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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.

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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.

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 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.