Nickel can dissolve a large amount of alloying elements such as chromium, molybdenum and copper while maintaining a ductile gamma phase. Therefore, a wide range of nickel based alloys is available for application in highly aggressive environments. Some of these alloys are versatile and some others are specific and may be used only in certain environments. Laboratory corrosion testing results are presented to substantiate the applicability of the many types of nickel based alloys in several common service conditions, especially in aggressive alkaline and acidic environments.
Nickel based alloys are commonly used to handle highly corrosive environments. Some of these alloys provide outstanding resistance to specific chemicals, and some are extremely versatile and able to handle complex process and waste streams. In particular, the versatile alloys are much less prone than the stainless steels (SS) to stress corrosion cracking (SCC), pitting, and crevice attack in hot, chloride-bearing solutions. Also, nickel alloys are among the few materials able to withstand hot hydrofluoric acid, a chemical which is very corrosive to the reactive metals (titanium, zirconium, niobium, and tantalum).
The aim of this paper is to describe the general characteristics of the different nickel based alloys and to examine the effect of different aggressive environments on the corrosion behavior of nickel based alloys.
NICKEL ALLOY TYPES
The nickel alloys can be categorized according to the main alloying elements, as follows:
1. Nickel (primarily for use in caustic solutions)
2. Nickel-Copper (primarily for mild, reducing solutions, especially hydrofluoric acid)
3. Nickel-Molybdenum (primarily for strong, reducing media)
4. Nickel-Iron-Chromium (primarily for oxidizing solutions)
5. Nickel-Chromium-Silicon (primarily for super-oxidizing media)
6. Nickel-Chromium-Molybdenum (versatile alloys for all environments)
The terms reducing and oxidizing refer to the nature of the reaction at cathodic sites during corrosion. Reducing solutions, such as hydrochloric acid, generally induce hydrogen evolution at cathodic sites. Oxidizing solutions, such as nitric acid, induce cathodic reactions with higher potentials.
To give an appreciation of the alloying element levels used in these materials, the composition of a few nickel alloys is given in Table 1. These are all wrought alloys, available in the form of plates, sheets, bars, pipes, tubes, forging, and wires.
With regard to the roles of the various alloying elements, copper, molybdenum, and tungsten all increase the inherent corrosion resistance of nickel. In addition, molybdenum and tungsten are significant strengthening agents, due to their large atomic sizes. The role of chromium in the corrosion-resistant nickel alloys is the same as that in the stainless steels; it enhances the formation of passive, surface films, in the presence of oxygen. These passive films impede the corrosion process. Iron, if added to the nickel alloys, also affects passivation. Silicon is beneficial at high corrosion potentials, where chromium-rich passive films cannot be maintained. It offers extended protection through the formation of protective (silicon-rich) oxides.
NICKEL ALLOY PERFORMANCE
Although the number of environments encountered within the chemical process industries is vast, the performance of metallic materials often boils down to their resistance to a few aggressive inorganic chemicals. Predominant among these are hydrochloric acid, sulfuric acid and hydrofluoric acid. Also very important are the effects of residuals, such as ferric ions, on performance in acids.
Caustic
