Oxidation-affected erosion

A low temperature imparts a metal erosion regime. With increase of temperature this metal erosion regime shifts to an oxide erosion regime via oxidation-affected erosion and oxidation-controlled erosion regimes. 

Schematic presentation of various erosion mechanisms at elevated temperature (a) metal erosion (b) oxide erosion (c) oxidation-affected erosion (d) oxidation-controlled erosion. 

The presence of an oxide scale is responsible for the existence of a variety of material removal mechanisms during elevated temperature erosion of metallic materials. These mechanisms have been carefully verified by experimental work. Four distinct material removal mechanisms, namely metal erosion, oxidation-affected erosion, oxidation-controlled erosion and oxide erosion, have been experimentally established. These experimental observations have been substantiated by theoretical considerations. A suitable criterion for transition from one erosion regime to another regime is yet to evolve. Attempts have been made to analyse such transitions using preoxidized sanities. 

The erosion of graphite in nozzle appHcations is a result of both chemical and mechanical factors. Changes in temperature, pressure, or fuel-oxidizing ratio markedly affect erosion rates. Graphite properties affecting its resistance to erosion include density, porosity, and pore size distribution... 

Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

Dishing can be also reduced by adding oxide studs in the two-dimensional wells (see Fig. 25). Since the two-dimensional well structures are usually probe pads for electrical measurements, the addition of oxide studs will not affect the pad function as long as the test probe can still touch metal materials. Although this method can reduce dishing, local oxide erosion may still exist. The mechanism of local oxide erosion due to dishing is still unclear at this point of time [26]. 

Oxygen Atom Recombination. Formation of molecular oxygen from the recombination of oxygen atoms on a spacecraft surface has been proposed to occur. The extent to which this recombination will affect the erosion and/or oxidation of a surface is only speculative at this time. 

In arid and semiarid regions, water restricts the production and decay of biomass. Consequently, the turnover of SOM is low but erosion prevents SOM accumulation in these regions. In boreal regions cold winters and hot and dry summers restrict biomass production and decay. Production of biomass is slow but decay is even slower and hence slow accumulation of SOM occurs in upper soil horizons. Radiation directly affects the decay of biomass, forming oxygen radicals from water. This mechanism has been found to be an important factor in the oxidation of DOM in northern peatlands (Bertilsson ef a/., 1999). 

In Regime 1, the oxide scale is not removed completely by erosion but is only thinned down and the erosive impacts do not affect the metal substrate. Consequently the surface of the specimen remains flat, but contains particle cutting features as shown in Figure 9.9. In Regime 2, the scale is removed and the metal substrate undergoes plastic deformation by the impact of the erosive particles. The plastic deformation of the metal surface results in the formation of surface features... 

In Equation (9.1), X is the instantaneous scale thickness, t is time, k is the scaling constant (cm s ) and eo is the erosion constant for the oxide (cm s ). This relationship is valid, so long as the erosive impacts affect only the outer surface of the scale. The scale will continue to grow, so long as the term > fceo- However, as the scale grows it reaches a critical thickness, X, given by Equation (9.2),... 

A special example is described by Fontana and Greene [7.1] The erosion corrosion rate on carbon steel in water may be higher at pH = 7-9 than at pH = 6 as well as at pH =10. The reason is assumed to be that the oxide layer at pH =7-9 consists of granular Fc304, which is less erosion resistant than the Fe(OH)2 or Fe(OH)3 dominating at the lower and higher pH, respectively. Increased content of oxidizers may also decrease or increase erosion corrosion rate depending on whether passivation is obtained or not. The temperature is another important factor. It may affect the properties of the deposits in addition to the shear stress and mass transport (see next section and Section 6.2.2). 

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