Corrosion Detection in Storage Tank Bottoms via Omni-Directional Magnetostrictive Guided Wave Inspection System

Abstract

Storage tanks are ubiquitous in various industries, and corrosion in tank bottoms is a major threat to their normal operation. A variety of nondestructive testing (NDT) methods have been proposed for corrosion detection, of which the ultrasonic guided wave (UGW) technique is widely considered efficient due to the large propagation distance and high sensitivity to defects. To obtain complete mapping of tank bottom defects, UGW tomography is usually preferred; however, the requirement to place transducers at fine angular increments can cause the measurements to be time consuming. In this paper, we report the usage of a newly designed omni-directional magnetostrictive guided wave system for tank bottom corrosion detection. Different from existing omni-directional system designs, which are typically based on sparse array elements, the system proposed here excites guided waves using a single probe in a single predominant direction that covers the area determined by its beam characteristics. The omni-directional coverage is achieved by rotating the probe with a servo motor and acquiring data at predefined angles. In this fashion, the beam directionality is improved due to a larger transducer aperture compared with those based on a sparse array of small elements, and the beam directionality remains the same at every angle. The performance of the proposed omni-directional system for corrosion inspection is evaluated experimentally by introducing drilled holes of different depths and locations in the bottom of a mock-up tank. The system was placed at several locations along the tank bottom chime plate edge (skirt), where triangulation using appropriate angular measurements can be applied to locate corrosion in the tank bottom.