Crevice cobalt-based alloys

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

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. 

Morral, F. R., Crevice Corrosion of Cobalt Based Surgical Alloys , Journal of Materials, 1, 384 (1966)... 

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. 

Table 2-16 shows the critical crevice and pitting temperatures for cobalt-based and other alloys. Data in Table 2-16 show that Ultimet alloy has an excellent resistance to localized attack, comparable with that of C-22 alloy. 

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