Super Duplex Stainless Steels (SDSS) are utilised extensively by the oil and gas industry offshore, on fixed platforms, production vessels and subsea developments because they provide an excellent combination of;
a) Strength, weldability, fracture toughness and structural integrity.
b) Corrosion resistance in natural and chlorinated seawater.
c) Corrosion resistance in CO2/ H2S/Cl- process environments.
d) Corrosion resistance in in marine atmospheres.
e) They have high design strength, allowing weight savings through the construction of lower wall thickness, pipe work systems, process equipment and vessels.
f) They are readily available in all cast and wrought product forms.
g) They provide long life/minimum maintenance functionality to facilities that means that they are not normally manned and personnel are not exposed to the arduous offshore conditions.
h) They provide lower level, cumulative, through life costs when compared with alternative material selection philosophies.
As such they find application in seawater cooling and fire protection systems, topside and subsea production pipe work, flow lines and pipelines. However, this presupposes that all items in the bill of materials are consistently and repeatedly properly processed, heat treated, pickled and passivated, fabricated, installed, commissioned, operated, maintained and deployed in environments within the limits of their application range. With over 30 years of wide scale use of these alloys by the oil and gas industry the overall experience has been good. However, intermittently problems recur. Some, like intermetallic precipitates in products are high profile and well known, but others are less well known. Based on alloy development and manufacturer independent supply experience of super duplex steel over a 30 year period the paper considers some of the less well known but recurring problems and the methods used to ameliorate them. From this information it is hoped that corrosion management strategies can be enhanced to sustain and or extended practical limits of use of this grade of steel in the future.
Key words: downloadable, Duplex Stainless Steel, Forging, Toughness, HISC, Welding, Sigma, Crevice Corrosion