Cathodic protection (CP) of carbon steel has been extensively studied for structures exposed to the open sea. However, the knowledge and data available for carbon steel cannot be directly applied to stainless steels, especially in the case of confined surfaces that are prone to crevice corrosion. In the context of stainless steels, confined surfaces (such as the contact surfaces of fasteners or valves) are critical zones as crevice corrosion represents the primary failure mode for passive alloys in seawater. With CP, the local potential achieved in confinement areas is highly dependent on various factors, including the actual geometries (crevice gap, length, local pH and Dissolved Oxygen (DO), ohmic drops, etc.). These factors can raise questions about the actual efficiency of CP if the current cannot reach the confined area. Conversely, if sufficient current can reach the confined area, the risk of hydrogen embrittlement (especially for strain-hardened or precipitation hardened alloys) should be taken into consideration. A specific experimental setup has been constructed to characterize the electrochemical behavior of stainless steel in a confined environment and the physicochemical properties of the confined seawater. The results have shown a complete deaeration of the confined seawater under all test conditions, along with an increase of the pH when CP is applied. The tests have also highlighted the significant impact of slight crevice gap variation on the current distribution. Based on the experimental findings, polarization curves representing confined environments have been generated. These curves have been integrated into a finite element model, allowing for the extrapolation of the experimental results to different crevice geometries. After a few centimeters, little to no current should be able to reach the confined surfaces if the crevice gap is inferior to 10µm. However, the risk of corrosion of stainless steels remains limited due to the local CP-induced chemistry at the interface. The CP also mitigates the ohmic drop in the confined area which also tend to reduce the risk of crevice corrosion. This work has enabled a deeper understanding of how cathodic protection prevents crevice corrosion on low-passive-grade stainless steels in natural seawater. It also provides additional information on the polarization behavior of stainless steels in confined areas on the crevice geometry.

Cathodic protection (CP) of carbon steel has been extensively studied for structures exposed to the open sea. However, the knowledge and data available for carbon steel cannot be directly applied to stainless steels, especially in the case of confined surfaces that are prone to crevice corrosion. In the context of stainless steels, confined surfaces (such as the contact surfaces of fasteners or valves) are critical zones as crevice corrosion represents the primary failure mode for passive alloys in seawater. With CP, the local potential achieved in confinement areas is highly dependent on various factors, including the actual geometries (crevice gap, length, local pH and Dissolved Oxygen (DO), ohmic drops, etc.). These factors can raise questions about the actual efficiency of CP if the current cannot reach the confined area. Conversely, if sufficient current can reach the confined area, the risk of hydrogen embrittlement (especially for strain-hardened or precipitation hardened alloys) should be taken into consideration. A specific experimental setup has been constructed to characterize the electrochemical behavior of stainless steel in a confined environment and the physicochemical properties of the confined seawater. The results have shown a complete deaeration of the confined seawater under all test conditions, along with an increase of the pH when CP is applied. The tests have also highlighted the significant impact of slight crevice gap variation on the current distribution. Based on the experimental findings, polarization curves representing confined environments have been generated. These curves have been integrated into a finite element model, allowing for the extrapolation of the experimental results to different crevice geometries. After a few centimeters, little to no current should be able to reach the confined surfaces if the crevice gap is inferior to 10µm. However, the risk of corrosion of stainless steels remains limited due to the local CP-induced chemistry at the interface. The CP also mitigates the ohmic drop in the confined area which also tend to reduce the risk of crevice corrosion. This work has enabled a deeper understanding of how cathodic protection prevents crevice corrosion on low-passive-grade stainless steels in natural seawater. It also provides additional information on the polarization behavior of stainless steels in confined areas on the crevice geometry. .

