Oxidation nickel-chromium alloys

For resistance against fatigue, Nimonic 75 has been used with Nimonic 80 and Nimonic 90. Nimonic 75 is an 80-20 nickel-chromium alloy stiffened with a small amount of titanium carbide. Nimonic 75 has excellent oxidation and corrosion resistance at elevated temperatures, a reasonable creep strength, and good fatigue resistance. In addition, it is easy to press, draw, and mold. As firing temperatures have increased in the newer gas turbine models, HA-188, a Cr, Ni-based alloy, has recently been employed in the latter section of some combustion liners for improved creep rupture strength. 

Barrett and his colleagues , and Kosakhave summarised existing information on the scales formed on nickel-chromium alloys. Up to about 10% Cr, the thick black scale is composed of a double layer, the outer layer being nickel oxide and the inner porous layer a mixture of nickel oxide with small amounts of the spinel NiO CrjOj. Internal oxidation causes the formation of a subscale consisting of chromium oxide particles embedded in the nickel-rich matrix. At 10-20% Cr the scale is thinner and grey coloured and consists of chromium oxide and spinel with the possible presence of some nickel oxide. At about 25-30% Cr a predominantly chromium oxide scale is... 

Table 7.16 Oxidation data for nickel-chromium alloys"...

Table 7.17 Results of oxidation tests on nickel-chromium alloys ...

Another factor that determines the long-term stability of the protective oxide layer is its ability to prevent sulphur penetration which would lead to the eventual formation of chromium sulphide beneath the external oxide layer. With most commercial nickel chromium alloys internal sulphidation... 

Extensive studies have been carried out by Giggins and Pettit and by Vasantasree and Hocking on a range of nickel chromium alloys with up to 50% alloying addition. Generally the principles outlined above can be used to interpret the experimental observations, where the thermodynamics of the reaction are a major factor determining the rate of attack, depending upon whether oxide or sulphide is the stable phase. 

While carburisation itself is not a normal corrosion process, in that there is no metal wastage, absorption and diffusion of carbon can lead to significant changes in the mechanical properties of the affected material and in particular to marked embrittlement. Furthermore, initial carburisation can produce an acceleration of the normal oxidation process, a phenomenon that is notable in nickel-chromium alloys. 

The main characteristic of attack by halogens at elevated temperatures is that most reaction products are volatile compared with the solid products that form in all cases considered hitherto in this chapter. Thus, in cases where metals are exposed to pure halogen gases large mass losses are usually reported with very little external scale formation. Li and Rapp " showed that internal chloridation occurred when nickel-chromium alloys were exposed to Ni + NiClj powders at 700-900°C. However, where oxide scales can also form, as in combustion gases, the oxide layer was usually highly... 

From among the methods mentioned above, iron has been determined with the use of Chrome Azurol S - in waters [176], Bromopyrogallol Red - in magnetic Fe-Co-Ni films [177], sulphanilic acid - in blood plasma [129] and in plants [109], Tiron - in geological materials [114], in aluminium alloys and copper [115], 2,2 -diquinoxalyl - in niobium oxide [128], PAN - in alloys and biological samples [79] and in waste waters [178], TAN - in geological samples [83], 5-Br-PADAP - in biological samples (by derivative spectrophotometry) [91] and in copper alloys [179], and morin - in copper-chromium and nickel-chromium alloys [122]. 

This type of behaviour has been observed for the nickel-chromium alloy system. In this work, pure nickel, Ni-20 wt% Cr and Ni-30 wt% Cr alloys were exposed to erosion-corrosion at 700 and 800 °C. An erosive stream, loaded at 400 mg min of 20 iJiva alumina particles and flowing at 75 and 125 m s , was used to impact normally on the specimens. Under simple oxidation in air at these temperatures, the alloys both developed protective scales of chromia and showed very low rates of... 

Giggins, C. S. Pettit, F. S. The effect of alloy grain size and surface deformation on the selective oxidation of chromium in nickel-chromium alloy at temperature of 900° and 1100°C. Transactions ofTMS-AIME 245 (1969) 2509. 

A. Atkinson and D.W. Smart, Transport of Nickel and Oxygen during the Oxidation of Nickel and Dilute Nickel/Chromium Alloy, J. Electrochem. Soc., 135 (11), p. 2886 (1988)... 

Molten carbonate fuel cells operate at temperatures around 650 °C and are tolerant to unlimited amounts of carbon monoxide. In most instances mixtures of lithium carbonate and potassium carbonate act as the electrolyte. The electrolyte is suspended in an insulating and chemically inert lithium aluminate ceramic. Nickel or nickel-chromium alloys serve as the anode catalysts, while nickel oxide is used as the cathode catalysts. 

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