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Some other reactions that are most commonly present in the chemical process are following.
Metal ion reduction
(1.7)
Metal ion deposition
(1.8)
The products of the anodic and cathodic reactions react to form solid corrosion products on the surface of the metal. The Fe2+ interacts with OH− ions as:‐
(1.9)
Fe(OH)2 is reoxidized to Fe(OH)3, an unstable product, and thus transforms to hydrated ferric oxide commonly called as red rust (Figure 1.3).
(1.10)
(1.11)
Figure 1.3 Mechanism of rust formation.
Figure 1.4 Classified forms of corrosion.
1.4 Classification of Corrosion
Seldom is a single class of corrosion discovered in corroding structures. Different metals in contact and contact with different environment hardly allow only one type of corrosion to occur even within a system. Each type of corrosion is caused by their specific reaction mechanisms and has their specific monitoring, prediction, and control methods. Figure 1.4 throws some light on classification of corrosion in a pictorial manner. None of the classifications is a universal standard, even the following classification is an adapted [4, 13].
1.4.1 Uniform Corrosion
This type of corrosion affects a large patch over the metal and causes overall reduction of metallic thickness subject to the fact that metal undergoing corrosion has a uniform composition and metallurgy too. What happens is that anode and cathode do not possess fixed sites; as such there are no sites preferable to corrosion, which occurs here in a uniform fashion. Corrosion rates are easily monitored by electrochemical measuring techniques or gravimetric analysis. A metal suffering from uniform corrosion can be protected using corrosion inhibitors or coatings and also by cathodic protection. Atmospheric corrosion is an example of uniform corrosion. When exposed to dry atmospheres with very less humidity, metals spontaneously tend to form an oxide film. This barrier oxide film acquires a thickness of 2–5 nm [14].
1.4.2 Pitting Corrosion
Pitting corrosion is highly destructive form and a kind of localized attack, which leads to little holes called pits in metal. Small cavities and holes, which are as deep as their diameter, are known as pits. They cause perforations by penetrating into the metal with least loss of weight [15]. Pitting is proportional to the logarithm of electrolyte’s concentration of chloride. The prerequisite for pitting to occur is that the electrolyte should be a strong oxidizer for onset of the passive state. The ferric and cupric halide ions are electron acceptors (cathodic reactants), and they do not need oxygen to initiate and propagate pitting. Other propagating factors causing pitting include localized damage chemically and mechanically to a passive oxide film, non‐metallic impurities/non‐uniformities of metal structure due to nonproportional inhibitor coverage. Pitting, however, can be evaded by reducing aggressiveness of the solution, decreasing the temperature of conductive solution, decreasing Cl− concentration and acidity.
1.4.3 Crevice Corrosion
It is also localized version of corrosion on a microenvironment level but related to a stagnant electrolyte caused by gasket surfaces, lap joints and holes, crevices under bolts, rivet heads, and surface deposits. To evade and limit this type of corrosion, it is suggested to (i) use of welds instead of bolt/rivet joints, (ii) a design to ensure complete draining, (iii) hydrofuging any interstices, which cannot be removed, and (iv) utilizing solely solid and nonporous seals, etc.
1.4.4 Galvanic Corrosion
This type of corrosion is also called “bimetallic corrosion” and “dissimilar metal corrosion,” because it occurs due to electrical contact with a more noble metal or maybe a nonmetallic conductor in the electrolyte. The active member of the metallic couple, i.e. less corrosion resistant bears an accelerated corrosion rate, while the noble member is protected by the cathodic effect. The joint between the metals is most corrosion affected. Moving away from this junction, the corrosive attack reduces. As it is already known that each metal has its unique corrosion potential in an electrolyte, the potential difference between the two dissimilar metals causes the less noble metal to corrode. To estimate the corrosion rate conductivity of metal and electrolyte, potential difference of relative anodic and cathodic areas might be accounted [16–18]. To prevent such an attack, the metals should lie close by on the electrochemical series, comparably area of anode should not be too small; if different metals are involved, insulation must be applied; and coatings, paints, and avoiding thread joints are good preventive measures.
1.4.5 Intergranular Corrosion
This corrosion can be referred to as “intercrystalline corrosion”/“interdendritic corrosion” as tensile stress causes it along the grain or crystal boundaries. It might also be known as “intergranular stress corrosion cracking” and “intergranular corrosion cracking.” These corrosive attack prefers interdendritic paths. A microstructure examination using a microscope is needed for recognizing this degradation; however, at times, it is recognizable with eyes as in weld decay. The composition’s local differences like coring in alloy castings lead to this type of corrosion. The mechanism includes precipitation in grain boundaries like in the case of precipitating chromium carbides in steel. Intermetallic segregation at grain boundaries in aluminum is called “exfoliation.” This corrosion type might be prevented and controlled by using mild steel,
