Zinc Coatings

Hot-dip-zinc-coated steel sheet, also called galvanized, is by far the most widely used coated sheet product. About 86% of the hot-dip-coated sheet produced in the United States (see Fig. 1) is zinc coated.

As evident in Table 1, hot dip zinc coatings for sheet are available in a broad range of coating thicknesses. For generalpurpose galvanized sheet, 19 pm (0.75 mil) per side is the usual thickness. This corresponds to a two-side coating mass of 275 g/m2 (0.9 oz/ft2). Heavier coatings are used in applications requiring maximum corrosion resistance, such as highway drainage culverts. In the automotive industry, where formability and weldability are key considerations, lighter coatings such as 90 g/m2 (0.3 oz/ft2) are more typical.

Microstructure. The surface and cross section of a galvanized coating are shown in Fig. 6. Most of the coating is nearly pure zinc. Near to the steel-coating interface are prismatic particles of i -phase, a zinc-iron intermetallic compound containing about 6% Fe (see Fig. 7).

Fig. 6 Galvanized coating microstructure. Scanning electron microscope cross section
Fig. 7 Iron-zinc equilibrium phase diagram. Source: Ref 22

Alloy Additions. Aluminum, typically in the range of 0.1 to 0.2%, is added to the zinc bath in order to prevent formation of a thick, continuous layer of zinc-iron intermetallic that could lead to poor coating adhesion during forming. Aluminum reacts preferentially with the steel to form a thin layer of an iron-aluminum intermetallic that acts as a barrier and delays the growth of the zinc-iron intermetallic layer.

Lead was originally present in galvanized coatings as an impurity from the smelting process. The presence of lead causes the formation of spangles, the familiar dendritic surface pattern visible on galvanized ductwork and garbage cans. Historically, users relied on the spangled appearance to distinguish hot dip galvanized from less corrosion-resistant thin electroplated sheet. In order to maintain this distinction, it became common practice to add lead (typically about 0.1%) to keep the spangles, even after refining methods for producing lead-free zinc were developed. With increasing environmental concerns, however, the use of lead is declining. Antimony is now increasingly often used to produce spangled coatings without the environmental drawbacks of lead.

Spangles are also prone to spangle cracking, a phenomenon in which cracking occurs along certain crystallographic planes during forming of the coated sheet. For painted end uses, such as automobiles and metal buildings, a smoother, nonspangled surface is desirable for appearance purposes. When this is the case, lead or the other spangle-promoting elements can be omitted from the bath. When it is desirable to produce both spangled and nonspangled galvanized on the same line without having to change the composition of the zinc bath, an alternate approach is to spray the surface of the molten coating with steam, water, or fine zinc powder just after wiping. The spray increases the number of sites for spangle nucleation, which minimizes the spangle size so that the coating surface is smoother. Further increases in surface smoothness are achieved by a light temper roll of the coated product.

Corrosion Resistance. Zinc coatings protect steel in three ways:

• Initially, a continuous film of zinc at the surface of steel serves as a barrier to separate the steel from the environment.

• At voids in the coating, such as scratches and cut edges, the zinc behaves as a sacrificial anode to provide galvanic protection.

• After anodic dissolution of the zinc metal, zinc hydroxide can precipitate at the cathodic areas of exposed steel, thus forming a secondary barrier.

In order for a coating metal to serve as an effective corrosion barrier, it should corrode at a slower rate than the steel substrate. As shown in Table 5, the corrosion rate of zinc varies greatly, depending on the location of the exposure (Ref 17). Time of wetness and chloride and sulfate level are among the environmental factors that affect the corrosion rate of zinc. Nevertheless, zinc is generally one to two orders of magnitude more corrosion resistant than steel in a wide range of outdoor environments.

Table 5 Corrosion of zinc and steel at 45 locations (10 * 15 cm test specimens)

Location

2-year exposure, grams lost

Zinc

Steel

Norman Wells, N.W.T., Canada

0.07

0.73

Phoenix, AZ

0.13

2.23

Saskatoon, Sask., Canada

0.13

2.77

Esquimalt, Vancouver Island, Canada

0.21

6.50

Fort Amidor Pier, Panama, Canal Zone (C.Z.)

0.28

7.10

Melbourne, Australia

0.34

12.70

Ottawa, Ontario, Canada

0.49

9.60

Miraflores, Panama, C.Z.

0.50

20.9

Cape Kennedy, 0.8 km (0.5 mile) from ocean

0.50

42.0

State College, PA

0.51

11.17

Morenci, MI

0.53

7.03

Middletown, OH

0.54

14.00

Potter County, PA

0.55

10.00

Bethlehem, PA

0.57

18.3

Detroit, MI

0.58

7.03

Manila, Philippine Islands

0.66

26.2

Point Reyes, CA

0.67

244.0

Trail, B.C., Canada

0.70

16.90

Durham, NH

0.70

13.30

Halifax (York Redoubt), N.S.

0.70

12.97

South Bend, PA

0.78

16.20

East Chicago, IN

0.79

41.1

Brazos River, TX

0.81

45.4

Monroeville, PA

0.84

23.8

Daytona Beach, FL

0.88

144.0

Kure Beach, NC, 240 m (800 ft) lot

0.89

71.0

Columbus, OH

0.95

16.00

Montreal, Quebec, Canada

1.05

11.44

London (Battersea), Eng.

