Volume 2, Issue 5 (May 2005)
Corrosion Behavior of Heat-Treated Duplex Stainless Steels in Saturated Carbon Dioxide-Chloride Solutions
Sweet and/or sour service environments require the use of corrosion resistant materials since conventional steels usually exhibit general corrosion, pitting attack, and stress corrosion cracking (SCC) under these conditions.
Long term performance and cost effectiveness must be considered when evaluating material selection. Duplex stainless steels (DSSs) may be considered useful materials under corrosive conditions. These materials are highly resistant to general corrosion and resistant to pitting attack. The low nickel content is an advantage from a SCC stand-point.
In this study, the pitting corrosion behavior of a lean duplex stainless steel (type Fe-15Cr-5Ni-1.9Cu) alloy as well as medium grade (type 3RE60 DSS) were evaluated in NaCl solutions saturated with CO2 (sweet environment), containing little or no thiosulfate species at 50°C. The effect of inappropriate heat treatment (e.g., formation of sigma phase in the 3RE60 alloy and sensitization in the lean grade DSS type) is also studied under such conditions.
The results revealed that these alloys are susceptible to chloride pitting corrosion with better performance for the 3RE60 alloy. The intensity of the chloride attack is remarkably increased with the addition of CO2, the presence of thiosulfate species, and with the application of inappropriate heat treatment.
Although chloride solutions containing saturated dissolved CO2 are more corrosive than those containing thiosulfate species, the presence of both species (CO2 and S2O32) has a more negative effect on the chloride pitting resistance than on each component separately. More severe attack was obtained for heat-treated samples immersed in chloride solutions containing both dissolved carbon dioxide and thiosulfate ions.