How Nickel based super alloys are superior to traditional stainless steels
A major limitation of stainless steel is that it is highly susceptible to chloride based localized attack like crevice, pitting and stress corrosion cracking. Hastelloy C276 is the most resistant nickel based super alloy to the chloride based localized corrosion that limits the applications of austenitic stainless steels. In various cases, stress corrosion cracking was found in various high alloys, although cracking was noticed only in the highly aggressive conditions for example at temperatures above 200oC, ph below 4 and in the presence of hydrogen sulfide. Samples of C2000, C22 and Hastelloy C276 are found to be immune to cracking in boiling 45% MgCl2 solution after 1008 hours. Hastelloy C276 also prevented corrosion in 25% NaCl solution at 232oC although it is found to be prone to cracking in MgCl2 solutions with same chloride concentration at the same temperature limit. Hastelloy C22 did not show any sign of corrosion to SCC up in the same solution and at the same temperature scale in fact in the 50% cold processed form and in the 50% cold processed and aged at 500oC for 100 hour condition.
Different materials such as stainless steels and Nickel alloys were evaluated in the lab for bolt applications. The tunnel air comprised gases such as oxides of sulfur, nitrous and hydrochloric acid. Here Inconel 625 and Hastelloy C4 were found to be resistant to cracking.
Upon aging of Hastelloy alloys above 600oC for approx 1000 hours, they developed TCP phases. Due to this the ductility of material decreased to a large level. For instance, annealed Hastelloy C276 has yield stress 360 MPa at room temperature but after aging for 16000 hours, its yield stress increased to 476MPa. Hastelloy C276 and Inconel 625 also lost their mechanical properties when they were aged for 4000 hours. C4 widely resisted aging and retains about 40% of its ductility even being aged for 649oC for 16000 hours. Its resistance to aging could be associated to its lower chemical composition as compare to other Nickel alloys.
When Hastelloy C276 was thermally aged, it became prone to hydrogen based cracking in conditions comprising hydrogen sulfide.
Nickel based alloys containing chromium, iron and molybdenum are used to produce tubes for steam generators in nuclear power plants. These alloys are Inconel 600, Incoloy 825, 800 and G30. Incoloy 800 is commonly used in nuclear power plants for offering higher resistance to cracking than Inconel 600, due to moderate composition of nickel in Incoloy 800. Precipitation hardened high strength Incoloy X750 is used in production of nuclear reactor components.
Incoloy 825 offers higher resistance to stress corrosion cracking in chloride solutions than SS 316 as it has higher nickel concentration. It has major applications in oil and gas production in sour plants. Meanwhile the properties of Hastelloy bar C276 and Hastelloy G are superior to Incoloy 825. These alloys are widely used in the cold processed condition for enhanced strength.