• **Pitting corrosion**

 Pitting corrosion is a localized form of corrosion that creates holes on the surface of metals. This type of corrosion takes place when a small area of metal becomes

#### *Self-Healing Coatings DOI: http://dx.doi.org/10.5772/intechopen.109500*

anodic while a vast area of the substrate acts as a cathode. The shape of holes may not be similar but mostly they are hemispherical and their size can vary from shallow and wide to deep and narrow. The formation and growth of holes on the surface of metals can have irreparable consequences, especially in critical points such as joint and welded parts [11]. **Figure 5** shows the pitting corrosion in metals.

#### • **Erosion-corrosion**

Erosion-corrosion is a form of corrosion that occurs as a result of chemical and mechanical stimuli. The mechanical effect of flow or velocity of a fluid combined with the corrosive action of the fluid causes accelerated loss of metal. This type of corrosion initiates by removing parts of the metal by erosion and is followed by contact with a corrosive medium. This usually happens when the velocity of the fluid is too high (turbulent flow) such as in tube blockages, tube inlet ends, or pump impellers. The process is a cyclic process that continues until the perforation of the metal occurs [12]. **Figure 6** illustrates the erosion-corrosion in metals.
