Nickel based alloys are designed for use in extreme conditions in the energy, power, chemical and petrochemical plants due to their excellent corrosion resistance. Many alloys are metallurgically based to austenitic stainless steels however have larger concentrations. These alloys in addition to having nickel, other elements like chromium, molybdenum, manganese are added to improve the performance of alloy as per the different service conditions. Generally, chromium is added to improve performance in oxidizing media while molybdenum increases strength and corrosion resistance in reducing media. Though costlier than iron based alloys, high strength nickel alloys are the suitable service material for hostile conditions in various high value applications where suitable combination of heat and corrosion resistance usually makes them the most economical and long term choice.
In this article, different grades of Nickel alloys such as Inconel 625, Hastelloy X and Nickel 200 are evaluated. Inconel 625 is a NiCrMo alloy that provides higher strength without the need of an additional heat processing. This alloy prevents corrosion in severe conditions and is particularly resistant to pitting and crevice corrosion. It is commonly used in chemical processing, aerospace, marine and nuclear plants.
Hastelloy X offers outstanding heat and oxidation resistance and is resistant to chloride stress corrosion cracking and carburization. It is one of the largely used alloys for gas turbine engine components. Nickel 200 is used in commercially pure wrought form offering good mechanical properties and resistance to a large range of corrosive media. It is used for a variety of processing systems, specifically to maintain product purity in handling foods, synthetic fibers. Hastelloy X is the costliest alloy among the stated alloys.
Performance of Nickel alloys in aqueous media
In aqueous media, an electrochemical process occurs between the material and its service condition. In a service condition of HI water solution, corrosion occurs due to oxidation of material and reduction of HI. The redox reaction occurs attacking the nickel alloy. The corrosion performance of nickel based alloys in different HI conditions, differentiated by temperature and exposure duration is evaluated. The selected temperatures are 70oC and 100oC to prevent the solution boiling in standard atmosphere pressure.
The corrosion rates of alloys increase with the rising temperature. For Inconel 625 and Hastelloy X, the corrosion rate increases and then decreases in flat form with passing time. This mechanism is considered as of their anti-corrosion layer. For Nickel 200, the corrosion rates do not vary significantly, due to weaker security layer as compare to other alloys. In Inconel 625, iron disappears in a stable ratio with respect to nickel. For Hastelloy X, iron, chromium and molybdenum are attacked in a constant ratio of rates with respect to nickel. With rising temperatures, the ratio of these elements to nickel decreases which states the corrosion rates of these elements increase.
Pitting resistance of Hastelloy bars of X grade is larger than Inconel 625 due to its more resistant oxide layer. After 48 hours of experiment, the potentials of Nickel 200 decreases nominally. Nickel 200 could not perform well in the initial 24 hours.
Corrosion Resistant Metals and Alloys 