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28 DECEMBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 12 FEATURE ARTICLE is based on the GIS source data and doesn't necessarily account for terrain shifts, this study carries a low confidence level. Testing actual soil samples taken around the structures' anchors and footings can measure the resistivity and corrosivity of the soil, which determines the soil's capacity to act as an electrolyte, as well as identifies the soil's corrosion performance parameters, which indicate how actively it will corrode the steel. This type of assess- ment can be used to estimate maximum galvanizing thickness loss and predict the structure's life expectancy under the worst possible conditions. Since the accuracy of the assessment depends on the location of the soil sample, the condition of the instru- ments, and the inspector's skill, this type of study has a moderate level of confidence. Knowledge-Based Assessment Both Gilpin-Jackson and Zamanzadeh note that a knowledge-based assessment evaluates the structure as well as defines the surrounding environment. Because all relevant corrosion and structural parame- ters are measured in addition to a visual inspection during the detailed assessment, the level of confidence in this type of assess- ment is high. Parameters reviewed in this type of assessment typically include the structure's characteristics, configuration, and expected performance; the site's fea- tures from a corrosion and materials point of view, including the atmospheric and soil characteristics; and other external influ- ences present, such as alternating and direct current interference (e.g., stray cur- rent), the presence of grounding, other land uses, etc. The galvanized structure is typically inspected visually to a below-grade depth of 36 in (0.9 m). If the structure shows signifi- cant signs of material loss at this depth, a more detailed condition assessment is per- formed using galvanizing thickness and adhesion measurements as well as defect characterization. Also, the maximum cor- rosion rate of the structural steel is mea- sured using linear polarization resistance and electric resistance probes. Concrete inspection and petrographic analysis (if required) are performed for concrete base structures that are damaged or degraded. "The knowledge-based assessment will let you know what mitigation techniques to use. Once you have all factual data, then you can make a recommendation—either implement corrosion mitigation, a repair, a replacement, or take no action," Zamanza- deh comments. Additionally, he adds, the knowledge-based assessment can guide the inspection schedule for the structure so the scope of work over several years can be planned, funded, and staged. A long-term corrosion mitigation and maintenance plan based on a knowledge-based assessment can cost-effectively extend the life of galvanized steel. Similarly, Gilpin-Jackson notes that owners want to be sure they invest upfront on corrosion mitigation for new structures that is appropriate for the in-service condi- tions. Because corrosion mitigation activi- ties are increasing, so is the recognition that knowledge-based mitigation is vital. Painting over galvanized steel transmis- sion and distribution structures that have been in service for many years can extend the life of the zinc coating. For all cases where a structure is buried in corrosive soil, the galvanized steel should at least have a suitable topcoat applied to protect it against the adverse effects of higher-than-normal soil moisture/corrosivity, says Zamanzadeh. He recommends that a factory-applied, organic coating, such as polyurethane or other environment-resistant coating, be specified for galvanized steel structures in contact with corrosive soil. For more robust corrosion mitigation results, he suggests a CP system be added for each such structure, and notes this corrosion mitigation combi- nation has proven to provide a stronger fail- ure-safe solution than either a protective coating or CP alone. Assessing and mitigating the corrosion before it causes a structural issue is vital. Monitoring the galvanized layer thickness will indicate the zinc coating's remaining service life and provide a time guide for applying a protective topcoat. Typically, a structure won't need to be topcoated until a significant portion of the galvanizing's sur- face zinc is depleted, which could take 30 to 40 years depending on the service environ- ment, says Zamanzadeh. Then, mainte- nance recoating should be considered when the galvanized steel's top-most intermetallic galvanized layers—the Eta, Zeta, and Delta layers—are corroded. Generally, if the galva- nizing's Gamma layer (the layer closest to the substrate) is depleted, then structural corrosion may have initiated and it could be too late to paint without further steel assessment and possible steel replacement, notes Gilpin-Jackson. If structural corrosion is present, the load-bearing members may need to be replaced to protect the struc- ture's integrity. In extreme cases, the entire structure may need to be replaced. Developing a long-term corrosion miti- gation and maintenance plan based on a knowledge-based assessment can cost- effectively extend the life of galvanized steel transmission and distribution structures and prevent catastrophic failures. Plans should include future inspections, coating, CP, mechanical repairs, and other corrosion mitigation procedures where feasible. More information on assessing galva- nized steel for electric power utility trans- mission and distribution structures, including a case study that describes the application of a system-wide CP system for aging galvanized poles with below-grade corrosion, can be found in CORROSION 2016 paper no. 7245, "Galvanized Steel Pole and Lattice Tower Corrosion Assess- ment and Corrosion Mitigation," by M. Zamanzadeh, C. Kempkes, D. Riley, and A. Gilpin-Jackson. Contact Mehrooz Zamanzadeh, Exova— e-mail: zee@exova.com; and Adelana Gilpin- Jackson, BC Hydro—e-mail: Lana Gilpin- Jackson@bchydro.com. Reference 1 "Utility Poles," Steelworks, the Online Resource for Steel, American Iron and Steel Institute, http://www.steel.org/the-new- steel/utility-poles.aspx (November 4, 2016).