Atmospheric Corrosion

Atmospheric corrosion is defined as the result of the interaction between a material and the surrounding natural atmosphere. The apparent simplicity of the definition does not convey the great complexity of the phenomenon, due in part to the difficulty of experimentally reproducing an atmospheric environment. Indeed, the latter is characterized not only by its chemical composition, but also by all the climatic parameters such as rainfall, winds, sunshine and temperatures, which vary from one geographical point to another and, above all, over time. And yet, the economic stakes involved in controlling atmospheric corrosion are considerable: while the estimated cost of corrosion-related expenditure represents between 2% and 4% of gross national product in some countries, it is estimated that atmospheric corrosion accounts for the bulk of this expenditure. This applies not only to infrastructure maintenance (bridges, railways, buildings, industrial facilities, etc.), but also to the restoration and conservation of heritage objects. In the United States, for example, the airborne deterioration of the Statue of Liberty, resulting from galvanic coupling between the iron armature and the copper skin, required US$230 million to restore in 1981. To prevent damage caused by atmospheric corrosion, the Eiffel Tower in Paris is repainted every seven years, requiring 60 tonnes of paint to treat 250,000 m ² of surface. The 19th painting campaign began in 2009 and will last 18 months.

In addition, atmospheric corrosion, which is often the cause of failures affecting electronic systems or means of transport, considerably reduces the reliability of devices and thus poses a serious threat to personal safety. For example, in 1988, an Aloha Boeing 737 lost part of its fuselage in mid-flight at 8,000 m altitude, as a result of the effects of atmospheric corrosion on an ageing structure.

This type of corrosion is not a new scourge, having originated with industrialization. Indeed, the main factors initiating corrosion are air humidity and the presence of oxygen, degradation being merely the manifestation of a return of metals to their thermodynamically stable state, in the form of oxide, hydroxide or salt. Pollutants of exogenous origin can interfere with the general corrosion mechanism, accelerating or slowing down reactions. A more precise definition of atmospheric corrosion would therefore consider it to be the result of the interaction between a material and atmospheric oxygen, in an electrolyte made up of humidity and any pollutants.

In the case of metallic materials, this interaction results in a film of oxide, hydroxide or metal salt, which can be either compact and protective (stainless steel, aluminum, titanium...),...