INTRODUCTION:
Pitting potentials (Ep) of super duplex stainless steel (SDSS) grade (UNS S32760) were determined using potentiodynamic polarization methods. The effect of chloride concentration on Ep was determined at 130oC in an autoclave at various chloride ppm levels (15, 100, 1000, 10000) for UNS S32760. This study also focussed on investigating the stress corrosion cracking (SCC) behaviour at + 200 mV from the measured pitting potential (EP), using slow strain rate testing (SSRT) methods in 15 & 1000 ppm chloride level for UNS S32760. Results show that below EP isolated small cracks initiate, around EP small cracks start to coalesce and above EP, the attack was very rapid and appeared to be galvanic/pitting in both phases. Slow strain rate tests indicate that applied potential (EAPP), and EP play a major role in the failure process. Electron Back Scattered Diffraction (EBSD) and Scanning Electron Microscopy (SEM) has shown that below Ep, SCC is associated with short cracks and selective phase attack and above Ep pits tend to link with cracks. It was also revealed that grain size distribution of both ferrite and austenite phases in SDSS played an important role in the final failure process. The phase morphology and the balance of the two phases was also found to be of significance with continuous single phase paths sometimes created along the rolling direction providing active corrosion pit paths.
Pitting and stress corrosion cracking (SCC) are important corrosion degradation modes, yet few studies exist on the new generation of super duplex stainless steels, UNS S32760 and UNS S32750. These corrosion resistant alloys are of relatively high cost and are used in process and heat exchanger applications in the Nuclear, Oil & Gas, Chemical Process and Desalination industry sectors. Various levels of chloride ion concentrations can be present in areas where these materials are exposed. The concentration of chloride ions varies with different industrial applications from 10 ppm in cooling water systems to 22,000 ppm (seawater) piping systems. SCC occurs predominantly when stainless steels are exposed to chloride containing environments. Also process tubing exposed to seawater can reach +600 mV (SCE) and there is therefore a need to evaluate the risk of SCC for SDSS under these conditions at the maximum service temperature of 130oC. Limited studies exist on these new materials, to explain the mechanism involved in the failure of these materials. Recently the need for a systematic study to define the boundary conditions of the usage of these materials was highlighted1, 2. Most of the previous laboratory tests reported 3,4,5,6 in the literature to study the design limits and understanding failure mechanisms of duplex and super duplex materials are conducted in very aggressive environments (boiling (150oC) MgCl2, CaCl2, and acidic NaCl solution) and can give little information about the reported failures in service. Also the qualification testing that is required for acceptance of these materials for usage in particular applications are carried out at low temperatures in concentrated solutions (Eg: ASTM G48A test for pitting susceptibility).
