Conditions of Aggressiveness

Environmental aggressiveness is a function of numerous factors that are not always independent of each other. They have, in fact, enormous and complex synergistic effects connected to both the macroclimate and to local microclimatic conditions that the structure itself helps create, such as humidity of the environment and its variability in time and place, the presence of chlorides and oxygen and the temperature. The following oudine summarises the environmental aggressiveness under the principal conditions of exposure. 

The environment is not aggressive if it is sufficiently dry. In the case of carbonated concrete that does not contain chlorides, the relative atmospheric humidity (R. H., expressed as percentage) below which the corrosion rate becomes negligible is about 70% and 60% R. H. in temperate and in tropical climates, respectively. If, on the other hand, the concrete contains chlorides, this is reduced to 60 % R. H. or, if the chloride level is very high, to even less than 50% R.H. 

In the presence of chlorides, the environment may be aggressive if the R. H. remains above 50 % (or even 40 % if the chloride content is very high and hygroscopic chlorides such as magnesium or ammonium chloride are present). Aggressiveness increases with humidity (until it reaches a maximum at R. H. of about 90-95 % for dense concrete and 95-98% for more porous concrete), with chloride content and with temperature. 

Conditions of exposure to marine atmosphere, even if not in direct contact with seawater, are aggressive. 

Conditions of contact with seawater and subsequent drying, as in the splash zone of marine structures or those found on the concrete slabs of viaducts where de-icing salts are used, are very aggressive. 

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Note The lifetime of a filament is several weeks under proper operating conditions. However, this may become drastically shortened by the harsh conditions of aggressive analytes or reagent gases in chemical ionization, too high emission current, and in particular sudden breakdown of the high vacuum. 

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