Development of High-strength Ni Alloy OCTG Material for Sour Environment

Abstract A new high-strength nickel alloy OCTG with 140ksi (965MPa) grade has been developed for use at high temperature and high pressure corrosive well, using a four point bend-beam test and a slow strain rate tensile test method in the solution containing hydrogen sulfide gas and chloride ion. The new alloy has exhibited equivalent stress corrosion cracking resistance to UNS N08535 of 110ksi (758MPa) grade. In addition, the influence of alloying nitrogen in austenitic alloy as a soluble element on SCC susceptibility has carefully been investigated in this study. Since alloying nitrogen is considered to affect not only on corrosion resistance but also on mechanical properties, the nitrogen content of the alloy and the strength level of cold-worked materials have been taken into account as the major factors affecting on the SCC susceptibility. The effect of nitrogen on localized corrosion resistance has been discussed from the results of SCC test and surface film analysis. Among the materials which have same yield strength level, the alloy with high nitrogen content has shown superior SCC resistance to those with lower nitrogen content. This is found to be achieved by nitrogen which enhances the workability and hence the ductility of the alloy, resulting in lower cold working to attain the same strength level of the alloys. Nitrogen is also found to be concentrated as an atomic state or a nitride around the interface between surface film and bulk metal after the corrosion testing. This is considered to be another effect of the nitrogen, because the dissolution of concentrated nitrogen would reduce the pH drop or act as a passivator. Further research for nitrogen effect should be studied in future work.