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To porcelain enamel coating operations

Stage

Solution composition

Temperature

Cycle time, seconds

°C

°F

1-Prewash

Tap water

Ambient to 71

Ambient to 160

30-60

2-Wash

Alkaline cleaner, 7.5-22.5 g/L (1-3 oz/gal)

60-71

140-160

60-90

3-Rinse

Tap water

60-71

140-160

30-60

4-Wash

Alkaline cleaner, 7.5-22.5 g/L (1-3 oz/gal)

60-71

140-160

60-90

5-Rinse

Tap water

Ambient to 71

Ambient to 160

30-60

6-Rinse

Tap water

Ambient

Ambient

30-60

7-Rinse(a)

Deionized water

Ambient

Ambient

10-30

(a) Deionized water rinse is required for a two-coat/one-fire finish with excellent appearance.

Fig. 8 "Clean only" metal preparation, spray process

In the acid etch/nickel deposition process, parts are placed on corrosion-resistant racks and dipped in or sprayed with a series of solutions. Mechanized equipment is required for high-production operations. The sequence of processing steps and the solutions used for the acid etch/nickel deposition method in production operations are indicated in Fig. 9. As is indicated in Stage 4, sulfuric acid is normally used to etch the metal. After drying at 93 to 150 °C (200 to 300 °F), the steel parts have a light straw color.

Alkaline titan

(dip or spray)

So luí i an 1

Pickle (dip Or" spray! Solution 4

Warm rinsi

Cold rin«

Cold rfnse

Solution 2

Solution 3

Solution 5

Neutralize Solution S

Deposit nicker Solution 6

Warm-air çtry

Cold rinse Solution 7

No.

Solution

Composition

Temperature

Cycle time, min

°C

°F

Dip

Spray

1

Alkaline cleaner(a)

Cleaner, 15-60 g/L (2-8 oz/gal)®

Ambient to

100(c)

Ambient to 212(c)

612

1-3

2

Warm rinse

Water

49-60

120-140

4

1-1 2

3

Cold rinse

Water

Ambient

Ambient

2-4

1-1 2

4

Pickle(d)

H2SO4, 6-8%

66-71

150-160

510

3-5

5

Cold rinse

Water, H2SO4(e)

Ambient

Ambient

4

1-1 2

6

Nickeldeposition®

NiSO46H2O, 5.6-7.5 g/L (0.75-1.0 oz/gal)(e)

60-82

140-180

510

4-6

7

Cold rinse

Water, H2SO4(e)

Ambient

Ambient

4

1-1 2

8

Neutralize

2/3 Na2CO3 and 1/3 borax, 0.60-2.10 g/L (0.008-0.28

49-71

120-160

1-6

1-2

(a) For spray cleaning, use a two-stage process.

(b) For spray cleaning, use 3.8 to 15 g/L (0.5 to 2.0 oz/gal).

(c) 60 to 82 °C (140 to 180 °F) for spray cleaner.

(d) Weight loss of metal is 3 to 5 g/m2 (0.3 to 0.5 g/ft2).

(e) Solution pH, 3 to 3.5, to prevent formation of ferric iron.

(f) Nickel deposit should be 0.2 to 0.6 g/m2 (0.02 to 0.06 g/ft2). Continuous filtration is commonly used to remove Fe(OH)3.

Fig. 9 Ground-coat enameling, acid-etch/nickel-deposition process (dip or spray application)

When low-carbon decarburized steel is enameled in a direct-on cover-coat operation, parts must be etched to remove 11 to 22 g/m2 (1 to 2 g/ft2) of metal surface, and they must receive a nickel deposit of 0.9 to 1.3 g/m2 (0.08 to 0.12 g/ft2) of surface. Table 10 indicates modifications of the solutions and operating conditions for the acid pickling and nickel deposition cycles shown in Fig. 9 that have been used to provide increased metal removal and an increase in the amount of nickel deposited. Also, a ferric sulfate etching solution is sometimes used.

Table 10 Acid-etch and nickel-deposition solutions for preparing decarburized steels for direct-on cover coating

Solution0"

Composition of solution

Operating temperature

Cycle time, min

°C

°F

Dip

Spray

Acid solutions(b)

1

H2SO4, 6-8 wt%

71

16Q

l5-3Q(c)

8-l5(c)

2(d)

H2PO4, 17-2Q wt%

6Q

14Q

4-8

2-5

3(e) First stage

Ferric sulfate(f), 5 wt%

65

15Q

2-4

11 -3 2

Second stage

H2SO4, 6-8 wt%

65

15Q

2-4

11 -3 2

4 Oxyacid

H2SO4, 6-9 wt%; Fe2(SO4)3, 3-5 wt%; FeSO4, 3-2Q wt%

74

165

11 -4 2

11 -3 2

Nickel deposition solution®

(a) Except for the use of these solutions, preparation entails processing as indicated in Fig. 9.

(b) Any of these solutions may be used in place of solution 4 in Fig. 9. Minimum metal removal required is 22 g/m2 (2 g/ft2) of metal.

Cycle time may be reduced to that indicated in Fig. 9 for ground coats by oxidizing the metal 16 g/m (1 g of iron/ft) at 680 °C (1250 °F) in an air atmosphere, or by blasting with sand or steel grit to remove metal prior to pickling.

