Cobalt-based alloys, localized corrosion

Materials classes that were tested included ceramics, nickel-based and cobalt-based alloys, refractory metals and alloys, reactive metals and alloys, noble metals and alloys, and high-temperature polymers, a total of 26 materials. Test periods varied between 37.5 and 47.5 hours. None of the materials was found to be suitable for all test conditions, and most exhibited moderate (equivalent to between 10 and 200 mil per year) to severe (>2()0 mil per year) corrosion. Titanium and titanium alloys (Nb/Ti and Ti-21S) exhibited the best performance, showing only slight corrosion in the presence of excess sodium hydroxide. Under acidic conditions, titanium showed increased rates of corrosion, apparently from attack by sulfuric acid and hydrochloric acid. Both localized pitting and wall thinning were observed. 

G61-86, Conducting cyclic potentiodynamic polarization measurements for localized corrosion susceptibility of iron-, nickel-, or cobalt-based alloys, Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 240, Vol. 3.02. 

ASTM G 61-86, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, ASTM International, West Conshohocken, PA, 2003. 

ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys— This standard presents a procedure for performing cyclic poten-tiodynamic polarization testing to determine the relative susceptibility of iron-, nickel-, and cobalt-based alloys to localized corrosion (pitting or crevice corrosion). It illustrates and presents test apparatus, reagents, and materials, test procedures, and interpretation of results. 

Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys... 

Electrochemical tests provide a means to understand the corrosion process, simulate service conditions, or accelerate evaluation of a material [27]. ASTM G 3, Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Polarization Measurements and ASTM G 61, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys provide background in some of these techniques. 

Cobalt-based alloys are usually less corrosion-resistant than nickel-based alloys, but are more resistant to wear. Co-Cr-Mo alloys such as Ultimet also have excellent resistance to chloride-induced localized attack. 

Cyclic potentiodynamic polarization for localized corrosion susceptibility of iron-, nickel-, or cobalt-base alloys G61... 

To evaluate the susceptibility of the cobalt-based eilloys to localized attack, the procedures described in ASTM G 48, Test Method for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by the Use of Ferric Chloride Solution can be used. When tested in 6 % ferric chloride, for example, at a temperature of 65°C, the pit densities and depths for samples of UNS R31233 were less than those for UNS N10276. 

The corrosion behavior of non-ferrous alloys such as those based on nickel, cobalt, copper, zirconium, and titanium has been reviewed in detail in this chapter. Besides exotic materials such as tantalum and platinum, nickel-based alloys are the most resistant to corrosion by mineral acids, and they are especially resistant to localized corrosion in chloride-containing environments, which troubles stainless steels. Nickel-based alloys can broadly be divided into alloys, e.g. Ni-Mo (B-2, B-3) and Ni-Cu (alloy 400), that do not contain chromium, and are not, therefore, passivated under oxidizing conditions, and alloys, e.g. Ni-Cr-Mo (C-22, C-2000,59,686, etc.) and Ni-Cr-Fe (G-30, 825, etc.), that form a chromium oxide passive film under oxidizing conditions. Ni-Mo alloys such as B-3 have excellent corrosion resistance in hot reducing acids such as hydrochloric and sulfuric. Ni-Mo alloys cannot withstand oxidizing conditions such as nitric acid and hydrochloric acid contaminated with ferric ions. Ni-Cr-Mo alloys such as C-2000 alloy are multipurpose alloys that can be used both in reducing and oxidizing conditions. 

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