Defining an Environment Where Internal SCC Occurs

Abstract This paper discusses and defines an environment where internal stress corrosion cracking (SCC) can form. Over the past few years, ADV performed several forensic analyses where internal SCC on gathering pipelines was identified. The identification of broad colonies of longitudinal cracks along the internal pipe surface, a quasi-cleavage fracture surface examined using a scanning electron microscope, and branched tight transgranular cracking were identified supporting the assertion of internal SCC. All SCC requires three conditions to be simultaneously present: stress, material, and environment. The stress to generate longitudinal cracking is largely a result of the operating pressure generating hoop stress. Carbon steel utilized during the pipeline construction is susceptible to SCC in specific environments. The environment to form internal SCC is dependent on the product/fluid within the pipe, which is drastically different than traditional external SCC. All incidents contained the same general product and added chemicals (inhibitor and biocide). Therefore, characterizing and mitigating the subject environment is critical to managing the internal SCC threat. In order to characterize susceptible conditions, a series of slow strain rate tests (SSRTs) utilizing varying environments, varying materials, and varying test durations were conducted to gain a better understanding of the internal SCC mechanism present. Based on these tests, the following was concluded: The SSRT cracking produced within a mixture of the chemicals resulted in the same mechanism identified within the forensic analyses. The individual fluids did not result in cracks. Therefore, the mixture of the inhibitor and biocide results in an environment that produces cracks; this crack producing environment appears to be exacerbated by the presence of 3% brine solution. Comparison of the results to a filmed SSRT indicated these cracks form in a submerged environment, such as that encountered in a low spot, rather than one where the chemicals are filmed on the steel surface. Tests performed on three materials and vintages indicated that all low carbon line pipe steels are susceptible to the cracking mechanism discussed in this paper.