Corrosion detection in welds and heat-affected zones using ultrasonic Lamb waves

The heat-affected zone in a welded structure is vulnerable to attack by corrosion. Unfortunately, if corrosion occurs, then detection of such areas via a local visual inspection is often difficult because of limited access, and such an inspection may also incur significant time and associated cost penalties because of downtime. One such example, where access is difficult and corrosion may occur in the heat-affected zone adjacent to a weld, is in storage tanks on ships and submarines. In these instances, it is often necessary to undertake an internal examination, requiring emptying of the tank and the entry of a person into a potentially hazardous area. Clearly, an alternative remote method for assessing the extent of corrosion in areas with restricted access, without emptying the tank, would be desirable. An alternative to local inspection is to use guided waves to inspect large areas from a single sensor. This approach has probably been most notably developed for corrosion detection in oil pipelines, rail testing and, more recently in the development of methods for inspection of large areas of plates (see, for example, the literature(1)). The work described here examines the use of ultrasonic Lamb waves for the remote detection of defects adjacent to a weld in steel plates of the type used in the ship industry for the manufacture of storage tanks. Corrosion defects were simulated by the introduction of a flat-bottomed hole successively drilled to give a hole with increasing depth in a region adjacent to the weld. Guided waves in plates will leak energy into the surrounding media if the plate is immersed in a liquid. This will be particularly marked if the media is viscous, and will occur much more where the mode type results in the surface deformation having out-ofplane motion, rather than in-plane motion. In general, this means antisymmetric modes have high attenuation, and symmetric modes with little out-plane surface displacement will show insignificant attenuation as a result of liquid loading(2), (3). Given that storage tanks will be filled with fluid, then the use of symmetric modes, such as the fundamental S0 mode, is preferable to the use of antisymmetric modes, such as the fundamental A0 mode. Complications associated with guided wave inspection are well documented(4), and arise because, in general, the waves propagate with different velocities at different frequencies (ie dispersion), and also because the excitation and reception of unwanted modes result in overlapping and confusing signals. The use of the fundamental S0 mode minimises the generation of unwanted higher order modes, and also permits easier discrimination of reflections because of the larger group velocity relative to the group velocity of A0 and other higher order modes. Typical practical problems associated with the use of guided waves also arise from overlapping and confusing reflections from plate edges, the presence of stiffeners and thickness changes. Of interest to this study, given that the weld might be imperfectly finished, or that the weld might itself obscure signals originating from a corroded area, was the extent to which reflections that originated from the weld could be discriminated from those which might originate in the heat-affected zone due to corrosion.