Pitting cobalt-based alloys

The literature contains a number of studies on the susceptibility of the cobalt-based alloys to pitting corrosion. In-vitro studies conducted by Mueller and Greener , involving static conditions, revealed no evidence of pitting having occurred. Syrett and Wing ", utilising cyclic polarisation analyses, observed that neither as-cast nor annealed Co-Cr-Mo alloy demonstrated hysteresis loops in their cyclic polarisation curves. They... 

Lucas, L. C., Buchanan, R. A., Lemons, J. E., and Griffin, C. D., Susceptibility of Surgical Cobalt-Base Alloy to Pitting Corrosion , Journal of Biomedical Materials Research, 16, 799-810 (1982)... 

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. 

In aircraft turbines, especially nickel-and cobalt-based alloys suffer from salt-induced corrosion. On these alloys, type II corrosion mechanisms were observed, depending on the surface temperature of the turbine blades. Studies were performed on Co-Cr, Co-Al, Ni-Cr, Ni-Cr-Al, and Co-Cr-Al-Y alloys [38, 39]. It was shown that on Co-Cr and Co-Cr-Al pits were formed on the alloy, filled with corrosion products. On Co-Cr and Co-Cr-Al, the morphology... 

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. 

Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibilily of Iron-, Nickel-, or Cobalt-Based Alloys, is quite effective [8,9,22]. A sample curve showing the effects of a corrosion inhibitor is in Fig. 1. An increase in the pitting (Ep), and breakdown (E),) potentials is indicative of a good anodic inhibitor. 

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. 

---