Effect of Tantalum Addition on Inclusion Formation and Pitting Corrosion Resistance of Super Duplex Stainless Steels

Duplex stainless steels have been applied in many different applications such as desalination plants, chemical plants, LNG carburetors, and off-shore petroleum facilities owing to their favorable combination of good mechanical properties and high resistances to pitting corrosion, crevice corrosion, and stress corrosion cracking.  The development of the duplex stainless steels with even higher corrosion resistance, however, is required to improve the product safety.  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.¹  In general, manganese sulfide (MnS) is known to act as the pitting initiation in environments containing chloride ions.  Recently, it was reported that other non-metallic inclusions such as oxides (i.e. MnCr₂O₄ and (Ti,Ca)-oxides) strongly affect the pitting initiation.²  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 exhibit the remarkably high pitting corrosion resistance compared to that of the duplex stainless steels without Ta.  The Ta-bearing duplex stainless steels contain no MnS inclusions but (Ta,Mn)-oxysulfides.  This indicates that sulfur is stabilized as (Ta,Mn)-oxysulfides instead of MnS which is the typical pitting initiation.  In order to examine the effect of Ta on the formation of MnS, we focus on the UNS S32750-type super duplex stainless steels with relatively-high sulfur content (20ppm S) where MnS is expected to form.

The super duplex stainless steels were prepared by a vacuum induction melting method.  Table I shows the chemical composition of the steels designated as Base and Ta.  The steels were solution heat-treated for 30 min at 1373K.  After the heat treatment, the steels were quenched in water and cut into 20×30 mm coupons.  Figure 1 shows the effects of the Ta addition on the pitting corrosion behavior in a 20wt% NaCl aqueous solution at 353K.  Comparing the polarization curves of the base and the Ta-bearing alloys, it is clearly seen that the current spikes due to the pitting initiation and the subsequent repassivation were suppressed by addition of Ta.  In addition, the pitting potential of the Ta-bearing alloy is 0.96 V which is much higher than that of the base alloy (0.30 V).  These results indicate that the pitting corrosion resistance is dramatically improved by addition of Ta. 

In order to understand the mechanism of the improvement of the pitting corrosion resistance by addition of Ta, the SEM observations were conducted before and after an electrochemical measurement.  Figure 2 shows the SEM images of inclusions in the base and the Ta alloys.  MnS inclusions were observed at the surface of the base alloy before the electrochemical tests, and the pitting initiation was observed from the MnS dissolution.  By contrast, no MnS inclusions but Ta-containing inclusions such as (Ta,Mn,Cr)-oxides and (Ta,Mn)-oxysulfides were observed at the surface of the Ta-bearing alloy.  No pitting was observed after the same electrochemical test as was conducted for the base alloy.  This demonstrates that the Ta-containing inclusions are electrochemically-stable. 

In conclusion, Ta addition to the super duplex stainless steels increased the pitting corrosion resistance because MnS, which acts as a pitting initiation, was modified to the electrochemically-stable Ta-containing inclusions. 

REFERENCES

1. I. Muto et al., J. Electrochem. Soc., 156, C395 (2009).
2. S.Zheng et al., Corros. Sci., 67, 20 (2013).