Nickel high-temperature corrosion

Tips of platinum, platinum—nickel alloy, or iridium can be resistance-welded to spark-plug electrodes for improved reHabiHty and increased lifetime. These electrodes are exposed to extremely hostile environments involving spark erosion, high temperature corrosion, thermal shock, and thermal fatigue. 

The melting temperature of sulfides is generally lower than that of oxides. For example, for NiS it is 797 °C, compared to 1990 °C for NiO. In addition, the sulfides of Ni, Co, Fe and Cr can form eutectics with the base metal lowering even more their melting temperature for example, in the system Ni-S the eutectic melts at 645 °C already. Nickel sulfides therefore melt easily under typical high-temperature corrosion conditions. On the other hand, chromium sulfides melt less easily the Cr-S eutectic having a melting temperature of 1350 °C. 

The presence of a molten salts favors the dissolution of the surface oxides that otherwise limit the rate of high temperature corrosion. The dissolution of nickel oxide in molten sodium sulfate provides a good example. The solubility of NiO in Na2S04, at a given temperature, depends on the activity of Na20, which is established by the equilibrium ... 

The nickel-based alloys show a wider range of application than any other class of alloys. These alloys are used as corrosion-resistant alloys, heating elements, controlled expansion alloys, creep-resistant alloys in turbines and jet engines, and high-temperature, corrosion-resistant alloys. 

Once a corrosion product layer is formed, the corrosion process may continue through the diffusion of at least one of the reactants through the corrosion product layer. Let us consider, for example, the case of nickel exposed to air at high temperature. Corrosion can theoretically continue through the nickel oxide layer by means of diffusion in either direction, alone or by counter-current diffusion, as illustrated in Fig. 15.9. 

Nanociystalline alloy coatings, oxide dispersive alloy coatings and metal oxide composite coatings show superior high temperature corrosion resistance. A Ni-Cr nanocomposite with an average Cr concentration of 11 wt% deposited onto nickel... 

Container materials. A material capable of being fabricated into various shapes and resistant to high-temperature corrosion by the fuel alloy is a necessity if practical use is to be made of the low melting temperature plutonium alloys. Since the transition metals readily form low melting point alloys with plutonium, the normal constructional materials, steels and nickel alloys, are eliminated. 

The simplest form of a solid corrosion product on a metal surface is a continuous homogeneous surface scale consisting of one phase. Such a situation is encountered, e.g., in the oxidation of pure nickel where only nickel oxide is formed. Therefore, in many textbook examples, this type of reaction has been used to illustrate the principles of high-temperature corrosion. In the present chapter, the initial stages of the oxidation process are not addressed as this has been part of another chapter in this book. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

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