Chloride-Stress Corrosion Cracking (ClSCC) of austenitic stainless steels has been one of the biggest challenges in the refining industry and one of the main reasons where upgrading to stainless steel may not be the miracle solution for battling corrosion problems. Even with all that is known about this mechanism, the industry still faces failures, mainly because chlorides show up when they are not expected and accounted for, leading to economic or worse, catastrophic failures.
Factors affecting ClSCC like pH, operating temperature and chloride ion concentration are considered in API 581 to designate a Severity Index. Other factors like the number of inspections, the effectiveness of the inspections and time since the last effective inspection etc. are considered to determine the Damage Factor. However, factors like presence of oxygen, effects of extreme pH and temperatures or stress relieving are not considered. A proposed Risk Assessment Tool using a new factor "susceptibility modifier" to API RP 581 Task Group was presented in November 2016 and has been incorporated in the newly released API 581 – Addendum 2 in October 2020.1
This paper will identify and document how these different factors affect the susceptibility of austenitic stainless steel to Chloride-Stress Corrosion cracking based on a review of currently available literature. A review of current industry best practices and a review of how the Oxygen content, the pH and application of stress relief affects Chloride-Stress Corrosion Cracking will be documented and presented.
Chloride promoted stress corrosion cracking (ClSCC) is a damage mechanism that can occur in a chloride containing aqueous environment. Damage can occur during in-service or during shutdown, if chloride containing solutions are present, especially at temperatures above 66°C (150°F). Austenitic stainless steels are highly susceptible to ClSCC. Damage can occur internally (from process, wash water or firewater etc.) or it can occur externally under insulation. ClSCC is typically transgranular and highly branched. Susceptibility to ClSCC is usually assessed based on the chloride content, pH and temperature. The formal risk assessment methodology used in API RP 581 Third Edition (1) provides a quantitative procedure to establish an inspection program using risk-based methods for pressurized fixed equipment. In API 581, Section 12 "SCC Damage Factor – Chloride Stress Corrosion Cracking," the basic assumption is that ClSCC of austenitic stainless steels can occur in a chloride-containing aqueous environment and the susceptibility to ClSCC is dependent on:
• the concentration of the chloride ions,
• the temperature, and