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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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Product Response Product Response
Cooking oil   Butter  
Olives   Margarine  
Anchovies in pickling brine   Menthol  
Sugar   Jam  
Brandy   Honey  
Cocoa   Mustard  
Coffee   Cream  
Caramel   Bread  
Meat   Gherkins  
Beer   Fish  
Cereal   Cheese  
Egg white   Rum  
Cognac   Sucrose  
Curd cheese   Sea salt  
Chocolate   Cider  
Fruit essence   Soda  
Spinach   Whey  
Creme caramel   Tea  
Biscuits   Runner beans  
Gin   Vinegar  
Glucose   Wine  
Flour   Whisky  
Ice cream   Yoghurt  
Ice   Onion juice  
Lactose   Lemon juice  
Milk   Apple juice  
Yeast   Orange juice  
Spirits   Tomato juice  
Lemonade   Carrot juice  

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Product Response Product Response
Alkali acetates   Phosphorus hexasulphide  
Arsenic acid   Hydrogen sulphide (anhydride)  
Boric acid   Sulphurous hydrogen  
Carbonic acid   Calcium hydrosulphide  
Chromic acid   Barium hydroxide (solution)  
Hydrobromic acid   Potassium hydroxide  
Hydrochloric acid   Sodium hydroxide  
Hydrofluoric acid   Calcium hypochlorite  
Nitric acid (C>80% a 20ºC)   Potassium hypochlorite  
Nitric acid (dilute)   Sodium hypochlorite  
Nitrous acid   Sodium hyposulphite  
Orthophosphoric acid   Iodide (anhydride crystals)  
Perhydrochloric acid   Iodide (in alcohol tincture)  
Sulphuric acid   Arsenic iodide  
Sulphuric acid (in dilute solution)   Bleach  
Sulphurous acid (in dilute solution)   Mercury  
Chlorinated water   Carbon monoxide  
Rainwater   Aluminium nitrate  
Seawater   Ammonium nitrate  
Distilled water   Potassium nitrate  
Ammonia (gas)   Sodium nitrate  
Sulphur   Potassium nitrite  
Sodium bicarbonate   Sodium nitrite  
Sodium bisulphite   Calcium oxalate  
Sodium borate (cold solution)   Alkaline oxalates  
Ammonium bromide   Chromic oxide  
Potassium bromide   Lithium oxide  
Sodium bromide   Zinc oxide (<10 %)d> 
Calcium carbonate   Phosphorus pentoxide  
Calcium carbonate (lime)   Ammonium perchlorate  
Ammonium carbonate   Potassium permanganate  
Potassium carbonate   Hydrogen peroxide (concentrate)  
Sodium carbonate   Hydrogen peroxide (dilute)  
Calcium carbide (Anhydride)   Nitrogen peroxide (wet)  
Cement   Nitrogen peroxide (dry)  
Cement (wet)   Sodium peroxide  
Aluminous cement   Ammonium persulphate  
Potassium chlorate   Mercury salts  
Sodium chlorate   Magnesium silicate  
Chloride (Anhydride)   Potassium silicate  
Aluminium chloride   Sodium silicate  
Ammonium chloride   Ammoniacal solution  
Barium chloride   Ammonia solution  
Calcium chloride   Calcium sulphate  
Tin chloride   Aluminium sulphate  
Magnesium chloride   Ammonium sulphate  
Mercury chloride   Copper sulphate  
Zinc chloride   Magnesium sulphate  
Ferric chloride   Potassium sulphate  
Potassium chloride   Sodium sulphate  
Sodium chloride   Zinc sulphate (<10 %)d> 
Potassium chromate   Ferric sulphate  
Potassium dichromate   Ferrous sulphate  
Sulphur dioxide   Aluminium potassium sulphate  
Carbon disulphide   Sodium sulphide  
Potassium ferrocyanide   Calcium sulphide (Pure)  
Sodium fluorosilicate (<1%)d>  Ammonium sulphide  
Ammonium formate   Lime sulphur  
Ammonium phosphate (dibasic)   Sodium sulphur  
Tribasic sodium phosphate   Indian ink  
Phosphides (anhydrides)   Potassium thiocyanate  
Inorganic herbicides   Nitrogen vapours (dry)  
Rust   Plaster  

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Product Response Product Response
Essential oils   Dichloroethane (Anhydride)  
Sunflower oil   Dichloroethylene (Anhydride)  
Olive oil   Ethylene dichloride (Anhydride)  
Vegetable oil   Carbon disulphide  
Acetaldehyde (wet)   Enamel  
Acetanilide   Nut extract  
Butyl acetate   Diethyl ether (non-medicinal)  
Cellulose acetate   Ether  
Acetylene   Glycol ethylene  
Acetone   Soapbark extract  
Acetic acid (dilute)   Phenylamine (cold)  
Anthranilic acid   Phenol (concentrate)  
Benzoic acid   Phenols (<100ºC)d> 
Butyric acid   Formaldehyde  
Citric acid (cold)   Aluminium formate  
Stearic acid   Fuel oil  
Formic acid   Mercury fulminate  
Phthalic acid (pure)   Furfural  
Gallic acid   Town gas  
Glycolic acid   Gelatine (dry)  
Hydrocyanic acid   Glycerine (pure)  
Lactic acid (hot)   Rubber  
Malic acid (<10 %, frío)d>  Animal fat  
Margaric acid   Herbicides  
Oleic acid   Hexamethylenetetramine  
Oxalic acid   Aniline hydrochloride  
Palmitic acid   Hydroquinone  
Picric acid, pure   Indole  
Salicylic acid   Iodoform  
Succinic acid   Soft soap  
Tannic acid   Latex  
Tartaric acid (10%, cold)   Mannitol  
Valeric acid   Metaldehyde  
Fatty acids   Methanol (<75%)d> 
Eau de cologne   Methylamine  
Camphor   n-butanol  
Ethyl alcohol, 98% (cold)   n-e-isopropanol  
Methyl alcohol (98%, cold)   Naphthalene  
Benzoic aldehyde   Naphthylamine  
Aromatic amines   Nicotine  
Acetic anhydride   Nitroglycerine  
Aniline (liquid), cold   Nitrocelullose  
Anthracene   Urine  
Anthraquinone   Ethyl oxalate  
Clay   Paraffin  
Asphalt   Paraldehyde  
Benzene   Perchlorethylene (anhydride)  
Benzaldehyde   Pyrrole  
Bitumen   Kerosene  
Bromoform   Photographic reagents  
Methyl bromide   Resins  
Coal (wet)   Resorcinol  
Coal (dry)   Salicylaldehyde  
Cellulose (dry)   Aniline sulphate  
Wax   Nicotine sulphate  
Aromatic ketones   Sulphonal  
Potassium cyanide   Tobacco  
Chloroform (boiling), pure   Tannin  
Chloroform (wet), at 20ºC   Synthetic tannin  
Benzene chloride (dry)   Carbon tetrachloride  
Ethanol Chloride (Anhydride), cold   Tetramine  
Methyl chloride   Dyes  
Glue (neutral)   Thiourea  
Cork (wet)   Toluene  
Cork (dry)   Tricresylphosphate  
Cresol (less than 80ºC)   Triethanolamine  
Crotonaldehyde   Urea  
Ethylene dibromide