Corrosion: kinetics types

In the simplest case where the oxidation reaction of a semiconductor material (42a) proceeds exclusively through the valence band and the reaction of reduction of the Ox component of the solution exclusively through the conduction band (see Fig. 13a), corrosion kinetics is limited by minority carriers for either type of conductivity. In fact, it can be seen from Fig. 12 that icorr(p) = i"m(p) and 1 ( ) = ipim(n), where ijj are the limiting currents of minority carriers (symbols in parentheses denote the type of conductivity of a sample under corrosion). Since the corrosion rate is limited by the supply of minority carriers to the interface, it appears to be rather low in darkness. The values of [Pg.283]

Relationships of other type are observed in the case where both the conjugated reactions proceed through the same band (Fig. 13b). For example, the cathodic reaction (42b) can take place with the participation of valence electrons rather than conduction electrons, as was assumed above. Thus, reduction of an oxidizer leads to the injection of holes into the semiconductor, which are used then in the anodic reaction of semiconductor oxidation. In other words, the cathodic partial reaction provides the anodic partial reaction with free carriers of an appropriate type, so that in this case corrosion kinetics is not limited by the supply of holes from the bulk of a semiconductor to its surface. Here the conjugated reactions are in no way independent ones. 

The coexistence of the above-mentioned four factors is a necessary but not sufficient condition for the development of the corrosion phenomenon. There are a number of resistances, in a generalized sense, of kinetic type that hinder the corrosive phenomenon and by means of which it is possible to act in order to prevent corrosion. 

Immersion Test—Immersion tests involve placing coupons in deep melts. This type of test is usually used for deep melts. Some investigators have attempted to use electrochemical techniques to measure corrosion kinetics. These techniques will be discussed in the following section on deep melts. The results from immersion tests must be used... 

Tran et al. studied the kinetics of carbonation of molten alkali hydroxides [228], the air flow pattern [229], and the corrosion of Type 304L stainless steel by molten hydroxides at air port openings [230], The presence of oxygen was found to make the corrosion inevitable in the presence of molten hydroxides. Electrochemical testing showed that a galvanic cell is set up between carbon steel and stainless steel with the latter protecting the former. 

The discussion of Section 17.2 treated the corrosion of metallic materials in terms of electrochemical reactions that take place in aqueous solutions. In addition, oxidation of metal alloys is also possible in gaseous atmospheres, normally air, in which an oxide layer or scale forms on the surface of the metal. This phenomenon is frequently termed scaling, tarnishing, or dry corrosion. In this section, we discuss possible mechanisms for this type of corrosion, the types of oxide layers that can form, and the kinetics of oxide formation. 

High temperature (Type 1) hot corrosion (HTHC) is normally observed in the temperature range of about 825-950°C when the condensed phase is clearly liquid. The typical microstructure for HTHC shows the formation of sulphides and a corresponding depletion of the reactive components in the alloy substrate. The external corrosion products consist of oxide precipitates dispersed in the salt film. The presence of the pore, crevice or crack across a protective film can lead to the sulphidation of the alloy substrate. This results in a significant shift in the basicity of the salt film. Once the fused salt contacts the alloy substrate, the rate and duration of the rapid corrosion kinetics are decided by the magnitude and gradient of salt basicity relative to the local solubilities for the oxide scale phases (Rapp and Zhang, 1994). 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Multiinformational Prohes Corrosion probes can provide more information than just corrosion rate. The next three types of probes yield information about the type of corrosion, the kinetics of the corrosion reaction, as well as the local corrosion rate. 

Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 g. or less) is lower than that of a venturi scrubber. Spray towers can handle larger gas flows with minimal pressure drop and are therefore often used as precoolers. Because wet scrubbers may contribute to corrosion, removal of water from the effluent gas of the scrubbers may be necessary. 

Kassner used a rotating disc, for which the hydrodynamic conditions are well defined, to study the dissolution kinetics of Type 304 stainless steel in liquid Bi-Sn eutectic. He established a temperature and velocity dependence of the dissolution rate that was consistent with liquid diffusion control with a transition to reaction control at 860 C when the speed of the disc was increased. The rotating disc technique has also been used to investigate the corrosion stability of both alloy and stainless steels in molten iron sulphide and a copper/65% calcium melt at 1220 C . The dissolution rate of the steels tested was two orders of magnitude higher in the molten sulphide than in the metal melt. 

