Dissolution kinetics reaction order

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. 

Alternatively, if the reactions at the surface are slow in comparison with diffusion or other reaction steps, the dissolution processes are controlled by the processes at the surface. In this case the concentrations of solutes adjacent to the surface will be the same as in the bulk solution. The dissolution kinetics follows a zero-order rate law if the steady state conditions at the surface prevail ... 

The kinetic parameters for an Fe dissolution reaction, according to the BDD mechanism, are transfer coefficient a = 1.5 and a reaction order with respect to OH ions of pOH- an = 1, while the kinetic parameters for an H2 evolution reaction on Fe in acid solutions are OC = 0.5 and pH+cath = 1. Using these data, work with the pH dependence of the corrosion potential and the corrosion current density of Fe in acid solutions. (Gokjovic)... 

The relationship of the stirring rate in these experiments to the rates of hydrolysis reactions of basalt phases is indicative of surface-reaction controlled dissolution (21). First order kinetics are not inconsistent with certain rate-determining surface processes (22). Approximate first order kinetics with respect to dissolved oxygen concentration have been reported for the oxidation of aqueous ferrous iron (23) and sulfide (24), and in oxygen consumption studies with roll-type uranium deposits(25). 

Phosphate has been found to be an extremely strong inhibitor of carbonate reaction kinetics, even at micromolar concentrations. This constituent has been of considerable interest in seawater because of its variability in concentration. It has been observed that phosphate changes the critical undersaturation necessary for the onset of rapid calcite dissolution (e.g., Berner and Morse, 1974), and alters the empirical reaction order by approximately a factor of 6 in going from 0 to 10 mM orthophosphate solutions. Less influence was found on the log of the rate constant. Walter and Burton (1986) observed a smaller influence of phosphate on calcite... 

Dissolution Kinetics. Pseudo first-order reactions are widely employed in the field of soil-water environmental science for evaluating physical, chemical, or biochemical events. A pseudo first-order dissolution example is given below to demonstrate the use of kinetics in identifying or quantifying minerals in simple or complex systems. Consider a metal carbonate solid (MC03s) reacting with a strong acid (HC1) ... 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

Morse (30) carried out an examination of the near-equilibrium dissolution kinetics of calcium carbonate-rich deep sea sediments. His results are summarized in Figure 14. The sediment samples from different ocean basins have distinctly different reaction orders and empirical rate constants. The dissolution rate equations for the different sediment samples are ... 

Dissolution of most primary silicates is pH dependent, as illustrated by the release of Si from anorthite (CaAl2Si2O8) after four successive pH-adjusted, 3-d rinses (Fig. 7-7) (Amrhein and Suarez, 1988). For anorthite there is little or no pH dependence at pH values from 5 to 9. At higher and lower pH values, however, the dissolution rate increases. In a review of feldspar hydrolysis kinetics, Helgeson et al. (1984) utilized the data of Chou and Wollast (1984) for albite and Siegel and Pfannkuch (1984) for microcline to show that for pH < 2.9 the rate of feldspar dissolution approaches first-order with respect to [H At pH values greater than 8, the reaction has... 

In contrast, the study of calcite dissolution kinetics in CaCOj-poor sedi-ments of the equatorial Atlantic, Adler et al. (2001) again favored higher reaction orders. In this sense, the observed dissolution rate constants are highly variable, which seems to be mainly dependent on differences in the physical (e.g. surface area) and chemical properties (high/low Mg-calcite) of the calcite mineral phase. 

For secondary redox reaction comparatively large rate constants (magnitude order 10" to 10 " (L mol yr ) are reported. This confirms that these reactions are fast compared to primary redox reactions and can be assumed to be in local equilibrium. Non-redox precipitation/dissolution reaction result from literature data as slow kinetic reactions with rate constants ranging from 10 to 10 " [1 moF yr ]. 

The propagation of crevice corrosion, cannot he sustained at rates on the order of A cm for long periods of time. Like all corrosion, the anodic reaction, cathodic reaction, or solution properties can control the rate. For example, if the dissolution kinetics in the occluded solution are sluggish, the polarization by the cathodic reaction has a more limited effect. On the other hand, diffusion... 

As in /l, the last term in Eq. (47) vanishes and c,Fe = 1 - h. From the measurements of hydrogen evolution kinetics in the same system, the transfer coefficient an was fotind to be 0.51. Hence, ac,Pe = 0.49 and the cathodic Tafel line would have a slope of 2RT/F (shown in Figure 10 in the form of the corrected Tafel plot), if the pH in the vicinity of the electrode were constant with changing current density. Considering this result, as well as the reaction order of 1 for OH ions, mechanism (38) is indicated for the deposition and dissolution of iron, with FeOH as the electroactive species and its discharge as the rds. 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

Tafel slope and a decrease in the reaction order with respect to OH have been observed and mark the start of processes specific to the transition range of the overall active range of iron dissolution among them, the formation of crystallized ferrous and ferric solid species including anions and their blocking effect on the metal dissolution superimpose and change the mechanism and the kinetics. 

The next iterative analysis of the influence of adsorbed hydrogen on the iron dissolution kinetics included the possible correlation between the surface structure and the reactions occurring at specific sites, as revealed by experimental and theoretical studies by Allgaier and Heusler, " Lorenz and co-workers, and Keddam andco-workers. " All these authors agreed that the dissolution rate constant should be proportional to the weakness of the binding of the surface atoms to the bulk metal, thus decreasing in the order kink > step > plane. The rate of metal dissolution is proportional to the rate constant and to the number of atoms in the position concerned, which decreases in the order plane > step > kink. 

The rate order is given by a charge balance of protons at the surface this result is, however, not observed for complicated silicate minerals. There are many potential reasons for this, including the selective leaching of cations from the surface [e.g., 41, 45, 46], self-poisoning of the reaction [e.g., 47], or migration of protons deep into the mineral. We yet have virtually no fundamental understanding of the dissolution kinetics of complicated mixed oxide and aluminosilicate polymerized structures. 

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