Predicting the service life of galvanized steel

Zinc, which has been used to hot-dip-galvanize steel for 250 years, provides 50 to 75 years of corrosion protection in many environments. Empirical data collected about hot-dip galvanized (HDG) steel field performance from 1940 to 1980—in environments ranging from industrial to marine to suburban—indicates that zinc can prevent base steel corrosion more than other surface treatments. Because of zinc's long-lasting protection, projects require no maintenance and therefore no maintenance costs.

Much of the industrialized world has become cleaner and safer over the last 20 years, mainly because of progress in environmental protection. Sulfur and chloride emissions have been reduced. Because both sulfur and chloride compounds increase the corrosion rate of most metals, including zinc, it reasonably can be assumed that galvanized steel should last longer now than in previous decades.

This was confirmed in a 2001 study funded by the International Lead Zinc Research Organization (ILZRO) and conducted by Gregory Zhang, Ph.D., of Teck Cominico Metals Ltd.

Zinc Coating Life Predictor

Figure 1

A computer program, the Zinc Coating Life Predictor, was developed to estimate the corrosion rate of zinc in various environments. The program performs calculations based on models developed using statistical methods, neural network technology, and an extensive worldwide corrosion database. The environmental data input required to estimate a corrosion rate includes temperature, airborne salinity, sulfur dioxide concentration, relative humidity, rainfall, and sheltering conditions for the project (indoor, rain-sheltered, or outdoor).

Once these values are known, the software calculates and reports a corrosion rate and also gives an option either to calculate the predicted life given the coating thickness or the coating thickness required to achieve a specified life.

This software was used to calculate the performance of HDG steel in several North American cities that represent the five different types of corrosive climates (see Figure 1). These data points then were used to develop a graph of anticipated service life of HDG steel of varying coating thickness for each of the climates (see Figure 2).

Figure 2

The environmental data required to use the Zinc Coating Life Predictor was obtained from resources on the World Wide Web. All of the predictions made were assumed to be in open atmosphere and subject to all environmental conditions. The temperatures and annual precipitation, or rainfall, were gathered from The Weather Channel's Web site. The relative humidity data was gathered from the National Weather Service, which has annual relative humidity data for many North American cities. The sulfur dioxide concentrations were determined from a report produced by the Environmental Protection Agency (EPA) that lists the peak air quality statistics for the major pollutants. This file is available online at the agency's Web site.

It should be noted that some cities, particularly those in rural areas, do not monitor their levels of sulfur dioxide because of its low concentration. These values were estimated to be the lowest of all the concentrations.

Air salinity data does not readily exist for numerous environments. Some data exists for areas close to the sea, and values for inland areas were estimated based on their distance from the ocean and from other known data. Airborne salinity information and data are provided with the Zinc Coating Life Predictor. Definitions of the other four environmental parameters are provided also.

The Zinc Coating Life Predictor is available at the American Galvanizers Association's Web site.

Predictor Relies on Environmental Variables

The model requires just five environmental variables to determine the long-term corrosion protection that galvanized steel can provide in specific geographic locations. This protection translates into even lower life-cycle costs for manufacturing facilities, water and wastewater projects, and myriad buildings and other structures that are constructed with HDG steel.

After entering the five variables into the program, the user has a choice: Determine the coating thickness and request the program to calculate the expected life cycle or determine the life cycle and request the program to calculate the coating thickness required.

After completing the calculation, the program generates a printable report that displays the parameters provided by the user and the results generated by the program.

Cleaner Air, Longer Protection

Stricter environmental regulations over the past 20 years have improved the micro- and macroenvironment by reducing sulfides and chlorides in the air from sources such as automobiles, power plants, and industry in general. A draft of the EPA's 2003 strategic plan, released March 5, reveals that the EPA has many strategies for further pollution reduction. For instance, one specific goal is to reduce sulfur dioxide emissions from electric generating units by 4.6 million tons by 2010. It is expected that further reductions in such pollutants will help galvanized structures last even longer in the future.

Three-way Protection From Corrosion

The hot-dip galvanizing process prevents steel from corroding by providing cathodic, barrier, and patina protection.

• Cathodic. Zinc is more anodic than steel. Thus, when a corrosion cell forms—when the zinc and steel have both an electrolyte and return current path present—the zinc readily gives up electrons that protect the steel from corrosion. Zinc will protect the base steel until all of the galvanized coating is consumed.

• Barrier. Zinc metal is very dense and thus does not allow moisture, or electrolytes, to penetrate the galvanized coating. Therefore, the base steel does not corrode.

• Patina. When exposed to the atmosphere immediately after the galvanizing process is complete, zinc metal reacts with oxygen in the air to form a very thin zinc-oxide powder on the galvanized coating surface. After a few days the zinc oxide reacts with hydrogen in the air to form zinc hydroxide. As the zinc hydroxide is exposed to moisture in the air over a period of months, a thin film of zinc carbonate forms. Zinc carbonate is a passive patina film that is tightly bound to the remaining zinc of the galvanized coating and gives a HDG coating its durability.

Thomas J. Langill, Ph.D., is technical director and Philip G. Rahrig is executive director of the American Galvanizers Association, 6881 S. Holly Circle, Suite 108, Centennial, CO 80112, 720-554-0900, fax 720-554-0909, aga@galvanizeit.org, www.galvanizeit.org.

National Weather Service, 1325 East West Highway, Silver Spring, MD 20910, www.nws.noaa.gov.

Teck Cominco Metals Ltd. Products Technology Centre (PTC), 2380 Speakman Drive, Sheridan Park, Mississauga, ON L5K 1B4, 905-822-2022, fax 905-822-2882, www.teckcominco.com.

U.S. Environmental Protection Agency, 1200 Pennsylvania Ave. N.W., Washington, DC 20460, 202-566-1667, www.epa.gov.

The Weather Channel Interactive Inc., P.O. Box 724554, Atlanta, GA 31139, www.weather.com.