Effect of Tantalum Addition on Pitting Corrosion Behavior of Super Duplex Stainless Steels

－Development of High Corrosion Resistant Duplex Stainless Steels by Inclusion Control (1)－

Abstract

To improve the corrosion resistance of the duplex stainless steels, great efforts have been made to clarify the pitting initiation in order to suppress the pitting corrosion which is one of the most hazardous forms of the stainless steels' failure. In general, manganese sulfide (MnS) is known to act as the pitting initiation in environments containing chloride ions. Thus, the inclusion control is essential for the development of duplex stainless steels with high corrosion resistance.
In this study, the effects of the inclusion control by Ta addition on the pitting corrosion resistance of the duplex stainless steels were investigated. The Ta-bearing duplex stainless steels exhibited the remarkably high pitting corrosion resistance compared to that of the duplex stainless steels without Ta. The Ta-bearing duplex stainless steel contains no MnS inclusion but (Ta, Mn)-oxysulfide. This indicates that sulfur is stabilized as (Ta, Mn)-oxysulfide instead of MnS which is the typical pitting initiation. Ta addition to the super duplex stainless steel improved the pitting corrosion resistance because MnS, which acts as a pitting initiation, was modified to the electrochemically-stable Ta-containing inclusions.