1.07

23.0

Pittsburgh, PA

1.14

14.90

Waterbury, CT

1.12

11.00

Limon Bay, Panama, C.Z.

1.17

30.3

Cleveland, OH

1.21

19.0

Dungeness, Eng.

1.60

238.0

Newark, NJ

1.63

24.7

Cape Kennedy, 55 m (60 yd) from ocean, 9 m (30 ft) elevation

1.77

80.2

Cape Kennedy, 55 m (60 yd) from ocean, ground level

1.83

215.0

Cape Kennedy, 55 m (60 yd) from ocean, 18 m (60 ft) elevation

1.94

64.0

Bayonne, NJ

2.11

37.7

Pilsey Island, Eng.

2.50

50.0

Kure Beach, NC, 25 m (80 ft) lot

2.80

260.0

London (Stratford), Eng.

3.06

54.3

Halifax (Federal Building), N.S.

3.27

55.3

Widness, Eng.

4.48

174.0

Galeta Point Beach, Panama, C.Z.

6.80

The corrosion loss of a hot dip zinc coating is generally considered to be linear. Because of this, the life of a zinc coating is proportional to its thickness. In the industrial environment of Bethlehem, PA (see Fig. 8), the corrosion loss is approximately linear with time at an average rate of about 2 pm/year (Ref 18). Near-linear behavior is also observed in marine and rural environments.

Fig. 8 Corrosion losses of hot dip coatings in the industrial environment of Bethlehem, PA. Source: Ref 18

The sacrificial properties of zinc coatings derive from the position of zinc relative to steel in the galvanic series. The corrosion potential of zinc is usually about 0.4 V less noble than steel in most environments at normal ambient temperatures. Thus, zinc will sacrifice itself to provide galvanic protection to steel exposed at voids in the coating. The effective distance of the sacrificial protection increases with the conductivity of the environment, but it is generally limited to a few millimeters in most atmospheres.

Other Properties. Zinc coatings provide some protection to steel against high-temperature oxidation. However, their usefulness for this purpose is limited to a maximum temperature of about 260 °C (500 °F) because of the tendency of zinc at higher temperatures and long exposure times to diffuse into the grain boundaries of the steel and cause embrittlement.

Zinc coatings may impair the formability of steel sheet under certain conditions. For example, in stretch-forming operations, the increased frictional coefficient of the zinc coating against the punch tends to concentrate the strain within a smaller area and thus result in less total stretch before fracture. Galling and coating pickoff can also occur in severe forming operations. The buildup of particulate material on die surfaces may cause impressions and poor appearance on the surface of formed parts.

The life of spot welding electrodes is reduced by zinc coatings, as shown in Table 6. This reduction occurs as a result of alloying of the copper electrode with zinc, which leads to higher local resistance, greater heating, and increased pitting and erosion of the electrode contact surface. Lower tip life reduces productivity and increases manufacturing costs because of more frequent interruptions to the welding operation to redress tips.

Table 6 Effects of hot dip coatings on tip life during spot welding of steel sheet

Type of coating

Coating mass, g/m2

Electrode tip life, number of spots

None

>10,000

Zinc

197

2,500

Zinc-iron

110

6,000

Zn-55Al

150

700

Aluminum type 1

120

Source: Ref 15, 19

Galvanized sheet is used in both bare and painted conditions. In order for paint to have good adhesion to a hot-dip-zinc-coated surface, it is important that the surface is properly pretreated. Zinc phosphate or complex oxide pretreatments are the usual pretreatments for coil-line prepainted sheet. For galvanized components that are painted after fabrication, such as automobile body components, zinc phosphate or zinc phosphates modified with nickel or manganese are commonly used.

In the automotive industry, resistance to electrophoretic paint (e-coat) cratering is an important property. After phosphating, most automobile bodies produced today are primed with cathodic e-coat. In the e-coating process, the phosphated automobile body is immersed in an aqueous bath containing suspended, positively charged paint particles. A negative potential of several hundred volts is applied to the part, and the positively charged paint particles are attracted to the metal surface. Here the paint particles contact hydroxyl ions produced by the cathodic breakdown of water and coalesce to form a paint film. At higher voltages, the dielectric properties of the deposited paint film may be exceeded, with the result that sparking occurs. The heat generated by the sparks causes localized film disruption and premature curing of the paint. After overall curing of the paint, the sparked areas form pinpoint-size craters that are detrimental to the appearance of the painted surface.

Resistance to e-coat cratering can be expressed in terms of a threshold voltage below which cratering does not occur. As shown in Table 7, zinc-coated surfaces have a lower cratering threshold voltage than bare steel and hence are more prone to cratering (Ref 20). Although cratering can be avoided by reducing the voltage applied during e-coating to a level below the threshold, this may result in less productivity due to lower rates of coating deposition and slower line speeds.

Table 7 Effects of hot dip coatings on threshold voltages for cratering of cathodic electrophoretic primer

Type of surface

Cratering threshold, V

Uncoated bare steel

>400

Zinc

275

Zinc-iron

225

Zn-55Al

375

Additional information about galvanized coatings is available in the article "Batch Hot Dip Galvanized Coatings" in this Volume.

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