Anodic reactions, inhibitors

The polarization characteristic of a corroding metal can be controlled by various additives to the solution, called corrosion inhibitors, which adsorb on the metal and lower the rates of the cathodic and/or anodic reaction. Inhibitors are used primarily for acidic electrolyte solutions, sometimes also for neutral solutions. Various organic compounds with -OH, -SH, -NHj, -COOH, and so on, as the functional groups are used as inhibitors. The effects of an organic inhibitor, tetradecylpiperidinium... 

Nitrogenous organic components such as toluidine, quinoline, aniline, etc. all act as inhibitors to the anodic reaction between metal and acid and thereby favour the cathodic reaction and accelerate the process. 

Anodic or cathodic inhibitors This classification is based on whether the inhibitor causes increased polarisation of the anodic reaction (metal dissolution) or of the cathodic reaction, i.e. oxygen reduction (near-neutral solutions) or hydrogen discharge (acid solutions). 

Anodic inhibitors limit the oxidation of iron by sharing the lone pair electrons on the nitrogen with a metal ion or atom and supressing the anodic reaction. Examples are benzotriazole (at high concentrations), pyridines, thiols, and quinolines. 

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In addition, they are anodic inhibitors, depositing iron metaphosphate, and forming y-Fe2C>3, thus stifling the anodic reaction. High and low pH or high temperatures will hasten the reversion (hydrolysis) of polyphosphate to orthophosphate. 

In the mechanism of inhibitor action so far considered, it has been assumed that monolayer adsorption is the position in which the organics reduce the rate of the cathodic or anodic reaction, and indeed in Figs. 12.42 and 12.43, two positions of octynol, lying down and standing up, are shown. Time-resolved automatic ellip-... 

Depending upon whether the cathodic reaction or the anodic reaction is suppressed by the added inhibitor, inhibitors have been further classified as follows ... 

Corrosion inhibitors are commonly used to prevent corrosion. There are many hundreds of different inhibitors in commercial use. Some act by slowing the cathodic reaction and others inhibit the anodic reaction. Some are ionic and some are neutral. In choosing a suitable corrosion... 

Further oxidation results in the formation of hydrated ferric oxide or Fe(ni) hydroxide, i.e. rust. The corrosion potential (Ec) and corrosion current (/c) for the cathodic and anodic reaction can be represented by an Evans-type polarisation diagram, Eig. 6.6. Corrosion inhibitors interfere with the anodic or the cathodic partial reaction, or with both, resulting in a reduction in the corrosion current. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The difference between the anodic external current and the independently determined anodic partial current (dissolved Fe) is the cathodic partial current density. The results as obtained by Hoar and Holiday are shown in Fig.6. The dashed curve represents the external polarization behavior in the absence of inhibitor and the black lines are the Tafel slopes for the anodic partial current density (the metal dissolution) for different inhibitor concentrations. The cathodic partial current density (hydrogen eveolution) is found for all values of the inhibitor concentrations in the shaded area. Therefore, it is obvious that the inhibitor in this case acts exclusively by reducing the anodic reaction rate but not the cathodic one. 

Anodic Inhibitors Inhibitors that directly affect the anodic reaction, that is, the metal dissolution process, are termed anodic inhibitors. Addition of an anodic inhibitor to the corrosion system (dashed and dashed-dotted lines in Fig. 1) can either lower the rate (i.e. the exchange current density) of the anodic process... 

Some substances inhibit corrosion by reducing simultaneously the rate of the anodic and cathodic reactions involved in the corrosion process and are therefore called mixed inhibitors. Mixed inhibition not only requires that both of the electrochemical reactions are influenced by the inhibitor, which indeed is often the case, but also that the corrosion rate is actually limited by anodic as well as cathodic reactions. As an example, again a diffusion-limited cathodic reaction may be considered, in which inhibition may rely solely on the reduction of the cathodic reaction rate (see Fig. 2b, curve III), even if the anodic reaction is also affected by the inhibitor (see Fig. lb). In this case, the inhibitor effectively is cathodic in its action. Furthermore, it is noted that a substance may also inhibit one partial reaction, but accelerate the other. 

Inhibiting properties Because the barrier properties are often insufficient for corrosion protechon, corrosion inhibitors are often used. Inhibitors can act by limihng the cathodic or the anodic reaction, or both. They are typically added to the primer, where they are close to the metal surface. This subject was treated in Chapter 5.2. 

In the absence of solid corrosion products, the corrosion rate can be Urnited by reducing the availability of the cathodic reactant, such as in the removal of oxygen from the water for boilers substituting for the anodic reaction an external current, as in the case of impressed current cathodic protection or inducing artificially the formation of surface films, as in the case of the application of corrosion inhibitors. 

Forming a resistance in movement or diffusion of ions to or from the metal surface, thus decreasing the rates of either the cathodic or anodic reactions. Hence, we can classify inhibitors as either anodic or cathodic inhibitors, in other words, according to the way they act on the corrosion reactions in the corroding system. 

Electrochemical inhibitors retard or prevent the anodic and/or cathodic partial reactions (i.e they influence the reaction at the metal/corrosive medium interface). Chemical inhibitors can react both with the material and form protective coatings and with the medium itself or its constituents and thus diminish its aggressiveness. Physical inhibitors form adsorption layers on the metal surface, which block the corrosion reaction. Inhibitors that influence the electrochemical electrode reactions are subdivided according to their mode of action and site of action in the area of the metal/ medium phase boundary, with the subdivision being between interface inhibitors, electrolyte film inhibitors, membrane inhibitors, and passivators. 

Small organic molecule inhibitors often function by an adsorption mechanism, the extent of adsorption being dependent on a number of variables including the potential of the metal relative to its potential of zero charge (pzc) (for oxide surfaces pzc is dependent on pH value), the structure and the charge (or polarizability) of the inhibitor (adsorbate), the structure of the metal (oxide) surface, and the presence of other species in the electrolyte [22]. Such inhibitors are often of the mixed type, reducing the rate of both cathodic and anodic reactions [22]. It is interesting to note that aniline [24], pyrrole [25], thiophene [26], and the functionalized forms of these molecules exhibit the ability to inhibit... 

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