Control of Coating Weight

Figure 8 compares processing stages involved in manganese phosphating to two different ranges of coating thickness, 2.5 to 7.6 pm (0.1 to 0.3 mil) for light coatings and 7.6 to 15 pm (0.3 to 0.6 mil) for heavy, or conventional, coatings. The same phosphating compound is used in each line. Difference in coating weights depends on cleaner used and time in the phosphating solution. For conventional, heavy manganese phosphate coatings, parts are cleaned in an alkaline cleaning solution, providing a surface that permits good contact between metal and the phosphating bath. The resulting coating is heavy and coarse-grain, and it can readily absorb oil. For light coatings, a kerosene-based or similar solvent emulsion cleaner is used. A thin residue of oil left on the metal after two rinses acts as a buffering agent or grain-refiner, to produce a thinner, finer-grain coating. Usually, less lubricating oil is desired in conjunction with a fine-grain coating. Consequently, an additional step is involved for removing excess oil. The additional step is not usually necessary with a coarse-grain coating.

Solvent clean (dip) Solution 1

Light manganese phospflatfi ooatinp 10 Q um, or 0.1 to 0.3 ml thick) 1

fSP'ay) SolullDJI 2

Hm rinse

I'dJpl

Solution 3

Immersion manganese phosphate Solution 4

Hof rmse

Idipl Solution 3

Hof rmse

Idipl Solution 3

Heavy manganese phosphate coaling (8 m IS jim. or 0.3 to 0.6 mil thick)

AiKaiine clear

*

Ko! r nso

(dip)

idipj

Solution &

Solulion 3

Immersion manganese phosphate Solution 4

Heavy manganese phosphate coaling (8 m IS jim. or 0.3 to 0.6 mil thick)

Immersron manganese uricisph ate Solution 4

Hol nnss? (dip) Solu lion 3

O.i dip Solution 5

Solution

No.

Type

Composition

Operating temperature

Cycle time, min

°C

°F

1

Solvent cleaner

3-10

2

Warm rinse

Water

38

100

1-3

3

Hot rinse

Water

82

180

1-3

4

Manganese phosphate

(a)

93

200

(b)

5

Oil

Soluble oil, 5%

60

140

1-3

6

Alkaline cleaner

93

200

3-10

(a) Contains 12 points total acid, as measured by titration of a 2 mL (5.3 x 1Q"4 gal) sample.

(a) Contains 12 points total acid, as measured by titration of a 2 mL (5.3 x 1Q"4 gal) sample.

(b) For light coating, 8 to 12 min; for heavy coating, 1Q to 2Q min

Fig. 8 Sequence of operations for light vs. heavy applications of manganese phosphate coatings. Coating weight is function of specific cleaner used and immersion time in phosphating solution.

A phosphating line for spray zinc phosphating of automobile bodies is shown in Fig. 9. The bodies average 80 m2 (860 ft2) in area and the line speed ranges from 0.12 to 0.14 m/s (24 to 28 ft/min). Coating weights range from 1.6 to 2.4 g/m2 (5.2 x 10-3 to 7.9 x 10-3 oz/ft2). Production rate may reach 75 bodies per hour. The table accompanying Fig. 9 gives details of solutions used in this line and lists cycle times for various stages.

Prçwiptf 1

Alkaline clean 2

Acidulated ■rinse a

Deiomzed water 9

aif dry 11

Fresh deionizeö wale' rinse to

Stage

Type

Composition

Temperature

Time,s

°C

°F

1

Organic solvent

Mineral spirits

30

86

60

2

Alkaline cleaner

Titanated, alkali 6.0 g/L (5.0 x 10-2 lb/gal)

60-65

140-150

70

3

Alkaline cleaner

Titanated, alkali 6.0 g/L (5.0 x 10-2 lb/gal)

60-65

140-150

70

4

Hot rinse

Water

55-60

130-150

150

5

Activated water rinse

Titanated, 7.5-8.5 pH 0.5 g/L (4 x 10-3 lb/gal)

40-45

104-115

35

6

Zinc phosphate

ClO3 accelerated(a)

50-55

122-130

70

7

Rinse

Water

35

95

15

8

Acidulated rinse

Partially reduced chromic acid (150 to 200 ppm Cr6+)

35

95

35

9

Rinse

Deionized water (100 ^mho max)

35

95

70

10

Rinse

Deionized water (10 ^mho)

35

95

15

11

Dryer

Hot air

(a) Total acid 20 mL (5.3 x 10-3 gal), free acid 0.9 mL (2.4 x 10-4 gal), nitrite accelerator 1.5 mL (4.0 x 10-4gal); acid checked with 10 mL (2.6 x 10-3gal) sample, accelerator checked with gas evolution apparatus

(a) Total acid 20 mL (5.3 x 10-3 gal), free acid 0.9 mL (2.4 x 10-4 gal), nitrite accelerator 1.5 mL (4.0 x 10-4gal); acid checked with 10 mL (2.6 x 10-3gal) sample, accelerator checked with gas evolution apparatus

Fig. 9 Sequence of operations for spray zinc phosphating of automotive bodies

Tables 3, 4, and 5 present phosphate coating applications and weights. Table 3 deals only with spray application, but it covers both iron and zinc phosphate coatings as bases for paint films. By comparing the area of the parts and the production per hour controlled to obtain the uniform coating weights shown, it is easy to see the interrelation of size, production time, and coating weight. In all applications, the material being coated was low-carbon steel sheet. Table 5 lists applications for manganese phosphate coatings for wear resistance.

As indicated by the curve in Fig. 10, based on the experience of one processor of small threaded parts, the consumption of phosphating solution concentrate is directly proportional to the area and thickness of the coating applied. These parts were immersion zinc phosphated, processed in batches in a rotating drum, to a coating weight of approximately 10.8 g/m2 (3.5 x 10-2 oz/ft2). Figure 10 shows that the direct proportionality of area coated to concentrate consumed does not begin until an initial coat is deposited. At the time when parts are immersed, there is an immediate reaction in which an irregular coating is quickly deposited. Because the maximum area of bare steel is exposed to the bath at that time, maximum efficiency takes place. The remaining time in the bath serves to refine the coat by depositing crystals to fill gaps between existing crystals and to increase coating weight to uniform thickness by depositing crystals over previously deposited crystals.

Are# cpjtect, 1000 ft"

Are# cpjtect, 1000 ft"

A-'-jj cofllfM 1000 Fii"

Fig. 10 Plot of zinc phosphating concentrate consumed vs. area covered for small threaded components coated to 10.8 g/m2 (3.5 x 10-2 oz/ft2) with barrel phosphating

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