Aluminum and its alloys are by nature a metal that protects itself from oxidation. In the presence of atmospheric oxygen, the metal reacts chemically and almost immediately produces a uniform layer of aluminum oxide, which is resistant, does not easily detach, and is also transparent. However, when the metal is in contact with sufficient water for a prolonged period of time, the oxygen present in it produces a chemical reaction that generates a layer of aluminum hydroxide that is usually white in color. Although on some occasions it can be brown, black, or show a combination of shades.
Moisture stains on aluminum
The moisture stains that appear on the surface of the aluminum do not have any effect on the chemical or mechanical properties of the aluminum. However, they are sometimes inconvenient for aesthetic reasons or for subsequent surface treatments.
Moisture stains on aluminum are undoubtedly due to the presence of water. This can be present for various reasons, some very obvious such as exposure to rain or snow during transport or storage, roof leaks, leaks in storage area pipes, water splashes from nearby machines or processes, among others. Therefore, it is of utmost importance to take precautions when packing, shipping, transporting, unloading and storing aluminum, always considering the necessary precautions to avoid contact with water.
In the case of flat aluminum in roll or sheet presentation, if contact with water occurs, it will remain hidden between the surfaces that form the coil or between the sheets of a lot, the presence of moisture for a sufficient time will produce the chemical reaction described at the beginning, which will subsequently generate a symmetrical stain on the surfaces of both sides.
Stains in dry environments
On some occasions, stains may appear in an apparently dry environment and without the presence of water sources as described above, this can occur due to the physical phenomenon of Water Condensation, this source of moisture is the most difficult to understand and control.
The atmospheric air contains water in the form of vapor, the amount of water vapor that the air can contain depends on its temperature, the higher the temperature, the higher the percentage of humidity that it can keep dissolved. The proportion of water vapor that the air contains at a given moment compared to the maximum it can keep dissolved is named Relative Humidity and is measured by a hygrometer as a %.
Dew point of the air
The Dew Point is the temperature at which water vapor begins to convert from vapor to liquid (condensation) and this point is affected by the relative humidity and air temperature.
So, if a sufficiently cold roll is "exposed" to air conditions of a warehouse with a relative humidity and temperature equal to or lower than the dew point, drops of condensed water will appear on all the exposed surfaces of the roll, even the water in the form of drops that is generated at the edges of the roll would be absorbed by capillarity into the coil, generating the moisture stains.
Condensation is more likely during the cold winter months; let's explain more with an example: if a truck carries a shipment of aluminum rolls on a cold day where the temperature is -1°C, the aluminum rolls will take that temperature after at least 12 hours of transfer, when the truck arrives at the destination it is immediately taken to the unloading warehouse which is at a temperature of 16°C and a relative humidity of the environment of 50%. Observing the table in figure 3 we can see that the dew point for these conditions is 5°C, therefore, as the metal rolls are at a temperature of -1°C and this value is lower than the dew point of the air, then the conditions are ideal for the water vapor contained in the air of the environment to condense on all the surfaces of the roll; the condensed water would begin to appear in the form of microscopic drops that will increase in size as time passes.
The phenomenon in the previous example is exactly the same as what happens when we take a soda can out of the refrigerator and a few moments later the surface of the can begins to show water drops on the outside of it.
Recommendations to avoid moisture stains on aluminum
With the above information and explanation we can make some recommendations that will help to avoid the appearance of moisture stains on aluminum and protect the valuable metal:
- Always protect aluminum packages from the environment by means of a suitable plastic packaging that prevents the metal from coming into contact with humid air.
- Pack the metal together with desiccant capsules (silica gel) that will help to prevent the presence of moisture
- Store and transport the metal out of reach of water sources, leaks, splashes or any other contact with water.
- In cold and humid places, consider precautions in the movement of the metal between cold and hot areas of the plant to avoid condensation in the packages
- When using a package partially, ensure an efficient re-packaging that prevents the appearance of moisture in the metal.
- In the event of wet or damp material, immediate drying and cleaning of the metal should be done to prevent the appearance of stains.
The purpose of this document is to help users and personnel in the aluminum supply chain; the recommendations should be considered under the judgment of the person applying them.
This information is supported by commercial experience, as well as publications from the Aluminum Association.
