The advantages of duplex stainless steels are increasingly recognized in the chemical and petrochemical industries. Their superior SCC resistance over austenitic stainless steels has been known for years. It is now clear that these steels can be utilized in many more areas and that they can often be an alternative to much more costly materials.

This paper discusses a case study and laboratory tests to illustrate the superior SCC resistance of duplexes in comparison with high-nickel alloys in caustic environments with and without chlorides.

After three years’ service, some internals of a duplex stainless steel (X2CrNiMoN 25 6 2) reactor containing a strongly alkaline, chloride-containing solution at temperatures up to 180ºC exhibited serious cracking. Investigation revealed that the affected internals had been fabricated of type NiMo16Cr15W steel (Werkstoff 2.4819), a high-nickel alloy. They had been added after the reactor had been in service for a year. Neither the duplex stainless steel internals nor the ditto reactor vessel showed any signs of cracking. The failed NiMo16Cr15W internals were replaced with duplex stainless steel ones. Today, twelve years on, no corrosion of any significance has been found on the duplex stainless steel internals and the reactor vessel.

Microscopic examination indicated that the cracks in the internals were due to SCC. Laboratory tests were conducted in order to gain a better understanding of the failure mechanism. The material’s stress corrosion cracking performance was assessed by the drop evaporation test (DET).

In this test a standard tensile specimen (effective length 30mm, diameter 3mm) is inserted in, and electrically insulated from, a yoke of highly compact design that occupies very little additional space and allows the specimen to be tested in a tensile testing rig under constant uniaxial loading, the rig being designed for vertical loading. The corrosive fluid is applied to the specimen drop by drop while the specimen is electrically heated in order to evaporate the fluid. The fluid we used was a dilute caustic solution to which in a number of instances we added chlorides. In these tests, SCC is initiated fairly quickly by concentration of the fluid in combination with the tensile stress subjected to thermal fluctuations.

In the examination, the performance of high-nickel alloys was compared with that of duplex stainless steels.

The results indicate that the SCC should be characterized as caustic-induced SCC. Also, duplex SS appears to be more resistant to caustic environments, with or without chlorides, than the high-nickel alloys tested.