Stress corrosion cracking behaviour of lean duplex stainless steels in the presence of chloride and thiosulphate

In comparison to conventional duplex stainless steels (DSS), lean duplex SS are characterized by restricted nickel and molybdenum contents and by incremented manganese and nitrogen percentages, added to stabilize the austenitic phase. A lower nickel percentage in these materials determines cost savings when they are used in place of conventional duplex grades or austenitic stainless steels, such as AISI 304 and 316. They may also substitute advantageously carbon steels when maintenance costs are significant. Besides, the decrement of molybdenum content can reduce or inhibit the precipitation of deleterious χ (chi) and σ (sigma) phases, during production or welding operations. In this context, the evaluation of SCC susceptibility of lean duplex alloys in oil and gas and petrochemical applications, where they can substitute ordinary austenitic stainless steels or DSS (for example 2205 DSS), has becoming an interesting research topic. In this work, a comparative study of the susceptibility to SCC of three lean duplex grades (DSS 2304, LDX 2101® and LDX 2404®) was carried out, on the basis of slow strain rate tests (SSRT). The aggressive solutions were 20wt% NaCl solution at 80 °C and standard NACE TM-0177 solution at 25 °C, containing different thiosulphate ion concentrations, ranging from 10 -4 to 10 -2 M. The cracks propagation due to the effect of the applied stress and aggressive environments was studied by Optical Microscope (OM) observations.