(d) Equipment containing lead or Monel cannot be used.

(e) Equipment containing lead cannot be used.

(f) Convert ferrous to ferric by adding hydrogen peroxide, sulfuric acid, and water.

(g) Nickel deposit should be 0.6 to 1 g/m2 (0.06 to 0.10 g/ft2).

(h) pH of solution, 3.2 to 3.5. Sulfuric acid or sodium hydroxide is used to adjust pH. The addition of 0.3 to 0.8 g/L (1 to 3 g/gal) of sodium hypophosphite to solution will increase the rate of nickel deposition and permit the use of the lower end of the temperature range without excessive cycle time.

A modification of the above-described system, identified as oxyacid, is also used. Oxyacid etchant solution is a mixture of sulfuric acid and ferric sulfate. The sulfuric acid and the ferric sulfate etch the metal, producing ferrous sulfate. The ferrous sulfate is then oxidized to ferric sulfate in the presence of sulfuric acid. In this system, all of the reactions take place in only one etching tank.

Mechanical preparation requires abrasive blasting using steel shot or steel grit. Grit or shot blasting is used on parts designed without pockets or crevices and with configuration and thickness that permit blasting without distortion. Parts with flat areas should be fabricated from sheet steel thicker than 16-gage, 1.52 mm (0.0598 in.) to avoid excessive distortion when cleaned by this method.

Abrasive blasting is often used for preparing hot-rolled steel and parts that are to be enameled on one side only, as for water heater tanks. The process is also used for preparing large parts and parts having enamels with poor bonding characteristics. Before blasting, oil and drawing compounds may be removed by alkaline cleaning or by heating at 425 to 455 °C (800 to 850 °F) to burn off the organic contaminants.

Preparation of Cast Iron and Aluminum for Porcelain Enameling

Cast iron is prepared by blasting to remove adhering mold sand and the thin surface layer from chilled iron. Because the surface contains more combined carbon than is present in the remainder of the casting, this carbon-heavy layer must be removed to prevent excessive evolution of gas during firing of the enamel.

Quartz sand of nearly spherical grains, propelled by compressed air, is commonly used for abrasive cleaning of cast iron; however, steel shot, steel grit, and chilled cast iron grit propelled centrifugally from rotating wheels are generally used for cleaning sanitary ware. Zircon sand and fused alumina grit are used for special purposes.

After blasting, the casting should be inspected for cracks, sand holes, slag holes, blowholes, fins, and washes. Cracks and larger holes need to be filled by welding, and the welds require spot grinding to blend the area with the surrounding surface. Also, fins and washes must be removed by grinding. The repaired casting is blasted a second time prior to enameling. Small holes need not be repaired by welding; usually they can be filled with a ceramic paste after final blasting.

Cast iron parts should be enameled within a few hours after cleaning, especially during periods of high humidity; even a very thin layer of rust will reduce the adherence of the enamel. Parts that have rusted excessively can be reconditioned by being heated to a red heat, cooled to room temperature, and cleaned abrasively.

Aluminum. The preparation of parts made of heat-treatable aluminum alloys for porcelain enameling involves the removal of soil and surface oxide and the application of a chromate coating. Figure 10 shows the sequence of these surface preparation treatments and gives operating conditions. Final drying removes all surface moisture; drying must be accomplished without contaminating the cleaned surface of the aluminum. Parts made of non-heat-treatable aluminum alloys or aluminum sheet require only the removal of soil, which can be done by alkaline cleaning or vapor degreasing.

AlkaNne clean Solution 1

Remove o*nJe

Ciirom-ate dip Solution 3

Cold vwaier nn»

Cold water rinse

Cold water rinse

-

Solution 2

Air dry

No.

Type

Composition of solution

Operating temperature

Cycle time, min

Constituent

wt%

°C

°F

1

Alkaline cleaner(a)

(b)

(b)

60-82

140-180

2-5

2

Oxide removal

Chromic acid

3.5

82

180

3-10

Sulfuric acid

18.0

3

Chromate dip

Chromic sulfate

0.2

Ambient

Ambient

1-6

Potassium dichromate

14.4

(a) Vapor degreasing may be used instead of alkaline cleaning.

(b) Either inhibited or mildly etching (uninhibited) cleaners can be used.

Fig. 10 Process for preparing heat-treatable aluminum alloys for porcelain enameling The Porcelain Enameling Process

Porcelain enamel may be applied to the base metal by either the wet process or the dry process. Wet process methods include manual spraying, electrostatic spraying, dipping, flowcoating, and electrodeposition (electrophoresis). Dry process methods are electrostatic dry powder spraying for application to sheet steel and, for application of the cover coat to cast iron, sprinkling the dry powder by means of a vibrating sieve onto the hot cast iron part.

The best method of application for a particular part is determined by quantity and quality requirements, the type of material being applied, units produced per hour, capital investment, labor cost, environmental considerations, and, ultimately, cost per part. Application techniques can be manual or mechanized. Manual application is necessary for parts having a variety of sizes and shapes that must go through the same enameling process system. Mechanization is essential for high-volume production of parts of the same or similar shape.

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