Nickel-base alloys anodic polarization

The effects of acid concentration and temperature on the anodic polarization of a commercial nickel-base alloy (Hastelloy C, nominal composition 54 wt%Ni, 2.5 wt% Co, 15.5 wt% Cr, 16 wt% Mo, 4 wt% W, 5.5 wt% Fe, 0.06 wt% C maximum) are shown in Fig. 5.32 (Ref 29). Qualitative conclusions from these curves indicate that the changes in corrosion rate on increasing the acid concentration from 1 to 10 N should be relatively small but that the effect of increasing the tempera-... 

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

ASTM G 61 Standard Test Method for Conducting Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys Provides information on conducting cyclic anodic polarization tests. Useful procedure for evaluating pitting and crevice corrosion susceptibility. Can be modified for other alloys that exhibit passive behavior. 

Other DC methods that are quite simple to use and provide important information to the corrosion scientist include polarization resistance (ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements), potentiostatic and potentiodynamic polarization measurements (ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements), cyclic polarization measurements (ASTM G 61, Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-based Alloys), and galvanic current monitoring. These DC techniques can be used to estimate the reactivity of a mateiieJ in a peurticular environment, to determine the corrosion rate of a materieJ in a particular environment, and/or to determine the susceptibility of a material to localized corrosion. 

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