Cobalt-based corrosion-resistant alloys

Cobalt aluminum blue, transparent, 19 412 CobaltCII) aminobenzoate, uses, 7 239t CobaltCII) ammonium sulfate, uses, 7 239t CobaltCII) arsenate, uses, 7 241t Cobalt-base corrosion-resistant alloys,... 

The name Wallex designates a line of cobalt-base hard-surfacing alloys. All of them resist corrosion well, but they vary in their ability to resist abrasion and impact, and in the way they can be applied. Wallex alloys would seldom be recommended for protection against corrosion alone. In most cases, they are chosen for their ability to fight the twin hazards of corrosion and abrasion. The specific alloy choice depends on a careful analysis of the extent of the problem presented by each hazard. 

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. 

Nonferrous alloys account for only about 2 wt % of the total chromium used ia the United States. Nonetheless, some of these appHcations are unique and constitute a vital role for chromium. Eor example, ia high temperature materials, chromium ia amounts of 15—30 wt % confers corrosion and oxidation resistance on the nickel-base and cobalt-base superaHoys used ia jet engines the familiar electrical resistance heating elements are made of Ni-Cr alloy and a variety of Ee-Ni and Ni-based alloys used ia a diverse array of appHcations, especially for nuclear reactors, depend on chromium for oxidation and corrosion resistance. Evaporated, amorphous, thin-film resistors based on Ni-Cr with A1 additions have the advantageous property of a near-2ero temperature coefficient of resistance (58). 

As of this writing the 2inc alloys are too new to have actual corrosion resistance data, except for that based on accelerated tests. Zinc—nickel usually shows better results than 2inc-cobalt in salt spray tests. The reverse is tme when the Kesternich test is used. Tin—2inc performs well in both salt spray and Kesternich tests, but appears only to equal 2inc plating and 2inc—nickel in humidity tests. 

However, under more realistic test conditions Hancock and Islam showed that in burner rig tests with contaminant flux rates greater than about 0-1 mgcm h" the corrosion rate of nickel- and cobalt-base superalloys was largely independent of alloy composition in the temperature range 7(X)-850 C. However, in burner rig tests at 6(X) C, simulating diesel engine combustion, Saunders et reported that Nimonic 80A (20% Cr) had superior resistance to Stellite 6 (Co-28%Cr) and EN 52 (Fe-8%Cr-3%Si). 

The most important application of chromium is in the production of steel. High-carbon and other grades of ferro-chomium alloys are added to steel to improve mechanical properties, increase hardening, and enhance corrosion resistance. Chromium also is added to cobalt and nickel-base alloys for the same purpose. 

Yttrium is also used in other areas of metallurgy notably as a component of certain nickel-base and cobalt-base superalloys of the NiCrAlY and CoCrAlY type.(3) These alloys possess excellent corrosion and oxidation resistance, properties that have attracted the attention of the aero-engine industry where they are used as protective coatings on turbine blades. The alloys, when applied by vapour deposition, form an oxide coating that exhibits remarkable adhesion, a property attributed largely to the yttrium component acting to prevent the formation of voids at the oxide/substrate interface.(4)... 

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