Materials Performance

OCT 2018

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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OCTOBER 2018 MATERIALSPERFORMANCE: VOL. 57, NO. 10 15 environment where we would not expect to see corrosion in 100 years, then there is no need to make the expenditure for providing extra corrosion pro- tection. We avoid over protecting because it's not a good use of funding," Lasa notes. Corrosion-Resistant Materials A common factor in bridge deterioration is the corrosion of carbon steel (CS), whether it is used for structural components or for concrete reinforcement, comments NACE member Frank Smith, an independent consultant based in Kingston, Ontario, Canada. CS is particularly prone to corrosion in chloride- containing environments, he adds. A very large percentage of reinforced concrete fails due to corrosion of the embedded CS rebar. Smith explains that moisture, oxygen, and chlorides penetrate the concrete cover via pores or cracks. Corrosion of the CS rebar produces corrosion products that occupy a larger volume than the original steel. These products then exert pressure on the surrounding concrete that causes it to crack and eventually delaminate or spall off. SS rebar is resistant to corrosion because it contains chromium (between 10.5 to 23 percent), which helps to form a protective passive film. Molybde- num and nitrogen are two alloying elements that are also added to improve the chloride resistance of SS, Smith says. Adding additional alloying elements, however, drives up the cost of the material, so SS is typically used to replace CS only where there is a risk of chloride ingress, such as the bridge's concrete deck and side barrier walls in geographical regions where road deicing salts are used, or in marine envi- ronments where bridge supports, piers, and columns are exposed to sea - water. One way to assess a SS alloy's resistance to chlorides is the pitting resistance equivalence number (PREN), Smith notes. The PREN is a predictive measurement of a SS alloy's localized pitting corrosion resistance based on its chemical composition (e.g., the amount of chromium, molybdenum, and nitrogen). Generally, a SS with a higher PREN value is more resistant to local- ized pitting corrosion from chlorides. Many bridges have structural components that are made from CS. These components are visible and can be easily inspected and maintained by recoating. Addressing corrosion on bridges with structural members made of reinforced concrete can be much more challenging because the embedded CS reinforcement cannot be readily inspected and repaired. This is why many DOTs have moved from CS rebar to SS rebar in locations where the concrete will be exposed to chlorides, Smith says. Across North America, and around the world, hundreds of bridges have been built or rehabilitated with SS rebars made from alloys such as UNS S31653, S31803, and S32304. The Virginia DOT (VDOT) uses corrosion-resistant reinforcing steel in the bridge decks, which helps with the corrosion issues from deicing salts, says NACE member Stephen R. Sharp, senior research scientist with the Virginia Transportation Research Council (Charlottesville, Virginia, USA). He com- ments that early corrosion resistance research work at the VDOT focused on bridge decks since they essentially can act as a roof. If roadway salts are kept from trickling down onto the bridge structures below the deck, then the chloride exposure to those components can be reduced. "We started with the deck, improved the corrosion resistance of the deck, and then we moved on to other areas of the bridge," he says. To implement corrosion protection in an economically responsible way, VDOT has developed a tiered system that states which materials are consid- ered the most corrosion resistant and where they should be used. Corrosion- resistant reinforcing (CRR) steel Class I includes alloy Type 1035 CS with a minimum chromium content of 9.2 percent and UNS S24100. CRR steel Class II includes UNS S32101 SS, and CRR steel Class III includes UNS S24000, S30400, S31603, S31653, S31803, and S32304 SS. The U.S. Route 340 Bridge, 8 constructed in 2016-2017 in Waynesboro, Virginia, USA, is the first highway bridge in the United States to be constructed completely out of SS, including ASTM A709 Grade 50CR (UNS S41003) SS for the girders and cross frames and SS fastener assemblies for the bolted splices. Photo courtesy of Stephen Sharp, VDOT.

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