Anodic control

Fig. 1.27 Evans diagrams illustrating (a) cathodic control, (b) anodic control, (c) mixed control, (d) resistance control, (e) how a reaction with a higher thermodynamic tendency ( r, ii) may result in a smaller corrosion rate than one with a lower thermodynamic tendency and (/) how gives no indication of the corrosion rate...

Fisher, A. O., Magnesium Anodes Control Pitting in an Inhibited Circulating Cooling Water System , Mater. Protect., 3, 64 (1964)... 

The corrosion of tin by nitric acid and its inhibition by n-alkylamines has been reportedThe action of perchloric acid on tin has been studied " and sulphuric acid corrosion inhibition by aniline, pyridine and their derivatives as well as sulphones, sulphoxides and sulphides described. Attack of tin by oxalic, citric and tartaric acids was found to be under the anodic control of the Sn salts in solution in oxygen free conditions . In a study of tin contaminated by up to 1200 ppm Sb, it was demonstrated that the modified surface chemistry catalysed the hydrogen evolution reaction in deaerated citric acid solution. 

The primary function of a coating is to act as a barrier which isolates the underlying metal from the environment, and in certain circumstances such as an impervious continuous vitreous enamel on steel, this could be regarded as thermodynamic control. However, whereas a thick bituminous coating will act in the same way as n vitreous enamel, paint coatings are normally permeable to oxygen and water and in the case of an inhibitive primer (red lead, zinc chromate) anodic control will be significant, whilst the converse applies to a zinc-rich primer that will provide cathodic control to the substrate. 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

Even if systems under pure anodic control were to be found in plentiful amounts, the kinetics could not yet be clearly worked out because of the variables resident in the metal itself, hi general, the thermodynamic concept of the solid befog in its standard state has been used in kinetic reasoning, only the surface area being considered as a variable. In other words, the effect of crystal imperfections and the quantitative relationship... 

Conditions Pt anode, controlled current, MeCN (from ref. 59). 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

There are several references reporting on template-assisted approaches for nanofabrication such as Hulteen and Martin. They are regarded as one of the pioneer groups for functional nanowire array fabrication. With the use of a periodic structured template, one-dimensional nanostructures can be prepared thanks to the confinement effect of the porous template. The templates can be prepared easily with anodization. Control of the aspect ratio and the area density of one-dimensional nanostructures can be achieved by changing the diameter and length of the template, and by changing the anodization voltage. 

Conversely, when the anode is highly polarized in comparison to the cathode, the mixed potential lies close to the cathode equihbrium potential and such systems are anodically controlled. This type of corroding system is described in Fig. 3.11b. 

Fig. 3.11 Effect of the polarization curve on the mixed potential of a corroding system (a) cathodically controlled, (b) anodically controlled.

There is an additional constraint on the nature of the mixing, namely that the holetransporting p-type phase be continuous to the cathode, while the electron-transporting n-type phase connect to the anode. Controlling the morphology of such bi-continuous networks is the goal of recent research on self-assembled block copolymers. 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

Being amphoteric, lead is corroded by alkalies at moderate or high rates, depending on aeration, temperature, and concentration. Nevertheless, lead resists corrosion in many environments because the products of corrosion are insoluble and form self-heahng protective films. Because of these protective films, the corrosion rate of lead is usually under anodic control [2]. 

Anode (controls electron energy) Electron beam Electron collector plate... 

Corrosion controlled by the properties of the passive film. The reaction is then under anodic control and the average corrosion rate is often quite small (although locally it may be substantial, see Chapter 7). Stainless steels in aqueous solutions behave in this manner. 

Anodic control protection Increase in anodic overpotential nJi... 

Noble metal coatings provide anodic control protection. They are usually used where corrosion protection and decorative appearance are required. Nickel, chromium, tin, lead, and their alloys are the coating metals that provide anodic protection. 

For example, a low-carbon steel was not found to be susceptible to SCC in a solution in which was 2.8. In this case, repassivation occurred very rapidly. However, the steel was found to be susceptible when that ratio was 26, and it was not susceptible again in a solution in which was 75 [121], In the latter case, widespread pitting occurred instead. Hence, a balance between repassivation and widespread dissolution is required for anodically controlled SCC. Only under intermediate conditions is SCC possible. 

FIG. 12—Type of control of galvanic corrosion reactions (a) cathodic control (b) anodic control (c) mixed control. 

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