In-service corrosion evaluation in pipelines using gamma radiography – a numerical approach

Wall thickness measurement and deterioration determination of 6 and 10 inch pipes due to corrosion/erosion/pitting was evaluated by using radiographic film density measurements. Special reference blocks were prepared with defined step wall reductions and artificial defects. Gamma radiography with a Ir-192 source was used. A double-wall technique with longitudinal film arrangement was used for this purpose. Formulae were developed from the experiments for numerical calculations. It was observed that this method can determine remaining wall thickness as well as pitting corrosion in insulated and non-insulated pipes with differential and absolute density measurements. The purpose of the work was the optimisation of techniques, quality control, evaluation of radiographs and parameters affecting the results. Furthermore, the studies included investigation on the limitations of each technique and stability of test conditions. Results show good precision – above 97% – between real and calculated values.