What Test Environment for the Qualification of OCTG Sour Service Grades – Means and Consequences of pH Control

Abstract

Abstract Many worldwide Oil & Gas fields are sour with H2S partial pressures (PPH2S) above 0.05 psi. For such situations, it is critical to employ Oil Country Tubular Goods (OCTG) qualified for sour service, meaning resistant to Sulfide Stress Cracking (SSC). More especially when very high strength grades are required for producing high pressure O&G reservoirs. For instance, a quenched and tempered sour service low alloyed steel with 125 ksi Specified Minimum Yield Strength (SMYS) has been investigated through various standardized test solutions. As corrosion processes in acidic environment with dissolved H2S make the testing solution to drift, some of the standardized test solutions are highly buffered with the acid-base couple acetic acid / acetate. Another method for limiting the pH drift consists by frequent additions of strong acid as hydrochloric acid during the test. The objective of this work is to compare the electrochemical of such high strength steel in NACE TM0177 Solution B, EFC 16 solution with and without pH adjustment. Thermodynamic calculations for predicting the saturation pH, hydrogen permeation tests, corrosion rate measurements and fine characterizations of iron sulfide scales (composition, morphology and defectology) were performed for being correlated to SSC test results. Build a better understanding of test methodology impact on the hydrogen uptake and the protectiveness of corrosion products is also an aim of this work.