Superalloys high-temperature corrosion

The association of the high temperature corrosion of superalloys with contaminants has been reported by Hancock (1987). He proposed that to compare contaminant conditions the contaminant flux rate (CFR) rather than the contaminant level in the fuel or environment should be considered. He further suggested that at temperatures above 700°C where vanadates cause fluxing of the protective oxide scales, corrosion could be determined by the CFR and temperature rather than by material selection. 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

Alloys that retain high strength at high temperatures (>1000°C in some cases) are known as superalloys. Some of these materials are also highly resistant to corrosion (oxidation). These alloys are difficult to make, contain metals that are not readily available, and are expensive. They are used in situations where the conditions of service make them essential, such as in aircraft engines, where certain designs require as much as 50% by weight of some of these special alloys. 

The most imporant use of cobalt is in the manufacture of various wear-resistant and superalloys. Its alloys have shown high resistance to corrosion and oxidation at high temperatures. They are used in machine components. Also, certain alloys are used in desulfurization and hquefaction of coal and hydrocracking of crude oil shale. Cobalt catalysts are used in many industrial processes. Several cobalt salts have wide commercial apphcations (see individual salts). Cobalt oxide is used in glass to impart pink or blue color. Radioactive cobalt-60 is used in radiography and sterihzation of food. 

Corrosion Resistance of Nickel-based Alloys. Nickel-based alloys are solid solutions based on nickel. Nickel-based alloys used for low-temperature aqueous or condensed systems are generally known as corrosion-resistant alloys (CRA), and nickel alloys used for high-temperature applications are known as heat-resistant alloys (HRA), high-temperature alloys (HTA), or superalloys. The corrosion performance could change due to the presence of second phase or a weld seam. (Rebak)5... 

The United States has to import all the cobalt it uses. One of the most important applications of cobalt is in the production of superalloys. These superalloys consist primarily of iron, cobalt, or nickel, with small amounts of other metals, such as chromium, tungsten, aluminum, and titanium. Superalloys are resistant to corrosion (rusting) and retain their properties at high temperatures. Superalloys are used in jet engine parts and gas turbines. 

Superalloys Consist primarily of iron, cobalt, or nickel, with small amounts of other metals that are resistant to corrosion (rusting) and retain their properties at high temperatures. 

More Mo is consumed annually than any other refractory metal. The major use for Mo is as an alloying element in alloy steels, tool steels, stainless steels, Ni-based and Co-based superalloys. In these materials, it increases the hardenability, toughness, high-temperature strength, and corrosion resistance. Molybdenum is important in the missile industry, where it is used for high-temperature structural parts, such as nozzles, leading edges of control surfaces, support vanes, struts, reentry cones, radiation shields and heat sinks. In the electrical and electronic industries. Mo is... 

Oxides are widely used and encountered in technical applications, such as high-temperature ceramics, nuclear fuels, glasses, and corrosion layers on superalloys. 

F. S. Pettit and G. H., Meier, The effects of refractory elements on the high temperature oxidation and hot corrosion properties of superalloys. In Refractory Alloying Elements in Superalloys, eds. J. K Tien and S. Reichman, Metals Park, OH, ASM, 1984, p. 165. 

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