Localized Corrosion of Mooring Chain Steel in Seawater

Abstract

Corrosion of mooring chains is regarded as one of main threats to the offshore mooring systems. Localized corrosion is even more dangerous than uniform corrosion because it may not show significant mass loss but it can cause stress concentration and initiate cracks under force, leading to accelerated degradation of mooring chains. Localized corrosion of steel in seawater is influenced by many factors such as the local heterogeneities of the steel, and the local electrochemical and microbiological environments. It is 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 in the search for corresponding monitoring tools and mitigation methods. The corrosion behavior of chain steel grade R4 was studied in artificial seawater and artificial seawater containing microorganisms collected from a practice field. The corrosion behavior of the steel was studied using different techniques such as potentiodynamic polarization, linear polarization resistance measurements and electrochemical impedance spectroscopy. The microstructures such as inclusions and compositions of the chain steel were studied using SEM and EDS. The microbial cells were observed using epi-fluorescence microscopy. The corrosion morphology and pit geometry were investigated using photo-microscopy. The localized corrosion rate has been found to be much higher than the uniform corrosion rate of the steel in the seawater in the presence of bacteria. In the case of localized corrosion, applying uniform corrosion measurement techniques and formulas is not considered representative. The representative areas have to be introduced to match physical results with the measurements. Inclusions, such as MnS and TiVCr found in the steel have a critical influence on localized corrosion. The corrosion mechanism of the steel in seawater is discussed.

The Role of Metallurgical Features in the Microbially Influenced Corrosion of Carbon Steel: A Critical Review

“…While there have been various reports on the potential effects of inclusions and abiotic corrosion of carbon steels, there only a relatively small number of papers discuss how inclusions may affect/relate to MIC. The reports can largely be split into the following two categories: anecdotal evidence (e.g., references [163][164][165][166]) and more detailed laboratory studies. Anecdotal links between MIC of steels and the presence of inclusions have been discussed in field failure reports or tests with limited evidence to definitively verify whether inclusions were actually the cause of the observed corrosion.…”

Effect of long periods of corrosion on the fatigue lifetime of offshore mooring chain steel

“…The applied current density of 6.29×10 −4 A/cm 2 was calculated [24] to give an average material removal rate of 20 μm/day across the corroding section of a specimen. The current density corresponds to a potential of about 100 mV above the open circuit potential [25]. The nominal surface area for application of current on the specimens was recalculated after the precorrosion phase.…”

Non-Contact Acoustic Emission Monitoring of Corrosion under Marine Growth

“…Given the electrochemical nature of the corrosion process, electrochemical techniques find wide applications in the monitoring and controlling of corrosion [ 17 ]. Applications of potentiodynamic polarization (PDP) are reported in the literature [ 18 , 19 , 20 , 21 ]. However, the PDP is not considered a non-destructive technique and not fit for long-term monitoring [ 21 ].…”

“…Applications of potentiodynamic polarization (PDP) are reported in the literature [ 18 , 19 , 20 , 21 ]. However, the PDP is not considered a non-destructive technique and not fit for long-term monitoring [ 21 ]. Applications of less destructive (compared to PDP) techniques, such as Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS), are reported in the literature as alternatives to the PDP [ 17 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 ].…”

“…However, the PDP is not considered a non-destructive technique and not fit for long-term monitoring [ 21 ]. Applications of less destructive (compared to PDP) techniques, such as Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS), are reported in the literature as alternatives to the PDP [ 17 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 ]. Nonetheless, in order to retrieve the corrosion rate at the monitored area, EIS have to be interpreted with the help of Tafel slopes that are usually obtained from the PDP curves [ 17 , 21 ].…”