INTRODUCTION
Alloy 22 is a nickel base alloy highly resistant to all forms of corrosion. In conditions where tight crevices exist in hot chloride containing solutions and at anodic potentials, Alloy 22 may suffer crevice corrosion, a form of localized attack. The occurrence (or not) of crevice corrosion in a given environment (e.g. salt concentration and temperature), is governed by the values of the critical potential (Ecrit) for crevice corrosion and the corrosion potential (Ecorr) that the alloy may establish in the studied environment. If Ecorr is equal or higher than Ecrit, crevice corrosion may be expected. In addition, it is generally accepted that as Alloy 22 becomes passive in a certain environment, its Ecorr increases and its corrosion rate (CR) decreases. This paper discusses the evolution of Ecorr and corrosion rate (CR) of creviced Alloy 22 specimens in six different mixtures of sodium chloride (NaCl) and potassium nitrate (KNO3) at 100°C. The effect of immersion time on the value of Ecrit was also determined. Two types of specimens were used, polished as-welded (ASW) and as-welded plus solution heat-treated (ASW+SHT). The latter contained the black annealing oxide film on the surface. Results show that, as the immersion time increases, Ecorr increased and the CR decreased. Even for highly concentrated brine solutions at 100°C the CR was<30 nm/year after more than 250 days immersion. Some of the exposed specimens (mainly the SHT specimens) suffered crevice corrosion at the open circuit potential in the naturally aerated brines. Immersion times of over 250 days did not reduce the resistance of Alloy 22 to localized corrosion.
Alloy 22 (N06022) is a nickel (Ni) based alloy that contains nominally 22% chromium (Cr), 13% molybdenum (Mo), 3% tungsten (W) and 3% iron (Fe) (ASTM B 575). By virtue of its high level of Cr, Alloy 22 remains passive in most industrial environments and therefore has an exceptionally low general corrosion rate. The combined presence of Cr, Mo and W imparts Alloy 22 with high resistance to localized corrosion such as pitting corrosion and stress corrosion cracking even in hot high chloride (Cl-) solutions. It has been reported that Alloy 22 may suffer crevice corrosion when it is anodically polarized in chloride containing solutions. It is also known that the presence of nitrate (NO3 -) and other oxyanions in the solution minimizes or eliminates the susceptibility of Alloy 22 to crevice corrosion. The value of the ratio ([Cl-]/[NO3 -]) has a strong effect of the susceptibility of Alloy 22 to crevice corrosion. The higher the nitrate to chloride ratio the stronger the inhibition by nitrate.
From the general and localized corrosion point of view, it is important to know the value of Ecorr for Alloy 22 under different environmental conditions. The corrosion degradation model for the Yucca Mountain nuclear waste container assumes that localized corrosion will only occur when Ecorr is equal or greater than a critical potential (Ecrit). That is, if Ecorr<Ecrit or ÄE = Ecrit ? Ecorr >0, general or passive corrosion will occur and localized corrosion is not expected. Passive corrosion rates of Alloy 22 are generally exceptionally low. In environments that promote localized corrosion, Ecrit is the lowest potential that would initiate crevice corrosion. The value of Ecrit is generally ascribed as the repassivation potential for crevice corrosion obtained using the cyclic potentiodynamic polarization (CPP) curve described in ASTM G 61. From the CPP, the repassivation potential is taken as the potential at which the reverse scan line crosses over the forward scan. This potential is called the repassivation potential cross over (ERCO). In short, by knowing
