Sensitization duplex steels

T.M. Devine and B.J. Drummond, Use of Accelerated Intergranular Corrosion Tests and Pitting Corrosion Tests to Detect Sensitization and Susceptibility to Intergranular Stress Corrosion Cracking in High Temperature Water of Duplex 308 Stainless Steel, Corrosion, Vol 37, 1981, p 104-115... 

There are important differences between the thermal conditions that produce IGC susceptibility in ferritic, austenitic, and duplex stainless steels [61,62]. Cr diffusion is slow in austenitic stainless steels and this minimizes the risk of sensitization below about 425 C for short heating periods. 

Selection of material. As dealt with in previous sections, conventional stainless steels, with martensitic, ferritic, austenitic or ferritic-austenitic (duplex) structure, are sensitive to crevice corrosion (Table 7.4). Newer high-alloy steels with high Mo content show by far better crevice corrosion properties in seawater and other Cl-containing environments (see Section 10.1). 

Type 304 and 316 stainless steel are used in the high-pressure pipes and pipe fittings in RO plants. The 300 series SS are also sensitive to chlorides at a pH of 6.5—8 and at temperatures below 60°C. Type 316 SS tolerates chlorides up to 1000 ppm, but concentrations can reach 26,000 ppm in dry zones (for MgCl2), resulting in failure. In seawater RO desahnation plants, duplex (2205) or super-duplex (2507) steels are used instead of 316 SS. The compositions of these steels are detailed below [25] ... 

Duplex, austenitic stainless steels containing more than 5FN delta ferrite (weld metal, cast metal, weld deposit overlay), are not considered unstabilized since these alloys do not sensitize, that is form a continuous network of chromium-iron carbides. Specifically, alloys in this category are ... 

In duplex, austenitic/ferritic alloys, chromium-iron carbides are precipitated preferentially at the ferrite/austenitic interfaces during exposure to temperatures ranging from 800-l500°F. This precipitate morphology precludes intergranular penetrations associated with sensitized Type 300 series stainless steels exposed to oxygenated or fluoride environments. 

No duplex cast stainless steels, which are sensitive to thermal ageing, are used in German NPPs. Besides, the mechanical properties of steels which are used in German NPPs are investigated by some destructive testing. Time-dependent influences on the mechanical properties even after 100.000 hours of service have not been detected up to now. The properties are in most cases within a 10% margin of the original properties. 

The duplex stainless steels are not as ductile as the austenitic family of stainless steels. Welding requires more care than with the austenitic alloys due to a greater tendency towards compositional segregation and sensitivity to weld heat input. 

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