Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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65 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 Galvanized Steel Structural Members Galvanized steel generally has good atmospheric corrosion resistance. When exposed to the atmosphere, the zinc in gal vanized steel converts to oxide, then to hydroxide, and finally to carbonate. Zinc oxide and hydroxide are not dense layers so they can't protect steel against corrosion. In contrast, zinc carbonate is a dense and pro tective layer. It has good continuity and adhesion to the substrate and can provide corrosion resistance for steel for a long time. 4 In Dolatabad and Habibabad, the thickness of galvanized steel on electrical fit tings was ~100 ┬Ám. Its service life was very short, however, and after about five to six years, the fittings became rusty (Figure 2). Atmospheric corrosion of galvanized steel is related to humidity and corrosive agents in the air, such as SO 2 . 5 Table 1 lists the AQI of Isfahan's air at different loca tions. High levels of air pollution occur at the end of December and beginning of Jan uary (during the winter season). Unfortu nately, there were no air pollution measur ing devices in Dolatabad and Habibabad; however, the levels of pollution would be expected to be greater than levels listed for other areas of Isfahan (Table 1) because of the sources of pollution near these suburbs. In polluted air, humidity and SO 2 form sul furic acid (H 2 SO 4 ), which can dissolve the zinc carbonate layer on galvanized steel. The carbonate layer has an important effect on the corrosion resistance of galva nized steel. When that layer is removed, underlying layers of the galvanizing are ex posed to polluted air ; therefore, the corro TABLE 1. AQI OF ISFAHAN CITY IN DIFFERENT LOCATIONS ON DIFFERENT DATES Date Ahmadabad Square Azady Square Emam Hossein Square Khajo Bridge Kharrazi Freeway 12-25-2013 248 261 235 238 203 12-29-2013 302 310 273 301 356 1-16-2014 61 64 72 67 70 1-21-2014 162 163 171 159 177 1-24-2014 92 86 97 88 98 FIGURE 2 A rusty galvanized cross arm in the polluted atmosphere of Dolatabad. FIGURE 3 (a) Melted contact surface of a clamp by fault current and (b) a typical new seal clamp. sion rate increases rapidly. 5 After a short time rusty conditions appear (Figure 2). Because the zinc carbonate layer plays a major role in the corrosion resistance of galvanized steel in polluted air, when that layer is removed, corrosion of the galva nized coating accelerates. Increasing the thickness of the galvanized coating isn't a suitable solution for high corrosion rates in polluted air. W h en th e zinc carb onat e l ayer on clamps made of galvanized steel is removed by polluted air and the other layers of clamps made of galvanized steel corrode, ferrous oxides are formed on the surface of the clamp and migrate between the clamp and the power line. Although the oxides are electrically conductive, this increases the electrical resistance of the clamp in contact with the power line. When fault current occurs, electrical heat in the corroded clamp is much higher than it would be oth erwise. This heat has been high enough to melt the contact surface of the clamp and cause the power line to fall to earth. If an aluminum clamp is used, then its melting point from electrical heat will be lower than a steel clamp. Figure 3 shows a dam aged clamp and a new clamp. The solution to this problem is to seal the contact surface of the clamp against polluted air. Polymer Composite Structural Members Polymer composite cross arms have good corrosion resistance in polluted air. They are nonconductors and can increase resistance against current creep across insulators. Therefore, they were proposed as a suitable alternative material selection for galvanized cross arms in Dolatabad and Habibabad. Figure 4 shows a typical poly mer composite cross arm.

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