Passivating inhibitors

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

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. 

There are basically three main types of inorganic inhibitors used in industry anodic passivating inhibitors, cathodic inhibitors and cathodic precipitators. 

Nitrite formulations are employed for both hot and cold water closed loops (and also occasionally for open cooling systems). Unfortunately, nitrite is easily oxidized to nitrate and is very susceptible to microbiological attack (by Nitrobacter agilis and other microorganisms). Nevertheless, it is a good low-cost passivating inhibitor. 

Cathodic inhibitors are passivating inhibitors they raise the hydrogen overvoltage, thus impeding the reduction of hydrogen ions at the cathode. They are suitable for inhibiting sulfuric or phosphoric acids. Examples are benzotriazole (at low concentrations) and alkylamines. 

Roubal J and Krivucova M Hygienic problems in the application of tertiary butyl chromate as a passivation inhibitor of metal corrosion. Arch Bewerbepath Gewerbehyg 17 589-596, 1960... 

The heart of corrosion science has been identified as electrochemical science coupled with the thermodynamic and kinetic values. Other limbs are oxidation and high-temperature oxidation of metals, protective coatings, passivity, inhibitors, microbial-induced corrosion, corrosion fatigue, hydrogen embrittlement and corrosion-resistant alloys. Having identified the limbs of corrosion science, it is instructive to examine how the various aspects came into existence over a period of time. 

Corrosion protection by an inhibitive primer may be envisaged as shown in Figure 1.67. Inhibitive metal primers are designed with relatively high pigment volumes to allow sufficient water absorption to dissolve the passivating inhibitor and yet prevent aggressive ions such as chloride which interfere in the passive oxide formation process. 

Some inhibitors produce films on the anode and hence stifle the corrosion reaction (iron in chromate or nitrite solutions). Several authors consider the presence of a thick barrier of corrosion products, relatively protective, on the metallic surface as passivation. Inhibitors may enhance the formation of passive films on top of the substrate, such as benzotriazole on copper or benzoate on iron, or they may form monomolecular... 

Passivation Inhibitors. Examples of passivators (anodic inhibitors) include chromate, nitrite, molybdate, and orthophosphate. AH are oxidizers and promote passivation by increasing the electrical potential of the iron. Chromate and nitrite do not require oxygen, and thus, can he the most effective. Chromate is an excellent aqueous corrosion inhibitor, particularly from a cost perspective. However, owing to health and environmental concerns, use of chromate has decreased significandy and will probably he outlawed soon. Nitrite is also an effective inhibitor, but in open systems it tends to be oxidized to nitrate. 

It is shown in the next chapter that nitrites can be used as passivating inhibitors for corrosion of iron in near-neutral solutions. Since the basis for accomplishing this is related to the polarization characteristics of the reduction of the nitrite ion, brief consideration is given here to the reaction and to the form of the experimentally determined polarization curve forthis ion. The curve is shown in Fig. 3.21. Although several reactions have been proposed for the reduction of this nitrite ion, the following is considered here ... 

As shown in Fig. 3(a), the efficiency of passivating inhibitors depends strongly on their presence at a sufficient (critical) concentration. Below this critical concentration (curve I) a bistable situation may arise, in which the corrosion potential can either sit in the passive (Ej.) or active ( ) region, resulting generally in pitting. Because of this problem, which can even cause an increase in the corrosion... 

Fig. 3 -log 1/1 diagrams for a passivating inhibitor for the case of a reaction-controlled cathodic process. Passivation is achieved (a) by increasing the cathodic current density and (b) by decreasing the critical current density for passivation. 

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. 

Non-passivating inhibitors include both some anodic (Figure 10.5c) and the cathodic ones (Figure 10.5d). The latter are exemplified by those that remove free oxygen by a reaction, as is the case for hydrazine and sodium sulphite ... 

There are two types of passivating inhibitors oxidizing anions such as chromate, nitrite, and nitrate, which can passivate steel in the absence of oxygen, and the nonoxidizing ions such as phosphate, tungstate, and molybdate, which require the presence of oxygen to passivate steel. Examples of passivators (anodic inhibitors) include chromate, nitrite, and orthophosphate (Dihua et al. 1999). 

Inhibitors can also be classified on the basis of their functions. For instance, chromates and nitrates are called passivating inhibitors because of their tendency to passivate the metal surface. Some inhibitors, such as silicates, inhibit both the anodic and cathodic reactions. They also remove undesirable suspended particles from the system, such as iron particles, by precipitation. Certain types of inhibitors make the surrounding environment alkaline to prevent corrosion. Such inhibitors in the gas phase are called vapor phase inhibitors, and they consist of heterocyclic compounds, such as cyclohexylamine. These inhibitors are used within packing crates during transportation by sea. 

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