Behavior

Aluminium and all its alloys generally have an excellent response to all types of external agents. Its layer of natural aluminium oxide, which is self-passivating, protects it against corrosion. In the following texts and tables you can find out more details about the response of aluminium to corrosion, organic and inorganic substances and food products.

> Response to corrosion 
> Response to food products 
> Response to inorganic substances 
> Response to organic substances

The marine environment is an aggressive one for most materials, including metal, wood, plastic, etc. with maintenance costs being higher for some than others.

This is why better results are obtained from products with "marine quality" because this "label" means that its quality has been proven in marine environments, which is why there are marine paints, marine bronze and also, for half a century, marine aluminium alloys that offer excellent resistance to corrosion in hostile environments, such as marine ones.

CHARACTERISTICS OF MARINE ENVIRONMENTS

The aggressiveness of marine environments for metals is due to the abundance of chlorides ("Cl") in seawater, with amounts of around 19 grammes per litre, in the form of sodium chloride, salt, magnesium chloride, etc. In fact, it is in marine environments where they are balanced, being made up of:

• Dissolved mineral salts, around 30 to 35 grammes per litre.
• Dissolved gases, which are 5 to 8 ppm oxygen.
• Decomposing organic materials.
• Mineral substances in suspension.

As a whole, it constitutes a highly complex medium where the influence of each chemical factor (composition...), physical factor (temperature, pressure...) and biological factor (fauna...) on the corrosion response of metals is not really separable or quantifiable independently.

The aggressiveness of the marine atmosphere is accentuated by humidity and splashes from fine drops of seawater carried by the wind. The effect of the marine atmosphere depends on the direction and intensity of prevailing winds and is much higher a few kilometres from the coast.

Salinity varies from one sea to another, for example, 8 grammes per litre in the Baltic Sea (which allows it to easily freeze) and 41 grammes per litre in the Mediterranean Sea, although this does not have a significant influence on the response to corrosion of aluminium alloys. The same happens with the seawater temperature on the surface, which varies according to the season and latitude, from a few degrees centigrade in the North Sea to 25ºC in the tropics.

Experience shows that resistance to corrosion is similar in the tropics to what it is in the North Sea and in Spain to what it is in the Pacific. Nothing makes it possible to differentiate the mere fact of marine environment from foreign elements that contaminate it and that locally modify the composition of the seawater or the local atmosphere as well as effluent or gaseous emissions. Knowledge of the basic data on the corrosion of aluminium and its alloys in the marine environment, as well as with respect to some rules, which are very easy to apply, will avoid certain classic disadvantages in the use of aluminium in the marine environment.

To this effect it is necessary to remember the importance of the natural oxide layer in the response to corrosion of aluminium and its alloys. Below we will discuss the forms of corrosion that can be observed in the marine environment with particular emphasis on galvanic corrosion.

_The purpose of the aluminium oxide layer

The good response to corrosion of aluminium is due to the permanent presence on the metal of a layer of natural oxide made up of aluminium oxide (alumina) that makes it passive to the action of the environment.

Although very small in thickness, between 50 and 100 Angstroms (or 50 to 100 billionths of a metre), the oxide film forms a barrier between the metal and the environment and is formed instantaneously from the time when the metal comes into contact with an oxidising medium: oxygen in the air, water, etc. The physical and chemical stability of the oxide layer is therefore of great importance in the corrosion resistance of aluminium. It depends on the characteristics of the medium, one of which is pH and also the composition of the aluminium alloy.

INFLUENCE OF pH

The dissolution speed of the oxide layer depends on the pH. This is high in an acid medium and in an alkaline medium but is weak in media close to neutrality (pH 7). Seawater has a pH of 8 - 8.2. The oxide layer is therefore very stable in seawater and in the marine environment.

Contrary to a widespread idea, pH is not only a criterion to take into account to predict the response of aluminium in an aqueous medium: The nature of the acid or base plays a predominant role. This is very important when choosing a cleaning or stripping product for aluminium.

In this way, while hydrazides such as sulphuric acid strongly attack aluminium (especially if they are in a concentrated solution), concentrated nitric acid, on the other hand, has no action on aluminium, since it contributes by its oxidising function to slightly strengthening the oxide layer and can be used in a concentration higher than 50% for the stripping of aluminium and its alloys. Organic acids have only a slight action on aluminium. This is also true in an alkaline medium: caustic soda and potash severely attack aluminium. Concentrated ammonia has a much more moderate action.

INFLUENCE OF ADDITIVES

Certain additives of aluminium alloys reinforce the protective properties of the alumina film.

Others, on the other hand, weaken it. In terms of the former, magnesium should be mentioned, the oxide of which, magnesia, is combined with the alumina. The improvement of the protective properties of the natural oxide film is what explains the optimum performance of the response to corrosion of aluminium-magnesium alloys from the EN W 5000 (Magnealtok) family, such as 5005 (Magnealtok 10), 5052 (Magnealtok 25), 5754 (Magnealtok 30), 5154 (Magnealtok 35), 5086 (Magnealtok 40) and 5083 (Magnealtok 45).

On the contrary, copper is one of the elements that weaken the properties of the oxide layer. This is the reason why its use in a marine environment is completely advised against, without special protection, the aluminium-copper alloys from the EN AW 2000 family (Cobrealtok 07-11-14-17 and 24) and the aluminium-zinc alloys from the 7000 family with the addition of copper.

FORMS OF CORROSION

Here, we will only mention the forms of corrosion that can be found in the marine environment in the extrusion and rolling alloys of the following families: 1000 (Pure Aluminium), 3000 (Aluminium-Manganese), 5000 (Aluminium-Magnesium) and 6000 (Aluminium-Magnesium-Silicon) and moulding alloys with silicon or magnesium.

_Uniform corrosion

This type of corrosion takes the form of a decrease in regular and uniform thickness over the entire surface of the metal. The dissolution rate may 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 a marine environment, whether immersed in water or subjected to the marine atmosphere, uniform corrosion is negligible. It cannot be measured.

_Pitting corrosion

This is a very localised form of corrosion common to many metals. It consists of the formation of cavities in the metal, in which the geometry varies according to a number of factors inherent in the metal (nature of the alloy, manufacturing conditions...) or 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 media: surface water, seawater, air humidity, etc.

Unlike most other metals, this form of corrosion draws attention because the corrosion holes are always covered with very voluminous white pustules of Al(OH)3 gelatinous hydrated alumina. The volume of the pustule is larger than the underlying cavity.

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

What it interests the user to know is the penetration speed of the pitting where it started. Contrary to other metals whose corrosion products are soluble, such as case of zinc, those of aluminium, alumina Al(OH)₃, are insoluble in water, although once formed, they remain attached to the metal in the pitting cavities. Hydrated alumina significantly slows down exchanges between seawater or humidity in the air and the metal.

The pitting corrosion rate of aluminium and its alloys therefore decreases very rapidly in most media, even in seawater. The penetration measurements of the pitting made at regular intervals shows that the attack speed of the pitting is linked to time by a ratio of the type V = Kt 1/3.

Extensive experience in the use of unprotected aluminium in construction near the sea (roofs, flat roofs, etc.) and in marine construction confirms the results obtained in the laboratory or in natural exposure to corrosion over a long period: The depth of the pitting, once it has formed during the first few months, does not continue to evolve. This slowing down of the pitting corrosion rate explains that aluminium products can be used in some natural environments (rural atmosphere, marine atmosphere, seawater, etc.) without any protection for decades.

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

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