Hot Dip Galvanizing

Hot dip galvanizing is a process in which an adherent, protective coating of zinc and iron-zinc alloys is developed on the surfaces of iron and steel products by immersing them in a bath of molten zinc. Most zinc coated steel is processed by hot dip galvanizing. In general, steels with the following maximum alloying/impurity levels are best suited to galvanizing: 0.05% Si, 0.05% P, 0.25% C, and 1.3% Mn.

Batch Galvanizing. One method of hot dip galvanizing is the batch process, which is used for prefabricated steel items. This method involves cleaning the steel articles, applying a flux to the surfaces, and immersing them in a molten bath of zinc for varying time periods to develop a thick alloyed zinc coating.

The advantage to galvanizing after fabrication is that the zinc completely seals edges, overlaps, rivets, and welds; establishes liquid tightness; and prevents corrosion from starting. Iron and steel in all shapes and sizes can be coated with zinc by batch galvanizing. The process is simple, extremely versatile, and has been used to provide protection to articles ranging from small items, such as bolts, nuts, and miscellaneous hardware, to large items like structural beams for bridges or buildings. The virtually unrestricted size range of parts that can be galvanized and the ability to bolt or weld prefabricated sections after galvanizing enables almost any structure to be built from galvanized steel. Shape is not a restriction to batch galvanizing. Tubes, open vessels, drums, tanks, and complicated shapes such as large heat exchangers are readily galvanized on the inside and outside in one operation.

The thickness of the coating is controlled by the composition of the steel substrate and the immersion time. Part withdrawal rate and any postgalvanizing treatments also influence the coating thickness. Process details (zinc bath temperature, steel surface preparation, degree of wiping, shaking, or centrifuging, and rate of cooling) are described in the article "Batch Hot Dip Galvanized Coatings" in this Volume.

The zinc coating on batch galvanized parts is generally specified in ounces or grams per unit of surface area, measured in either square feet or square meters on a single surface or on one side of the part. With proper coating techniques, the coating weight can usually be controlled between 610 and 1220 g/m2 (2 and 4 oz/ft2), equivalent to a coating thickness of approximately 43 to 86 pm (1.7 to 3.4 mils) per side (Ref 6).

Continuous Galvanizing. Steel sheet and wire are coated by a continuous hot dip process; that is, they enter the coating bath in an unending strip. In theory, all continuous hot dip processes are similar in that the steel sheet or wire is subjected to successive cleaning, coating, and postcoating steps.

Typical cleaning steps may include alkaline cleaning or acid pickling (both of which may be electrolytic), oxidation (usually gaseous for sheet, but often in molten lead for wire), and reduction (gaseous). If a gaseous reduction is the final cleaning step, the steel must enter the molten coating bath directly without being exposed to air. When the final cleaning step is an acid pickle (this is usually the case for wire), the steel is then immersed in a liquid flux, which dissolves any remaining oxides, before entering the molten bath. Similarly, gaseous reduction can also be considered a flux treatment. All-gaseous cleaning is used on about 60% of the steel-coating lines. The remaining 40% is approximately half liquid cleaning/flux and half liquid cleaning/gaseous cleaning. Most wire-coating lines use the liquid/flux technique. More detailed information on surface cleaning and fluxing can be found in the article "Surface Preparation for Continuously Applied Coatings" in this Volume.

The clean steel is then immersed in the molten coating bath long enough to allow the coating metal to wet and react with the steel surface. As the coated sheet or wire emerges from the molten bath, it pulls coating metal up from the surface which can then be smoothed or wiped to the desired thickness by a variety of methods. Most sheet-coating lines use a gas-wiping technique in which a jet of steam, air, or gas (such as nitrogen) is directed against the emerging sheet.

The coated steel can be given any number of subsequent mechanical, thermal, or chemical post-treatments designed to impart specific properties. Typically, a coated sheet might be oiled or coated with a chromate solution to inhibit staining or superficial corrosion during storage and transit (see the previous discussion of chromate conversion coating in this article). Waxing would serve the same purpose on wire and would facilitate handling during subsequent processing.

