behaviour

We will only mention here the forms of corrosion that can be found in the marine environment in extrusion and rolling alloys of the families 1000 (Pure Aluminum), 3000 (Aluminum-Manganese), 5000 (Aluminum-Magnesium) and 6000 (Aluminum-Magnesium-Silicon) and the alloys of molding with silicon or magnesium.

Uniform corrosion

This type of corrosion results in a decrease in the regular and uniform thickness over the entire surface of the metal. The rate of dissolution can vary from a few microns per year in a non-aggressive medium to many microns per hour depending on the nature of the acid or the base of the solution in the water. In the marine environment, whether immersed in water or under the effects of the marine atmosphere, uniform corrosion is negligible. It is not measurable.

Pitting corrosion

This is a very localized form of corrosion and common to many metals. It consists of the formation of cavities in the metal, in which the geometry varies according to a certain number of factors inherent to the metal (nature of the alloy, manufacturing conditions…) or to the environment: concentration of mineral salts, etc.

Aluminium is sensitive to pitting corrosion in media where the pH is close to neutrality, i.e. in fact in all natural environments: surface water, seawater, air humidity, etc.

Unlike other common metals, this form of corrosion is striking because the corrosion pits are always coated with very bulky white pustules of gelatinous hydrated alumina Al(OH)3. The volume of the pustule is more important than the underlying cavity.

Pitting corrosion develops in places where the natural oxide layer has defects: thickness reductions, local breaks, gaps, etc. caused by various causes related to the conditions of transformation or defective handling and alloying elements, etc. Experience shows that sanded, scratched areas in boilermaking, bending, welding operations are places where pitting can develop during the first weeks of immersion in seawater.

What interests the user is to know the speed of penetration of the bites where they have started. Unlike other metals whose corrosion products are soluble, such as zinc, those of aluminium, alumina Al(OH)₃, are insoluble in water, although once formed, they remain fixed to the metal in the pitting cavities. Hydrated alumina slows down the changes between seawater or air moisture and metal.

The rate of pitting corrosion of aluminium and its alloys therefore decreases very quickly in most environments, including seawater. Penetration measurements of the stings made at regular intervals show that the attack velocity of the stings is time-bound by a relationship of the type V= Kt 1/3.

The extensive experience in the use of unprotected aluminium in construction at the seaside (roofs, decks, etc.) and in shipbuilding, confirms the results obtained in the laboratory or in natural exposure to corrosion over a long time: The depth of the pits, once formed during the first months, does not continue to evolve. This slowing down in the rate of pitting corrosion explains why aluminium products can be used in some natural environments (rural atmosphere, marine atmosphere, seawater, etc.) without any protection for decades.

Corrosion occurs both in the marine atmosphere and in immersion in seawater. In both cases, the depth of any bites rarely exceeds one millimeter after several years.