Impact of time of exposure on HIC testing of very high strength steel in low H2S environments.
Résumé
The methodology for testing hydrogen induced cracking (HIC) resistance of carbon steel in mild sour service is discussed. When H2S content is low, the use of steel grades with intermediate HIC resistance often presents many advantages, such as higher mechanical properties and economical benefits. As a counterpart, fit-for- purpose testing procedures are required in order to assess the HIC performance at the intended service conditions. With regards low H2S environments, time of exposure is recognised as the critical testing parameter. The environment encountered in the annular space of flexible pipelines is very often mildly severe. Due to the high confinement, pH level is higher than in bulk conditions with the same gas composition but a larger V/S ratio between the volume of electrolyte and the exposed steel surface. H2S content in the annulus is also less than in the fluid transported in the bore of the pipeline, due to permeation through inner thermoplastic sheaths and consumption. Qualification tests of very high strength steel wires are therefore usually conducted with one month exposure at the predicted pH and PH2s in the annulus, while standard testing procedures referenced in ISO 15156 specify 96 h immersion but a higher H2S level. ln the present study, different grades of high strength steel wires were tested in environments containing as low as 100 ppmv H2S, with exposure times varying from 96 hours and up to 1 year. For the steels grades that have been studied, it is confirmed that the actual qualification procedure consisting in 1 month exposure is appropriate to detect HIC susceptibility.