Once installed at the seabed, subsea rigid pipes in Carbon steel (CS), Low Alloys Steels (LAS) or in Corrosion Resistant Alloys (CRAs) can be wet stored for various durations. During this idle period, the lines can be filled with natural seawater generally treated with different types of chemicals, to mitigate localized corrosion initiation of stainless steel or the general corrosion of carbon steel. The chemicals are usually oxygen scavengers combined with or without biocides and corrosion inhibitor. Field experiences show that satisfying efficiency is obtained with some chemical’s combination. However, only few data are available in the literature to quantify separately the actual benefit of these chemicals and their combination. The needs to investigate sustainable alternative chemicals for these wet storage operations are also anticipated. In order to quantify the individual and synergistic effects of the selected chemicals, API 5L X65 carbon steel and UNS S31603 stainless steel were exposed for one year in seawater treated by different available combinations of oxygen scavenger, biocides, and corrosion inhibitors. Environmental-friendly chemicals have also been assessed as possible alternatives to conventional biocides. The corrosion rate of carbon steel coupons immersed in specific exposure cells, that simulate the confined exposure conditions during the wet storage, have then been determined by weight loss measurements after 3 weeks, 2, 6, and 12 months. The impact of the treatments on the corrosion rate and the bacterial activity have also been studied by Dissolved Oxygen Content (DOC) and Open Circuit Potential (OCP) monitoring, Electric Resistance (ER) measurements, and bacterial analysis. The current investigation shows that low DOC levels have been achieved which inhibited the so-called “biofilm ennoblement” of stainless steel. Thus, no corrosion occurred for UNS S31603. Such results are also applicable to most CRAs under similar exposure environment. The synergic effects of oxygen scavenger, biocides and corrosion inhibitor on the corrosion rate of carbon steel are also discussed.

Once installed at the seabed, subsea rigid pipes in Carbon steel (CS), Low Alloys Steels (LAS) or in Corrosion Resistant Alloys (CRAs) can be wet stored for various durations. During this idle period, the lines can be filled with natural seawater generally treated with different types of chemicals, to mitigate localized corrosion initiation of stainless steel or the general corrosion of carbon steel. The chemicals are usually oxygen scavengers combined with or without biocides and corrosion inhibitor. Field experiences show that satisfying efficiency is obtained with some chemical’s combination. However, only few data are available in the literature to quantify separately the actual benefit of these chemicals and their combination. The needs to investigate sustainable alternative chemicals for these wet storage operations are also anticipated. In order to quantify the individual and synergistic effects of the selected chemicals, API 5L X65 carbon steel and UNS S31603 stainless steel were exposed for one year in seawater treated by different available combinations of oxygen scavenger, biocides, and corrosion inhibitors. Environmental-friendly chemicals have also been assessed as possible alternatives to conventional biocides. The corrosion rate of carbon steel coupons immersed in specific exposure cells, that simulate the confined exposure conditions during the wet storage, have then been determined by weight loss measurements after 3 weeks, 2, 6, and 12 months. The impact of the treatments on the corrosion rate and the bacterial activity have also been studied by Dissolved Oxygen Content (DOC) and Open Circuit Potential (OCP) monitoring, Electric Resistance (ER) measurements, and bacterial analysis. The current investigation shows that low DOC levels have been achieved which inhibited the so-called “biofilm ennoblement” of stainless steel. Thus, no corrosion occurred for UNS S31603. Such results are also applicable to most CRAs under similar exposure environment. The synergic effects of oxygen scavenger, biocides and corrosion inhibitor on the corrosion rate of carbon steel are also discussed. 

