Atmospheric corrosion (Topic 11013)

Aluminium alloys in the 6000-group are normally very corrosion resistant in the atmosphere and will not experience any severe corrosion even without a surface treatment. There can however be an aesthetic effect on the surface that makes it necessary to anodise or lacquer in order to protect the surface. In extremely corrosive industrial atmospheres or at severe marine sites, the corrosion rate can be significant.

The corrosion rate in the atmosphere develops similar to pitting. There is first a period of rapid corrosion and then the corrosion almost stops, unlike steel , where the corrosion will be more or less constant during the whole exposure. The decrease in the corrosion rate is however dependent on that corrosive species and dirt are not allowed to accumulate on the surface and thus create a poultice which is constantly wet. This creates a situation similar to a crevice with possibility for decreased pH and high concentration of corrosive species like chlorides in a marine atmosphere, where corrosion can proceed uninterrupted.

The atmosphere is normally divided into three different classes; rural, industrial and marine. The corrosivity of the atmosphere can vary a lot within each class depending on climate, level of pollution, distance to the sea, etc. In addition some atmospheres may be combinations of marine and industrial. This makes it difficult to compare results from different tests. Another problem is that the results are reported differently in different test programs and that only one alloy of interest is included in each test.

The results from two different tests are included in order to give an idea of the atmospheric corrosion rates and the influence on mechanical properties.

An artificially aged alloy AW-6063 has been tested for 12 years in different environments in the Netherlands. The changes in mechanical properties and the weight loss are shown.

The change in mechanical properties for an alloy AW-6082 in T4 and T6 tempers are shown. The specimens have been exposed for 20 years in marine environments in France. The maximum and average pit depths from the same test are given. The pit depths have been measured separately on the surfaces facing skywards and downwards, the specimens are mounted with a 45ƒ angle to the horizontal. On the AW-6082 alloy there are not great differences on the two surfaces, but generally both on other alloys in this test and in other tests the surfaces facing downwards have the deepest attacks. This is due to condensing moisture that does not evaporate as quickly on the downwards facing surfaces as on the upwards facing which are exposed to sun and wind. In addition will the rain have a cleaning and diluting effect on the skywards facing surface, this shows the importance of cleaning aluminium surfaces exposed to marine or polluted atmosphere.

References [41,42]