Scale-forming oxidation

Metal Cleaning. About 204 thousand metric tons of HCl (100% basis) was consumed in 1993 for steel pickling, wherein the hydrochloric acid readily dissolves all of the various oxides present in the scale formed during the hot rolling process. Using suitable inhibitors such as alkyl pyridines, HCl reacts very slowly with the base metal rendering the surface so clean that it must be passivated with a mild alkaline rinse. 

Cobalt cannot be classified as an oxidation-resistant metal. Scaling and oxidation rates of unalloyed cobalt in air are 25 times those of nickel. The oxidation resistance of Co has been compared with that of Zr, Ti, Fe, and Be. Cobalt in the hexagonal form (cold-worked specimens) oxidizes more rapidly than in the cubic form (annealed specimens) (3). 

The scale formed on unalloyed cobalt during exposure to air or oxygen at high temperature is double-layered. In the range of 300 to 900°C, the scale consists of a thin layer of the mixed cobalt oxide [1308-06-17, Co O, on the outside and a cobalt(Il) oxide [1307-96-6] CoO, layer next to the metal. Cobalt(Ill) oxide [1308-04-9] maybe formed at temperatures below 300°C. Above 900°C, Co O decomposes and both layers, although of... 

Actually, in many cases strength and mechanical properties become of secondaiy importance in process applications, compared with resistance to the corrosive surroundings. All common heat-resistant alloys form oxides when exposed to hot oxidizing environments. Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film. Thus, mild steel is seldom used above 480°C (900°F) because of excessive scaling rates. Higher temperatures require chromium (see Fig. 28-25). Thus, type 502 steel, with 4 to 6 percent Cr, is acceptable to 620°C (I,I50°F). A 9 to 12 percent Cr steel will handle 730°C (I,350°F) 14 to 18 percent Cr extends the limit to 800°C (I,500°F) and 27 percent Cr to I,I00°C (2,000°F). 

Mill Scale—an oxide layer on metals produced by metal rolling, hot forming, welding or heat treatment. 

Fig. 3.35 Oxidation rate at 535°C of I8Ni250 maraging steel compared with a generally-available tool steel. These tests were performed on 7 in (6-35 mm) cubes placed in refractory cubicles and exposed to still air for total times of 5, 25 and 100 h. The weight gain includes the scale formed during heating and cooling (after Reference 9)...

Generally, the most important reaction is that of tantalum with oxygen, since it tends to form oxides when heated in air. Reaction starts above 300°C and becomes rapid above 600°C . The scale is not adherent, and if the oxidised material is heated above 1000°C oxygen will diffuse into the bulk of the material and embrittle it. At 1200°C catastrophic oxidation attack takes place at a rate of about 150 mm/h Oxygen is not driven off by heating alone, but in vacuum above 2300°C it is removed as a suboxide. The first step of the conversion mechanism of tantalum into oxide was shown to occur by the nucleation and growth of small plates along the 100) planes of the BCC metaP. ... 

For iron in most oxidising environments, the PBR is approximately 2.2 and the scale formed is protective. The oxidation reaction forms a compact, adherent scale, the inner and outer surfaces of which are in thermodynamic equilibrium with the metal substrate and the environment respectively, and ion mobility through the scale is diffusion controlled. 

In 1929 Pfeil" published a most interesting account of the way layered structures form and the manner in which they influence oxidation rates. From detailed studies of the growth and composition of scales he was able to show clearly how the formation of barrier layers reduced scale formation by hindering outward diffusion of iron through the scale. Naturally, this work had to be largely based on the study of scales of sufficient thickness so that the mechanism of the early stages of oxidation could not be studied in this way. Pfeil analysed the outer, middle and inner layers of scales formed... 

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... 

Fig. 7.38 Diagram. showing the type of oxide scales formed as continuous layers upon a number of superalloys at temperatures of about 1 100°C (after Pettit and Meier...

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... 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

When mild steel is heated in air at between 575 and 1 370°C an oxide or scale forms on the steel surface. This scale consists of three well-defined layers, whose thickness and composition depend on the duration and temperature of heating. In general, the layers, from the steel base outwards, comprise a thick layer of wiistite, the composition of which approximates to the formula FeO, a layer of magnetite (FejOJ, and a thin layer of haematite (FejO,). 

A rapid and clean oxidation of sulphides to sulphoxides can also be carried out using the titanium(III) trichloride/hydrogen peroxide reagent35. On a milimole scale, the oxidation takes place in a time shorter than 20 min upon addition of a solution of hydrogen peroxide to a solution of the sulphide and titanium(III) trichloride in methanol at room temperature. It was suggested that the formation of a sulphoxide in this reaction resulted from a direct coupling of the hydroxy radical with cation radical 20 formed at the sulphur atom of the sulphide (equation 6). 

Acid treatment is used to remove rust, scale, and oxides from the base and may be carried out in the form of acid cleaning, pickling, or etching. Each option involves a slightly stronger acid solution. Generally, sulfuric acid is used for this treatment, although other acids may be applied. 

Observations of the same clay sample in a very finer scale (500 nm) by TEM, may help to identify the potential Fe-oxyhydroxide surfaces attached on a sediment grain (Fig.6). Moreover, abundances of wide spread oxides that may have formed oxide minerals after binding with other elements such as Si, Fe and Al can easily be recognized from the right part of the TEM image (Fig. 7). 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

Methylbenzenes lose a proton from a methyl group to form a benzyl radical. In aqueous M-percbloric acid this reaction is fast with a rate constant in the range 10 lO s and the process is not reversible [24]. The process becomes slower as the number of methyl substituents increases, Hexaethylbenzene radical cation is relatively stable. When the benzyl radical is formed, further reactions lead to the development of a complex esr spectrum. Anodic oxidation of hexamethylbenzene in trifluoroacetic acid at concentrations greater than 1 O M yields the radical-cation I by the process shown in Scheme 6.1 [14], Preparative scale, anodic oxidation of methylbenzenes leads to the benzyl carbonium ion by oxidation of the benzyl radicals formed from the substrate radical-cation. Products isolated result from further reactions of this carbonium ion. 

Preparative scale electrochemical oxidation of phenothiazine in aqueous acetonitrile, with no added acid, leads to the radical formed by proton loss from the radical-cation. The radical dimerizes and further oxidation leads to the green qui-nonoid cation 67 [230]. 

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