Search

This article presents a synthesis of recent studies focused on the corrosion product layers forming on carbon steel in natural seawater and the link between the composition of these layers and the corrosion mechanisms. Additional new experimental results are also presented to enlighten some important points. First, the composition and stratification of the layers produced by uniform corrosion are described. A focus is made on the mechanism of formation of the sulfate green rust because this compound is the first solid phase to precipitate from the dissolved species produced by the corrosion of the steel surface. Secondly, localized corrosion processes are discussed. In any case, they involve galvanic couplings between anodic and cathodic zones of the metal surface and are often associated with heterogeneous corrosion product layers. The variations of the composition of these layers with the anodic/cathodic character of the underlying metal surface, and in particular the changes in magnetite content, are thoroughly described and analyzed to enlighten the self-sustaining ability of the process. Finally, corrosion product layers formed on permanently immersed steel surfaces were exposed to air. Their drying and oxidation induced the formation of akaganeite, a common product of marine atmospheric corrosion that was, however, not detected on the steel surface after the permanent immersion period.

In line with the recent industrial trends of hyperconnectivity, 5G technology deployment, the Internet of Things (IoT) and Industry 4.0, the ultimate goal of corrosion prevention is the invention of smart coatings that are able to assess their own condition, predict the onset of corrosion and alert users just before it happens. It is of particular interest to tackle corrosion that occurs in non-accessible areas where human inspectors or handheld devices are useless. To accomplish this, a variety of technologies that are embedded or could potentially be embedded into the coatings are being developed to monitor coating condition, which are based, for instance, on the evolution of electrochemical or mechanical properties over time. For these technologies to be fully embedded into the coatings and work remotely, solutions are needed for connectivity and power supply. A paradigm shift from routine prescheduled maintenance to condition-based preventive maintenance could then become a reality. In this work, the technologies that enable the in-service monitoring of organic anticorrosion coatings were compiled. Soon, some of them could be integrated into the sensing elements of autonomous, connected neural-like networks that are capable of remotely assessing the condition of the anticorrosion protection of future infrastructures.

International audience

Corrosion product layers formed on mild steel coupons after 6 years of permanent immersion at shallow depth in marine sediments were characterized by X-ray diffraction and μ-Raman spectroscopy. They proved mainly composed of FeS which testifies for an intense sulfate-reducing bacteria activity. Other identified compounds were siderite FeCO3 and four varieties of green rust associated with local variations of composition of the environment. The corrosion rates determined by residual thickness measurements were higher on the lower half of the coupons (average 40 μm/yr, local maximum 140 μm/yr) and smaller on the upper half, suggesting that differential aeration effects took place.

Corrosion product layers formed on carbon steel coupons after 6–8 years of permanent immersion in natural seawater were characterized by X-ray diffraction and μ-Raman spectroscopy. Some of the coupons showed blisters of corrosion products on a surface covered elsewhere by a much thinner layer. In anodic zones, the formation of the sulfate green rust proved to be favored at the expense of that of magnetite. In contrast, the formation of magnetite was favored in cathodic zones, together with that of aragonite and carbonated corrosion products. The galvanic effects are then self-sustaining via their influence on the corrosion products.

International audience; Submerged harbor steel structures often employ cathodic protection using galvanic anodes to guard against corrosion. A laboratory experiment, with three different cathodic protection config-urations by galvanic aluminum-based anodes, was performed to evaluate the potential metal trans-fer from the anodic alloy dissolution into the surrounding marine water. The anode dissolution rate is proportional to the imposed current demands and induced a significant Al, In, and Zn trans-fer in the dissolved and particulate fractions of the corrosion product layers covering the anode surface. These layers were poorly adherent, even under low hydrodynamic conditions. Conse-quently, at the anode vicinity, the suspended particle matter and dissolved fraction of surrounding marine waters showed strong enrichments in Al and Zn, respectively, the values of which could potentially affect the adjacent biota. After the anode activation period, however, the metal inputs from galvanic anode dissolution are rapidly diluted by seawater renewal. At regional scale, these metal fluxes should be negligible compared to river and wastewater fluxes. These results also showed that it is difficult to assess the impact of the anode dissolution on the concentrations of metals in the natural environment, especially for metals included in trace amounts in the anode alloy (i.e., Cu, Fe, In, Mn, and Si) in the aquatic compartment.

An experimental study was conducted to determine the influence of temperature on crevice corrosion initiation for Alloy 625 (UNS N06625) in natural seawater. These tests showed that there was a critical potential-temperature-time relationship needed to initiate crevice corrosion. The potential necessary to cause crevice corrosion on Alloy 625 decreased (became less noble) when the temperature was increased from ambient to 40°C. The crevice initiation potential decreased from 300 mV for ambient temperature seawater to 100 mV for 40°C seawater. Crevice initiation potentials were essentially unchanged between 40°C and 65°C, while the time required to initiate crevice corrosion decreased as temperature increased. In a second aspect of this work, natural seawater exposure studies were conducted to determine if there is a mechanistic connection between ennoblement (the gradual elevation of corrosion potential that occurs during long-term continuous immersion in natural seawater) and crevice corrosion i...

Publisher Summary This chapter mentions that Delegation Generale pour l' Armement (DGA) and DCN have developed a new Al–Ga low voltage anode that is in use to protect marine structures. After the development of the first industrial anode version with plain rod inserts, additional studies were launched with the aim of optimizing the insert–anode bond. Some of the conclusions drawn from both laboratory studies and field experience are: (1) Use of plain rods as inserts has proved to be unsatisfactory as penetration of seawater at the insert–anode interface occurs, thus reducing the performance of the cathodic protection system. All the attempts regarding modification of the insert surface preparation, nature or geometry did not improve the behavior of such anodes used in severe cyclic pressure conditions. (2) Although expensive, treating the anodes with plain inserts by Hot Isostatic Pressure could be a promising orientation. However, more tests are necessary to reach a final conclusion. (3) A new anode design with stainless steel fittings replacing the insert was developed, qualified, and installed on a new marine structure. Adequate continuity between the anode and the structure to be protected was achieved; thus ensuring a good protection level. If this option has proved to be an efficient one, it is, however, a costly solution. (4) Laboratory tests have shown that the use of tube inserts is a promising option. Further investigations on Al–Ga industrial versions as well as supplier qualification are necessary before any use in service.

Publisher Summary This chapter focuses on the 70/30 copper–nickel seawater piping systems and highlights the use of descaling agents and their effects on corrosion properties. Use of copper–nickel alloys in seawater-cooled heat exchangers is common due to the inherent properties of these alloys—namely, a relatively low corrosion rate, antifouling properties, and erosion–corrosion resistance relative to other alloys. The main causes of failure of copper–nickel alloy components in seawater are usually attributed to either erosion–corrosion at extreme velocities or the pitting phenomena. It is well established that these failures are generally due to detrimental seawater conditions, such as the presence of sulphides or poor designs resulting in extreme flow velocities. Periodic cleaning is necessary on seawater cooling systems due to the gradual formation of scaling and macro-fouling during operation. Several chemicals and procedures can be used to remove these deposits and growths, the most common products being hydrochloric, citric or sulfamic acids. Hydrolancing and mechanical cleaning are in use by the fleet, but these have major drawbacks in terms of costs and labor hours. In addition, most of these methods raise some safety and environmental issues. Three commercial descaling solvents, two hydrochloric acid-based and one phosphoric acid-based, have been identified that claim to be safe, environmentally friendly, and inexpensive and exhibit no detrimental effects on the materials.