Corrosion resistance of super duplex stainless steels in chloride ion containing environments by means of a new microelectrochemical method. I. Precipitation-free states

It has been suggested that the corrosion resistance of super duplex stainless steels (SDSS) is determined by the corrosion resistance of the weaker phase. An optimised alloy should have equal pitting resistance equivalent numbers (PREN) for both phases, ferrite and austenite. In this work, five different alloys with varying PREN of the single phases have been investigated with respect to the general corrosion resistance, and the corrosion resistance of the single phases, in chloride containing environments. Macroelectrochemical and microelectrochemical (reduced size electrochemical cells) measurements were carried out using electrolytes containing HCl, NaCl and varying concentrations of lithium chloride solution. The results of the macroelectrochemical measurements were not able to provide a clear ranking of the duplex alloys with respect to the corrosion resistance. The polarization curves appear as if the specimens consisted of only one phase, and do not reveal the distinct contributions from the austenite and ferrite phases in the active range. However, microelectrochemical investigations in the region of the phase boundary clearly exhibit different contributions from the individual phases. These measurements showed that the PREN are quite reliable to assess the corrosion resistance of the individual phases of a SDSS. (Abstract quotes from original text)