Materials Performance Supplements

October Calendar 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 WWW.MATERIALSPERFORMANCE.COM 18 performance history for the coating. TXDOT is amenable to trying shop metal- izing for a new bridge but hasn't had the opportunity to do so yet. "But it's coming down the road—it's a matter of taking the next step," he says. On bridges with steel girders that are exposed to atmospherically corro- sive environments, FDOT also relies on protective coatings, Lasa adds. Simi- larly, the FDOT has experimented with thermal spray coatings—using zinc and aluminum as well as 85/15 zinc/aluminum alloys, he said. All three of these coatings are included in the NACE International standard for applying metallic coatings. 9 "On older corroding steel bridges in very corrosive envi- ronments, we have implemented the use of dual coatings, where we use a thermal spray coating on the steel with a seal coat over it," Lasa explains. "We feel this is helping us considerably on extending the service life of struc- tural steel components." After much experi- mentation, Lasa notes that the FDOT is using more aluminum thermal spray coating along with a topcoat. "This has given us the best results," he says. The research was pre- sented at CORROSION 2018. 10 Like many other state DOTs, TXDOT is looking at a service life of 100 years for its bridges. Many of the structural components can be designed for a life of 100 years before maintenance is required, Miller says, but a 100-year service life for a coating as a corro- sion protection system is not realistic. The general industry consensus is that the esti- mated life for a coating system is between 15 to 20 years—with an estimated life closer to 15 years for coating systems along the coast and in highly corrosive areas—and up to 30 years with aluminum thermal spray metalizing. While the coatings industry is always formulating and creating new prod- ucts, and Miller's group is continually looking at different coatings, TXDOT is not yet ready to add any new products to its existing lineup of bridge coating systems. "While we don't want to give the impression that we are not willing to look at any new products or try them, it's a matter of the product estab- lishing a reputation that it works," Miller says. "We're going to be very cau- tious about adopting a particular product into our general use until we know with confidence that this product is going to work as intended." Cathodic Protection If a state DOT has a structure that is already experiencing corrosion, and every state has many of these, then probably the most common corrosion protection technique being employed is cathodic protection (CP) in one form or another, says Whitmore. Some DOTs use a combination of impressed cur- rent CP (ICCP) and galvanic CP. In some projects, provisions for a CP system are installed on a new bridge but not connected. This is an effective strategy when the new bridge is constructed in a harsh environment since it is easier than trying to retrofit a CP system into an existing structure. The FDOT has implemented a CP program to extend the service life of its older bridges. "If we know we have corrosion on a bridge or that corrosion is extremely close to initiating, we implement a CP system," Lasa says. "CP is mostly used on reinforced concrete bridges, but we also use it on steel bridges on por- tions that are submerged." In Florida, there typically aren't prob- lems with corrosion of the bridge decks. The bridge substructure—the pilings and piers that hold up the bridge—experiences most of the corrosion issues, comments Lasa. CP is used to protect prestressed concrete piles as well as steel piles if they are exposed to corrosive environments. For bridges in extremely aggressive environments where corrosion concerns develop during construction, such as the Escambia Bay Bridge near Pensacola, FDOT will install CP on the reinforced concrete components—either ICCP or galvanic CP depending on the structure and location of the piers—particularly where chlorides have penetrated the concrete cover and corrosion of the steel rebar has developed or will most probably develop in the future. An in-house group within the FDOT's corrosion laboratory mon- itors the performance of CP systems for it districts. Not all DOTs, however, have the same level of in-house technical capabil- ity as FDOT and tend to use more galvanic CP techniques because there is less maintenance and monitoring required to keep the systems functioning. Sharp comments that CP used by VDOT is typically sacrificial. An advan- tage with ICCP, however, is that the system will last a long time vs. a

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