Lean duplex stainless steels are becoming attractive for applications in oilfield and marine environments due to their economic advantages, very good mechanical properties and relatively good corrosion resistance. One such application is in the production of the carcass of flexible pipes. However, materials selection for such oilfield applications becomes more complex as a result of the interactions between corrosion and erosion. Much effort has been directed towards the study of erosion-corrosion behaviour of carbon steels and other passive alloys. However, the subject of erosion-corrosion of lean duplex stainless steels is still rarely reported. Moreover, data available in the literature on the localized corrosion resistance of the lean duplex stainless steels are limited to alkaline environments.
Efforts have been made in this thesis to add to the existing data and to the understanding of the subject of localized corrosion and erosion-corrosion resistance of lean duplex stainless steels UNS S32101, UNS S32304 and UNS S82441 in oilfield environments. The lean duplex alloy UNS S32101 has been studied in detail because of its combination of high strength and good corrosion resistance. This research also compared the corrosion and erosion-corrosion resistance of lean duplex stainless steels with standard austenitic stainless steels UNS S30403 and UNS S31603 as well as duplex stainless steel alloy UNS S32205. Aerated 3.5% NaCl and synthesized CO2-saturated oilfield brines were considered as the corrosion media. Extreme erosion-corrosion conditions were simulated to design for severe environments often encountered in sand-containing oilfield pipeline systems.
Breakdown potentials, under static conditions, were found to be more positive in the aerated 3.5% NaCl than the CO2-saturated oilfield brine solution. Also, lean duplex stainless steels and standard austenitic stainless steels exhibited similar resistance in both environments. X-ray Photoelectron Spectroscopy (XPS) analysis of the passive film indicated higher chloride incorporation in the CO2-saturated oilfield brine. This, in addition to lower pH of the CO2-saturated oilfield brine appeared to be the reason why the breakdown potential was more negative in this environment.
Erosion-corrosion results showed that lean duplex stainless steels, UNS S32101 and UNS S32304, have higher resistance to pure-erosion damage than UNS S30403 and UNS S32205; better erosion-corrosion resistance than UNS S30403 austenitic stainless steel; and equivalent erosion-corrosion resistance to UNS S32205 standard duplex stainless steel. There was also a correlation between the erosion-corrosion resistance of the alloys and the sub-surface crystallography, microstructure and phase transformation. This, together with repassivation kinetics of the passive film, may be used to explain the erosion-corrosion behaviour of UNS S32101 and UNS S30403 in the oilfield slurry
