Corrosion inhibitors anodic

Fig. 12.49. Action of corrosion inhibitors, (a) Anodic inhibitors. Examples Chromate, nitrite, molybdate, tungstate, orthophosphate, silicate, benzoate, (b) Cathodic inhibitors. Examples Ca(HC03)2, ZnS04, Cr2(S04)3, NiS04 phosphate, aminoethylene phosphate (AMP), Ag3+, Sb3+ (on iron) Hg (on zinc), (c) Mixed inhibitors. Examples organic inhibitors containing nitrogen and/or sulfur (e.g., amines, triazoles, thiazoles, alkylthioureas) inorganic inhibitors (e.g., arsenite, arsenate, selenate). (Reprinted from G. Ranglen, Corrosion of Metals, p. 165,1985 with permission from Chapman Hall.)...

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-... 

Corrosion, the degradation of a material s properties or mass over time because of environmental effects, is a costly reality that effects every industry. A study issued by the Federal Highway Administration (FHWA) in 2002 conservatively estimates the annual direct cost of corrosion in all U.S. industry sectors at US 276 billion. Costs associated with corrosion include cathodic/anodic protection coatings inhibitors corrosion-resistant alloys and materials and maintenance, repair, and depreciation of equipment. Indirect costs, such as lost productivity, environmental or product contamination, planning and design, and lost opportunities, can easily outpace direct costs by factors of two or more. 

It is well known that quite low concentrations of certain oxygen-containing anions, such as chromate, are effective inhibitors of aqueous corrosion of a number of metals. These results have been ascribed to specific adsorption of the inhibitor at anodic sites of the metal surface, or alternatively to continuous repair of the protective film. 

Corrosion control methods consist of protective coatings, corrosion-resistant metals and alloys, corrosion inhibitors, polymers, anodic and cathode protection, corrosion control services, corrosion research and development, and education and training. The total annual cost of corrosion estimated with this method for the average year of 1998 was 121.41 billion or 1.381% of the 8.79 trillion gross domestic product. Table 4.1 shows the distribution of corrosion control methods and services eosts. 

Anodic inhibitors increase anode polarization to the critical passivation potential of the metal or alloy. They are called passivating inhibitors because they drastically decrease the corrosion current. Figure 14.2a and b illustrate the polarization and passivation effect of anodic inhibitors. These inhibitors are strong oxidizing agents and shift the corrosion potential of the metal in the noble direction with the formation of a passive film. 

Dihua, W., Xianzhang, B.U., Fuxing, G., Jinyun, Z., Lu an, Y. 1999. A differential polarization curve method and its application in anodic desorption study of corrosion inhibitor. Corrosion Science and Protection... 

The effect of an inhibitor on the corrosion rate of iron in an acidic environment of pH 0.2 is studied. In the absence of the inhibitor, the anodic and cathodic partial reactions obey the following equations (logarithms are base-10) ... 

S] Berke, N. S., The Use of Anodic Polarization to Determine the Effectiveness of Calcium Nitrite as an Anodic Inhibitor, Corrosion of Rebars in Concrete, ASTM STP 906, ASTM International, West Conshohocken, PA, 1986, pp. 78-91. 

Most of these inhibitors have a binary compound base that enables them to block die corrosion anodes and cathodes at the same time. They act directly on electrochemical... 

A second example of the analysis of inhibitor-covered surface is shown in Figure 23. Iron substrates have been immersed in a solution containing the inhibitor ammonium benzoate at a concentration of 0.025 M [88]. After exposure, the samples are removed fixam the electrolyte and the corrosion potential map is recorded with the Kelvin probe. Large potential differences result for the surface immersed in the solution with a low concentration of inhibitor. Again, anodic potentials correspond to the inhibited area whereas negative potentials close to the free corrosion potential of the active iron electrode are representative of the noninhibited surface. Obviously, at a concentration of 0.025 M the inhibitor is not able to protect the iron surface completely. If the inhibitor concentration is increased to 0.05 M, no active corrosion is observed and the potential stays at very anodic values. Similar results have been obtained by Schultze et al. [89,90] for the inhibitor toloylanaline on iron. 

In this section, the relationship of corrosion inhibitors to anodic and cathodic polarization wiU be explained. Of the four components of a corrosion cell (anode, cathode, electrolyte, and electronic conductor), three may be affected by a corrosion inhibitor to retard corrosion. The inhibitor may cause ... 

In water with a pH near 7.0, a low concentration of chlorides, silicates, and phosphates cause passivation of steel when oxygen is present hence, they behave as anodic inhibitors. Another anodic characteristic is that corrosion is localized in the form of pitting when insufficient amounts of phosphate or silicate are added to saline water. However, both sUicates and phosphates from deposits on steel increase cathodic polarization. Thus, their action appears to be mixed, i.e., a combination of both anodic and cathodic effects. 

CoiTosion is a natural, spontaneous and thermodynamically favoured process. There are several methods by which corrosion can be controlled one such method is the use of corrosion inhibitors. Corrosion inhibitors are chemical compounds which, when added in small quantities, reduce or completely prevent the corrosion process. They may be anodic, cathodic or mixed type depending upon whether they control the anodic reaction, the cathodic reaction or both anodic and cathodic reactions. A good inhibitor is expected to have the following characteristics ... 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

Inhibitors The use of various substances or inhibitors as additives to corrosive environments to decrease corrosion of metals in the environment is an important means of combating corrosion. This is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, it has also proved to be economicaUv attrac tive for many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective as the result of their controlling influence on the cathode- or anode-area reactions. 

Chemical inhibitors, when added in small amounts, reduce corrosion by affecting cathodic and/or anodic processes. A wide variety of treatments may be used, including soluble hydroxides, chromates, phosphates, silicates, carbonates, zinc salts, molybdates, nitrates, and magnesium salts. The exact amount of inhibitor to be used, once again, depends on system parameters such as temperature, flow, water chemistry, and metal composition. For these reasons, experts in water treatment acknowledge that treatment should be fine tuned for a given system. 

Passivating inhibitors act in two ways. First they can reduce the passivating current density by encouraging passive film formation, and second they raise the cathodic partial current density by their reduction. Inhibitors can have either both or only one of these properties. Passivating inhibitors belong to the group of so-called dangerous inhibitors because with incomplete inhibition, severe local active corrosion occurs. In this case, passivated cathodic surfaces are close to noninhibited anodic surfaces. 

Inhibitors are materials that reduce either one or both of the partial corrosion reactions as in Fig. 2-5. Anodic or cathodic inhibitors inhibit the anodic or cathodic reaction respectively so that the rest potential becomes either more positive or more negative. Most inhibitors, however, inhibit the anodic partial reaction. This is because the transfer of metal ions can be more easily restricted than that of electrons. 

Hatch, G. B., Maximum Self-generated Anodic Current Density as an Inhibitor Pitting Index , III. State Water Surv., Circ. No. 91, 24 (1966) C.A., 66, 8l8l4f Herbsleb, G., Pitting Corrosion on Metals with Elearon-conductive Passive Layers , tVerksl. Korros., 17, 649 (1966) C.A., 66, 5337m ... 

The use of soluble inhibitors as a means of controlling bimetallic corrosion presents many technical problems. Apart from the fact that this method is limited in applicability to recirculating systems, efficient anodic inhibitors, such as chromates, are frequently quite specific in their action and so certain bimetallic couples, such as the Al-Cu couple in chloride solutions, are... 

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