High-strength steels (HSS) might undergo hydrogen embrittlement (HE) in seawater under cathodic protection (CP) with traditional Aluminum-Indium (Al-In) or zinc (Zn) galvanic anodes. The actual failure risk will also depend on metallurgy, mechanical properties and local loads. Even though super duplex stainless steels (SDSS) generally do not require CP, they can unintentionally become polarized due to electrical continuity with protected carbon steel structures. CP current demand and hydrogen evolution depend on various factors, like dissolved oxygen, temperature, flow, microbial activity, calcareous deposits, and depth. Hence, the risk of HE varies with exposure site and must be assessed. Aluminum-gallium (Al-Ga) is a low-voltage anodic material aimed at reducing protective potential to mitigate HE in high-strength steels in seawater. However, long-term field data is limited. This study aimed to gather long-term CP data for SDSS at different depths and seawater types, and to assess HE risk using Al-Ga vs. conventional anode materials. Instrumented anchored lines, including CP sensors, stressed samples, and environmental sensors were deployed for over a year in shallow, intermediary, and deep seawaters. Results showed varying cathodic current demands, with deep seawater having the highest. Biofilm formation on SDSS caused depolarization at all sites, with differences noted between sites. Al-Ga significantly reduced hydrogen-induced cracks in SDSS. ©

Many systems can be converted and used for different applications not initially planned. This is the case for production wells, sometimes converted into water injection wells and for which the production tubing material selection is clearly not adapted for prolonged contact with natural seawater. Oxygen removal treatment must be applied but precise control is not obvious and excursions above zero or close-to-zero oxygen can occur. The production tubing material S13%Cr are known to be sensitive to dissolved oxygen excursions in seawater, but data from the literature cannot precisely help in defining the allowed limits of use. Exploring the possibility to convert a production well into a treated seawater injection well then requires a careful assessment of the corrosion resistance limits of the involved alloys. A series of corrosion tests in treated seawater were designed to assess the limits of use of fast connector made of alloy UNS S41427. The corrosion tests were performed both at laboratory scale and on full-scale fast connectors in a treated seawater flow loops simulating service conditions. For all the performed tests, maintaining the dissolved oxygen content (DOC) at 15 ppb and below never led to localized corrosion and has been considered as a safe condition in terms of corrosion risk for alloy S41427 at ambient temperature. It was found that prolonged dissolved oxygen content (DOC) above 30±10 ppb may lead to initial crevice corrosion after only 4 h of exposure. Globally, a very good correlation between the laboratory and the full-scale test results was found. The critical crevice potential for alloy S41427 was significantly affected by the cleaning process of the tested coupons, while the stop of the corrosion was always measured for potentials reached at DOC < 10 ppb. The proposed methodology, involving both laboratory and full-scale tests, allowed to precisely quantify the limits of use of UNS S41427 in treated injection well. It could be used for any other material and applications to help at designing adapted and reliable engineering diagrams for material selections.

Many systems can be converted and used for different applications not initially planned. This is the case for production wells, sometimes converted into water injection wells and for which the production tubing material selection is clearly not adapted for prolonged contact with natural seawater. Oxygen removal treatment must be applied but precise control is not obvious and excursions above zero or close-to-zero oxygen can occur. The production tubing material S13%Cr are known to be sensitive to dissolved oxygen excursions in seawater, but data from the literature cannot precisely help in defining the allowed limits of use. Exploring the possibility to convert a production well into a treated seawater injection well then requires a careful assessment of the corrosion resistance limits of the involved alloys. A series of corrosion tests in treated seawater were designed to assess the limits of use of fast connector made of alloy UNS S41427. The corrosion tests were performed both at laboratory scale and on full-scale fast connectors in a treated seawater flow loops simulating service conditions. For all the performed tests, maintaining the dissolved oxygen content (DOC) at 15 ppb and below never led to localized corrosion and has been considered as a safe condition in terms of corrosion risk for alloy S41427 at ambient temperature. It was found that prolonged dissolved oxygen content (DOC) above 30±10 ppb may lead to initial crevice corrosion after only 4 h of exposure. Globally, a very good correlation between the laboratory and the full-scale test results was found. The critical crevice potential for alloy S41427 was significantly affected by the cleaning process of the tested coupons, while the stop of the corrosion was always measured for potentials reached at DOC < 10 ppb. The proposed methodology, involving both laboratory and full-scale tests, allowed to precisely quantify the limits of use of UNS S41427 in treated injection well. It could be used for any other material and applications to help at designing adapted and reliable engineering diagrams for material selections. .

