51315-5533-Analysis of Paint Integrity and Underfilm Corrosion of Transmission Tower Cross-arms Using Electrochemical Impedance Spectroscopy and Terahertz Imaging

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Product Number: 51315-5533-SG

ISBN: 5533 2015 CP

Author: Norikazu Fuse

Publication Date: 2015

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Electric transmission lines located near rivers or coastal areas are protected against atmospheric corrosion factors. Hot-dip galvanizing and anticorrosion painting for such measures degrade during long-term operation. The present study conducts bidimensional analysis of paint and steel degradation using electrochemical impedance and terahertz waves to steel cross-arms in a transmission tower used for approximately 25 years on a Pacific coast.Visual observation indicates that the underfilm corrosion is intense on the surface facing the sea breeze. Paint peeling and tarnishing are severe in materials installed at high altitude. Electrochemical impedance that evaluates the water interception ability agrees with such trends. Terahertz waves are types of far infrared waves. Paintings exhibit good transmissivity whereas steel rust strongly absorbs terahertz waves. Owing to such material properties the distribution of two-dimensional rust beneath the opaque paint film is observed without any destruction. This new technology shows that corrosion is severe near mountain-folded regions of the angle steels and expands toward the edge.Both resistance and capacitance obtained through electrochemical impedance measurement have their criteria on the paint aging. The set of these two data obtained varies from light to severe degradation states and judgments by the two criteria frequently agree with each other. Through such confirmation that the aging criteria are effective in in-service materials electrochemical impedance assessment can be used for the integrity evaluation of paintings. However the impedance measurement sometimes indicates good electrical insulating values even when underfilm corrosion has occurred. One of the reasons for this misleading result is that the measured object is primarily paint rather than rust. Another reason is that rust expands under the film without degrading the paint as investigated by the terahertz imaging. Terahertz imaging can then be expected to provide sophisticated rust diagnosis by providing supporting data for corrosion analysis via its "see-through"ability. Adequate underfilm corrosion could be subsequently provided through matching the integrity evaluations both for the paint and steel corrosion.