Resistance cobalt-based alloys

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. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

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. 

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. 

Cobalt-based alloys with a carbon content in the range of 1 to 3wt%C are widely used as wear-resistant... 

Nickel-base alloys respond well to most electrochemical test techniques and show active-passive behavior in many environments. Due to their rapid repassivation, however, the results obtained with potentiod3mamic techniques can sometimes be affected by scan rate and immersion time prior to starting the test [5,6], Electrochemical techniques are useful for investigating localized corrosion resistance, ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, and general corrosion resistance, ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements of nickel alloys. Electrochemical impedance measurement techniques have not been extensively applied to nickel alloys. 

By virtue of these crystallographic features, the cobalt-based alloys are inherently resistant to those forms of wear that involve a microfatigue constituent, namely sliding wear, fretting, cavitation erosion, and liquid droplet erosion [7],... 

From a corrosion standpoint, the roles of various alIo)dng elements in the cobalt-base alloys parallel those seen in the nickel-base alloys. Chromium, molybdenum, and tungsten, for example, are highly soluble in both atomic forms of cobalt. Chromium is added to most of the commercially important alloys, and provides passivity over a wide range of potentials. Molybdenum and tungsten enhance resistance to corrosion within the active regime. 

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