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    Wrought and Cast Iron Structures

    continued

    Deterioration

    Iron oxidizes rapidly when exposed to moisture and air. The product of the oxidation process is rust. The minimum relative humidity to promote rusting is 65 percent, but humidity levels lower than this can cause oxidization in the presence of pollutants. In addition, if chlorides are present, the corrosion process can become accelerated. Once a film of rust starts to develop, the natural porosity of the corrosion byproduct tends to act as a reservoir for moisture, resulting in even further acceleration of the deterioration process.

    Cast iron will develop a somewhat protective surface scale, which makes it slightly more resistant to corrosion than wrought iron; however, it is still recommended that cast iron be painted to prevent rusting. And because of the slag content of wrought iron, that material is more resistant to progressive corrosion than cast iron.

    Iron can also be corroded by acids, magnesium, and some sulfur compounds. It is also vulnerable to galvanic corrosion (dissimilar metal corrosion) from contact with copper, chromium, lead, stainless steel, and brass.

    Another type of deterioration common to cast iron is graphitization. This condition can occur in the presence of acidic precipitation or seawater. As the iron corrodes because of exposure to these types of environments, porous graphite residue is impregnated within the surface corrosion byproduct. The cast-iron element retains its original appearance and shape, but becomes gradually weaker internally. Graphitization typically occurs when cast iron is left unpainted for extended periods, or where the sealant has failed at the joints between adjoining components. This condition can be identified in the field by scraping the surface of the cast iron with a knife to see if deterioration of the iron is revealed beneath the surface.

    Repair and Restoration

    There are a number of methods available to remove paint and corrosion from cast and wrought iron, including manual scraping; chipping or wire-brushing; low-pressure grit- or sandblasting; flame-cutting; and chemical removal.

    Once any existing paint and corrosion have been removed, the most common method of protecting cast iron from further deterioration is to repaint the surface. Prior to repainting, it is first necessary to prepare or repair the surface.

    Proper preparation includes elimination of crevices or pockets that can collect moisture, to prevent accelerated deterioration; removal or smoothing of sharp corners, to prevent accelerated paint failure; hermetical sealing of hollow section, to prevent moisture intrusion and freeze-thaw damage; filling of joints, cracks, and bolt or screw holes with sealant, to prevent moisture intrusion and freeze-thaw damage; and taking the steps recommended in the paint manufacturer's specifications.

    In all cases, it is recommended that a test area be used to confirm that the selected cleaning, preparation, and painting techniques are effective prior to attempting to remediate the entire restoration area. It is also recommended that sheltered areas such as eaves, where evaporation of moisture can be inhibited, be coated with additional layers of the selected paint or coating.

    Additional protection and repair procedures can also include plating with metals or cladding with plastic or epoxy. Sections or entire portions of a significantly deteriorated area may be replaced with glass-fiber-reinforced concrete (GFRC), fiber-reinforced polyester (FRP), or aluminum.

    If additional structural retrofitting is required as a part of an adaptive reuse project, the following remedial work should be avoided if at all possible: welding, burning of holes, the use of impact drills, high-strength bolts, and filling of voids or posts with concrete.

    I encourage readers to become familiar with the reference material listed at the end of this article to gain additional insights into the development and use of wrought and cast iron in the United States and Europe. In particular, Cast Iron Architecture in America: The Significance of James Bogardus will provide a fascinating story of the early development and use of cast iron in the U.S.

    D. Matthew Stuart, P.E., S.E., F.ASCE, SECB, is a licensed structural engineer in 20 states. He currently works as a senior project manager at the main office of CMX, located in New Jersey, and also serves as an adjunct professor for the master's of structural engineering program at Lehigh University in Bethlehem, Pennsylvania.   More by D. Matthew Stuart

    This article is reprinted from the March 2009 issue of STRUCTURE magazine, with permission of the publisher, the National Council of Structural Engineers Associations (NCSEA).

    References

    A. & P. Roberts & Company. Wrought Iron and Steel in Construction: Convenient rules, formulae, and tables for the strength of wrought iron shapes used as beams, struts, shafts, etc., manufactured by the Pencoyd Iron Works, 2nd edition. New York: John Wiley & Sons, 1885. Available at Google Books.

    Gayle, Margot, and Carol Gayle. Cast Iron Architecture in America: The Significance of James Bogardus. New York: W.W. Norton & Company, 1998.

    Gayle, Margot, David W. Look, and John G. Waite. Metals in America's Historic Buildings: Use and Preservation Treatments. Washington, D.C.: U.S. Department of the Interior, National Park Service, Cultural Resources, Preservation Assistance, 1980. Updated in 1992 by John G. Waite. Available from the National Park Service for purchase.

    Waite, John G., with a historical overview by Margot Gayle. Preservation Brief 27: The Maintenance and Repair of Architectural Cast Iron. Washington, D.C.: U.S. Department of the Interior, National Park Service, 1991. Available from the National Park Service for purchase and online (in a slightly altered form).

     

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