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What is the significance of nickel on sulphide stress cracking in duplex stainless steels?

 

Nickel can have a beneficial effect on sulphide stress cracking (SSC) resistance in duplex stainless steels. For instance, increasing the Ni content (from 3.5 to 7.5%) of a 22%Cr duplex stainless steel increased the critical chloride concentration for SSC to occur in both the base metal and the simulated HAZ in a solution containing 0.01 MPa H 2S.[1] Furthermore, the threshold stress required in the simulated HAZ increased when the Ni content increased from 3.5 to 7.5%. However, a slight decrease in the threshold stress required for SSC to occur in the base metal was found when the Ni content was increased from 6.5% to 7.5%, suggesting that there is an optimum level.

The threshold stress for SSC and the pitting corrosion resistance are related to the ferrite content of a 22%Cr-3%Mo duplex stainless steel. In both cases, resistance peaks at 40-45% ferrite. The Ni content, together with N, Cr and Mo in particular, control the amount of ferrite and therefore the phase concentration of alloying elements such as Cr and Mo and N, which control the pitting corrosion resistance of the ferrite phase. This is important because sulphide cracks typically initiate at pits. Electrochemical interaction between the ferrite and austenite phases acts to inhibit sulphide stress cracking.[2]

References

Carbon-manganese and low alloy steels in sour service
Testing of superduplex stainless steel for sour service

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