The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part 1: Pitting and cracking occurrence

In this investigation, tensile test specimens were fabricated with increasing levels of compressive and tensile residual stress on the surface and through the thickness of the specimen. These residual stresses were then measured by neutron diffraction at multiple points along the length and through the depth of the specimens. The specimens were then exposed to a neutral pH aqueous soil environment in combination with an applied cyclic stress for various lengths of time in order to initiate and propagate stress corrosion cracking (SCC). The formation of micro-pitting was found to occur preferentially in areas where the tensile residual stresses were the highest (approximately 300 MPa), while SCC initiation occurred with a 71% normalized frequency in areas where the surface residual stress was in the range 150-200 MPa. The difference between residual stress levels occurring at SCC locations versus pitting locations resulted from both the change of residual stress during cyclic stress application during SCC testing and the residual stress gradient in the depth direction. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.