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, although crevice corrosion may occur at lower temperatures. A ‘soft start-up’ in cold, chlorinated seawater may increase these temperatures .
The operational limit stipulated in the Norsok Standard (M-001, Rev.5, September 2014) is one of the most conservative for unprotected superduplex service. The maximum operating temperature is set at 20°C for chlorinated and natural seawater. . 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 110°C have been set for 22%Cr duplex and 25%Cr duplex 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.
- Byrne G, Schulz Z, Schwartz A, Haman D, Warburton G, Fajimi A, 2016: ‘Corrosion Management of Assets Constructed Super Duplex Stainless Steels Topside and Subsea – A Suppliers Perspective’, Paper No. 6982, Corrosion 2016, NACE International.