Comparison of the Test Severity of HLP Test Solution and NACE TM0284-2016 Test Solution C in HIC Testing

For HIC testing under mildly sour test conditions at fixed pH, NACE TM0284 Test Solution C has been introduced in the 2016 revision of the test standard. For the same purpose the use of the HLP solution, a highly acetate-buffered test solution, has been permitted by NACE TM0284 as an alternative. This test solution is intended to become TM0284 Solution D. For this test solution a better pH stability at low H₂S concentrations has been reported, which reduces the amount of pH adjustment cycles. However, concerns have been raised about the effect of the highly concentrated sodium acetate. Within this paper, the actual test severity of both test solutions is compared. HIC tests in a wide range of mildly sour test conditions have been performed for two large-diameter line pipe steels intended for mildly sour environments in both test solutions. Based on the results of ultrasonic evaluation, HIC resistivity diagrams based on pH and partial pressure of H₂S have been defined. For the HLP test solution a higher test severity can be observed for some test conditions. Although the HLP test solution shows a better pH stability, one-to-one comparison of the results with NACE TM0284-2016 test solution C does not allow a general conclusion.

Steel pipelines designated for the transport of oil and gas containing wet hydrogen sulfide (H₂S) are faced with the risk of sudden and severe cracking. In sour environments containing water and H₂S, hydrogen atoms, originating from the anodic dissolution of the material, can diffuse into the steel and induce severe damage. Different forms of cracking may occur, such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) or Stress Oriented Hydrogen Induced Cracking (SOHIC). These cracks can often be difficult to detect in routine inspections and are thus regarded as a higher risk for integrity loss than weight-loss corrosion. Due to the sudden and unforeseeable appearance of these failure mechanisms, use of HIC resistant line pipe steel is necessary in general for all sour applications. Within this paper, the performance and severity of test solutions for HIC testing under mildly sour service environments that require pH stability and longer test durations, is discussed.