Corrosion reactions, simplest form

One of the reaction products (say, Q will be an oxidised form of the metal, and D will be a reduced form of the non-metal — C is usually referred to as the corrosion product, although the term could apply equally to D. In its simplest form, reaction 1.1 becomes... 

The corrosion of iron is one of the most widespread and technologically important examples of metallic corrosion. In the presence of water and oxygen, the corrosion of iron proceeds to form a complicated mixture of hydrated iron oxides and related species a complete description is beyond the scope of the present discussion, and the interested reader is referred to the previously cited general references on corrosion as well as to the well known descriptions of electrochemical equilibria in aqueous solution given by Pourbaix (8, 9,). Iron is a base metal, subject to corrosion in aqueous solutions. In the presence of oxidizing species, iron surfaces can be passivated by the formation of an oxide layer if the oxide layer formed is Imperfect, rapid corrosion may occur. In simplest form, the reaction of iron to form iron oxide can be written as ... 

Passive corrosion with scale formation on the substrate is often more complicated. The simplest form is that of reaction control. If a reaction is slow compared to the delivery of the agent to the reaction site, the concentration of the agent at the interface is as high as the external concentration of the corrosive medium. For a given temperature this reaction rate constant, k, dictates then the linear rate constant k in Eq. (6) and linear kinetics prevail. Only this time Ax is positive and denotes a layer growth or a mass gain. 

Corrosion of steel in concrete is probably the most serious durability problem of reinforced concrete in modem times, and therefore, a clear understanding of the phenomenon is of cmcial importance. The phenomenon itself is an electro-chemical reaction. In its simplest form, corrosion may be described as a current flow from anodic to cathodic sites in the presence of oxygen and water. This is represented by the following equations ... 

The cell shown in Fig. 1 illustrates the corrosion process in its simplest form. This cell includes the following essential components (a) a metal anode, (b) a metal cathode, (c) a metallic conductor between the anode and the cathode, and (d) an electrolyte in contact with the anode and the cathode. If the cell were constructed and allowed to function, an electrical current would flow through the metallic conductor and the electrolyte, and if the conductor were replaced by a voltmeter, a potential difference between the anode and the cathode could be measured. The anode would corrode. Chemically, this is an oxidation reaction. The formation of hydrated red iron rust by electrochemical reactions may be expressed as follows ... 

The simplest form of a solid corrosion product on a metal surface is a continuous homogeneous surface scale consisting of one phase. Such a situation is encountered, e.g., in the oxidation of pure nickel where only nickel oxide is formed. Therefore, in many textbook examples, this type of reaction has been used to illustrate the principles of high-temperature corrosion. In the present chapter, the initial stages of the oxidation process are not addressed as this has been part of another chapter in this book. 

Possibly the simplest example of the first approach would be the substitution of a sulphonated polystyrene-divinylbenzene polymer ion-exchange resin (in free acid form) for p-toluene sulphonic acid as the catalyst in an esterification or other acid-catalysed chemical reaction (see section 11.7.2). The ion-exchange resin may be used in bead form and packed into a tubular reactor, or may be introduced as a powder and subsequently filtered off. Furthermore, the solid resin may prove less corrosive to the metal walls of an industrial reaction vessel. 

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