Inhibition, corrosion anodic dissolution

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. 

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 ", ... 

In acidic solutions, the corrosion rate is relatively high. Studies on cadmium monocrystals and polycrystals in acidic chloride solutions revealed anodic dissolution independent of the crystallographic orientation the dissolution rate was controlled by the mass transport of CdCl" ions [331]. The inhibitive influence of adsorbed organic substances, for example, alcohols [332], phenotiazine [333], and some polymers (e.g. poly (vinyl alcohol), poly(acrylic acid), sodium polyacrylate. 

Most metals are covered with thin layer of oxide film which inhibits anodic dissolution. When corrosion does occur, it sometimes hollows out a narrow hole, or pit, in the metal. The bottoms of these pits tend to be deprived of oxygen, thus promoting further growth of the pit into the metal. Many oxides have semicon-ductive properties and thus do not interfere with the flow of electrons to O2. 

Low corrosion rates in NaOH, KOH and NH4OH solutions over wide concentration and temperature ranges anodic dissolution in 2CM-0 wt % NaOH results in hydrogen absorption leading to hydrogen embrittlement hydrogen absorption can be inhibited by adding nitrate or chlorate... 

Corrosion inhibitor - corrosion inhibitors are chemicals which are added to the electrolyte or a gas phase (gas phase inhibitors) which slow down the - kinetics of the corrosion process. Both partial reactions of the corrosion process may be inhibited, the anodic metal dissolution and/or the cathodic reduction of a redox-system [i]. In many cases organic chemicals or compounds after their reaction in solution are adsorbed at the metal surface and block the reactive centers. They may also form layers with metal cations, thus growing a protective film at the surface like anodic oxide films in case of passivity. Benzo-triazole is an example for the inhibition of copper cor-... 

In the case where the anodic dissolution is inhibited, e.g., by surface adsorption of a chemical species, the anodic curve becomes 2. This will result in a more positive corrosion potential (from E°o to if the cathodic reaction remains unchanged. In such a situation the corrosion current is reduced with a more positive potential relative to the original value. On the other hand, if the anodic dissolution kinetics remains unchanged but the rate of the cathodic reactions is changed from curve Ic to curve 2, the potential also becomes more positive (from 2orrto Ecorr). However, in this case the corrosion current is increased with a more positive potential. 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

It is worthwhile to mention that both the passivating film and the precipitate film, which are formed in the presence of foreign ions and molecules, usually inhibit not only the anodic metal dissolution but also the cathodic reaction of corroding metals. There are however some inhibitors, which are effective only to one of the anodic and the cathodic reactions of metallic corrosion. In the case of porous precipitate films loosely attached to the metal surface, the anodic metal dissolution may be accelerated at porous sites of the precipitate films. For instance, Zn2+ ions, Al3+ ions, Co2+ ions, and Ce3+ ions, which are hard or slightly hard Lewis acid, combine with hydroxide ions of hard base forming a porous precipitate film of metal hydroxide on metallic iron in neutral solution. The porous precipitate film thus formed effectively inhibits the cathodic oxygen reduction, but it may accelerate the anodic dissolution of metallic iron at the porous sites of precipitates [86],... 

It is generally assumed that ions which can accelerate either or both partial reactions in a corrosion process are capable of being adsorbed on the iron surface. Thus it is known that hydrogen sulfide ions which accelerate both partial reactions of acid corrosion (although predominantly the anodic one), and formic acid molecules which catalyze the cathodic partial reaction but inhibit the anodic one, as well as commercial inhibitors which reduce both partial reactions, are in fact adsorbed on the iron surface. As a consequence the mere fact that adsorption takes place cannot be used to predict an expected change in corrosion rate as it is also known that halide ions cat-alize the anodic dissolution of indium, while hydroxyl adsorption catalyzes the anodic dissolution of iron. Furthermore, it is also known that certain ions can act either as a catalyst or an inhibitor when adsorbed on the metal surface depending on the type of metal considered. Kolotyrkin (18) observed that the adsorp-... 

They become passive if they substantially resist corrosion under conditions in which the bare metal would react significantly. This behavior is due to the inhibition of active dissolution by the more or less spontaneous formation of a dense passive film of limited ionic conductivity, which is formed by an anodic reaction of the type... 

Nitrites are environmentally fiiendly anodic inhibitors. They form a passive film with ferric oxide and inhibit the corrosion of copper, nickel, and tin alloys in alkaline environments (pH levels 9-10), but aggressive ions such as chloride and sulfate ions attack and destroy the barrier film. They reduce the rate of anodic dissolution on steel as shown in Fig. 14.10 [61]. Nitrites are used only in closed systems because they oxidize to nitrates in the presence of oxygen. They are not as efficient inhibitors as chromates. 

