The susceptibility or Alloy 22 (N06022) to crevice corrosion may depend on environmental or external factors and metallurgical or internal factors. Some of the most important environmental factors are chloride concentration, inhibitors, temperature and potential. The presence of a weld seam and second phase precipitation in the alloy are classified as internal factors. The localized corrosion resistance of Alloy 22 has been extensively investigated in the last five years, however not all affecting factors were equally considered in the studies. This paper discusses the current findings regarding the effect of many of these variables on the susceptibility (or resistance) of Alloy 22 to crevice corrosion. The effect of variables such as temperature, chloride concentration and nitrate are rather well understood. However there are only limited or no data regarding effect of other factors such as pH, other inhibitive or deleterious species and type of crevicing material and crevice geometry. There are contradictory results regarding the effect of metallurgical factors such as solution heat treatment. inhibitors
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), Nimolybdenum (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 C-4 (N06455), C-276 (N10276), C-2000 (N06200), 59 (N06059) and 686 (N06686). 1,2 Alloy 22 belongs to the Ni-Cr-Mo family of nickel based alloys and contains nominally 22% Chromium (Cr), 13% Molybdenum (Mo) and 3% tungsten (W). 2 The Ni-Cr-Mo alloys were 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 protec- tion against oxidizing conditions and molybdenum is the beneficial alloying element to protect against reducing conditions. 1,3-4 The base element (nickel) protects the alloy against caustic conditions. 1,3-4 All three elements, Ni, Cr and Mo act synergistically to provide resistance to environmentally assisted cracking in hot concentrated chloride solutions. 1,3-4 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. 5 Ni-Cr-Mo alloys are practically immune to pitting corrosion but they may suffer crevice corrosion under aggressive environmental conditions.
