Handbook ASM International 1987 p 399418 52 Zinc Coatings for Corrosion Protection Zinc Institute 1978 p 14 Porcelain Enameling

Porcelain enamels are glass coatings applied primarily to products made of steel sheet, cast iron, or aluminum to improve appearance and protect the metal surface from corrosion. Porcelain enamels are distinguished from other ceramic coatings by their predominantly vitreous nature and the types of applications for which they are used, and from paint by their inorganic composition and the fusion of the coating matrix to the substrate metal. Porcelain enamels of all compositions are matured at 425 °C (800 °F) or above.

Several basic methods are used to apply porcelain enamels to the base metal. These include dipping, flow coating, electrodeposition, manual spray, electrostatic spray, and dry-powder spray. 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, and, ultimately, part cost. Details on application techniques for porcelain enameling are provided in the article "Porcelain Enameling" in this Volume.

Coating Materials The frits for porcelain enamel are designed and manufactured to meet specific end-use applications and processing requirements. Porcelain enamel frits for sheet steel are classified as either ground coat or cover coat types. Ground coat enamels contain metallic oxides that promote adherence to the steel substrate and may be used as a single functional coat or as a base coat for additional cover coats. Table 30 gives the composition of a typical ground coat enamel. Cover coat enamels (Table 31) are normally applied over ground coats for appearance and/or to improve the chemical and physical properties of the coating. Cover coats may also be applied directly to properly prepared decarburized steel substrates.

Table 30 Melted-oxide compositions of frits for ground coat enamels for sheet steel

Constituent

Composition, %

Regular blue-black enamel

Alkali-resistant enamel

Acid-resistant enamel

Water-resistant enamel

SiO2

33.74

36.34

56.44

48.00

B2O3

20.16

19.41

14.90

12.82

Na2O

16.74

14.99

16.59

18.48

k2o

0.90

1.47

0.51

Li2O

0.89

0.72

1.14

CaO

8.48

4.08

3.06

2.90

BaO

9.24

8.59

ZnO

2.29

AI2O3

4.11

3.69

0.27

ZrO2

2.29

8.52

TiO2

3.10

3.46

CuO

0.39

MnO2

1.43

1.49

1.12

0.52

NIO

1.25

1.14

0.03

1.21

C03O4

0.59

1.00

1.24

0.81

P2O5

1.04

0.20

f2

2.32

2.33

1.63

1.94

Table 31 Melted oxide compositions of frits used for cover coat enamels for sheet steel

Constituent

Composition, wt%

Titania white enamel

Semi-opaque enamel

Clear enamel

SiO2

44.67

44.92

54.26

B2O3

14.28

16.40

12.38

Na2O

8.27

8.67

6.55

k2o

6.99

8.12

11.32

Li2O

0.98

0.45

1.14

ZnO

0.74

ZrO2

1.98

3.34

1.40

AI2O3

0.31

0.16

TiO2

18.49

13.05

10.04

P2O5

1.32

0.88

MgO

0.5

F2

2.21

3.27

2.91

Because all porcelain enamels are variations of borosilicate glass, they are characterized by end use and not by chemical composition. Some common designations for those with particular characteristics are acid-resistant, alkali-resistant, heat-resistant, glossy, low-gloss, and matte.

Steel Substrates. Typical compositions of the various grades of low-carbon sheet iron or steel that are commercially available for porcelain enameling are listed in Table 32. Cold-rolled sheet steels used for porcelain enameling can be divided into three groups:

• Extra-low carbon steels (a maximum of 0.008% C), including HSLA steels in which the carbon is stabilized by the addition of titanium or niobium

• Low-carbon steels containing about 0.02% C (these steels are suitable for ground or two-coat enameling)

• Conventional cold-rolled sheets with higher carbon contents of about 0.06% (such sheets have a tendency toward primary boiling and sagging and are used in less critical ground coat and two-coat enameling applications)

Table 32 Compositions of low-carbon steels used for porcelain enameling

Type of steel

Composition, %(a)

C

Mn

P

S

Al

Ti

Nb

B

Replacement steel for enameling iron

0.02-0.05

0.15-0.3

0.015(b)

0.015(b)

0.03-0.07

0.006

Decarburized

0.005

0.2-0.3

0.01

0.02

(c)

Titanium-stabilized

0.05

0.30

0.01

0.02

0.05

0.30

Interstitial-free

0.005

0.20

0.01

0.02

0.04

0.09

Cold-rolled

0.06

0.35

0.01

0.02

(c)

(a) All compositions contain balance of iron.

(a) All compositions contain balance of iron.

(c) Some steels may be supplied as aluminum-killed products. Data from Porcelain Enamel Institute

Hot-rolled steels are generally used for porcelain enameled water heater tanks and for other applications where thickness and strength requirements dictate their use. When hot-rolled steels are used, components should be coated on one side only to minimize processing defects such as fishscales.

Coating Properties. Porcelain enamel is used extensively because of its resistance to household chemicals and foods. Mild alkaline or acid environments are generally involved in household applications. Special enamels or glass compositions are available to resist most acids--except for hydrofluoric or concentrated phosphoric--to temperatures of 230 °C (450 °F). Compositions may be formulated to resist alkali concentrations up to a pH of 12 at temperatures as high as 95 °C (200 °F).

The hardness of porcelain enamels ranges from 3.4 to 6.0 on the Moh's scale. Porcelain enamels show a high degree of abrasion resistance. Abrasion resistance can be increased by adding crystalline particles to the enamel composition by a devitrification heat treatment.

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