Use of alternating current field measurement (ACFM) technique for sizing internal surface defects on thin-walled austenitic stainless steel pipes

Putra, Wing Hendroprasetyo Akbar (1999) Use of alternating current field measurement (ACFM) technique for sizing internal surface defects on thin-walled austenitic stainless steel pipes. Masters thesis, Memorial University of Newfoundland.

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Abstract

The demand for more reliable materials to face corrosion problems in process plant equipment has led to the use of advanced materials such as stainless steel for many piping systems. As severe localised corrosion sometimes attacks these steels, a reliable inspection technique is needed to detect and characterise such damage, prior to failure of the system. This thesis explores the potential of Alternating Current Field Measurement (ACFM) as a suitable technique for addressing this problem. ACFM is a relatively new electromagnetic based non-destructive testing (NDT) technique that can be used for inspecting electrically conductive materials. The technique offers the potential to detect and size surface, subsurface, or remote surface defects in components made from either carbon or stainless steels. -- On the Hibernia platform, stainless steel piping systems that are used to transport freshwater or seawater are sometimes attacked by localised corrosion processes near welds. -- Experiments were carried out to develop models to predict the actual depth and length of defects on the inside surface of 6-Mo austenitic stainless steel pipes using the ACFM technique. Two different pipe wall thicknesses were considered in the experiments. Artificial defects of variable depth and length were made using a slitting wheel. Some of the defects were located in the parent metal and others were located at the weld toe. The experimental results indicate that it is possible to reliably detect and characterise backwall corrosion damage in 6-Mo piping systems using ACFM. Larger defects were noted to produce clearer indications, as would be expected for this NDT technique. It was also apparent that there was no significant difference between results obtained from defects located in the parent metal and results obtained from similar defects located at the weld toe, suggesting that changes in material properties near the weld (e.g. variations in magnetic permeability) have an insignificant influence on the ACFM signals.

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