lead or lead-tin anodes; C, cathode rod

Acid-resistant brick has been used as a lining material. Because of its electrical insulating characteristics, acid-resistant brick lining has the advantage over metal linings of reducing possible current losses or stray currents. Some installations combine a lead lining or plastic sheet lining with an acid-resistant brick facing. With fluoride-containing solutions, a brick lining is suitable only for temporary use.

Almost invariably, plasticized polyvinyl chloride is used for all three types of chromium plating solutions, provided that the solution temperature does not exceed 66 °C (150 °F). Sheets of this plastic are cemented to tank walls and welded at joints and corners. Other plastic materials are equally resistant to chemical attack but are more likely to fail at the welds when exposed to an oxidizing acid. Fiberglass utilizing either polyester or epoxy is unsatisfactory for use in mixed-catalyst solutions, because exposed fiberglass will be attacked by the secondary fluoride catalyst.

Design specifications for low-carbon steel tanks for chromium plating are given in Table 10. Lining materials for low-carbon steel tanks are given in Table 11. Steel tanks should be supported at least 100 mm (4 in.) from the floor; steel I-beams are used to provide this support and are mandatory when side bracing is required. To provide insulation, reinforced strips of resin-bonded glass fiber can be placed between the floor and the I-beams. Glass brick can be used as insulation between electrodes and the plating tank.

Table 10 Design specifications for low-carbon steel tanks for hard chromium plating

Size of tank

Thickness of low-carbon steel

Width of rim

Tank reinforcing

Length

Depth

m

ft

m

ft

mm

in.

mm

in.

Up to 1

Up to 4

Under 0.9

Under 3

5

3 16

50

2

No

Up to 1

Up to 4

Over 0.9

Over 3

5

3 16

50

2

Yes

1-4

4-12

All

All

6

1 4

75

3

Yes

Table 11 Lining materials for low-carbon steel tanks for hard chromium plating

Tank length

Lining material

Lead alloy(a)

PVC(b)

Brick(c)

m

ft

kg/m2

lb/ft2

mm

in.

mm

in.

Up to 2

Up to 6

40

8

5(d)

A(d) 16

100

4

2-4

6-12

50

10

5(d)

A(d) 16

100

4

Over 4

Over 12

60

12

5(d)

A(d) 16

100

4

(a) Antimonial lead, or lead-tin alloy.

(a) Antimonial lead, or lead-tin alloy.

(b) Plasticized polyvinyl chloride.

(c) Acid-resistant brick. For further protection, brick may be backed up with 39 kg/m2 (8 lb/ft2) of antimonial lead or lead-tin alloy, or with plasticized polyvinyl chloride sheet.

Lining should be 10 mm (— in.) thick at top to 0.3 m (1 ft) below top of tank. 8

Heating and Cooling. Steam heating coils and cooling coils can be made of antimonial lead or silver-bearing lead. Titanium coils are preferred for conventional and fluoride-free plating solutions because of their relatively low cost and long life. Tantalum or niobium-clad coils should be used for mixed-catalyst solutions due to the fluoride attack on titanium. These coils are mounted on tank walls behind the anodes. Steel pipes carrying steam and cooling water to the tank must have a nonconducting section in each leg, so that the coils cannot become an electrical ground back through the power plant system.

Electric immersion heaters sheathed in fused quartz are suitable for heating chromic acid solutions. The quartz is fragile and must be handled with care. Similar immersion heaters are sheathed in either tantalum, titanium, or lead alloy. It is sometimes feasible to heat and cool a chromic acid solution by piping the liquid to a tube bundle, concentric, or tube heat exchanger located outside the plating tank. Preferably, heat exchanger tubes should be made of tantalum or titanium. This method has the disadvantage of requiring pumping of the solution.

Temperature-control planning should begin with selection of the volume of solution required in the plating solution. An ideal volume consists of 1 L or more of solution for each 13 W of plating power (1 gal or more of solution for each 50 W of plating power). About 60% of this plating power (30 W) produces heat and maintains the solution at temperature in an uninsulated tank of standard design. Power applications in excess of 13 W/L (50 W/gal) require cooling of the plating solution and cause relatively rapid changes in solution composition.

Agitation. A chromium-plating solution should be agitated periodically, particularly when the solution is being started, to prevent temperature stratification. Air agitation is effective, but oil from an air pump must not be permitted to leak into the air system. Preferably, the air should come from an oil-free low-pressure blower. A perforated pipe of rigid polyvinyl chloride may be used to distribute air in the solution.

Busbars. Anode and cathode busbars are usually made of round or rectangular copper bar stock. These rods should be adequately supported to prevent them from sagging under the weight of anodes and work. Generally, selection of bar size is determined by allowing 1 cm2 of cross-sectional area for each 150 A (1 in2 of cross-sectional area for each 1000 A), although mechanical strength for load support is also a factor in determining rod size. Anode and cathode rods are supported above the tank rim by insulators, which may be made of brick, porcelain, or plastic. Even metallic supports can be used if a strip of electrical insulating material is placed between the plating tank and the busbar.