Hot-dipped galvanized coatings are applied by highly mechanized mass production methods at speeds of over 90 m/min (300 ft/min). Several designs of galvanizing lines have been developed for commercial use by the steel suppliers. Most steel suppliers can produce galvanized coils (sheets) in widths of 250 to 1830 mm (10 to 72 in.) and thicknesses of 0.43 to 4.2 mm (17 to 165 mils). Typical applications for mill coated sheets are roofing and siding panels, guardrails, appliance cabinets, automotive body parts, and ductwork.

Coating weights applied by continuous galvanizing vary from 150 to 840 g/m2 (0.5 to 2.75 oz/ft2) (Ref 7). The zinc coating may be on one side of the sheet only, of equal weight on both sides of the sheet, or differentially applied (one side has a thicker coating than the other side). Process details are given in the article "Continuous Hot Dip Coatings" in this Volume.

Nature of the Hot Dip Galvanized Coating. Figure 4 shows a photomicrograph of a typical hot dip galvanized coating consisting of a series of layers. These layers are also identified in Table 16. Starting from the base steel at the bottom of the section, each successive layer contains a higher proportion of zinc until the outer layer, which is relatively pure zinc, is reached. Therefore, there is no real line of demarcation between the iron and the zinc; instead, there is a gradual transition through the series of iron-zinc intermetallics, which provide a powerful bond between the base metal and the coating.

Table 16 Properties of alloy layers of hot dip galvanized steels

Layer

Alloy

Iron, %

Melting point

Crystal structure

Diamond pyramid microhardness

Alloy characteristics

°C

°F

Eta (n)

Zinc

0.03

419

787

Hexagonal

70-72

Soft, ductile

Zeta (■> )

FeZn13

5.7-6.3

530

986

Monoclinic

175-185

Hard, brittle

Delta (8)

FeZn7

7.0-11.0

530-670

986-1238

Hexagonal

240-300

Ductile

Gamma ( t )

Fe3Zn10

20.0-27.0

670-780

1238-1436

Cubic

Thin, hard, brittle

Steel base metal

Iron

1510

2750

Cubic

150-175

Fig. 4 Typical hot dip galvanized coating. Note the gradual transition from layer to layer, which results in a strong bond between base metal and coating.

Hardness and Abrasion Resistance. The layers that compose the galvanized coating, being discrete zinc-iron alloys, vary in hardness. The free zinc layer (n) is relatively soft, but the alloy layers are very hard, harder even than ordinary structural steels. Typical values for the microhardness and other properties of the various alloy layers are given in Table 16.

The alloy layers are from four to six times more resistant to abrasion than pure zinc. Galvanized coatings exhibit better abrasion resistance compared to paints with the same coating thickness and can be effectively used where excessive abrasive wear is expected, for example, floor gratings, stairs, conveyors, and storage bins.

Adhesion and Impact Resistance. Unlike other coatings, which are mechanically or chemically bonded to the steel, the galvanized coating is metallurgically bonded to and integral with the steel, making conventional measures of bond strength inappropriate for this coating. As a result of the metallurgical bond, the galvanized coating is very adherent.

The structure of the galvanized coating, particularly the relative thicknesses of the 8 and C layers (Table 16), is primarily influenced by the steel chemistry and, to a lesser extent, by the galvanizing temperature and the duration of immersion.

Coating structure has the greatest effect on the impact resistance of the coating. A high relative proportion of C phase in the iron-zinc alloy may result in localized flaking if the coating is subjected to heavy impact or excessive twisting or bending. Semikilled steels with silicon contents of 0.05 to 0.12% are the most susceptible to coating brittleness and less adherent coatings.

