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Metallic zinc coatings provide corrosion protection for steel by acting first as a barrier coating and secondly as a sacrificial anode. Whether exposed to the atmosphere or embedded in concrete, if the coating is scratched or compromised in any way, the zinc will still provide galvanic protection to the steel. In the same way, concrete structures reinforced with plain or bare steel showing signs of distress can be galvanically protected with external zinc anodes. Thermal sprayed metallic zinc on the exposed surface of the concrete can be electrically connected to the steel reinforcement and provide corrosion protection to the steel. The external zinc anode will corrode instead of the steel reinforcement, reducing formation of corrosion products which stress and degrade the concrete. Thermal sprayed zinc coatings were used as anodes in ICCP systems on the historic reinforced concrete Cape Creek Bridge, Yaquina Bay Bridge and Depoe Bay Bridge on US Route 101 in Oregon. The 20-plus-year performance confirms that the service life of reinforced concrete structures can be significantly and economically extended by using metallic zinc anodes to protect the plain steel reinforcement from further corrosion.
Key words: Thermal sprayed zinc, sacrificial anode, corrosion, rebar
Failure of water/wastewater mains can result in high visibility repairs customer inconvenience and replacement costs. Preventive measures such as condition assessment for early recognition of corrosion in aging infrastructures is crucial for agencies from resiliency safety and economic standpoints but is not regulated as in the oil & gas industry. Internally deployed tools/technology or external excavations for direct assessment techniques provide valuable insight on the existing condition of buried structures but at a significant cost in terms of shutdown and technology expenses in addition to safety concerns for manned entries into confined spaces. Because of the cost and safety implications large diameter cement mortar lined (CML) pipe extensive and recurring direct assessments are less common in the water and wastewater industries. Indirect assessment techniques particularly the over-the-line potential surveys for condition assessment of water/wastewater lines can be conducted to determine active external corrosion areas. Traditionally over-the-line potential surveys were applied to electrically continuous pipelines. Most water/wastewater pipeline designs utilize rubber gasket bell-and-spigot joints. Unless electrical continuity is intentionally designed for the pipeline such joints result in a pipeline with no electrical continuity. This paper presents multiple case studies where over-the-line potential surveys were successfully applied on electrically discontinuous water pipelines. The results of over-the-line surveys correlated well with direct assessment techniques. The paper presents the methodology and results of such assessments and findings for various pipe materials.
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