Power Sources. Although dynamos or motor-generator sets were once the usual sources of power for low-voltage direct current for plating, rectifiers are now regularly used. In general, use of motor-generator sets is now restricted to larger and more permanent installations. Originally, plating rectifiers were made of copper oxide or magnesium-copper sulfide, but these have been largely replaced by silicon rectifiers. Silicon is favored for plating rectifiers because of its high resistance to thermal overload and small space requirement. Hard chromium platers often start plating on a piece by sweeping up applied voltage and current from very low values to the high values used for plating. Because silicon-controlled rectifiers have high ripple at low output, the output should be filtered. Tap-switch controls, however, produce relatively low ripple over the entire output range.

A 6 V power source can be used for chromium plating, but it is generally desirable or necessary to operate with 9 to 12 V available. Chromium plating requires full-wave rectification with a three-phase input and full control, giving a ripple less than 5% and no current interruptions. If a rectifier becomes partially burned out, it may single phase to some degree, and this can cause dull or laminated, peeling deposits.

Fume Exhaust. A chromium-plating process produces a chromic acid mist, which is toxic. The maximum allowable concentration for 8 h continuous exposure is 0.1 mg of chromic acid mist per cubic meter of air. This concentration value is in accordance with recommendations by the American Conference of Governmental Industrial Hygienists. Because of the extreme toxicity of this mist, it is mandatory to provide adequate facilities for removing it. The minimum ventilation rate should be 60 m3/min per square meter (200 ft3/min per square foot) of solution surface area. (It should be noted that these regulations are presently under revision and are subject to changes.)

Generally, fumes are exhausted from a chromium plating tank by means of lateral exhaust vents along both long sides of the tank. For narrow tanks, up to 600 mm (24 in.) wide, a lateral exhaust on one side of the tank should be adequate unless strong cross-drafts exist. Velocity of the air at the lateral exhaust hood slots should be 600 m/min (2000 ft/min) or more.

In the design of ductwork, condensate duct traps should be included to capture chromic acid solution. Drains from these traps should be directed to a special container and not to the sewer. In this way, chromic acid solutions can be returned to the tank or recovery system or be safely destroyed. A fume scrubber or a demister should also be included in the system to remove most of the chromic acid fumes before exhausted air is emitted to the atmosphere. Many communities have air pollution regulations requiring fume scrubbers. Fume exhaust ductwork may be made of carbon steel and coated with acid-resistant paint. Modern construction uses chlorinated polyvinyl chloride.

Rinse Facilities. Rinsing the work after chromium plating prevents it from becoming stained or discolored. Insufficient rinsing can result in contamination of cleaning solutions during subsequent cycling of racks. Multiple rinsing facilities are recommended. After being plated, parts should be rinsed in a nonrunning reclaim tank, which can be used to recover part of the chromium solution dragout. After they are rinsed in the reclaim tank, plated parts should be rinsed in counterflowing cold water and hot water tanks. Water should cascade from the hot water tank to the cold water tank. A multiple counterflowing arrangement requires much less water than two separate rinsing tanks.

If rinse water is being returned to a chromic acid waste disposal unit, the flow of water into the hot water tank should be controlled automatically by a conductivity-sensing element in the cold water tank. At a predetermined concentration of chromic acid in the cold water, the water inlet to the hot water tank should flow, causing an overflow of cold water to the waste disposal unit. This arrangement decreases the amount of water consumed and minimizes the required capacity of the waste disposal unit.

Cold water rinse tanks may be coated, sprayed, or otherwise lined with plasticized polyvinyl chloride. Hot water rinse tanks may be constructed of types 347, 304, or 316 stainless steel, or they may be made of carbon steel and lined with lead. Reinforced polyester glass fiber also may be used for either hot water or cold water rinse tanks.

Spray rinsing also effectively removes residual chromic acid. Because spraying does not always reach recessed areas, sprays should be positioned above a dip rinse. As parts are removed from the dip rinse, they may be sprayed with clean water, which, in turn, is returned to the dip tank.

Maintenance. Following is a maintenance schedule for a still tank installation for hard chromium plating:

• Daily: Check temperature. Check concentration of solution by density measurements. Clean busbars and electrical connections. Remove any parts that fall from racks.

• Weekly: Analyze for chromic acid and sulfate contents.

• Monthly: Remove all sludge and parts from tank, using a hoe and dragging the bottom. If tank is used for plating inside diameters, analyze for trivalent chromium.

• Semiannually: Check tanks for leaks and condition of lining. Clean and inspect rectifiers or motorgenerating units. Check ammeter calibration.

• As necessary: Analyze for trivalent chromium, iron, nickel, copper, and zinc. Check condition of anodes.

This schedule is intended only as a guide; local conditions determine exact requirements. The rate of variation of solution constituents depends on the volume of solution, the method of operation for the solution, and the type and amount of work.

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