The pitting corrosion resistance of three types of lean duplex stainless steels (grades UNS S32003, UNS S32101 & UNS S32304) along with duplex stainless steel grades (UNS S31803 & UNS S32550) and super duplex stainless steel (grades UNS S32750 & UNS S32760) were investigated using the potentiodynamic polarisation technique. Results are discussed based on data generated in an autoclave for wide range of chloride concentrations (15-22,000 ppm) and temperatures (600C-1700C) in 8ppm dissolved oxygen. Super duplex stainless steel (SDSS) showed superior pitting resistance in all the conditions tested and demonstrated the highest pitting potentials (EP). But among all the lean duplex stainless steel (LDSS) grades tested, UNS S32003 with higher pitting resistance equivalent number (PREN) value has shown high resistance to pitting than corresponding LDSS (UNS S32304 & UNS S32101) for the conditions tested. A full scale evaluation of corrosion mechanisms was performed using Electron Back Scattered Diffraction (EBSD) techniques to characterise the damage initiation processes and the behaviour of austenite and ferrite phases in corrosive conditions. It is observed that the ferrite phase is preferentially attacked in LDSS. But in case of DSS the austenite phase is selectively attacked, whereas in SDSS both austenite and ferrite phases are attacked.
Lean Duplex, Duplex and Super Duplex stainless steels are growing in demand for wide range of applications where corrosion resistance and good mechanical properties are required. Duplex and Super Duplex grades are already well established in various industrial sectors including oil & gas, paper & pulp, desalination etc. The experience of newly developed LDSS (<4% Nickel) is still limited and they are a recent entry to the market. The main drivers for these newly developed LDSS are raw material costs. The lean grades are very attractive in periods of the high fluctuation of nickel and molybdenum prices and used as alternative grades to traditional grades such as 316L and 304L austenitic grades. But very limited corrosion evaluation data exists on these materials, which is insufficient to make an effective assessment for critical applications. For that reason it is very important to make a systematic study in more realistic conditions to closely assess the performance of these newly developed LDSS. Most of the reported studies [1-2] on these LDSS were alloy specific and the comparative data exists for limited conditions and in some cases the materials were tested in boiling acidic solutions [2].The present authors have already shown [3] the effects of temperature and chloride on pitting in DSS and SDSS, but this study will highlight the performance of LDSS along with DSS and SDSS and discuss the pitting corrosion mechanisms in detail for all grades.
The potentiodynamic polarization technique was used to study the pitting resistance of the materials investigated in this study. Experiments were conducted following the ASTM G5 recommended procedures using an ACM Instruments potentiostat. A conventional three electrode system was used inside in an autoclave with 2 litres capacity that can withstand temperatures of up to 200°C.
