Corrosion fatigue behaviour of X65 steel oil and gas pipelines

X65 steel oil and gas pipelines are typically subjected to cyclic loading and an aggressive corrosive environment. The combined action of these two can result in fatigue cracking induced from a type of localized corrosion called pitting. The transition from corrosion pit to fatigue cracks, the so-called pit-to-crack transition, is a significant part of the total fatigue life and yet the mechanism of this transition is not well understood. Several studies in the literature have used Scanning Electron Microscopy (SEM) during ex-situ analysis, but the disadvantage of this method is that SEM is only capable of characterising surface cracks and thus does not enable researchers to probe the crack initiation and evolution from pit base or pit wall during the corrosion fatigue test. To overcome this limitation, X-ray computed tomography can be used.

The main objective of this work is to investigate the pit-to-crack transition in sour corrosive environments. In this study, in-situ corrosion fatigue tests were conducted using a bespoke test cell (Fig. 1) that allows X-ray micro-tomography of the specimen during fatigue testing. The test environment comprised 3.5% NaCl solution saturated with a gas mixture of 12.5% H2S in CO2 at ambient pressure and room temperature. Prior to the start of the corrosion fatigue tests, a corrosion pit of the desired dimensions was created on the centre of each sample by using a VersaScan electrochemical instrument. An initial baseline S-N curve was obtained from testing smooth samples in the sour environment. The pit-to-fatigue crack transition was investigated using X-ray micro-tomography during corrosion fatigue tests of pre-pitted samples. Preliminary results show that the crack initiation site at the pit depends on the morphology of the pits and the level of applied stress.