From the fundamental theory, the time of wetness (TOW) of a corroding surface is a key parameter, directly determining the duration of the electrochemical corrosion processes. This is a complex variable, since all the means of formation and evaporation of the surface electrolyte solution must be considered. The TOW refers to the period of time during which the atmospheric conditions are favorable for the formation of a surface layer of moisture on a metal or alloy. This moisture film is extremely important from the point of view of the chemical mechanisms of the corrosion process. For the purposes of the standard this has been defined as the time period during which the relative humidity is in excess of 80% and the temperature is above 0 degrees Celsius. This measure can either be determined from weather data or measured directly through various means.
TOW categories range from "Internal microclimates (T1) with climatic control" to "Part of damp climates, unventilated sheds in humid conditions (T5)." TOW units are hours per year (hours/year) when relative humidity (RH) > 80% and the temperature > 0oC.
The TOW is obviously strongly dependent on the critical relative humidity. Apart from the primary critical humidity (associated with clean surfaces), secondary and even tertiary critical humidity levels may be observed, where the corrosion rate increases abruptly. Hygroscopic corrosion products and capillary condensation of moisture in corrosion products are thought to account for these effects. A capillary condensation mechanism may also explain electrolyte formation in microscopic surface cracks and metal surface/dust particle interfaces. Other sources of surface electrolyte include chemical condensation (by chlorides, sulfates an carbonates), adsorbed molecular water layers and direct moisture precipitation (ocean spray, dew, rain).
Back to ISO 9223
Serious discrepancies between that artificial threshold have been observed in cold countries of the world. The presence of salts on a surface is a well documented and useful freezing depressant. Stating that there is no corrosion activities below zero degree Celsius appears to be a troubling flaw in the ISO 9223 framework. (back)
Thin film "invisible" electrolytes tend to form on metallic surfaces under atmospheric corrosion conditions, when a certain critical humidity level is reached. For iron, this level is around 60%, in unpolluted atmospheres and can be much less when the presence of hygroscopic salts on a surface stabilize the surface wetness even more.