In the offshore oil and gas industry, extensive use is made of corrosion resistant alloys. These alloys are selected to perform without weight loss corrosion or corrosion cracking under expected operating conditions. Nevertheless, failures do sometimes occur, even with the "work horse" alloys of the industry. Two examples will be described, one for a Cr and Mo containing nickel base alloy (UNS N07718) and one for a 22 percent Cr duplex stainless steel (UNS S31803). The first case concerns a tubing hanger and the second topside condensate piping. Typical compositions of the alloys are given in Table 1. The failures occurred in a gas and condensate production operation with CO2 partial pressures in the range of 2 to 10 bar and bulk formation water with chloride levels up to 170,000 ppm (as chloride ions). The materials used are also suitable for mildly sour conditions, but till today, H2S levels have been very low. The described cases are different in nature in that in the nickel alloy case, the failure was possibly related to an improper microstructure of the alloy used, while in the duplex case, conditions were found more aggressive then expected at the time of selection of the steel.
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