Alloy cobalt

Chromium-containing cobalt alloys have been developed for use requiring wear resistance and high temperature corrosion resistance. The nominal composition of some wear-resistant cobalt alloys is given in Table 4.49. 

Stellite Contains significant amount of carbon in order to form carbides in the 

Triballoy High Mo and Si give rise to precipitates and provide corrosion resistance to acids 

In the case of liquid droplet impingement erosion, cavitation erosion and cyclic sliding resulting in microfatigue, the ability of cobalt alloys to absorb the stress is useful. The cobalt-rich matrix is useful in wear applications. Corrosion rate data for Haynes 6B alloy in sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid at different concentrations of acids and temperatures are given in Tables 4.50—4.53, respectively. 

The wrought version of Haynes 6B alloy has higher corrosion resistance in aqueous media than the cast and weld overlay version (Stellite 6) because of hot forging and rolling breakdown of the interconnected carbides into discrete particles. Haynes 6B can be used in sulfuric acid in the concentration range 2-77 wt % at 24°C and in dilute solutions at higher temperatures (see Table 4.50). 

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. 

Chromium-bearing cobalt alloys can be divided into three groups [2]  

At temperatures up to 417°C, cobalt has a hexagonal close packed (hep) structure, whereas above 417°C, it is face-centered cubic (fee). Alloying elements that are fee stabilizers (such as nickel, iron, and carbon) lower the transformation 

Some commercial cobalt-aUoy compositions are listed in Table 24.1. They typically contain chromium as well as molybdenum and/or tungsten, making them relatively resistant to both reducing and oxidizing conditions. High resistance to abrasion results from the precipitation of carbides, which can be carbides of chromium, molybdenum, and tungsten [3]. 

The alloys resist pitting in FeCls at room temperature, except Alloy 6B because of its high carbon content of 1.2%. At a lower carbon content—for example, 0.4%—the alloy is resistant. Vitallium, having a very noble critical pitting potential is also resistant to pitting in dilute NaCl solutions, a property that extends to Alloy 25 and MP35N as well. 

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Platinum-cobalt alloys have magnetic properties. One such alloy made of 76.7% Pt and 23.3%... 

Cobalt alloys, cast Cobalt aluminate Cobalt aluminate [1333-Cobalt alumimdes... 

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