Sour environments differ significantly in their aggressiveness to materials depending on such factors as total system pressure, partial pressures of hydrogen sulfide and carbon dioxide, pH of the aqueous phase, temperature, chloride ion concentration, etc. Materials that perform poorly in standard laboratory sulfide stress cracking and hydrogen-induced cracking tests and some field environments may give acceptable performance under other sour service conditions. This paper provides some additional guidance on the manner in which standard sour service testing methods should be selected and performed, and on the interpretation and application of the results. The relevant recommendations and requirements of NACE International (NACE), the European Federation of Corrosion (EFC) and the American Petroleum Institute (API) will be reviewed. The recommendations of NACE, EFC and API for the selection of the most appropriate test method(s), test frequency, and acceptance criteria for particular components and services are listed. Some alternative tests being used to qualify materials for general and specific sour gas service conditions will be described and reviewed.
Standardized test methods for evaluating materials for potential service in sour environments such as NACE Standard Test Methods TM0177-961, TM0284-962 and TM0198-983 provide rules on the manner in which environmental cracking (EC) and hydrogen-induced cracking (HIC) tests should be conducted in hydrogen sulfide (HzS)-containing environments. Little guidance is provided on the applicability (e.g. materials, components) of each of the five EC and one HIC test method described in these NACE standards. No data are provided on the interpretation of the results with respect to test method acceptance criteria. These topics are outside the scope of the standards. Standardization of test methods and their universal use can greatly assist the interpretation and comparison of material EC and HIC data generated at laboratories around the world, including those of component manufacturers, user companies (i.e., producers of sour oil and gas), and independent test laboratories. This is invaluable for those committees charged with the responsibility for deciding whether or not new materials or processes should be included in a material requirement standard such as NACE MR0175-994. Another important use of standardized test methods involves purchase specification quality assurance (QA) testing requirements. This is much easier for producer, user and test companies if everybody is using the same test method(s). The user knows what the service requirements are, and therefore what the test criteria should be. The producer knows or can determine whether or not they can meet the stated test requirements, and if so, what processing (e,g., chemistry, refining steps, heat treatment, etc) is necessary to meet them. The test company (if used) knows how to perform the test correctly.
An essential part of the test method is the environment. It must be at least as aggressive as the worst case service environment and must also be described in the test method in such a manner that it can be accurately reproduced in laboratories worldwide. For many years, the so-called "NACE environment" has been used for EC testing [i.e., sulfide stress cracking (SSC), hydrogen embrittlement cracking (HEC) and stress corrosion cracking (SCC)] of carbon steels (CS), low alloy steels (LAS) and corrosion-resistant alloys (CRAs). This environment is now called "Test Solution A" in TM0177-96 methods A, C and D, and in TM0284-96. A higher pH, less aggressive environment was selected for HIC testing CS linepipe materials when TM0284 was first introduced (1984). This en
