Anodic dissolution corrosion

Statement of the problem. Now let us consider mass transfer from a solid wall to a liquid film at high Peclet numbers. Such a problem is of serious interest in dissolution, crystallization, corrosion, anodic dissolution of metals in some electrochemical processes, etc. In many practical cases, dissolution processes are rather rapid compared with diffusion. Therefore, we assume that the concentration on the plate surface is equal to the constant Cs and the incoming liquid is pure. As previously, we introduce dimensionless variables according to formulas (3.4.5). In this case, the convective mass transfer in the liquid film is described by Eq. (3.4.1), the boundary condition (3.4.2) imposed on the longitudinal variable x, and the following boundary conditions with respect to the transverse coordinate ... 

Since metals have very high conductivities, metal corrosion is usually electrochemical in nature. The tenn electrochemical is meant to imply the presence of an electrode process, i.e. a reaction in which free electrons participate. For metals, electrochemical corrosion can occur by loss of metal atoms tluough anodic dissolution, one of the fiindamental corrosion reactions. As an example, consider a piece of zinc, hereafter referred to as an electrode, inunersed in water. Zinc tends to dissolve in water, setting up a concentration of Zn ions very near the electrode... 

Ores are mined and are then refined in an energy intensive process to produce pure metals, which in turn are combined to make alloys (see Metallurgy Mineral RECOVERY and processing). Corrosion occurs because of the tendency of these refined materials to return to a more thermodynamically stable state (1—4). The key reaction in corrosion is the oxidation or anodic dissolution of the metal to produce metal ions and electrons... 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

It must be emphasised that although, the rate of anodic dissolution of iron increases with,increase in. pH this will not necessarily apply to the corrosion rate which will be dependent On a number of other. factors, e.g. the thermodynamics and kinetics of the cathodic reaction, film formation, etc. 

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

Wilde, B. E. and Teterin, G. A., Anodic Dissolution of Copper-Zinc Alloys in Alkaline Solutions , Brit. Corrosion J., 2, 125 (1967)... 

Langenegger, E. E. and Robinson, F. P. A., Effect of the Polarisation Technique on Dezincihcation Rates and Physical Structure of Dezincihed Zones , Corrosion, 24, 411 (1968) Brooks, W. B., Discussion of the De-alloying Phenomenon , Corrosion, 24, 171 (1968) Pickering, H. W., Volume Diffusion During Anodic Dissolution of a Binary Alloy , J. Electrochem. Soc., 115, 143 (1968)... 

Greene, N. D. and Judd, G., Relation Between Anodic Dissolution and Resistance to Pitting Corrosion , Corrosion, 21, 15 (1965)... 

Hodge, F. G. and Wilde, B. E., Effect of Chloride Ion on the Anodic Dissolution Kinetics of Cr-Ni Binary Alloys in Dilute H2SO4 , Corrosion, 26, 146 (1970)... 

Zamin, M. and Ives, M. B., Effect of Chloride Ion Concentration on the Anodic Dissolution Behaviour of Nickel , Corrosion, 29, 319 (1973)... 

For situations controlled by anodic dissolution of a film P = 1/density of metal, but if the corrosion is controlled by the cathodic reaction P = 1/density of metal x nc Ma/na Me where n and M are the number of electrons and the molecular masses of anodic and cathodic reactants. 

As with most other metals, the anodic behaviour of nickel is influenced by the composition of the solution in which measurements are made, particularly if the solution is acidic. Acidic solutions containing d ions or certain sulphur compounds in particular have a pronounced influence both in increasing the rate of anodic dissolution in the active range and in preventing passivation, and in stimulating localised corrosion . Thiourea and some of its derivatives have a complex effect, acting either as anodic stimulators or inhibitors, depending on their concentration . 

Pitting of nickel and nickel alloys, as of other metals and alloys, occurs when passivity breaks down at local points on the surface exposed to the corrosive environment, at which points anodic dissolution then proceeds whilst the... 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

When stress corrosion involves very localised dissolution, with the geometrical requirements of a crack to be fulfilled, the rate of anodic dissolution may be expressed as a rate of crack propagation... 

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope The reaction path through... 

Fe-HS )jds has been postulated to lead to easier anodic dissolution than that through (Fe-014)3 5. This effect of hydrogen sulphide is thought to be responsible for the acceleration of corrosion of iron observed with some inhibitive sulphur compounds, e.g. thioureas , at low concentrations, since hydrogen sulphide has been identified as a reduction product. However, the effects of hydrogen sulphide are complex, since in the presence of inhibitors such as amines , quaternary ammonium cations , thioureas ", ... 

Fig. 19.15 Schematic representation of range of corrosion potentials expected from various chemical tests for sensitisation in relation to the anodic dissolution kinetics of the matrix (Fe-l8Cr-IONi stainless steel) and grain boundary alloy (assumed to be Fe-lOCr-lONi) owing to depletion of Cr by precipitation of Cr carbides of a sensitised steel in a hot reducing acid (after Cowan and Tedmon )...

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

Considering the similarity between Figs. 1 and 2, the electrode potential E and the anodic dissolution current J in Fig. 2 correspond to the control parameter ft and the physical variable x in Fig. 1, respectively. Then it can be said that the equilibrium solution of J changes the value from J - 0 to J > 0 at the critical pitting potential pit. Therefore the critical pitting potential corresponds to the bifurcation point. From these points of view, corrosion should be classified as one of the nonequilibrium and nonlinear phenomena in complex systems, similar to other phenomena such as chaos. 

Figure 6. Bipolar precipitates consisting of an inner anion-selective layer and an outer cation-selective layer.19 When the electrode is polarized to the more noble side, protons and chloride ions are kept from permeating through the film, so that anodic dissolution of the substrate metal is blocked. (Reproduced from N. Sato, Corrosion, 45 354, 1989, Fig. 24 with permission of NACE International.)...

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