Alternating Periods of Dampness and Dryness

It is well known that the corrosion rate of common metals and alloys globally increases when submitted to repeated cycles of immersion and emersion in seawater, compared to permanent immersion. This also applies to atmospheric corrosion when the metals are submitted to alternating cycles of rain and sunshine. However, in this case corrosion is not as severe as observed in the case of alternated immersion-emersion in salt solutions. 

Whether the samples are placed on inclined stands in outdoor corrosion testing stations or in constructions (buildings, etc.), on sheet products, a significant difference in surface aspect is always observed between the faces oriented upside and downside. 

As an example, an alloy 6060 exposed at a 45° angle to industrial atmosphere for 1 year shows a pitting depth of 

The influence of alternate periods of dampness and dryness depends heavily on the climate when the relative moisture is high, the evaporation rate of the moisture films is slowed down, and in humid climates, the effect is therefore less pronounced than in a hot, dry climate the higher the frequency of the dampness and dryness cycles, the greater the surface of the metal affected by pitting corrosion (Rguie C.2.11). To put it differently a high frequency of dampness and dryness will yield a more uniform surface appearance. 

In general, aluminium and aluminium alloys, except alloys of the 2000 series and copper-containing 7000 series alloys, are much less sensitive to the effect of repeated dampness and dryness cycles than most other common metals. 

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