Materials Performance

NOV 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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A test program determined that corro- sion rates of the structural steel ranged from 0.2 to12 µm/year. "Although the steel corro- sion product from a corrosion engineer's perspective is pretty minor, it is an expan- sive process that puts an extremely high amount of stress on the very weak limestone material," Noyce explains. He adds that limestone has limited tensile strength and cracks very easily, and the buildup of corro- sion scale on the steel can be detrimental to the structure's masonry. Once a crack initiates on the exterior of the stone, it becomes a conduit for moisture and oxygen, allowing them to get to the embedded steel much faster than by pure diffusion through the limestone material. Moisture and oxygen fuel corrosion of the steel, and more corrosion products are formed, which put stress on the stone and expand the crack. The expanded crack allows even more moisture and oxygen to reach the steel and increase the corrosion rate, which produces additional corro- sion products that expand the crack even further. The cycle repeats itself until major masonry damage is imminent and sheer failure occurs. In addition to creating a safety hazard from the risk of falling pieces of stone, minute amounts of corrosion can cause millions of dollars' worth of damage to the historic decorative façade—an expensive and aesthetically valuable portion of the structure. The Cathodic Protection System Based on durability calculations from on-site data, microcrack-related damages to the structure were expected to occur within six years or less if conditions remained the same and a proactive approach to corro- sion mitigation wasn't pursued. As a means to protect the lug anchors, steel frame, and other steel support components from future corrosion, Crevello and Noyce were asked to explore the feasibility of ICCP. They initiated a test program with a trial ICCP system installed in one of the bays to protect ~9 linear ft (2.7 m) of steel at both the exterior and interior face of the entab- lature. The anodes, ballasted mixed metal oxide (MMO)-coated titanium expanded mesh probes, were installed within the same masonry joint line as the steel, and the lug anchors were accessed at the same joint line to determine the feasibility of bonding them with the steel for electrical continuity. When the trial ICCP system was energized, the steel was successfully polarized after 24 h and the 4-h, 100 mV depolarization criteria were achieved. All potentials were more negative than –250 mV vs. silver/silver potassium chloride (Ag/AgKCl) reference electrodes, and the decision was made to move forward with an ICCP system as part of the overall scope of the masonry repair project. To determine the feasibility of using CP to protect the steel components from corrosion, a trial system was installed in one of the rotunda's bays. Photo courtesy of Gina Crevello, Echem Consultants. To ensure all steel support components were electrically continuous, steel anchors supporting the soffit stones were bonded to the steel frame. Photo courtesy of Gina Crevello, Echem Consultants. 29 MATERIALS PERFORMANCE NOVEMBER 2014 NACE INTERNATIONAL: VOL. 53, NO. 11 Mitigating Structural Steel Corrosion Protects Decorative Façade of the Historic Elks Veterans Memorial

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