Page 196 - 2024-Vol20-Issue2
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192 | Al-mtory, Alnahwi & Ali
Fig. 1. Structure of the corrosion cell. 2) Cathode: represents the metal or the site in which the reac-
tion of reduction (gaining electron) occurs.
wastage, depletion in nature resources, and human discomfort 3) Electrolyte: is conductive medium between the anode and
[28]. In general, the dry corrosion and wet corrosion are the cathode.
main two types of corrosion. The dry corrosion occurs when 4) Electrical connection between anode and cathode.
the metal surface is exposed to dry hot gas. The wet corrosion 5) Potential difference should be available between the anode
refers to the exposure of the metal surface to aqueous solution and cathode to move the electrons from anode to cathode.
acid or alkali which represents an electrochemical reaction The conditions that may lead to an increase in the corrosion
[29]. The chemical corrosion results from a direct reaction rate and may lead to a decrease in the corrosion rate are as
between a metal and its surrounding environment or another follows [33]:
metal without any catalyst. An example of this corrosion is • Effect of oxygen and oxidizer: The corrosion rate increases
the corrosion of the reaction of iron with acid chloride hy- with the increase of oxygen.
drogen to generate the corrosion of the iron and liberation of • Effect of velocity: the velocity has the same effect as the
hydrogen. On the other hand, the electrochemical corrosion oxygen since the velocity controls the polarity of corrosion.
takes place through an electrochemical interaction between • Effect of temperature: since corrosion is a chemical reac-
the metal and the surrounding environment. In fact, the elec- tion, the rate of corrosion increases exponentially when the
trochemical corrosion depends on oxidation and reduction. temperature increases.
The oxidation is the metal’s loss of electrons, whereas the • Effect of corrosive concentration.
second interaction is gaining an electron from the surrounding • Effect of galvanic coupling [34].
environment. The two reactions can also be described by the
following two chemical equations [30], B. Pitting Corrosion
where M represents an arbitrary metal and the surrounding The corrosion can take different forms depending on the emer-
environment is represented by X. gence and the environmental conditions that led to its occur-
rence [35]. Some of the most common forms of corrosion
M ? Mn+ + ne (1) are uniform corrosion, intergranular corrosion, galvanic corro-
sion, selective corrosion, crevice corrosion, erosion corrosion,
Equation 1 represented Oxidation reaction (2) stress corrosion, and pitting corrosion. Our proposed algo-
Nx+ + ne- ? Xn rithm is inspired from the pitting corrosion.The pitting cor-
rosion has the mechanism of the phenomenon of the electro-
Equation 2 represented Reduction reaction chemical corrosion that is explained previously in this section.
The corrosion can be expressed by a corrosion cell whose A break occurs in the area of a passive layer so that a small
components are illustrated in Fig. 1. The corrosion does part of the metal surface is exposed to the external environ-
not occur if any of the corrosion cell components is missing ment. If the environment conditions are suitable to produce a
[31],[32]. These components are as follows: 1) Anode: rep- potential difference to drive the current between the metal and
resents the metal or the site in which the oxidation reaction the surrounding environment, the electrochemical reaction
(loss of electron) occurs. does exist. In other words, the base metal is considered as
the anode, and the surrounding environment is considered as
the cathode of electrochemical cell. Therefore, the electron
moves from the metal to the environment [36].
The pitting corrosion passes through three steps. These steps
are the initiation or nucleation of pits, pit growth or pit propa-
gation process, and re-passivation of pits [33]. The stability
of pit growth depends on the electrolyte, type of metals, and
pit-bottom potential. Finally, the spontaneous occurrence and
absence of the corrosion can be determined with the aid of
the Gibbs free energy [33]. This energy is mathematically
expressed in the following formula [31]:
?G = -nFEcell (3)
where ?G is the Gibb’s free energy, n represents electron trans-
fer from anode side to cathode side, F is faraday constant, and
