Anodic inhibitors passivation

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. 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

Fig. 10 24 Modes of inhibition, shown by the effect an polirizeittofi for charge-transfer-comrolted eicctrode reactions, (a) Decrease in the apparent anodic Tafet slope, (b) Decrease in the anodic exchange current (in the case of anodic inhibitors, passivation may also occur), (c) Decrease in the apparent cathodic Tafel slope, (d) Decrease in the cathodic exchange current.

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. 

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). 

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. 

Nitrite-based formulations are available either as 100% powdered or granular products, or as liquids varying from 20 to 40% available sodium nitrite. Sodium nitrite (NaN02) is most commonly used and acts as an anodic inhibitor and passivator. 

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 ... 

The conversion of a reactive metal surface into a lower energy state that does not readily further react or corrode. Usually carried out by anodic inhibitors producing a passive oxide film on a clean surface. A vital component of any program for long-term protection of a metal waterside surface. 

Anodic inhibitors such as nitrites, chromates and molybdates are strong oxidizing passivators. They strengthen the protective oxide layer over the steel which otherwise would break down in the presence of chloride ions. The mechanism involves a redox reaction in which the chloride and nitrite ions engage in competing reactions the inhibitor is reduced and steel becomes oxidized to iron oxide as follows ... 

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. 

Sodium nitrite is incorporated into formulations for both open and closed cooling systems and acts as an anodic inhibitor. It is a good passivator but requires a relatively high dose rate to ensure that all anodic areas within a system are protected from the risk of pitting corrosion. The dose rate has to be increased when high chlorides or sulfates are present. 

Passivation of steel in cooling systems does not occur naturally and thus anodic inhibitors are generally employed. This step can be relatively expensive in large cooling systems (as the consumption of chemicals can be quite high in a short period of time), or seen to be a waste of time and therefore unnecessary. This is not the case and proper passivation should always be undertaken. 

Corrosion-inhibitive properties of the compound Na2P03F have been tested by Andrade et al., either by incorporating it in a mortar or as a penetrant[68]. This compound, which is currently available as a proprietary product, is reported to act as an anodic inhibitor, possibly with some cathodic action. The minimum required ratio of phosphate to chloride was suggested as 1 1. The mechanism of action of this admixture is to stabilize the passive layer of iron oxide on the steel and also increase the density of concrete, thus decreasing the permeability... 

The mitigation of corrosion can be achieved economically by the use of corrosion inhibitors. Chromate has been extensively used in an aqueous environment for the protection of aluminium, zinc and steel. Although chromates are cheap and effective, they are not acceptable because of their toxicity. Alternate inhibitors such as molybdates, organic inhibitors such as phosphonates, mixtures of phosphates, borates and silicates and surfactants like sulfonates have been used in place of chromates. Chromates are anodic inhibitors and help to form passive oxide on the metal surface. 

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. 

Sodium carbonate is another widely used solution for the conservation of ferrous artefacts. This maintains the pH in the zone of passivity (pH > 9.5) and is also said to act as an anodic inhibitor. A concentration of 3 g dm 3 (0.2 M) was used in the conservation of Holland 1 in Gosport Submarine Museum. It is advisable to use the sesquicarbonate form as it is far easier to dissolve than the simple carbonate. The use of this solution is said to avoid formation of solid sodium chloride in the rust. 

The precise mechanism resppnsible for the passivity conferred on metals by anodic inhibitors, such as chromate, is not known. While some early workers thought that a protective salt film (e.g., chromate) was formed, this view is not generally applicable, since passivity can occur in a system where the salt film would be freely soluble (e.g., iron in nitric acid). It is, however, generally accepted that passivity is associated with the formation of a protective film, and current views ascribe the action of anodic inhibitors either to adsorption at anodic sites or to continuous repair of the protective film. The former view has received attention in recent publications by Cartledge ), while the latter is favored by Evans (2). However, work on aluminum has suggested that true passivity is associated with the crystal structure of the film, which in turn determines its stability. This principle has recently been introduced by one of the authors (3) and is developed below into a general theory of passivity. 

PASSIVATING (ANODIC) INHIBITORS - A material capable of forming a protective oxide film on metal surfaces. 

Apart from a few exceptions, notably chromate, the protection ability of inhibitors depends strongly on the nature and composition of the metal or alloy and even small differences may cause substantial changes in the inhibition efficiency. A well-known example is sodium benzoate, which in neutral solution is an anodic inhibitor for mild steel but not for cast iron, although the anodic corrosion reaction is the same for both materials [5]. Similarly, the state of the surface, in particular the presence and nature of passive films and... 

Anodic inhibitors are, in most cases, species that can form insoluble salts with the ions of the corroding metal, thus rendering the surface passive. Therefore, their effectiveness usually depends on the metal. As an example, again the protection of copper by benzotriazole (BTAH)... 

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