Field Experience with Chloride Stress Corrosion Cracking of Stainless Steels Below 60°C in Condensate Stabilization Unit

Abstract

Abstract Objectives/Scope This paper provides further understanding of chloride stress corrosion cracking (Cl-SCC) at temperatures lower than the conventional threshold value of 60°C in an operating unit and in a Condensate Stabilization unit. There has been work demonstrating conditions where Cl-SCC can initiate; however, there isn't enough information on the potential Cl-SCC of stainless steel at temperatures lower than 60°C, and no sufficient information on actual Cl-SCC crack growth rate (CGR) under an operating environment exists. Metallographic tests and corrosion tests were performed for stainless steel 321 & 316 to characterize the cracking for various exposure temperatures in current service. A comparison was made of the cracking resistance of austenitic 321 and 316 stainless steels and that of the more resistant corrosion resistant alloy e.g., Alloy 825, which is the next best alternative to resist cracking in the Condensate Stabilization Units. Extensive field surveys, employing various non-destructive & destructive testing, were conducted to determine the expected crack growth rate at various stages of the process in the Condensate Stabilization Unit. Stress corrosion cracking can be a serious threat to the integrity of natural gas and petrochemical plants – particularly when it occurs in service. Dolphin Energy responded to this threat by performing a comprehensive review to determine the cause(s) and investigate various techniques for managing and mitigating integrity threats arising out of Cl-SCC at temperatures lower than 60°C. A detailed site survey and subsequent investigations confirmed that a certain combination of H2S, chlorides and acidity of water could result in chloride stress corrosion cracking of stainless-steel metallurgy in the Condensate Stabilization Unit, over a range of temperatures lower than 60°C, which was not adequately referenced in literature. Documented service experience in Condensate Stabilization confirms that 300 series Stainless Steel could be subject to chloride stress corrosion cracking at temperatures below 60°C, PH2S>1.0 Bara & pH<3.5 (i.e., within the permissible environmental limits under Table A.2 of ISO 15156 / NACE MR0175). It was concluded that there is correlation between chloride concentration and the time taken to failure at temperatures below 60°C, PH2S>1.0 Bara & pH<3.5. Also, a reduction in H2S partial pressures delays the time taken for failure due to Cl-SCC(at temperatures below 60°C, unlimited chloride and pH). Expected time to failure / crack growth rates for Cl-SCC typical of a gas processing unit is highlighted. Also shared are various ways and means to tackle Cl-SCC in a running unit using short and long-term actions to manage and mitigate the risks. It is important that metallurgical investigations and non-destructive testing of stainless-steel assets, operating at temperatures below 60°C, PH2S>1.0 Bara & pH<3.5, be prioritized within 120 months of operation, to detect onset of chloride cracking, if any.