# Review of Structural Health Monitoring Techniques in Pipeline and Wind Turbine Industries
## 3 Health Monitoring in Corrosion  
The estimated corrosion-related cost is about $5.8 billion annually to monitor, replace, and maintain these assets. It has been estimated that corrosion costs a sum of $1.4 billion annually to the United States in production and exploration alone, with $589 million used in surface pipeline and facility costs, $463 million in downhole tubing expenses, and $320 million in capital expenditures related to corrosion [51]. One out of every four natural gas transmissions and gathering incidents has been the consequence of corrosion in the last three decades according to the Pipeline and Hazardous Materials (PHMSA) database, and more than half of these were due to internal corrosion [52, 53]. However, it is even more difficult to inspect the insides of a pipe, and more so when the pipe is thousands of kilometers long and the problem could be anywhere. Hence, steps have been taken to achieve easier detection in these scenarios. Corrosion is an electrochemical process and usually requires an anode, a cathode, and an electrolyte to occur. These components are fulfilled by the fluids found in the reservoirs, thereby promoting corrosion [54, 55, 56, 57, 58, 59].  
\[\text{Anode: }\text{Fe}\rightarrow\text{Fe}^{2+}+2\text{e}^{-} \tag{24}\]  
\[\text{Cathode: }2\text{CO}_{2(\text{aq})}+2\text{H}_{2}\text{O}_{(\text{I})}+2 \text{e}^{-}\rightarrow\text{H}_{2(\text{g})}+2\text{HCO}_{3}{}^{-}{}_{(\text{aq})} \tag{25}\]  
\[2\text{H}_{2}\text{S}_{(\text{aq})}+2\text{e}^{-}\rightarrow\text{H}_{2( \text{g})}+2\text{HS}{}^{-}{}_{(\text{aq})} \tag{26}\]  
\[2\text{H}^{+}{}_{(\text{aq})}+2\text{e}^{-}\rightarrow\text{H}_{2(\text{g})} \tag{27}\]  
\[0.5\text{O}_{2}+\text{H}_{2}\text{O}_{(\text{I})}+2\text{e}^{-}\to 2 \text{OH}^{-}{}_{(\text{aq})} \tag{28}\]  
This reaction causes a decrement in mass and material and also unexpected cracking and catastrophic failures in oil and gas industries. Corrosion is thermodynamically favorable, so it occurs, but its kinetics can be brought under control. Localized corrosion or pitting occurs due to hydrogen sulfide and chloride ions, which provide structural weak points, resulting in cracking without the force being enough to cross the mechanical threshold. Microbes are also another causative agent of corrosion, especially the sulfate-reducing ones [60, 61, 62]. More than one-quarter of the damages done by corrosion could have been prevented in the United States. Widely used carbon steel is prone to corrosion; monitoring it to plan maintenance can be helpful in corrosion prevention [51]. The carbon steel is used in many places including drill pipes, transmission pipes, and casing tubing [63] owing to its excellent mechanical properties and reasonable prices. SHM is required to detect signs of risk that could cause catastrophic events. Monitoring allows for the identification of corrosive elements, and this especially comes in handy when thousands of miles of pipes are being discussed. Corrosion sensors are broadly classified as direct or indirect based on detection through environments or consequences. The optimum type of sensors needs to be selected in this scenario (Figure 1).  
Figure 1: Categories of corrosion sensors and their characteristics with the causes and consequences.