In the presence of water most metals corrode: iron and steel rust, galvanized steel forms “white rust” when the zinc coating is exposed to moisture in the absence of air, copper turns green, and aluminum will stain under certain conditions. Sources of moisture can be external (obvious sources such as precipitation and water leaks) or related to humidity causing condensation.
Humidity is a measure of the amount of water vapour in the air. Specific humidity is expressed as grams of water vapour per kilogram of air. In the atmosphere the ratio can vary from nearly zero (in deserts or polar regions) to as much as 30 grams per kilogram (in warm, tropical climates).
Relative humidity, expressed as a percent, also measures water vapour, but relative to the temperature of the air. In other words, it is a measure of the actual amount of water vapour in the air compared to the total amount of water vapour that can exist in the air at a given temperature. Warm air can possess more water vapour than cold air, so with the same amount of humidity, air will have a higher relative humidity if the air is cooler and a lower relative humidity if the air is warmer.
The Dew Point Temperature is the temperature to which the air must be cooled in order for that air to be saturated and for condensation to start to form. Condensation will form on any object when the temperature of the object is at or below the dew point temperature of the surrounding air.
Dew point, relative humidity and temperature are all related. There are extensive tables, easily accessible on the internet, that provide the specifics. An example:
- At 27°C, and relative humidity of 75%, the dew point is 22°C
- At 27°C, and relative humidity of 45%, the dew point is 13°C
The most reliable way to anticipate conditions conducive to condensation is to measure the dew point temperature in the area of interest. There are units that will do this using full time monitoring and portable indicators that can be utilized on a spot basis.
Fundamentally, controlling condensation can be done by heating the ambient area to keep the objects above the dew point temperature or by reducing the dew point temperature of the air by implementing an air-drying system. Airflow helps by reducing layers of stagnant cool air surrounding cold objects and raising their surface temperature. It can also help to replace humid air with drier air from outside under appropriate conditions. When condensation can’t be prevented elevated airflow can dry the water that forms on surfaces.
In practical terms, when storing metal, it is detrimental to leave warehouse doors open especially during the spring and fall months when there may be significant differences in temperature between day and night. During the night, cold air enters and starts cooling the metal. During the day, if the temperature of the air increase rapidly the dew point rises quickly. But the temperature of the metal increases at a much slower rate and this sets up the condition where water begins to condense. Similar circumstances can occur during loading and unloading. If wrapped material is being transported it is prudent to leave the wrapping on until the material has reached ambient temperature.
Dessicants are substances which draw and retain moisture from the environment and prevent it from reacting with the metal surface. VCI’s (volatile corrosion inhibitors) are substances that slowly release a corrosion preventative compound that is capable of protecting metal surfaces. The suitability of these methods depends on the application and the circumstances.