Corrosion Protection Mechanism. Galvanized coatings protect steel in corrosion service in two ways: barrier protection and cathodic protection. Barrier protection is provided by the galvanized coating and is further enhanced by the formation of a thin, tightly adherent layer of zinc corrosion products on the coating surface. Upon initial weathering of a freshly galvanized surface, ZnO is formed, and it is converted to ZnOH2 in the presence of moisture. Further reaction with CO2 in the air results in the formation of basic ZnCO3, which is relatively insoluble and impedes further corrosion. The gray patina normally associated with weathered galvanized coatings is the result of this thin layer of basic ZnCO3.

Cathodic protection is provided to the steel by the fact that zinc is anodic to steel in most environments. Minor discontinuities or small areas of exposed steel resulting from drilled holes or cut edges are protected from corrosion by the sacrificial protection afforded by zinc. The corrosion products that result from this action provide further protection.

Atmospheric Exposure. Zinc, steel, and hot dip galvanized coatings have been the subject of long-term atmospheric studies conducted throughout the world (Ref 8, 9). From these studies, the behavior of these materials in a specific atmospheric environment can be reasonably estimated. An exact determination of corrosion behavior is complicated by several factors: the frequency and duration of exposure to moisture (rain, sleet, snow, and dew), the type and concentration of corrosive pollutants, the prevailing wind direction and velocity, and exposure to sea spray or windborne abrasives. All atmospheres contain some type of corrosive agent, and the concentration of these agents, as well as the frequency and duration of moisture contact, determines the corrosion rate of galvanized coatings.

Figure 5 shows the results of outdoor atmospheric-exposure tests designed to measure the protective life of galvanized coatings in various atmospheric environments. The sites were selected as representative of various broad environmental classifications: heavy industrial, moderate industrial (urban), suburban, rural, and marine. Within these broad classifications, the following factors are most significant in influencing the rate of corrosion of the galvanized coating.

Coating thickness, mils 2.0 3.0

Coating thickness, mils 2.0 3.0

-/

Tropical marineS

Aural /

^/Su bu rban^^

Temperate m:

trine y J*

Moderate iodustri

j|

/7//

Heavy induslrjsi

Coding Ihictness, |*rn

50 15

Coding Ihictness, |*rn

Fig. 5 Service life versus coating thickness for hot dip galvanized steel in various atmospheres. Source: Ref 8

Industrial and Urban Environments. Corrosive conditions are most pronounced in areas with highly developed industrial complexes that release sulfurous gases and corrosive fumes and mists to the atmosphere. These corrodents react with the normally impervious basic ZnCO3 film to produce ZnSO4 and other soluble zinc salts that, in the presence of moisture, are washed from the surface. This exposes fresh zinc to the atmosphere, and another corrosion cycle begins.

Rural and Suburban. The corrosion rate of zinc in these areas is relatively slow compared to that in industrial settings. Once the original weathering occurs, there is little in the atmosphere to convert the basic zinc salts to water-soluble compounds.

Marine Atmospheres. The corrosion rate of zinc and galvanized steel in marine atmospheres is influenced by several factors. Zinc forms a soluble corrosion product, zinc chloride (ZnCl2), in marine atmospheres; therefore, the corrosion rate is influenced by salt spray, sea breezes, topography, and proximity to the coastline. For example, one investigation found that the corrosion rate for zinc exposed 25 m (80 ft) from the ocean was three times that for zinc exposed 250 m (800 ft) from the ocean (Ref 9).

The corrosion product formed in a given type of atmosphere (industrial, marine, and so on) determines the corrosion rate of zinc in that atmosphere. Results from exposures in a variety of atmospheres show that zinc is 20 to 30 times more resistant to corrosion than is steel (Table 17).

Table 17 Weight losses of steel and zinc in various locations

Results are from 2-year atmospheric exposures.

Table 17 Weight losses of steel and zinc in various locations

Results are from 2-year atmospheric exposures.

