Download This PaperEffect of Chloride Concentrations and Temperature on Prestressed Austenitic Stainless Steel 304 Based on Pressure Vessel in Oil and Gas Industry
78 Pages Posted: 7 May 2020
Date Written: April 12, 2020
Abstract
Chloride-induced stress corrosion cracking (CISCC) of austenitic stainless steel type 304 (SS 304) under thermal insulation is a classical pressure vessel failure case, but recurrent failures require further analysis of possible mechanisms which lead to the damage particularly on the effect of leachable chloride from the thermal insulation. The objective of the research was to investigate the effect of chloride leached from thermal insulation material on CISCC of SS 304. The study was based on actual pressure vessel failure case sample whereby proposed parameters such as temperature and chloride concentrations were derived. SS 304 samples were immersed in sodium chloride solution to identify the pitting corrosion mechanism which leads to CISCC failure of a pressure vessel. CISCC test was carried out by using ASTM G30 standard for the U-bend sample preparations to simulate CISCC of SS 304 under chloride environment. The test parameters used were various chloride concentrations of 200 to 30,000 ppm as per ASTM C692 at 40, 60, and 85°C temperature as per ASME B31.3 standard [1]. Visual and stereomicroscope results revealed no external surface macro-crack appearance found on the U-bend SS 304 samples tested at 40, 60 and 85°C in chloride concentrations of 200 to 30,000 ppm. FESEM and EDX analysis revealed the surface morphology micro-void cracks on the corrosion products at 40 and 60°C and revealed CISCC initiation at 85°C of the SS 304 U-bend samples. The metallographic examination in cross-section revealed transgranular cracking across the grain boundaries by film dissolution mechanism has occurred only in the U-bend SS 304 tested at 85°C (3000, 8000 and 30,000 ppm chloride concentrations of 9 months duration). In conclusion, general corrosion has occurred at 40°C, localized pitting corrosion has occurred in a failed vessel at 60°C and the CISCC was initiated by the leachable chloride from the insulation material perlite at temperature above 85°C in the presence of moisture. Corrosion consideration in materials selection for vessel operating above 85°C is crucial and suggested to install a hydrophobic waterproofing thermal insulation material to minimize the accumulated chloride ions over years in-service.
Keywords: Stress, Corrosion, Chloride, Stainless steel, Pressure vessel
Suggested Citation: Suggested Citation