Abstract
There are a great variety of commercial nickel alloys mainly because nickel is able to dissolve a large amount of alloying elements while maintaining a single ductile austenitic phase. Nickel alloys are generally designed for and used in highly aggressive environments, for example when stainless steels may not perform well. Nickel alloys are generally resistant to pitting corrosion in chloride containing environments, but may be prone to crevice corrosion attack. Addition of chromium, molybdenum and tungsten increase the localized corrosion resistance of nickel alloys. A review on the resistance of nickel alloys to pitting corrosion includes specific environments such as the upstream oil & gas, chemical process industry and seawater.
Introduction
Nickel (Ni) is a popular metallic element since it is used as a component in hundreds of alloys; it is used as a corrosion resistant plating product and also as a catalyst. Approximately 61 % of the Ni produced worldwide is used in the fabrication of stainless steels, which contain by weight approximately 10% Ni [1]. Only about 12% of the world production of Ni is used in the fabrication of Ni-based alloys or Ni-rich alloys. Over 90% of Ni-containing products are recycled at the end or their useful life, and there are no limits how many times the Ni metal can be recycled.
Ni alloys are solid solutions of the element Ni and other alloying elements. In general, the minimum amount of Ni in Ni alloys is in the order of 50% by mass; however, some alloys, such as alloy 800 (N08800) and alloy 28 (N08028), are classified in the family of nickel alloys even though they may contain less than 35% Ni. Large percentages of alloying elements can be added to Ni to produce a vast variety of alloys, some of these alloys are tailored for specific applications [1]. The resulting Ni alloys still maintain the face centered cubic, gamma or austenitic single phase microstructure of pure Ni. In contrast, iron alloys (austenitic stainless steels) cannot dissolve as much alloying elements as Ni without precipitating secondary phases. For example Fe can accommodate as a maximum approximately 6% of molybdenum (Mo) in solid solution, while Ni can dissolve up to 30% Mo and still maintain an fcc single phase ductile microstructure.
