The susceptibility of Alloy 22 (N06022) to crevice corrosion may depend on environmental and metallurgical variables. This paper summarizes the current findings regarding the effect of many of these variables on the susceptibility of Alloy 22 to crevice corrosion. The effect of variables such as temperature, chloride and nitrate concentrations are rather well understood. Recently, some works had added some information about less known processes such us the effect of pH, other inhibitive species and type of crevicing material, geometry and applied torque. There are still contradictory results regarding the effect of metallurgical factors such as the presence of weld seams. Crevice corrosion stifling and arrest must be considered in evaluating the life expectancy of components made of Alloy 22.
Several corrosion resistant nickel-based families of alloys exist. These include commercially pure nickel (Ni) (e.g. Ni-200 or N02200), Ni-copper (Cu) alloys (e.g. Alloy 400 or N04400), Ni- Molybdenum (Mo) alloys (e.g. B-2 or N10665), Ni-Chromium (Cr)-Iron (Fe) alloys (e.g. Alloy 600 or N06600) and Ni-Cr-Mo alloys.1 The family of Ni-Cr-Mo is rather large and continuously growing. They include alloys such as Alloy 4 (N06455), Alloy 276 (N10276), Alloy 2000 (N06200), Alloy 59 (N06059) and Alloy 686 (N06686).1,2 Alloy 22 belongs to the corrosion resistant Ni-Cr-Mo family of nickel based alloys. The required composition and mechanical properties of this family is described in ASTM B575.2 Alloy 22 has all the required alloying elements for protection in a variety of environments.3 It was designed to withstand the most aggressive industrial applications, including reducing acids such as hydrochloric and oxidizing acids such as nitric. Chromium is the beneficial alloying element added for protection against oxidizing conditions and molybdenum is the beneficial alloying element to protect against reducing conditions. The base element (nickel) protects the alloy against caustic conditions. All three elements, Ni, Cr and Mo act synergistically to provide resistance to environmentally assisted cracking in hot concentrated chloride solutions.1,4,5 The alloying elements Cr and Mo also provide resistance to localized corrosion such as pitting and crevice corrosion in chloride containing solutions. Some of the Ni-Cr-Mo alloys also contain a small amount of tungsten (W), which may act in a similar way as Mo regarding protection against localized corrosion.6 Ni-Cr- Mo alloys are practically immune to pitting corrosion. However, they may be susceptible to crevice corrosion in chloride-containing solutions. This paper is a review of previously published data in the literature regarding crevice corrosion of Alloy 22.
Localized corrosion (crevice corrosion) is a type of corrosion in which the attack progresses at discrete sites or in a non-uniform manner. Localized corrosion will only occur when the corrosion potential ECORR is equal or greater than a critical potential (ECRIT) for localized corrosion, generally called as repassivation potential for crevice corrosion ER;CREV. That is, if ECORR<ER,CREV, only general corrosion will occur. ER,CREV can be defined as a certain potential above which the current density or corrosion rate of Alloy 22 increases significantly above the general corrosion rate of the passive metal. In environments that promote localized corrosion, ER,CREV is the lowest potential that would trigger localized (e.g. crevice) corrosion.
