This chapter contains the corrosion theory of grounding electrodes and basic electrochemistry in corrosion reactions. It contains also the forms of substation grounding grid corrosion (uniform corrosion, pitting corrosion, galvanic corrosion, microbial influenced corrosion), survey on corrosion rate of substation grounding grid, copper and steel corrosion rates, corrosion protection methods (coating, cathodic protection [CP]). The chapter contains also the methods of applying cathodic protection in grounding grids, anode selection, anode spacing, and impressed current in the grounding grid cathodic protection. Finally it contains the required information for design grounding system cathodic protection and sacrificial anode (galvanic) cathodic protection system design steps.
TopCorrosion Theory And Basic Electrochemistry In Corrosion Reactions
In this chapter, corrosion of substation grounding grid or any surround metallic structure due to current discharge buried in soil is fully introduced. Survey on calculating rate of corrosion and factors affecting the rate of corrosion are also introduced in this chapter.
Corrosion is defined as destruction or deterioration of material because of reaction with its environment. Some insists that the definition should be restricted to metals but often corrosion engineers must consider both metal and nonmetal for solution of a given problem as reported by Mars G. Fontana (1987). It is also considered the exothermic chemical transformation of a metal or metal alloy to a non-reactive covalent compound such as an oxide or silicate that is often similar or even identical to the mineral from which the metals were extracted as given by Howard (2004)
Most of the corrosion in soil is the result of an electrochemical reaction. Each galvanic corrosion cell comprises as shown in (Figure 1):
- 1.
Anode and cathode areas on metal surface,
- 2.
Soil electrolyte to enable occurrence of cathodic and anodic processes and current flow between anode and cathode and,
- 3.
Conducting path for the flow of current from cathode to anode.
In the absence of any of these components, the corrosion cell would stop to operate as reported by Zhou, Wang, Liu et al. (2008)Mitolo, M., Pettinger, A., (2016), investigated the interactions between cathodically protected pipelines and grounding systems
Figure 1. Basic galvanic corrosion cell
TopThere are number of corrosion forms of substation grounding grid. These forms can be summarized as follows:
Uniform corrosion refers to corrosion damage occurring evenly over the metal/alloy surface as shown in (Figure 2), and the rate of corrosion is nearly the same over the entire surface. Although uniform corrosion is the most important form of corrosion, it is relatively easily measured and predicted, making disastrous failures relatively rare as reported by Tullmin and Roberge (1995)
Pitting Corrosion
Pitting corrosion is a form of localized corrosion that produces pits (holes) in the metal/alloy as shown in (Figure 3). It occurs when discrete areas of a material undergo rapid attack while the vast majority of the surface remains virtually unaffected. This is in sharp contrast to uniform corrosion in which all parts of the exposed surface recede at approximately the same rate. Pitting of a given material depends strongly upon the presence of an aggressive species in the environment as reported by Zhou, Wang, Yang et al. (2008)