**7. Forms of corrosion**

Corrosion can manifest itself in many forms such as uniform corrosion or general corrosion, galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, selective leaching, erosion corrosion, stress corrosion, corrosion fatigue and fretting corrosion [18]. In order to improve the understanding between corrosion and design engineers it is classified into two broad categories. They are expressed in flow chart (**Figure 2**)**.**

#### **7.1 General corrosion**

This general corrosion also called as a uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area [19]. The metal becomes thinner and eventually fails.

#### **7.2 Galvanic corrosion**

It occurs when a potential difference exists between two dissimilar metals immersed in a corrosive solution. This potential difference produces a flow of electrons between the metals. Several investigations have shown that, galvanic corrosion is directly proportional to the area of the cathodic to the anodic metal [20]. A schematic diagram for galvanic corrosion is shown below(**Figure 3**).

**7**

corrosion.

**Figure 4.**

*Corrosion Inhibitors*

**Figure 3.**

*DOI: http://dx.doi.org/10.5772/intechopen.80542*

*Schematic representation of galvanic corrosion.*

**7.3 Crevice corrosion or deposit corrosion**

*Schematic representation of crevice corrosion.*

**7.4 Pitting corrosion or localized corrosion**

• Impurities at the grain boundaries

• Enrichment of one of the elements in the alloy

• Depletion of one of the elements in the boundary area.

**7.5 Intergranular corrosion**

most destructive and insidious forms of corrosion [22].

This kind of corrosion is attacked generally within crevices associated with small volumes of stagnant solution trapped in holes, surfaces, joints and crevices under bolt and rivet heads (**Figure 4**) [21]. It is also known as deposit or gasket

Pitting corrosion is a localized attack resulting in the formation of holes in the metals. These holes are relatively small and they are looks like a rough surface (**Figure 5**), they were sometimes isolated or so close together. Pitting is one of the

Most of the metals and alloys are susceptible to intergranular corrosion, when exposed to specific corrosion environment which is shown in the **Figure 6**. Grain boundaries are usually more reactive than grain matrix. Hence localized attack occurs at and adjacent to grain boundaries with relatively little corrosion of the matrix. This type of attack is usually rapid and penetrates deep into the metal resulting in loss of strength and causes catastrophic failures. It is caused by,

**Figure 2.** *Forms of corrosion.*

*Corrosion Inhibitors*

At Cathode:

ated with high temperature.

expressed in flow chart (**Figure 2**)**.**

The metal becomes thinner and eventually fails.

**7. Forms of corrosion**

**7.1 General corrosion**

**7.2 Galvanic corrosion**

+ 2e<sup>−</sup> → H2↑ (in acid solution)

2H2O + 2e<sup>−</sup> → H2↑ + 2OH<sup>−</sup> (in alkaline solution) O2 + 2H2O + 4e<sup>−</sup> → 4OH<sup>−</sup> (in neutral solution)

Dry corrosion takes place mainly through the direct chemical action of atmospheric gases and vapors present in the environment 30. This is most often associ-

Corrosion can manifest itself in many forms such as uniform corrosion or general corrosion, galvanic corrosion, crevice corrosion, pitting corrosion, intergranular corrosion, selective leaching, erosion corrosion, stress corrosion, corrosion fatigue and fretting corrosion [18]. In order to improve the understanding between corrosion and design engineers it is classified into two broad categories. They are

This general corrosion also called as a uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area [19].

It occurs when a potential difference exists between two dissimilar metals immersed in a corrosive solution. This potential difference produces a flow of electrons between the metals. Several investigations have shown that, galvanic corrosion is directly proportional to the area of the cathodic to the anodic metal [20].

A schematic diagram for galvanic corrosion is shown below(**Figure 3**).

2H+

**6**

**Figure 2.** *Forms of corrosion.*

**Figure 3.** *Schematic representation of galvanic corrosion.*

**Figure 4.** *Schematic representation of crevice corrosion.*
