Abstract
Since the advent of deep sour gas wells (in 1970-80s), the effects of elemental sulfur on increased pitting and stress corrosion cracking (SCC) susceptibility of nickel-based alloys have been documented in the literature. At the same time, methods for testing these materials for resistance to SCC have evolved in several forms that include under and over saturation of elemental sulfur in test environments over a range of temperatures usually in the range 150 °C to over 200 °C. Today, qualification of a variety of corrosion resistant alloys for inclusion into NACE MR0175/ISO 15156 Part 3 references these methods. However, there is little agreement as to the nature of the chemical environments (in terms of elemental sulfur and its speciation) in the test environment and their role in defining severity with respect to SCC.
This paper reviews both available SCC data and test methodologies involving additions of elemental sulfur using the abovementioned procedures. It utilizes thermodynamic modeling to assess the chemical speciation of elemental sulfur under selected test conditions. The goal of this study is to gain a better understanding of the connection between the method and quantity of sulfur addition and SCC of corrosion resistant alloys and to give better guidance for selection of test methods to simulate desired service conditions in HPHT wells required by the aforementioned NACE/ISO standard.
