Variations in Stress Concentration Factors Near Simulated Corrosion Pits as Monitored by Magnetic Flux Leakage, Magnetic Barkhausen Noise and Neutron Diffraction

The conditions under which a pit defect is formed in a pipe can influence local stress concentrations which, in turn, affect the Magnetic Flux Leakage (MFL) signal. In this study MFL, Magnetic Barkhausen Noise (MBN) and neutron diffraction (ND) techniques are used to monitor and compare the local stress distributions surrounding simulated pits in plate and pipeline samples. Our study shows two important findings. Firstly, mechanically machining of simulated corrosion pits creates considerable machining stresses around the defect Conversely, electrochemical machining (ECM) produces no measurable residual stresses. Secondly, all three techniques indicate that, provided stresses are high enough hr produce local yielding, there are significant differences in local stress concentrations depending on whether the pit was electrochemically machined prior to stress application or while the sample was under stress. The latter case is more relevant to pipelines which corrode whilst in service since operating line pressures normally produce pipe wall hoop stresses of up to 70% yield strength.