Corrosion and protection of metals in marine media

Corrosion is an interaction between a metallic material and its environment causing damage to the material, meaning changes to its properties that may lead to impairment of the function of the system of which it forms a part (NF EN ISO 8044 standard). At ambient temperatures, the phenomenon arises most often from the combined action of an aqueous environment and dioxygen in the air. It is thus clear, and must therefore be borne in mind at all times, that a metal’s resistance to corrosion is not an intrinsic property, in the same way as density, for example, but depends on a great many parameters, including those associated with an aggressive environment.

The term “marine corrosion” thus encompasses all physicochemical and mechanical interactions between materials in a totally specific environment: the marine environment. This is not simply a special case of aqueous corrosion and, typically, likening marine corrosion to the phenomena taking place in a solution of 0.5 mol.L ^–1 NaCl would be a very rough approximation. This distinction arises from the particular, unique properties of the marine environment.

As a general rule, that environment must be seen as a dynamic aqueous environment, containing dissolved salts, gases, organic compounds, undissolved solids and living microorganisms. One of our objectives will be to show, though the study of a number of real-life situations, how the mechanisms of corrosion and the various properties of the marine environment are linked. Protection against that damage, which must be thought about when the system is designed, and the selection of materials, are also intimately linked to the specific characteristics of the environment.

Thus the article starts by describing the marine environment in all its different aspects: physicochemical, biological and hydrodynamic. The main mechanisms of the marine corrosion of metallic materials and their relationship with the specific characteristics of the environment are then presented. Control measures to be taken when a structure is being designed, the rules to follow when selecting materials, and protective measures against corrosion in the marine environment are discussed in the third and final part.