Location

Weight loss, g

Zinc

Steel

Steel/zinc loss ratio

Norman Wells, N.W.T., Canada

0.07

0.73

10.4

Phoenix, AZ

0.13

2.23

17.2

Saskatoon, Sask., Canada

0.13

2.77

21.3

Esquimalt, Vancouver Is., Canada

0.21

6.50

31.0

Fort Amidor Pier, Panama C.Z.

0.28

7.10

25.4

Ottawa, Ontario, Canada

0.49

9.60

19.6

Miraflores, Panama C.Z.

0.50

20.90

41.8

Cape Kennedy, 0.8 km (0.5 mile) from ocean

0.50

42.0

84.0

State College, PA

0.51

11.17

21.9

Morenci, MI

0.53

7.03

13.3

Middletown, OH

0.54

14

25.9

Potter County, PA

0.55

10

18.2

Bethlehem, PA

0.57

18.30

32.1

Detroit, MI

0.58

7.03

12.1

Point Reyes, CA

0.67

244.0

364.2

Trail, B.C., Canada

0.70

16.90

24.1

Durham, NH

0.70

13.30

19.0

Halifax, NS (York Redoubt)

0.70

12.97

18.5

South Bend, PA

0.78

16.20

20.8

East Chicago, IN

0.79

41.10

52.0

Brazos River, TX

0.81

45.40

56.0

Monroeville, PA

0.84

23.80

28.3

Daytona Beach, FL

0.88

144.0

163.6

Kure Beach, NC (244 m, or 800 ft), site

0.89

71.0

79.8

Columbus, OH

0.95

16.00

16.8

Montreal, Quebec, Canada

1.05

11.44

10.9

Pittsburgh, PA

1.14

14.90

13.1

Waterbury, CN

1.12

11.00

9.8

Limon Bay, Panama C.Z.

1.17

30.30

25.9

Cleveland, OH

1.21

19.0

15.7

Newark, NJ

1.63

24.7

15.2

Cape Kennedy, 55 m (180 ft) from ocean

Ground level

1.83

215.0

117.5

9 m (30 ft) elevation

1.77

80.2

45.3

18 m (60 ft) elevation

1.94

64.0

33.0

Bayonne, NJ

2.11

37.70

17.9

Kure Beach, NC (25 m, or 80 ft) site

2.80

260.0

92.9

Halifax, NS (Federal Building) (25m, or 80 ft)

3.27

55.30

16.9

Galeta Point, Panama C.Z.

6.80

336.0

49.4

Seawater and Salt Spray Performance. Table 18 gives the approximate corrosion rates of zinc in various waters, and Fig. 6 illustrates the expected service life of galvanized coatings in areas exposed to salt spray influences. Sea salts are mainly NaCl, with small amounts of calcium, magnesium, and manganese salts. Typical pH is about 8. Compared to other metals and alloys, galvanized coatings provide considerably more protection to steel than many other metals and alloys, even though the anticipated coating life is shorter in seawater and salt spray exposures than a number of other environments.

Table 18 Corrosion of zinc in various waters

Water type

Approximate material loss

^m/yr

mils/yr

Seawater

Global oceans, average

15-25

0.6-1.0

North Sea

12

0.5

Baltic Sea and Gulf of Bothnia

10

0.4

Freshwater

Hard

2.5-5

0.1-0.2

Soft river water

20

0.8

Soft tap water

5-10

Fig. 6 Time to first maintenance versus coating thickness for hot dip galvanized coatings in seawater immersion and sea spray exposures. Source: Ref 11

Freshwater Performance. The corrosion protection mechanism of zinc in freshwater is similar to that in atmospheric exposures. The corrosion rate depends on the ability of the coating to develop a protective layer of adherent basic zinc salts. This layer denies access to the coating by oxygen and slows the rate of attack. The ability of the water to form scale depends on a number of variables, such as the pH of the water, hardness, total alkalinity, and total dissolved solids. Table 19 demonstrates the effects of various water chemistries on the relative corrosion rates of zinc.

