22%Cr duplex stainless steels are not suitable for elevated temperature unprotected seawater applications because they suffer from the crevice and pitting corrosion at even moderate temperatures.
Super duplex 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 super duplex 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 super duplex grades respectively by Norsok.
Thermally sprayed aluminum has been used to prevent pitting corrosion and chloride SCC at higher temperatures and is also of interest with respect to increased operating temperatures under immersed conditions.
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