## 1 Introduction  
Structural steel is the most widely used material of choice for the structures used in coastal and offshore locations. It offers many benefits, including durability and simplicity of fabrication, and facilitates relatively cost-effective solutions [1, 2]. Although structural steel offers many advantages, corrosion is a major problem for many industries, including the marine and offshore industry [3, 4]. It is the main cause of deterioration for waterfront steel structures. There are five other causes that deteriorate structural steel in the offshore industry: overloading, fatigue, abrasion, material loss and weakening of structural connections [5]. Recently it has been acknowledged that sheet piles made of structural steel are significantly more prone to advanced corrosion rates due to accelerated low water corrosion (ALWC) [2]. ALWC rapidly compromises the integrity of affected structures with major implications for safe operation and leads to significant costs for repair or replacement. ALWC is a type of microbiologically induced corrosion (MIC) that arises due to the presence of sulphates in marine environments. Sulphate reducing bacteria (SRB) convert sulphates into hydrogen sulphide (H\({}_{2}\)S), causing anaerobic corrosion. The H\({}_{2}\)S is converted by sulphide oxidising bacteria (SOB) into sulphuric acid (H\({}_{2}\)SO\({}_{4}\)). This electrolysis process never reaches the state of equilibrium due to the oxidation of hydrogen formed on the steel surface. The non-equilibrium state caused by this biochemical process accelerates corrosion [6]. The corrosion rates on steel pile surfaces vary non-linearly with increasing depth [2]. In order to reliably access sheet piles, determining the location of the most extreme values of thickness loss is of utmost importance [7]. Therefore, many researchers have adapted sheet pile terminology as shown in Figure 1[2; 6; 8]. Marine structure designers have considered corrosion conditions at five different vertical exposure zones in relation to environmental conditions. These exposure zones are defined below:  
1. Atmospheric zone: The atmospheric zone is located between the top of the pile and the splash zone as illustrated in Figure 1. In this zone, corrosion can take place at the encapsulation point where the pile is covered with concrete. 2. Splash zone: This exposure zone is most vulnerable to corrosion. Observations and findings to date suggest that the greatest corrosion rates and highest thickness loss of steel occurs at the splash zones. This is due to ALWC as explained earlier. 3. Tidal zone: This exposure zone is found between the mean high water (MHW) and mean low water (MLW) tides. The tidal zone is alternately submerged and emerged, and therefore the corrosion rate is relatively slow due to partial atmospheric corrosion. This zone may also be prone to concentrated corrosion on fixings such as ladder brackets due to the presence of dissimilar metals. 4. Mid-structure zone: This exposure zone is located just below the MLW. Corrosion is particularly active in this zone due to the differential aeration that is established with the upper structure. The highly oxygenated metal top, in the tidal zone, acts as a cathode, whereas the constantly submerged metal below acts as an anode, leading to corrosion. 5. Mud zone: The mud zone corresponds to the area just below the mid-structure zone where the pile is buried. Observations and findings to date suggest that this area usually requires very little maintenance. In this exposure zone, corrosion can advance in anaerobic conditions where the soils are either acidic or contain SRB, but such conditions rarely arise. This paper focuses on the corrosion defects in the tidal zone and mid-structure zone as discussed in Section 3.3. Currently, there are a variety of inspection and assessment techniques available to measure or monitor the deterioration of steel piles and obtain information on the mechanism of onset of corrosion.