Passive films Inhibitors

Passivating (anodic) inhibitors form a protective oxide film on the metal surface they are the best inhibitors because they can be used in economical concentrations and their protective films are tenacious and tend to be rapidly repaired if damaged. 

Precipitating (cathodic) inhibitors are simply chemicals that form insoluble precipitates that can coat and protect the surface precipitated films are not as tenacious as passive films and take longer to repair after a system upset. 

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

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. 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

Other corrosion inhibitors also enhance passivity without electrochemical reduction, by depositing insoluble oxidation products into the passivating film. For example benzoate ions cause deposition of ferric benzoate into the oxide, but do not provide any cathodic reaction. 

Another class of inhibitors in near-neutral solutions act by stabilising oxide films on metals to form thin protective passivating films. Such inhibitors are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate and borate. Passivating... 

When the boiler is placed back online, certain types of anodic inhibitors (which are generally employed to act as polishing treatments in the maintenance program) also may prove beneficial in further strengthening the passive film. 

Although orthophosphates are themselves passivating, anodic inhibitors (and also cathodic inhibitors, forming a calcium phosphate barrier film), the film strength is weak, even in simple HW systems and they are not used for this purpose. Nevertheless, despite the thermal instability of sodium hexametaphosphate and other polyphosphates, phosphates in general have several important properties that make them useful in boiler plant operations. These properties include ... 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

Anodically colored electrochromic inorganic films, 6 579-580 Anodic cleaning, 9 783, 785 Anodic (passivating) corrosion inhibitors, 26 144... 

The usual chemical employed is sodium molybdate (dihydrate), which acts as an anodic inhibitor and forms, in the presence of some oxygen, a passivating film composed of a ferrous-ferric-Mo-oxides complex. 

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... 

Add efficient inhibitors for certain systems since it is believed to assist the formation of a stronger, more stable or more readily repaired passive film. (Miller)24 Phosphates and other inorganic or organic corrosion inhibitors, used in fairly corrosive surroundings, decrease the effects of the SCC. A minimal critical concentration of some oxidizing inhibitors such as nitrites is absolutely necessary to avoid pitting. 

Stem (77) has recently discussed the mechanism of passivating-type inhibitors on the basis of the theory presented here. He points out that passivity may be accounted for operationally by anodic polarization phenomena irrespective of whether films are assumed to be responsible (79). This approach was also suggested by Mears (80). Tomasfcov (76) has proposed the use of the ratio of the extent of anodic polarization, - Ej p to the extent of the cathodic polarization,... 

As shown in Fig. 8.4, to form an effective passivating film on copper, a corrosion inhibitor must adsorb onto the surface. The adhesion to the copper surface must be strong enough to protect against the shear force impinged by slurry flow. The coordinate bond found in a typical copper(I) or copper(II) complex is usually sufficient to serve as an anchor. The static charge attraction between a surfactant and a charged surface may be too weak to perform such... 

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