The susceptibility of Alloy 22 (N06022) to crevice corrosion may depend on environmental and metallurgical variables and it was mostly tested to obtain crevice repassivation potentials. Regardless of all the combinations or variations of the electrochemical test methods Cyclic Potentiodynamic Polarization (CPP), Tsujikawa - Hisamatsu Electrochemical (THE), Potentiostatic (POT), and Mixed Poentiodynamic - Potentiostatic (PD - POT - PD), whenever crevice corrosion readily occurs, the values of repassivation potential for Alloy 22 seem comparable if all environmental and metallurgical variables remain the same. This finding suggests that the crevice repassivation potential is a property of the alloy in each testing condition and does not depend greatly on the manner it is obtained provided that highly aggressive conditions are used. Under less aggressive conditions, when crevice corrosion is more difficult to initiate, the value of repassivation potential may differ from method to method. In this work three different electrochemical methods (CPP, THE, and PD - POT - PD methods) were used to evaluate the susceptibility of Alloy 22 to crevice corrosion in two different solutions: one highly aggressive (1M NaCl at 90ºC), and one weakly aggressive (1M NaCl + 0.15M NaNO3 at 90 ºC). The effect of applied torque was also investigated in these systems.
Alloy 22 belongs to the corrosion resistant Ni-Cr-Mo family of nickel based alloys. 1,2 The required composition and mechanical properties of this family are described in ASTM B575.2 Alloy 22 has alloying elements for protection against localized corrosion initiation in a variety of environments.3 Chromium is added for protection against oxidizing conditions while molybdenum protects 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 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.7 There are several methods to determine the susceptibility of Alloy 22 and other engineering alloys to crevice corrosion (Table I).8,9 Immersion tests are generally used to find critical crevice temperatures, for example in standard ASTM solutions.1,8,10 For Alloy 22, the most popular electrochemical test used is the cyclic potentiodynamic polarization (CPP) test or ASTM G 61.8 There are two ways to quantify the repassivation potential from a CPP curve: the potential at which the anodic current in the reverse scan attains the passive current value (ER1, ER10, etc) or the cross over of the reverse and forward scans (ECO). Other tests are the Tsujikawa - Hisamatsu Electrochemical (THE) method (ASTM G 192)11, which does not have ASTM standard. In the THE method the repassivation potential (ER,CREV) is the potential at which the anodic current ceases to increase in the potentiostatic decreasing steps.
