Aqueous Corrosion Resistance of Nickel-based Alloys

Nickel offers a range of properties that make it a very important metal. It is relatively abundant on earth, highly ductile, has a high melting point and is capable of dissolving large quantities of additive elements while retaining its austenitic structure. As a result, a large number of nickel alloys have been developed and patented since the early 20th century. Some of them, with their remarkable resistance to high-temperature corrosion, have enabled the development of aeronautical or land-based turbines, while others, with their resistance to aqueous corrosion, have been important elements in the development of modern chemistry. Today, nickel and its alloys are often used, along with exotic metals, in difficult corrosion situations when stainless steels - whether ferritic, austenitic (duplex, superduplex, hyperduplex), austenitic or super- austenitic - are unsuitable for the application in question. Metallurgists have been creative, particularly over the last few decades, and this article aims to guide the potential user towards the best solution. Not all alloys are created equal, and a judicious choice must be made in order to select the best nickel alloy(s) for a given application.

Nickel alloys can be classified into several families according to their content of additive elements, mainly chromium, molybdenum, tungsten and copper. Chromium is essential for passivation in oxidizing media, while molybdenum and tungsten ensure resistance to reducing media. These families can be used to group together alloys with more or less equivalent passivation characteristics, depending on the redox potential of the media. Nickel alloys can be used to resolve various types of corrosion:

• generalized corrosion: the numerous comparisons of resistance to generalized corrosion given in this article show that in hydrochloric, hydrofluoric, hydrobromic, nitric and organic phosphoric acids or basic media, not all these alloys are equivalent, but that there is almost always a conceivable industrial solution;

• localized corrosion: they provide a range of solutions to the problems posed by halide-initiated localized corrosion, such as pitting and crevice corrosion, or stress corrosion cracking.

Their installation poses no particular problems for the skilled craftsman, but remains an essential element of their good behavior.

In this article, we look at corrosion resistance in the most corrosive media, such as acids (sulfuric, hydrochloric, hydrofluoric, hydrobromic, nitric, phosphoric and some organic acids) and basic media. We will show that nickel alloys are resistant to localized corrosion (pitting and crevice corrosion) in saline solutions, particularly seawater....