Corrosion double layers

For many practically relevant material/environment combinations, thennodynamic stability is not provided, since E > E. Hence, a key consideration is how fast the corrosion reaction proceeds. As for other electrochemical reactions, a variety of factors can influence the rate detennining step. In the most straightforward case the reaction is activation energy controlled i.e. the ion transfer tlrrough the surface Helmholtz double layer involving migration and the adjustment of the hydration sphere to electron uptake or donation is rate detennining. The transition state is... 

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

Radioactivation Techniques Neutron and thin layer (TLA) activation are non-intrusive techniques ofi ering the prospect of continuous, direct component monitoring, in addition to coupon or probe, monitoring. In principle, localised corrosion can be monitored using a double-layer technique. Process plant applications of the technique have been limited to date. ... 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

The potential difference across the electric double layer A. This cannot be determined in absolute terms but must be defined with reference to another charged interface, i.e. a reference electrode. In the case of a corroding metal the potential is the corrosion potential which arises from the mutual polarisation of the anodic and cathodic reactions constituting the overall corrosion reaction see Section 1.4). 

The capacitance. The electrical double layer may be regarded as a resistance and capacitance in parallel see Section 20.1), and measurements of the electrical impedance by the imposition of an alternating potential of known frequency can provide information on the nature of a surface. Electrochemical impedance spectroscopy is now well established as a powerful technique for investigating electrochemical and corrosion systems. 

The potential for electrochemical corrosion in a boiler results from an inherent thermodynamic instability, with the most common corrosion processes occurring at the boiler metal surface and the metal-BW interface (Helmholtz double layer). These processes may be controlled relatively easily in smaller and simpler design boilers (such as dual-temperature, LPHW heating, and LP steam boiler systems) by the use of various anodic inhibitors. 

The system developed by O Grady is reproduced in Fig. 9. A key element of this arrangement is the electrochemical thin layer cell, using a combined Pd-hydrogen reference and counter electrode, thus minimizing the amount of electrolyte necessary for the electrochemical treatment. This type of cell is particularly useful for double layer studies but cannot be used for gas evolution or corrosion experiments at higher current densities. For a collection and discussion of other transfer systems the reader is referred to the review article by Sherwood [43]. 

Thus, it is interesting to note that high-purity aluminum rests at a potential at which corrosion is at its minimum and is, indeed, relatively very small. It is also largely independent of the anions present in the electrolyte.69 This may be attributed to the coulombic repulsion of anions away from the surface by the negative charge on the metal. The latter seems not to be completely compensated in a thin oxide film, as shown schematically in Fig. 9, so that the solution side of the double layer formed at the O/S interface contains excess cations, anions being repelled. The anions could approach the O/S interface either at thicker films or at potentials more positive than the OCP. 

The rate at which corrosion occurs is expressed as the current density (A m" ), i.e. the ionic flux across the electrical double layer of the metal and at equilibrium, it is termed the exchange current density. The Tafel equation relates the exchange current density to the charge transfer overpotential. 

Figure 3.18 Formation of the electrical double layer of a surface in solution, showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP). Reprinted, by permission, from B. D. Craig, Fundamental Aspects of Corrosion Films in Corrosion Science, p. 4. Copyright 1991 by Plenum Press.

Corrosion inhibitors are solutes that blanket the electrochemically active surfaces of the corrosion-prone metal and suppress corrosion either by physically blocking the flow of ions or molecules to or from these surfaces or by altering the electrical double layer at the metal surface in such a... 

A different approach to blanketing anodic areas involves dissolving organic anions with appropriate hydrophobic (i.e., water-repellent) substituents. The anionic heads of the molecules specifically seek out a positively charged anodic spot, and the hydrophobic tails serve to isolate it from the aqueous solution and so block the ionic part of the corrosion circuit or, at least, modify the electrical double layer (Fig. 16.13). Sodium benzoate and especially sodium cinnamate are particularly effective in this regard. 

The previous section discussed the structure at the junction of two phases, the one a solid electron conductor, the other an ionic solution. Why is this important Knowledge of the structure of the interface, the distribution of particles in this region, and the variation of the electric potential in the double layer, permits one to control reactions occurring in this region. Control of these reactions is important because they are the foundation stones of important mechanisms linked to the understanding of industrial processes and problems, such as deposition and dissolution of metals, corrosion, electrocatalysis, film formation, and electro-organic synthesis. 

Current and potential distributions are affected by the geometry of the system and by mass transfer, both of which have been discussed. They are also affected by the electrode kinetics, which will tend to make the current distribution uniform, if it is not so already. Finally, in solutions with a finite resistance, there is an ohmic potential drop (the iR drop) which we minimise by addition of an excess of inert electrolyte. The electrolyte also concentrates the potential difference between the electrode and the solution in the Helmholtz layer, which is important for electrode kinetic studies. Nevertheless, it is not always possible to increase the solution conductivity sufficiently, for example in corrosion studies. It is therefore useful to know how much electrolyte is necessary to be excess and how the double layer affects the electrode kinetics. Additionally, in non-steady-state techniques, the instantaneous current can be large, causing the iR term to be significant. An excellent overview of the problem may be found in Newman s monograph [87]. 

Z. Nagy and R. F. Hawkins, J. Electrochem. Soc. 138 1047 (1991). Analysis of the correction of the corrosion measurement kinetics for double-layer effects. 

A single layer of bright nickel of thickness 5 to 10 //m will supply good durability in less corrosive environments but for more exacting conditions thicknesses of 15 to 20 /urn are necessary. An example of the latter is exterior trim for motor vehicles, where specifications demand a double-layer system of nickel. 

The response to the applied perturbation, which is generally sinusoidal, can differ in phase and amplitude from the applied signal. Measurement of the phase difference and the amplitude (i.e. the impedance) permits analysis of the electrode process in relation to contributions from diffusion, kinetics, double layer, coupled homogeneous reactions, etc. There are important applications in studies of corrosion, membranes, ionic solids, solid electrolytes, conducting polymers, and liquid/liquid interfaces. 

Figure 3 Electrical equivalent circuit model commonly used to represent an electrochemical interface undergoing corrosion. Rp is the polarization resistance, Cd] is the double layer capacitance, Rct is the charge transfer resistance in the absence of mass transport and reaction intermediates, RD is the diffusional resistance, and Rs is the solution resistance, (a) Rp = Rct when there are no mass transport limitations and electrochemical reactions involve no absorbed intermediates and nearly instantaneous charge transfer control prevails, (b) Rp = Rd + Rct in the case of mass transport limitations.

Consider the application of a small sinusoidal potential ( AE sin cut) on a corroding sample, which results in a signal along with the current flow of harmonics 2cq, 3co, etc. Then the impedance A/ sin ( of + r/j) is the relation between AE/ Al and phase (j). In the case of corrosion studies, the sample is made part of a system known as equivalent circuit,24 which consists of the solution resistance Rs, charge transfer resistance Rqt and the capacitance of the double layer Cdi- The measured impedance plot appears in the form of... 

---