Sulfide Stress Cracking Resistance of 95 and 110 ksi OCTG Sour Service Grades Under High Partial Pressure of H2S - A Contribution to Corrosion Mechanism Understanding

Abstract

Abstract The limit of Sour Service (SS) grades regarding Sulfide Stress Cracking (SSC) resistance is defined by ANSI NACE MR0175 / ISO 15156 standard. For instance, API T95 grade is acceptable to be used in all regions of environmental severity and for all temperatures. As per Annex B of the same standard, SSC laboratory testing in accordance with NACE TM0177 Methods A and D in Solution A saturated by 1 bar H2S partial pressure (PH2S) qualify a material for all SSC regions, up to 10 bar PH2S. To evaluate this statement and verify if the existing NACE guidelines apply also for SS 110 ksi Specified Minimum Yield Strength (SMYS), a procedure to assess the SSC resistance in high H2S pressure was developed. This procedure was applied to enhanced SS 95 ksi and 110 ksi SMYS materials. The methodology consists in regular purging and filling of the gas phase in order to keep constant the PH2S and to remove hydrogen gas produced by generalized corrosion. Investigations combined NACE TM0177 Method A and D tests, characterizations of the iron sulfide scales and hydrogen electrochemical permeations. This methodology enables to give some insights on the corrosion mechanism and the influence of high partial pressure of H2S on the SSC resistance.