Blackening of Aluminium

Blackening is not a form of corrosion it is only an alteration of the visual properties of the outmost oxide layer [6]. It does not alter the corrosion resistance of aluminium in water. 

This unavoidable phenomenon has been observed since kitchen utensils made of aluminium began to be used [7]. To prevent blackening, it used to be recommended to boil 

On the other hand, water composition is a factor of paramount importance blackening occurs if the water contains bicarbonates HCO and if its pH is between 8 and 9. Blackening is said to be caused by the adsorption of bicarbonate ions at the porous natural oxide film. Blackening may also appear when aluminium is coupled with a less electronegative metal in water silver, stainless steel, or copper. On the other hand, aluminium that is cathodically protected by a sacrificial magnesium anode does not blacken in water [10]. 

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Blackening of aluminium is due to the structure of the uppermost layer of the natural oxide film. Upon immersion of a sample of 3003 in boiling distilled water, a layer of well-crystallised boehmite fibres will form (Figure D.1.4). When doing the same experiment with natural tap water, the layer will have a totally different aspect it will be amorphous, black, and built from porous cells [12] (Figure D.1.5). 

Blackening of aluminium is only an alteration of the surface appearance. While in certain cases it may degrade the esthetic aspect of an equipment, it does not imply a lack of maintenance and cleaning of the equipment. It does not favour bacterial growth. 

Like blackening (see Section D.1.5), this is an alteration of the visual properties of the natural oxide layer, due to its progression under the influence of atmospheric humidity. This is not corrosion. Water staining does not affect the resistance of aluminium to weathering. Anodising is a reliable means to avoid water staining of aluminium that is exposed to weathering. 

The development of applications of aluminium in constructions such as metallic fittings, foldable walls, shop equipment, urban furniture stems from the excellent resistance to atmospheric corrosion of anodic layers. These layers aim at maintaining the surface appearance over a long period of time by preventing blackening and pitting corrosion. 

Methyl formate HCO2CH3 does not attack aluminium. This has been shown by immersion tests over 4 months with several alloys such as 1050A, 6062 and 5754. Only a slight blackening of the metal was observed. The presence of traces of water does not modify the resistance of aluminium. Methylformate may be stored and transported in aluminium vessels. 

Diluted acetate solutions of sodium acetate CH3C02Na, ammonium acetate CH3CO2-NH4 and potassium acetate CH3CO2K have no action on aluminium at room temperature. At temperatures between 60 and 70 °C, the metal blackens and will suffer superficial pickling the degree of pickhng depends on the acetate. For example, the dissolution rate in a solution of 10% potassium acetate amounts to 1.25 mm per year. 

Since 1950, in certain food industries such as dairies, aluminium has been dropped in favoiu of stainless steel for two reasons blackening and cleaning (see Chapter G.6). 

Aluminium does not resist well in such a medium the thickness of the attack depends on concentration, contact time, and temperature. In addition, aluminium can also blacken (see Section D.1.5). 

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