Frequently Asked Questions
22%Cr duplex stainless steels are not suitable for elevated temperature unprotected seawater applications because they suffer from crevice and pitting corrosion at even moderate temperatures.
Superduplex stainless steels present superior corrosion resistance to duplex stainless steels in seawater applications, and provided weld quality is maintained, pitting should not occur in seawater up to about 40°C.
The operational limit stipulated in the Norsok Standard (M-001, Rev.2, November 1997) is one of the most conservative for unprotected superduplex service and is more restrictive than most operators' own requirements. The maximum operating temperature is set at 15°C and the maximum residual chlorine concentration allowed is 1.5 ppm for service where crevices are present. For service in situations where no crevices are present, the maximum operating temperature is set at 30°C. Mitigating techniques can be employed to extend the service temperatures of ferritic-austenitic alloys and an example is given below. It may be noted that for external atmospheric corrosion, e.g. at offshore facilities, temperatures of 100°C and 120°C have been set for 22%Cr duplex and superduplex grades respectively by Norsok.
Thermally sprayed aluminium has been used to prevent pitting corrosion and chloride SCC at higher temperatures and is also of interest with respect to increasing operating temperatures under immersed conditions.
TWI continues its research effort to study the effect of thermally sprayed aluminium on prevention of stress corrosion cracking (SCC), pitting and crevice corrosion of duplex and superduplex stainless steels with respect to both internal and external environments. It is envisaged that the corrosion performance of the coated steels will remain intact for service temperatures well above the currently specified application limits.
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