Alloys internal oxidation

There are no significant high-temperature applications for alloys of nickel with iron. The scales formed in air consist of nickel oxide and iron oxide and the latter is usually present in the form of the spinel, NiO-FejOj . In the case of the more dilute nickel alloys, internal oxidation of nickel was Observed S. Substitution of a substantial proportion of nickel by iron results in a deterioration in the oxidation resistance of nickel-chromium... 

Silver-base alloys containing less noble solutes, such as In or Al, represent another useful case in this classification. In this case the solubility of oxygen in Ag is significant so that, for dilute alloys, internal oxidation can occur. 

Figure 5.6 Optical micrograph showing the microstructure of a Cu-0.47 wt% Ti alloy internally oxidized at 800 °C for 97 h.

Figure 5.8 Optical micrograph of a Co-5 wt%Ti alloy internally oxidized for 528 hat 900 °C.

Inhibition and passivation Corrosion of amorphous alloys Internal oxidation... 

Oxide Dispersion StrengthenedSuperallojs, Inco Alloys International, Huntington, W. Va., 1990. 

Rapp (1961) has confirmed this equation in a study of the oxidation in air of Ag-In alloys at 550°C. The reaction proceeds with tire internal formation of In203 particles over a range of indium concenuations, but at a critical mole fraction of indium in the alloy, external oxidation occurs with the growdr of a layer of In203 covering the alloy. The n airsitioir from internal to external oxidation was found by Rapp to occur at the mole fraction of indium cone-sponding to... 

Maak (1961) has obtained the equation governing the oxidation rate of a metal to form both an external oxide and an internally oxidized dilute solute, as for example in Cu-Be alloys, coiTesponding to tire equation given earlier... 

The effects on oxidation resistance of copper as a result of adding varying amounts of one or more of aluminium, beryllium, chromium, manganese, silicon, zirconium are described in a number of papers Other authors have investigated the oxidation of copper-zincand copper-nickel alloys , the oxidation of copper and copper-gold alloys in carbon dioxide at 1 000°C and the internal oxidation of various alloys ". ... 

In the case of alloys having one constituent considerably more reactive to oxygen than the others, conditions of temperature, pressure and atmosphere may be selected in which the reactive element is preferentially oxidised. Price and Thomas used this technique to develop films of the oxides of beryllium, aluminium, etc. on silver-base alloys, and thereby to confer improved tarnish resistance on these alloys. If conditions are so selected that the inward diffusion of oxygen is faster than outward diffusion of the reactive element, the oxide will be formed as small dispersed particles beneath the surface of the alloy. The phenomenon is known as internal oxidation and is of quite common occurrence, usually in association with a continuous surface layer of oxides of the major constituents of the alloy. 

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... 

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

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... 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

Figure 5.82 The strength of different dispersion-strengthened alloys relative to the pure metal strength as a function of relative temperature. Preparation techniques include sintered aluminum powder (SAP), internal oxidation, and salt decomposition. Reprinted, by permission, from A. Kelly, Composite Materials, p. 62. Copyright 1966 by American Elsevier, Inc.

Systematically speaking, so-called internal oxidation reactions of alloys (A,B) are extreme cases of morphological instabilities in oxidation. Internal oxidation occurs if oxygen dissolves in the alloy crystal and the (diffusional) transport of atomic oxygen from the gas/crystal surface into the interior of the alloy is faster than the countertransport of the base metal component (B) from the interior towards the surface. In this case, the oxidation product BO does not form a stable oxide layer on the alloy surface. Rather, BO is internally precipitated in the form of small oxide particles. The internal reaction front moves parabolically ( Vo into the alloy. Examples of internal reactions are discussed quantitatively in Chapter 9. 

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