# Review of Structural Health Monitoring Techniques in Pipeline and Wind Turbine Industries
## 2 Health Monitoring Techniques in Pipeline Industry  
### In Pipeline Industry  
Corrosion is a common phenomenon in places that contain oil. Recently, concentrations of corrosive substances have increased dramatically in crude oil. Materials such as sulfur and chloride and the increasing acidity are some of the common reasons. Several types of degradations inside refineries can take place with the varying environmental effects on naphthenic acids present in oils within the temperature range of 150-400 \({}^{\circ}\)C. These types include non-uniform, uniform, or localized pitting. This corrosion needs to be correctly and efficiently measured with the application of improvised techniques, and thereafter, the risk is to be calculated and minimized [11; 12; 13; 14]. The production of fuel is also becoming increasingly difficult to this enhanced corrosive nature. The presence of corrosive elements in the oil is somewhat difficult to predict as it depends on a number of factors especially when it is caused by the naphthenic acids [15; 16; 17; 18; 19; 20]. Some parameters that affect naphthenic acids corrosion are given in Table 1.  
Hydrogen cracking is another common problem in the pipeline industry. It could also be termed as cold cracking or delayed cracking. In carbon or low-alloy steels, hydrogen cracking occurs as atomic hydrogen diffuses and forms molecular hydrogen in it. Molecular hydrogen formation can be facilitated by inclusions or trap sites. Hydrogen cracking may also occur in the absence of any tensile stress. Molecular hydrogen formation pressurizes the material internally and starts cracking. A hard brittle structure, hydrogen produced by welding, and the tensile stress that acts on the welding joint are the three major factors that give rise to cracking. Hydrogen cracking is also caused by hydrogen strain (change in dimension of the pipeline due to tensile stress). Hydrogen cracking is a form of embrittlement of hydrogen common to carbon steels that originates in a step-like way from the propagation and linkage of small laminar cracks resulting from hydrogen pressurization trap sites. The strain of the internal hydrogen pressure inside laminar cracks increases as the hydrogen concentrations increase, causing adjacent cracks to be connected. The crack generally initiates in the heat-affected zone in the C-Mn steel, but it could also propagate into the weld metal. These cracks are linked with the coarse-grained region and could be intergranular, transgranular, or a mixture. Five types of hydrogen-assisted impairment for metals and alloys are being considered by the ASM materials handbook; these are: (1) \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Potential Target** \\ \hline Temperature & Up to 400 \({}^{\circ}\)C \\ Thickness Precision & 0.05 mm \\ Spatial Resolution Precision & 0.05 mm width and 0.05 mm length \\ Pipe Wall Thickness & 3–25 mm \\ Pipe Diameter & \textgreater{}100 mm \\ Metallurgy & Low-Alloy Steel (\(<\)9\% C \& \texttexttextless{ }\textless{}2.5\%\) Mo) \\ \hline \end{tabular}
\end{table} 
Table 1: The parameters that affect the napthenic acids corrosion. hydrogen attack, (2) hydrogen-induced blistering, (3) hydride formation, (4) cracking from precipitation of internal hydrogen, and (5) hydrogen embrittlement [21]. Internal hydrogen embrittlement (IHE) is a part of hydrogen-assisted cracking (HIC), which is defined as the accretion of hydrogen, which already exists near large stress concentration sites and hydron environment embrittlement (HEE), elucidating cracking because of hydrogen sulfide or the exposure of hydrogen [22]. In the case of high-strength steels, hydrogen embrittlement is indeed a challenge, as the saline water along with other corrosive chemicals come into contact with it, which promotes the absorption of atomic hydrogen [23].