Table 19 Corrosion of zinc in different types of water

Water type

Attacking substances

Passivating substances

Properties of corrosion products

Relative corrosion rate

Solubility

Adhesion

Hard water

Oxygen, CO2

Calcium, magnesium

Very low

Very good

Very low

Seawater

Oxygen, CO2, Cl"

Calcium, magnesium

Low

Very good

Moderate

Soft with free air supply

Oxygen, CO2

High

Good

High

Soft or distilled with poor air supply

Oxygen

Very high

Very poor

Very high

Source: Ref 10

Source: Ref 10

Water Temperature. The corrosion rate of zinc in water, and therefore that of the galvanized coating, increases with temperature to between 65 and 70 °C (150 and 160 °F), at which point the rate begins to decrease (Fig. 7). At temperatures near 70 °C (160 °F), a reversal in potential may occur where zinc coatings become cathodic to iron. Low oxygen and high bicarbonate contents favor reversal, but the presence of oxygen, sulfates, and chlorates tends to maintain the natural anodic state of the zinc.

Temperature, °F

10 40 70 100 130

Temperature,

10 40 70 100 130

Temperature,

Fig. 7 Influence of water temperature on the corrosion rate of zinc in distilled, aerated water. Source: Ref 12

Water pH. Zinc is an amphoteric metal with the capacity to passivate by means of protective layers. The corrosion rate of zinc decreases with increasing pH and reaches a minimum at 12.0 to 12.5. Most waters are in the pH range of 6 to 8. The scale-forming ability of the water and the concentration of dissolved ions in the water are more important influences on the corrosion rate than pH in this practical exposure range.

Performance in Soils. The corrosion rate and performance of galvanized steels in soils are a function of the type of soil in which the steel is located. Soils can vary considerably in composition and can contain bound and unbound salts, organic compounds, products of weathering, bacteria and other microorganisms, dissolved gases (such as hydrogen, oxygen, and methane), acids, and alkalies. Soils vary in permeability, depending on the soil structure. Although the concentration of oxygen is lower in soils than in air, the CO2 concentration is higher. Variation among soils is high, and corrosion conditions are complicated.

In general, soils in coarse, open textures are often aerated, and the performance of galvanized steel would be expected to be similar to that in air. In soils with fine textures and high water-holding capacities, such as clay and silt-bearing soils, corrosion rates are likely to be higher. Soil resistivity is recognized as a reliable method of predicting the corrosivity of soils. High-resistivity (poor conducting) soil would be less corrosive than low-resistivity (good conducting) soil. Dry soils are poor conductors and are the least corrosive to zinc.

Painting Galvanized Steel. Galvanized coatings, when used without further treatment, offer the most economical corrosion protection for steel in many environments. The galvanized coating makes an excellent base on which to develop a paint system. Painting of galvanized steel is desirable for aesthetics, as camouflage, as warning or identification markings, to prevent bimetallic corrosion, or when the anticipated environment is particularly severe.

In corrosive atmospheres, a duplex system of galvanized steel top coated with paint has several advantages that make it an excellent system for corrosion prevention:

• The life of the galvanized coating is extended by the paint coating

• The sacrificial and barrier properties of the zinc coating are used if a break occurs in the paint film

• Undercutting of damaged paint coatings, a major cause of failure of paints on steel, does not occur with a zinc substrate

• Surface preparation of a weathered zinc surface for maintenance painting is easier than that for rusted steel

The galvanized coating prevents rusting of steel by acting as a barrier against the environment and by sacrificially corroding to provide cathodic protection. Painting the galvanized coating extends the service life of the underlying zinc because the barrier property of the paint delays the reaction of zinc with the environment. If a crack or other void occurs in the paint and exposes the galvanized coating, the zinc corrosion products formed tend to fill and seal the void; this delays further reaction.

