Pitting Corrosion Mechanisms of Lean Duplex, Duplex and Super Duplex Stainless Steels in Chloride Environments

An investigation was conducted to evaluate the pitting corrosion resistance of four types of lean duplex grades (UNS S32003, UNS S32101, UNS S32304 & Zeron 21CLD) along with duplex (2205 & Ferallium Alloy 255) and super duplex grades (UNS S32750 & UNS S32760). Results are discussed based on corrosion test data generated in an autoclave for wide range of chloride concentrations (15-22,000 ppm) and temperatures (25⁰C-170⁰C). The pitting potential (E_P), a measure of localised corrosion resistance is varied by more than 0.40  at 130⁰C in 10000 ppm chloride solutions and the magnitude of difference decreased with increase in chloride concentration and temperature, for the range of stainless steels studied. UNS S32003 and Zeron 21 cast lean duplex, with higher pitting resistance equivalent number (PREN) values performed better than corresponding lower nickel grades. This study will also discuss the relative performance of lean duplex grades, to those of traditional stainless steel grades such as 316L and 304L.

A full scale evaluation of pitting corrosion mechanisms was performed using Electron back scattered diffraction techniques (EBSD), and Scanning Electron Microscope (SEM), to characterise the damage initiation and behaviour of austenite and ferrite phases in chloride environments. It is observed that grain size, strain due to misorientation between phases, and local pH of the solution tends to determine the selective phase attack observed in duplex materials. The effect of Intermetallic phases (Sigma and Chi) on pitting corrosion mechanisms of super duplex stainless steels will also be discussed in detail.