Tank and Solution Maintenance

The following maintenance schedule should be observed to ensure efficient phosphating operations for the setup described in the flowchart in Fig. 3.

• Daily: empty and clean stages corresponding to solutions 4 and 6.

• Weekly: empty and clean stages corresponding to solutions 7 and 8.

• Twice monthly: empty and clean stages corresponding to solutions 1 and 2.

• Monthly: empty and clean stage corresponding to solution 3.

• Quarterly: cleaner, water rinse, and phosphate stages should receive heated acidic cleanout. Blocked nozzles should be removed and cleaned or replaced. Heated acidic cleanout may involve inhibited hydrochloric acid.

Solution

No.

Type

Composition

Operating temperature

Cycle time, min

°C

°F

1

Acid pickle

H2SO4, 25 wt%

71

160

2

2

Cold rinse

Water®

RT

RT

1 1 4

3

Alkaline rinse

NaNO2, 2.4 g/L (2.0 * 10-2 lb/gal)(b)

66

150

1 1 4

4

Alkaline cleaner

Alkali, 0.7 g/L (5.8 * 10-3 lb/gal)

71

160

2

5

Hot rinse

Water

66

150

2

6

Zinc phosphate

NO2, accelerated(c)

60

140

1 I 2

7

Hot rinse

Water®

60

140

2

8

Acid rinse

Chromic and phosphoric acids(d)

71

160

2

(a) Purity maintained by overflow.

(b) Sodium hydroxide added to establish pH of 11.

(c) Total acid, 10 points; free acid, 0.7 to 1.1 points; acid checked using 10 mL (2.6 * 10-3 gal) sample. NO2 accelerator, 1.5 to 2.0 points, determined using 25 mL (6.6 * 10-3 gal) sample.

(d) Free acid, 0.4 to 0.6 points; total acid, less than 5 points; checked using 25 mL (6.6 * 10-3 gal) sample

Fig. 3 Sequence of operations for spray zinc phosphating of 80 mm (3.2 in.) mortar shell casings before painting. Total area, inside and outside, of each shell was 0.1 m2 (1 ft2); coating weight ranged from 1.7 to 2.1 g/m2 (5.6 x 10-3 to 6.9 x 10-3 oz/ft2).

Phosphating Tank Maintenance. The phosphate tank should be desludged on a continuous, automatic basis. Depending on work appearance, nozzles and spray pressure at the nozzle may require checking on a monthly basis, rather than quarterly. Phosphate heat exchangers require a heated acidic cleanout to maintain heating efficiency. Acidic cleanout usually involves the following procedure:

1. Pump out solution to holding tank.

2. Flush tank and spray piping with water.

3. Fill to pumping level with water; add hydrochloric acid (1 N or 10% volume acid/volume water, Va/Vw); add inhibitor.

4. Heat to 50 °C (120 °F); circulate spray system for 1 h.

5. Empty tank and flush with water.

6. Fill to pump level; add sodium hydroxide to pH of 10 to 12; circulate 5 to 10 min.

7. Empty tank; flush with water; and restore phosphate solution.

Break-in of Phosphating Solutions. Some zinc and manganese phosphating solutions, although mixed to recommended concentrations, must be broken in by the addition of ferrous salt, such as ferrous sulfate, before they can operate properly. Iron phosphating solutions require no break-in. After being mixed to proper concentration, iron phosphating solutions need only be raised to operating temperature to be ready for use. Most zinc phosphate processes used for paint base or for metal forming operate free of ferrous iron, and the break-in of these phosphating solutions is not a factor.

Zinc Phosphating Solutions. One method of breaking in a zinc phosphating solution is to tolerate a poor phosphate coating until some iron has gone into solution from the chemical reaction between the bath and the parts being coated. Some iron also may be present in sludge that has settled to the bottom of the tank or crusted on the sides from a previous bath. The coatings on first parts are of poorest quality; the coating quality gets progressively better as more iron goes into solution. A simple method is to suspend clean steel wool or scrap in the bath, or to introduce a small quantity of clean iron powder. Another method is to add 170 g (6 oz) of salt, such as ferrous sulfate, to each 380 L (100 gal) of solution. This is applicable to bath spray and immersion baths.

Manganese Phosphating Solutions. Careful attention should be given to breaking in a manganese phosphating bath because of its higher acid concentration in comparison to that of a zinc bath. For the best quality of manganese phosphate coatings, 0.2 to 0.4% Fe+2 in solution is the proper range. Usually, breaking in of a new bath is begun by the addition of 170 g (6 oz) of a ferrous salt, such as ferrous sulfate, powder to each 380 L (100 gal) of bath. This is followed by treatments using clean steel wool, powdered iron, or scrap iron to build up ferrous iron content. Manganese phosphating baths operate to best advantage when they have a steady, heavy work load. This permits considerable dissolution of iron, which usually maintains the ferrous iron content at a suitable level.

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