Kinds of Corrosion

Electrochemistry of the metal corrosion (dissolution) under an applied potential is presented in the form of a typical polarization curve. The origin of the common three regions (activation, passivation, and transpassivation) is explained. 

Using the common mixed potential definition, the background of the corrosion potential is presented. An example of the corrosion potential for Fe(s) is discussed. 

When corrosion occurs, the cathodic current density is equal to the anodic current density, and both of them are equal to the corrosion current,y .orr- The corrosion current cannot be experimentally measured but can be calculated. On the contrary, the corrosion potential can be measured and its value is somewhere between the equilibrium potentials of the cathodic and anodic reactions. 

Faraday s law allows calculating the corrosion rate if the anodic half-reaction and density of the corroding metal are known. The most common unit for the rate of the corrosion penetration is mm year . 

Two methods of the corrosion protection are discussed (1) inhibition of corrosion by providing a protective film and (2) cathodic protection with using another metal and forming a galvanic couple. 

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One must always bear in mind that potential dependence is not the same in different types of corrosion. Thus critical potential ranges for different kinds of corrosion can overlap or run counter to one another. This is particularly important... 

Equipment fabricated from such a steel may undergo corrosion by condensation of even mild corrosives unless it is possible to keep it above the dew-point or to neutralize acidic condensates. This kind of corrosion can be prevented by a preliminary heat-treating at temperatures of 815-915°C. The niobium-stabilized steels respond best to this treatment. 

The presence of tensile stress in a metal surface renders that surface more susceptible to many kinds of corrosion than the same material in a non-stressed condition. Similarly, the presence of compressive stress in the surface layer can be beneficial for corrosion behavior. 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

The data presented in this section illustrate that, with the exception of those accelerating water-reducing admixtures containing calcium chloride, there is an abundance of evidence to support the conclusion that water-reducing admixtures of lignosulfonate chemical form certainly will not accelerate any kind of corrosion with reinforcement and, when used to reduce the water-cement ratio, will form a more permeable and durable protective cover for the reinforcement. In view of the chemical nature of the other types of materials such as the hydroxycarboxylic acids and hydroxylated polymers, it seems most likely that these materials too would have no deleterious effect in this respect. 

These are most frequent kinds of corrosion, although erosion corrosion and galvanic corrosion can be encountered. 

The environment that causes this kind of corrosion is often rather inert the polymer does not dissolve or swell, but there is some diffusion of liquid or gas, in particular at the surface. If the diffusion rate is small localised plasticization is possible and subsequently deformation is caused by the stresses. This leads to increased plasticization and to the formation of crazes and even of cracks. 

T depends on the kind of corrosive agent (liquid or gas) and decreases with decreasing solvent quality... 

For example, the acid will uniformly corrode a piece of mild steel sheet immersed in dilute sulfuric acid. Corrosion by uniform attack accounts for the greatest loss in practice. The incidence of this kind of corrosion can easily be detected and remedial measures taken. 

RubCon can be used in specialized structures instead of traditional building materials (reinforced concrete, steel, wood). Comparison of technical and economic parameters of RubCon with the most widespread kinds of corrosion-resistant concretes, in particular polymer concrete based on epoxy and polyester resins, showed that the cost of a useful loading unit of the RubCon structure is more than twice as effective as epoxy polymer concrete and 2.2 times as effective as polyester polymer concrete. 

Stainless steel is corrosion resistant because a protective oxide layer naturally forms on top of the surface in the presence of oxygen and humidity. This protective oxide layer typically has a thickness in the order of nanometers, depending on the present environmental conditions. XPS studies of oxide films formed in air on AISI 316 revealed that not only oxidation of the material takes place, but also chromium and metallic nickel accumulate at the interface between oxide layer and bulk material [1]. The protective film is, of course, not perfect but contains defects like inclusions and grain boundaries. At these defects the film may locally break down and dissolution of the bulk material may start [2]. This kind of corrosion is called pitting corrosion and is estimated to cause a third of all chemical plant failures in the United States [3]. 

The metal has a hexagonal close-packed lattice and resembles other transition metals such as iron and nickel in being hard, refractory (m.p. 1680°+10°, b.p. 3260°), and a good conductor of heat and electricity. It is, however, quite light in comparison to other metals of similar mechanical and thermal properties and unusually resistant to certain kinds of corrosion and has therefore come into demand for special applications in turbine engines and industrial chemical, aircraft, and marine equipment. 

The behaviour of PAni is best characterised by that of an organic metal, a kind of noble metal. But it is a very special kind of metal, namely one that can exist in at least three stages of oxidation. These three steps show three different colours the normal PAni-(green), a reduced yellowish and an oxidised blue form. The three properties, the conductivity, the noble behaviour and the redoxability, allows the new kind of corrosion protection. 

The technical importance of this kind of corrosion has attracted many research groups for many decades, and various... 

The corrosion of a mold causes a great economical loss because the mold is expensive. Moreover, it is then difficult to precisely mold. In order to solve corrosive problems involved in the PAS resins, it has been proposed to blend various kinds of corrosion inhibitor. 

The basic kind of corrosion damage, which is the most dangerous for structural materials both in Pb-Bi and in Pb coolants, is local corrosion of materials appearing as the separate corrosion-erosion centres ( pittings ) Local through corrosion damages of structural elements may appear at temperatures over after holding for some hundred hours under... 

This oxygen reduction, which determines the flow of current and hence the corrosion of the iron at the uncoated spot, continues as long as oxygen can diffuse unhindered to the cathode. This local dissolution process can even perforate sheet and piping. This kind of corrosion can be avoided by completely removing the rolling scale. 

In reality the mechanism of concrete deterioration as a consequence of acid corrosion, if it is sulphuric acid, is the same as in case of sulphate corrosion. For this reason in both environments the matrix based on Portland cement with reduced Cj A content is more resistant. This example shows the imperfection of kind of corrosion classification, presented above. 

Starting in 1986, we tried to coat steel that was not prepassivated, under nonelectrochemical conditions, but with a paint containing dispersed polyaniline. We wondered if some kind of corrosion protection—by whatever mechanism—could be created by an interaction between a dispersion paint and a normal metal surface. This would be, in contrast to previous approaches, a nonelectrochemically applied PAni on a non-prepassivated metal surface. In 1987, we achieved the first promising results [70]. Subsequent work (also published in various patents) confirmed the previous findings, but did not show an exciting quantum leap in corrosion protection. Moreover, it was hardly reproducible and did not convince any paint manufacturer. 

To design the appropriate surface finishing process against corrosion, we need to determine what kind of corrosion would cause an item to fail and how much material would be damaged. Therefore, we need appropriate and precise corrosion tests. 

There are many kinds of corrosion tests. To make things simple, the two main types are shown in Fig. 9. 

Addition of Inhibitors. It is possible to add inhibitors for controlling two kinds of corrosion in boiler systems, namely, stress-corrosion cracking and return-line corrosion. The first can be minimized by addition of phosphates, as mentioned previously. 

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