The corrosion risk for stainless steel components is not the same in all seawaters, with more failures generally reported in tropical seas. In this study, the influence of biofilm on electrochemical behavior and corrosion resistance of passive films of high-grade alloys was studied in different seawaters, including temperate seawater (France-Brest, North Atlantic Ocean), tropical seawater (Malaysia-Kelatan, Meridional China Sea), and intermediate conditions in terms of temperature (Brazil-Arraial do Cabo, South Atlantic Ocean). The stabilized open-circuit potentials and the polarization behavior of high-grade stainless steels were measured as a function of temperature in all of the tested field marine stations, providing quantified data and direct comparison of the biofilm-enhanced corrosion risks. Significant differences were measured in tropical and in temperate seawaters in heated conditions. Above 37°C, the biofilm activity was much more pronounced in tropical seawater compared to Atlantic Ocean sites, leading to much higher localized corrosion risk. Crevice corrosion of eight high-grades passive alloys was also studied with the use of crevice formers specifically developed for tube geometries. Duplex UNS S32205, superduplex UNS S32750, hyperduplex UNS S33207 and S32707, and 6Mo stainless steels UNS S31266 have been evaluated together with Ni-based alloys UNS N06845 and N06625. In the more severe conditions, the high-grade alloys UNS S32707 and the 6%Mo UNS S31266, both with pitting resistant equivalent number (PREN) around 50, showed better performance than commonly used superduplex UNS S32750 and UNS S39274 (PREN 40). The corrosion results are discussed regarding the monitored biofilm-induced depolarization measured in the different test conditions.

The use of organic coatings in conjunction with cathodic protection (CP) for buried structures is the usual method for protecting steel against corrosion. When the organic coating loses its protective ability, regardless of the reason, the CP becomes the active protection, leading to a specific local environment. This environment can be characterized by high alkalinity, which can be detrimental for the coated structure, either by weakening the steel–coating interface or by the chemical aging of the coating. Thus, the coating must be compatible with CP and able to sustain aging under an alkaline environment. In this study, the susceptibility to alkaline aging and its consequences in regards to coating performance have been investigated for two commercial coatings used for buried structures—fusion bonded epoxy (FBE) and liquid epoxy (LE)—in free membrane and coated steel configurations. The results showed a clear impact of alkaline aging on the studied LE, leading to a significant reduction in coating resistance and ultimately, failure of the steel–coating interface, whereas the studied FBE remained stable. The presented results relate to a precise formulation of LE and FBE; however, the proposed chemical method appears to be relevant and shows the necessity of considering such specific aging results for coating specifications and improvements.

Chlorination is widely used in seawater systems to avoid fouling and associated microbial-induced corrosion. Free chlorine is a strong oxidizing agent that prevents biofilm formation on immersed surfaces when used above a certain content. However, the presence of residual chlorine associated with the relatively high chloride content in seawater significantly increases the risk of localized corrosion for most stainless steels. In the present study, a module initially developed to quantify the formation of electroactive biofilms on stainless steels has been used to assess the corrosiveness of chlorinated seawater. Both the electrochemical potential and the cathodic current were measured on super-duplex stainless steel as a function of residual chlorine levels and seawater temperatures. In parallel, long-term localized corrosion tests have been performed in simulated environments to assess the environmental limits for the safe use of high-grade stainless steels in chlorinated seawater. It includes crevice corrosion exposure tests using adapted ISO 18070:2015 crevice formers and internal tube pitting corrosion exposure tests in model tube heat exchangers simulating heat flux from 35°C to 170°C. The synergetic effect of residual chlorine content and temperature on the risk of localized corrosion has been quantified. Corrosion resistance properties are correlated to the electrochemical monitoring data, and the environmental limits of selected base materials stainless steels have been established for duplex stainless steel UNS S32205, super-duplex stainless steel UNS S32750, hyper-duplex stainless steels UNS S32707 and UNS S33207, and the high-grade austenitic stainless steel UNS S31266.