or 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.