Stress Corrosion Cracking of Super Duplex Stainless Steels above and below Pitting Potentials

Pitting and stress corrosion cracking (SCC) are important corrosion degradation modes, yet there are few studies which exist on the new generation of super duplex (2507, Zeron100) materials. Most of the previous laboratory tests reported in the literature to study the design limits of these materials are in very aggressive environments (boiling (150⁰C) MgCl₂, CaCl₂, and acidic NaCl solution) and can give little information about the reported failures in service. Process tubing exposed to seawater can reach +600mV (SCE) and there is therefore a need to evaluate the risk of SCC under these conditions at the maximum service temperature of 130⁰C.  

This study conducted in an autoclave, focussed on investigating the SCC behaviour at ± 200mV from the measured pitting potential (E_P), under the same test conditions. Results show that below E_P isolated small cracks initiate near second phases in austenite, around E_P small cracks start to coalesce and above E_P, the attack is very rapid and appears to be galvanic. This study also investigates the influence of heat treatment, and annealing on SCC behaviour using SEM & EBSD. Results of SSRT tests indicate applied potential (E_APP), and E_P play a major role in the failure of these materials.