Corrosion cobalt-based alloys

The effect of H2S concentration on total corrosion of selected alloys in 1000 hr at 1800°F is illustrated in Fig. 2. A variety of different oxidation-corrosion behaviors were observed. Ferritic alloys, like AISI 446, generally showed increased corrosion rate with decreasing H2S concentration, whereas 300 series austenitics typified by AISI 310 generally exhibited maxima at both 0.1 and 1.0 v/o H2S. IN-800 had high corrosion only above 0.5 v/o H2S. Aluminized AISI 310 and IN-800, IN-671, and several high-chromium alloys did not indicate a strong dependence of H2S concentration in 1000 hr total corrosion. Cobalt-base alloys also generally performed as shown for the aluminized AISI 310 and IN-671 specimen. 

Titanium alloy systems have been extensively studied. A single company evaluated over 3000 compositions in eight years (Rem-Cm sponsored work at BatteUe Memorial Institute). AHoy development has been aimed at elevated-temperature aerospace appHcations, strength for stmctural appHcations, biocompatibiHty, and corrosion resistance. The original effort has been in aerospace appHcations to replace nickel- and cobalt-base alloys in the 250—600°C range. The useful strength and corrosion-resistance temperature limit is ca 550°C. 

With cobalt historically being approximately twice the cost of nickel, cobalt-base alloys for both high temperature and corrosion service tend to be much more expensive than competitive alloys. In some cases of severe service their performance iacrease is, however, commensurate with the cost iacrease and they are a cost-effective choice. For hardfaciag or wear apphcations, cobalt alloys typically compete with iron-base alloys and are at a significant cost disadvantage. 

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)... 

Carbon steel is the predominant construction material for carbonate and amine solution containers. Corrosion in the overhead lines (hydrogen sulfide or carbon dioxide plus water from the regenerator) is prevented by adding corrosion inhibitors. Although amine carry-over can act as a corrosion inhibitor in the overhead line, SCC of carbon steel has occurred when amine added as a corrosion inhibitor became concentrated. Copper and copper base alloys should be avoided in amine service and are questionable in carbonate seivice. Nickel or cobalt base alloys (e.g., Monel00 400 and Inconel 600) except for Stellite01 should be avoided in carbonate service. Monel 400 should be avoided in amine service if UCC Amine Guard02 corrosion inhibitor is used. 

Black J (1988) In vivo corrosion of a cobalt-base alloy and its biological consequences. In Hildebrand HF and Champy M, eds. Biocompatibility of Co-Cr-Ni alloys. NATO-ASI Series 158 A, pp. 83 — 100. Plenum, London-New York... 

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... 

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... 

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. 

Van Orden, A. 1985. Corrosive response of the interface tissue to 316L stainless steel, Ti-based alloy and cobalt-based alloys. In The Dental Implant, R. McKinney and J.E. Lemons (Eds.), pp. 1-25, Littleton, PSG. 

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. 

Gas turbine blades are essentially nickel-base or cobalt-base alloys containing substantial amounts of chromium, several percent aluminum, and a few hundredths percent yttrium. Their susceptibility to hot corrosion and sulfidation has already been discussed. Applied coatings of aluminum or of aluminum-chromium-yttrium increase resistance to attack. 

Being less plentiful and more expensive than nickel, cobalt is usually alloyed with chromium for applications where the alloys have practical advantages over similar nickel- or iron-base alloys. The cobalt-base alloys, for example, are better resistant to fretting corrosion, to erosion by high-velocity hquids, and to cavitation damage. 

Corrosion of cobalt and cobalt-base alloys, in ASM Handbook, Vol. 13B, Corrosion Materials, ASM International, Materials Park, OH, 2005, pp. 164-176. 

Lorenz, M. Semlitsch, M., Panic, B., Weber, H. and Willhert, H.G. (1978) Fatigue Strength of Cobalt-Base Alloys with High Corrosion Resistance for Artificial Hip Joints. Engineering in Medicine, 1 (4), 241. 

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