In the description of an environment and explanation of corrosion phenomena the most important data are represented by the combination of relative humidity (RH) and temperature (T), often described as temperature-humidity complex (T-RH). It is well known that in open air the metallic corrosion correlates with the time during which a thin film of electrolyte covers the metal surface (i.e., TOW), which determines the duration of the electrochemical corrosion process. According to ISO 9223 TOW can be calculated from the T-RH complex or measured with appropriated sensors. The metal surface real temperature, which effect is not included in ISO 9223:92, can provide very important information about the kinetics of the corrosion process.
During five years, metal samples (low carbon steel, copper and zinc) were exposed to tropical humid climate of the Peninsula of Yucatan, Mexico, in two typical environments, marine-coastal (MC) and rural-urban (RU). The registered metal temperature (Tm) and TOW (by copper/gold sensors, reported by ASTM Practice G84-89 (1999)) strongly depend on the kind of environment. Our registers show that each of the studied metals could be at different temperatures, even though they are exposed in the same location. It was also found that the distribution of TOW in different temperature ranges was needed in order to explain metal corrosion process and rates. SEM images reveal that the Tm and TOW influence the morphology of the corrosion products formed.