Development of an Improved Version of UNS N08825 with Higher Corrosion Resistance

Abstract Corrosion resistant alloy UNS N08825 was originally developed for application in the chemical process industry. In oil and gas exploration it is used because of its high resistance to stress corrosion cracking and excellent resistance to CO2- and H2S corrosion. However, due to its low molybdenum concentration of 3%, this alloy has a limited resistance to pitting and crevice corrosion in chloride containing environments. The main target of a new alloy development was to increase the critical pitting and crevice corrosion resistance while maintaining mechanical strength. In this paper the development of the new alloy will be presented. To improve the corrosion resistance the molybdenum content was increased from about 3% to 5.5-6%. This improvement results in a higher critical pitting temperature in chloride containing media. While the current alloy UNS N08825 withstands a temperature of 35°C in a ferric chloride test solution, the alloy NiCr22Mo6 showed pitting not below 50°C. Results from corrosion test according to ASTM G28[1] and G48[2] on welded and unwelded samples confirmed these first results. Normally ASTM G48[2] could not be considered as a valid test for UNS N08825. Upscaling of the alloy was successful, cold worked strip material and down hole control line tubing were produced.