When painted steel is exposed to the environment, rust forms at the steel/paint interface. Because rust occupies a volume several times that of the steel, the expansion resulting from rusting leads to rupture of the steel/paint bond. Further, rust is porous; it accumulates moisture and other reactants, and this increases the rate of attack on the steel. The result is undercutting, flaking, and blistering of the paint film, leading to failure of the paint coating (Fig. 8). Zinc corrosion products occupy a volume only slightly greater (20 to 25%) than zinc; this reduces the expansive forces and conditions that lead to paint failure.

Fig. 8 Illustration of the mechanism of corrosion for painted steel (a) and painted galvanized steel (b). (a) A void in the paint results in rusting of the steel, which undercuts the paint coating and results in further coating degradation. (b) A void in the coating of a painted galvanized steel is sealed with zinc corrosion products; this avoids the undercutting seen in (a) and prevents further deterioration of the painted coating.

Fig. 8 Illustration of the mechanism of corrosion for painted steel (a) and painted galvanized steel (b). (a) A void in the paint results in rusting of the steel, which undercuts the paint coating and results in further coating degradation. (b) A void in the coating of a painted galvanized steel is sealed with zinc corrosion products; this avoids the undercutting seen in (a) and prevents further deterioration of the painted coating.

A coating system consisting of painted galvanized steel provides a protective service life up to 1.5* that predicted by adding the expected life times of the paint and the galvanized coating in a severe atmosphere (Ref 13). This is demonstrated in Table 20. The synergistic improvement is even greater for mild environments (Ref 14, 15).

Table 20 Synergistic protective effect of galvanized steel/paint systems in atmospheric exposure

Type of Atmosphere

Galvanized steel

Paint

Galvanized plus paint

Thickness

Service life(a), years

Thickness

Service life(a), years

Thickness

Service life(a), years

^m

mils

^m

mils

^m

mils

Heavy industrial

50

2

10

100

4

3

150

6

19

75

3

14

150

6

5

225

9

29

100

4

19

100

4

3

200

8

33

100

4

19

150

6

5

250

10

36

Metropolitan (urban)

50

2

19

100

4

4

150

6

34

75

3

29

150

6

6

225

9

52

100

4

39

100

4

4

200

8

64

100

4

39

150

6

6

250

10

67

Marine

50

2

20

100

4

4

150

6

36

100

4

40

100

4

4

200

8

66

100

4

40

150

6

6

250

10

Source: Ref 14

(a) Service life is defined as time to about 5% red rust.

Paint adhesion is the primary concern in painting galvanized steel. The surface of the zinc is nonporous and does not allow mechanical adhesion of the paint. Surface contaminants, such as oils, waxes, or postgalvanizing treatments, also effect adhesion. A fresh zinc surface is reactive to certain paint ingredients, such as fatty acids; this can produce zinc soaps and disrupt the zinc-paint bond.

Galvanized coatings can be successfully painted immediately after galvanizing or after extended weathering. The deliberate use of weathering is not recommended, because weathering may not be uniform, the time required is long (6 to 17 months), hygroscopic impurities can form that may be difficult to remove, and there is exposure to atmospheric pollutants.

Chemical etchants, such as acids or copper sulfate, should not be used for surface preparation. The action of these chemicals is difficult to control, the resulting surface may be nonuniform, and the galvanized coating could be damaged if allowed to remain in extended contact with the chemicals. Long-term adhesion will suffer with this type of treatment, although initial adhesion may be obtained.

Mechanical roughening of the zinc surface through the use of a light blast can provide a good surface for painting. However, careful control of the blast pressure and flow rate must be exercised to avoid excessive removal of the galvanized coating.

Initial adhesion of the paint can be achieved through the use of a pretreatment primer to provide an adequate base for further coating. Long-term adhesion is obtained by the selection of a top coat that is compatible with the primer and galvanized steel. Additional information on various paints can be found in the sections "Organic Coatings" and "Painting With Zinc-Rich Paints" in this article.

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