**1. Introduction**

Mooring chains are widely used to fix a floating production, storage and offloading (FPSO) system. Mooring chain steel has to withstand seawater corrosion and cyclic force loading during service. Marine corrosion, in particular localized corrosion, combined with mechanical loading is the main reason for mooring chain failures [1–4]. Ma et al. compiled a historical review of integrity issues of mooring systems [1]. They found that the chain, connector and wire rope are the top three components causing more incidents. Fontaine et al. undertook an industry survey of past failures and degradations for mooring systems of floating production units. They found that almost half of all failures were associated with chains and two out of three chain failures were related to corrosion and fatigue [2].

Many factors affect the corrosion of metals in marine environments. Marine environments include a number of zones, such as atmospheric, splash, tidal, submerged and bottom sediment areas. Some recovered chain links showed uniform corrosion in the splash zone and pitting corrosion in the submerged near-surface zone [5]. Microbial activity in concentrations around and below the low water zone leads to an aggressive form of con-

**Citation:** Zhang, X.; Noël-Hermes, N.; Ferrari, G.; Hoogeland, M. Localized Corrosion of Mooring Chain Steel in Seawater. *Corros. Mater. Degrad.* **2022**, *3*, 53–74. https:// doi.org/10.3390/cmd3010004

Academic Editors: Philippe Refait and Scott Wade

Received: 12 October 2021 Accepted: 8 February 2022 Published: 16 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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centrated corrosion, known as accelerated low water corrosion, which has been identified as microbiologically influenced corrosion (MIC) [6,7].

The steels for making mooring chains are classified so far by specified minimum ultimate tensile strength into five grades (R3, R3s, R4, R4s and R5) [8]. To achieve the required strength the chemical compositions and manufacture processes have to comply with the approved specifications [8,9]. Even so, the steels showed different corrosion rates in different seawater areas. It was reported that 120 mm diameter R4 chain serviced in the North Sea located in the Pierce field for 13 years has a corrosion rate of 0.53 mm/y of diameter reduction in the pitted area (for the worst case) [10]. Fontaine al. inspected 76 mm-diameter chain links of type R3 and ORQ grades in West African waters [11]. The long-term corrosion rate of these links was approximately 1.5 mm/y of dimeter reduction in the pitted areas, which is significantly higher than the corrosion wear allowance of 1 mm/y required for tropic waters [12,13].

Local corrosion attack may initiate at inclusions or grain boundaries due to a local electrochemical potential difference [14–16]. Avci et al. investigated MnS-mediated pit initiation and propagation in carbon steel in an anaerobic sulfidogenic media. They found that pitting on 1018 carbon steel was initiated within a 20–30 nm zone at the MnS inclusion boundary [17].

Jeffery et al. investigated the effect of microbiological involvement on the topography of corroding mild steel in coastal seawater and found that microbiological factors are responsible for the more severe pitting observed on the natural seawater coupons [18].

Melchers et al. investigated the corrosion of a working chain continuously immersed in seawater and developed a model to predict short- and long-term corrosion rates based on general corrosion loss [19]. For pitting corrosion, field data are necessary for calibration of the pit depth model [20].

A long-term field exposure test is a simple and valid method to verify the longterm corrosion performance of the steel but it is a time-consuming process. Mass loss gives an average corrosion rate which cannot reflect the localized corrosion rate. Sample surface analysis is necessary. Corrosion in seawater is an electrochemical process. Is there any electrochemical method that can be used to monitor the corrosion of offshore steel structures?

Potentiodynamic polarization (PDP) curve measurements under sliding were employed by <sup>L</sup>ópez et al. to investigate the tribocorrosion of mooring high–strength, low-alloy steels (grade R4 and R5) in synthetic seawater [21]. Based on the mass loss it was found that both the R4 and R5 steels have the same triboccorosoion behaviour in seawater. The PDP method is a destructive method that does not fit long-term monitoring.

Linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) are less destructive (compared to PDP) to the system to be studied since only a small dc or ac potential amplitude is applied. These techniques only give corrosion resistance of the steel. To convert the corrosion resistance to corrosion rate, Tafel slopes are necessary and are obtained from the PDP curves. Using these electrochemical techniques combined with microbiological and surface analytical techniques may be helpful in studying MIC [22].

So far, localized corrosion has been difficult to predict, and the mechanism is not fully understood. The aim of this work was to study the mechanism of localized corrosion on mooring chain steel in seawater which is helpful for the search of corresponding monitoring tools and mitigation methods.

Investigation into the mechanism of local corrosion was carried out by exposing samples of mooring chain steel grade R4 (named R4 according to the International Classification Society of Offshore Systems) in artificial seawater (SW) in the laboratory. During and after exposure, electrochemical and microstructure analyses were performed. Since MIC is expected to be one of the main causes of local corrosion, tests were also run with the addition of microorganisms cultured in the laboratory. These microorganisms, collected at Makassar Strait (Indonesia), contained different types of corrosive organisms.

The corrosion behavior of the steel in seawater was investigated using PDP, LPR measurements and EIS. Microbial attachment and biofilm formation were studied using fluorescent dye and epi-fluorescence microscopy [23]. The surface microstructures and compositions were analyzed using SEM and EDS, the corrosion morphology using photo-microscopy.
