8

Stainless steel may give longer life, but its greater cost generally is not justified for a zinc or iron phosphating line, unless acidic solutions contain high levels of chloride. Because of greater acid concentration in manganese phosphating solutions (6 to 10% Va/Vw as compared with 1 to 3% Va/Vw in zinc phosphating solutions) and higher operating temperatures used, low-carbon steel tanks for manganese phosphating solutions have a shorter life than those used with zinc or iron phosphating solutions. For this reason, stainless steel tanks may be economically practical for manganese phosphating solutions. Stainless steel should also be considered for the heating coils. Plastic, fiberglass, and rubber-lined tanks have also been used successfully.

Water Perfowect

Water Perfowect

Fig. 4 Immersion phosphating tank for batch coating of small components. Drum into which parts are loaded is shown in immersion position.

Table 8, representing data from the experience of one company, shows a comparison of expected tank life, in years, for the three types of phosphating solutions using both low-carbon steel and type 316 stainless steel. These figures are approximately correct for all solutions. Some solutions permit extended tank life, and others shorten the tank life. One tank made of 6 mm (1 in.) mild steel has been in continuous operation for 15 years in an iron phosphating line.

Table 8 Expected service life of low-carbon steel vs. type 316 stainless steel phosphating tanks

Process

Service life, yr

stainless steel(a)

Iron phosphating

10(b)

20

Zinc phosphating

4-5(c)

10-20

(a) Any thickness that provides mechanical strength required. May be used as liner for low-carbon steel tank.

Many phosphating tanks made of low-carbon steel are lined with glass fiber impregnated with polyester resins. Phosphating compounds have no effect on this material, and it will last indefinitely in normal service. It is, however, susceptible to damage from impact, and careless handling of equipment during loading or unloading of the tanks may cause fractures or cracks. Care must be exercised in the placement of heating coils when using polyester-impregnated glass fiber liners. The maximum temperature this material can withstand is about 105 °C (225 °F). Many polyester resins have little resistance to alkaline materials and should not be used where more than casual contact with strong alkaline cleaners is possible.

Tank accessories, including steam coils or other heating mediums, piping, screens, drum trunnions, and drum-rotating mechanisms, may be made of low-carbon steel or stainless steel. Electropolished stainless steel steam coils permit less sludge buildup on the coils.

Tank Design. Tanks should have sufficient capacity to stabilize solution temperature and solution concentration and to prevent rapid buildup of solution contamination. Tanks for the phosphating stage should have a sloping bottom, with at least 0.46 m (1.5 ft) of space below the lowest work level to accommodate sludge buildup.

Rinse Tanks. Water rinse tanks and associated equipment, including steam coils or other heating mediums, piping, and screens, may be constructed of low-carbon steel. Rinse tanks for certain parts sometimes require drum-rotating devices. Immersion rinse tanks should include a method for solution agitation to assist rinsing action. This can be accomplished by use of low-pressure air distributed through evenly spaced holes in pipes laid along the bottom of the tanks. Another method is to recirculate rinse water through a similar piping arrangement. For a clear water rinse, the pump housing, bearings, impeller, and any other part in contact with the water may be of normal material. Acidulated rinses containing chromium preclude the use of brass or bronze in any part of the pump or valving that is in contact with the solution.

Drying equipment for immersion phosphating systems can be of several types. For small parts, such as washers, a centrifuge may be used to spin off moisture. If parts are hot enough, no additional heated air is required. However, if parts are cold, heated air may be introduced into the centrifuge. When parts are centrifuged, the phosphate coating may be damaged on some parts, rendering them unacceptable. Such parts may be dried in a basket or on a rack, in the same manner that larger parts are dried. This is done in a final tank or enclosure in which the parts are held while heated air (at

120 to 175 °C, or 250 to 350 °F) is blown on them. Heat sources may be steam coils, gas burners, or electric heaters. Drying time usually ranges from 2 min for simple parts to 5 min for complex parts. If rinse solutions are retained in pockets or seams, drying requires additional time or temperature, a mechanical aid such as an air blast directed at the pocket or seam, or tilting of the part.

Drums for containing and rotating parts are usually made of low-carbon steel. To obtain longer life, stainless steels may be used; however, one company reports a life expectancy of approximately 10 years from similar drums made from low-carbon steel. This long life is attributed to a hard coating of phosphate that develops on surfaces of the drum. Drums should have a loading-and-unloading door with a positive latch to prevent accidental opening and loss of load during a processing cycle. A drum for containing small parts during batch phosphating is shown in Fig. 5.

Baskets for handling parts too large for drums or too small or too heavy for racks can be made of either low-carbon or stainless steel. The choice is dictated by cost-life relationships.

Conveying equipment for the immersion process may be of any type that can transport work from the loading to the unloading stage. It must be capable of lowering work into and raising it out of various tanks in the proper sequence and at the proper time, either automatically or manually. Various types of conveying equipment are:

• Overhead monorail conveyors with manual or electric hoists. For very small production, the work can be moved manually from tank to tank.

• Chain-driven conveyors that lower and raise work into and out of each tank while it is continuously moving

• Automatic equipment, similar to automatic plating equipment but without equipment necessary for supplying electric current

Conveying equipment can be of varied design, but it must allow sufficient time for solution to drain from the work as it is raised from the tank. This solution should drain back to the original tank so it will not contaminate the next tank. Drainage and transfer time should not exceed 30 s, or the work may become discolored because of partial or complete drying between stages. The conveyor need not be made of acid-resistant material.

Work-supporting equipment, such as racks, hooks, and baskets, is similar in design and function to that used in electroplating, with the exception that it need not be electrically insulated. For phosphating, however, racks, hooks, and baskets should be resistant to alkaline cleaners, acid phosphating solutions, and other materials used in a phosphating line. Low-carbon steel is usually satisfactory. Stainless steel may be used where its additional life justifies the greater cost. Work-supporting equipment does not need to have tight contact with the work to be phosphated. Light contact with work-supporting equipment is more desirable, particularly on significant surfaces of the work, because coating may be thin or nonexistent at the point of contact, depending on the degree of insulation of the surface by hook, rack, or basket.

Fig. 5 Drum used in batch phosphate coating of small components
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