Streicher s work indicates how useful the potentiostat has been in studying intergranular corrosion. Ideally, future data would be expanded to provide Pourbaix-type diagrams that also contain kinetic information showing various rates of attack within the general domain of intergranular corrosion. (Similar data for cases other than intergranular attack would be equally valuable.)... 

The first step is the evaluation of thermodynamic and kinetic data by quantitative energy calculations and qualitative considerations as discussed in Chapter 2. The results may provide a satisfactory answer as to whether the reaction can be performed in the open laboratory or requires a high-pressure cell arrangement on the small scale. Further evaluations are required for scale-up. Toxicity, corrosivity, type of apparatus, size, and other criteria must also be considered. 

Mixer-settlers have been the more common type of equipment and, with the development of hydrometallurgy over the past 20 years, designs have improved considerably. To select the appropriate equipment, a clear understanding of the chemical and physical aspects of the process is required. Also the economics must be considered relative to the type of equipment to suit particular conditions of given throughput, solution and solvent type, kinetics and equilibrium, dispersion and coalescence, solvent losses, number of stages, available areas, and corrosion. 

Pourbaix diagrams (Pourbaix, 1963) indicate graphically the conditions of redox potential (Eh) and pH under which different types of corrosion behaviour may be expected. These plots of potential vs. pH indicate the phase and species in equilibrium with iron under various conditions (see Chap. 8). The solid phases indicated are those that are thermodynamically the most stable owing to kinetic factors other phases may be present during the initial stages of the corrosion process. What the different regions show, however, are the predominant oxidation states to be expected. 

Corrosion is the deterioration of a material by reaction with its enviromnent. Although the term is used primarily in conjunction with the deterioration of metals, the broader definition allows it to be used in conjunction with all types of materials. We will limit the description to corrosion of metals and alloys for the moment and will save the degradation of other types of materials, such as polymers, for a later section. In this section, we will see how corrosion is perhaps the clearest example of the battle between thermodynamics and kinetics for determining the likelihood of a given reaction occurring within a specified time period. We will also see how important this process is from an industrial standpoint. For example, a 1995 study showed that metallic corrosion costs the U.S. economy about 300 billion each year and that 30% of this cost could be prevented by using modem corrosion control techniques [9], It is important to understand the mechanisms of corrosion before we can attempt to control it. 

For our purposes, the classification system is not as important as (a) recognizing that many different forms of corrosion exist and (b) understanding the fundamental kinetic processes behind these different types. To this end, we will first look at the important principles of corrosion and then see how they can be applied to some important types of corrosion. 

While thick film passivity has been documented and understood for many years, the difficulties in studying thin film passivity were daunting. It took many years to determine that indeed a film was responsible for the effect, as these films are so thin that they are invisible to the eye (i.e., transparent to radiation in the visible region). Two main types of theories were developed in order to explain the phenomena observed theories based upon the idea of adsorption reducing the corrosion rate, and theories based upon the formation of a new phase, an oxide of the base metal, on the surface. In all cases, an increased barrier to dissolution results upon the increase in potential. This increased kinetic barrier upon anodic polarization contrasts with the exponentially decreased barrier which develops during anodic polarization of an active material. 

In several cases it has been found that the oxidation of the redox system occurs entirely via hole transfer directly from the valence band to the reduced form of the couple. Then both processes, oxidation of the redox system and corrosion, proceed independently. This is usually not visible from measurements with an n-type electrode, because the photocurrent is entirely determined by the light intensity. As already mentioned above, p-type electrodes are more suitable, because the current is determined by majority carrier transfer (reaction rate Vf, in Fig. 21). From the thermodynamic point of view, the oxidation of Cu at GaAs is an interesting case. The corresponding current-potential curves are given in Fig. 22 [93]. The corrosion current is not changed upon addition of Cu, i.e. corrosion and redox process are completely independent. In this case, the kinetics of the direct hole transfer is obviously very fast, i.e. the redox current is considerably larger than the corrosion current. Both processes occur indepen-... 

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