Note, however, that there are conditions under which inhibitors can give rise to detrimental local corrosion, that is, pitting corrosion. This is the case when the amount of inhibitor is insufficient. Under these conditions, only part of the surface can be covered, thus giving rise to a local element. Corrosive attack is particularly extensive at the uncovered anode areas because of increased corrosion current density and deep cavities penetrating into the material. Similarly, if the inhibitor is too readily reduced at the cathodic areas of the metal surface, increased corrosion can result because compact protective films are not formed. Since there are no universally applicable inhibitors, they must be carefully selected and examined for each specific case. In doing so, inhibition of metal dissolution is not the only point to be considered—there is also hydrogen absorption. 

There are several mechanisms whereby mliibition occurs. A major group of inhibitors function by adsorption on the metal surface. The adsorption must occur in the region of the corrosion potential and decrease the rate of either or both the anodic and the cathodic reactions by one of the mechanisms discussed in Chapter 1. Figure 9.15 illustrates the case where an additive inhibits only the anodic dissolution of the metal. It can be seen, however, that the decrease in the rate of metal oxidation decreases the corrosion current and hence the rate of metal loss. 

Most pickling inhibitors function by forming an adsorbed layer on the metal surface, probably no more than a monolayer in thickness, which essentially blocks discharge of H+ and dissolution of metal ions. For example, both iodide and quinoline are reported to inhibit corrosion of iron in hydrochloric acid by this mechanism [19]. Some inhibitors block the cathodic reaction (raise hydrogen overpotential) more than the anodic reaction, or vice versa but adsorption appears to be general over all the surface rather than at specific anodic or cathodic sites, and both reactions tend to be retarded. Hence, on addition of an inhibitor to an acid, the corrosion potential of steel is not greatly altered (<0.1 V), although the corrosion rate may be appreciably reduced (Fig. 17.3). 

The adsorption of an inhibitor onto the metal surface slows the rate of corrosion by blocking part of the surface. The extent of inhibition depends on the equilibrium between the dissolved and adsorbed inhibitor species, expressed by the adsorption isotherm. This mechanism which is particularly important in acids will be discussed in the next section. (Sect. 12.4.2). Certain inhibitors promote the spontaneous passivation of a metal and thus drastically reduce the corrosion rate. Oxidizing species such as chromates fall in this category. Buffer agents that maintain a high pH at the metal surface also favor the passive state. Other inhibitors lead to the formation of surface films by precipitation of mineral salts or of weakly soluble organic complexes. These films reduce the ability of oxygen to reach the surface and, in addition, they may impede the anodic dissolution reaction. 

The corresponding corrosion potential (Ecorr), corrosion current density (icon), anodic Tafel slope (ba), cathodic Tafel slope (be) and CR for uninhibited and inhibited systems from PP measurement are listed in Table 3. The data demonstrates that the Ecorr values shift to more positive values as the concentration of added studied inhibitors are increased. On the other hand, the corrosion current densities are markedly declined upon addition of the studied corrosion inhibitors. The extent of its decline increases with increasing of the corrosion inhibitor concentration. Moreover, the numerical values of both anodic and cathodic Tafel slopes decreased as the concentration of inhibitors were increased. This means that the three natural products have significant effects on retarding the anodic dissolution of aluminium alloy and inhibiting the cathodic hydrogen evolution reaction. 

The electrochemical studies of the corrosion inhibition process of Al-Mg-Si alloy in seawater using three selected natural products as corrosion inhibitors show that the corrosion rate of the alloy significantly reduced upon the addition of studied inhibitors. PP measurement reveals that the studied inhibitors can be classified as mixed-type inhibitors without changing the anodic and cathodic reaction mechanisms. The inhibitors inhibit both anodic metal dissolution and also cathodic hydrogen evolution reactions. 

In the previously described work, low levels of lead were found in the rust layer near the paint-rust interface, within 30 tm of the rust-paint interface. Thomas suggests that because lead salts do not appear to reach the metal substrate to inhibit the anodic reaction, it is possible that lead acts within the rust layer to slow down atmospheric corrosion by interfering with the cathodic reaction (i.e., by inhibiting the cathodic reduction of existing rust [principally FeOOH to magnetite]) [33], This presumably would suppress the anodic dissolution of iron because that reaction ought to be balanced by the cathodic reaction. No conclusive proof for or against this theory has been offered. 

N. Sato, Mechanism of homogerreous and heterogeneorrs anodic dissolntiorr of metals, Proceedings Second Japan-USSR corrosion seminar Homogeneous and Heterogeneous Anodic Dissolution of Metals and Their Inhibition, Japarr Soc. Corros. Eng., Tokyo, 1980, p. 35. 

The important point is that once a film is formed, the corrosion rate sharply declines. The passivity on the metal surface which develops due to film formation on metal surface causes inhibition of the anodic dissolution process. 

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