Guidelines for Minimizing Water Staining
Aluminum is naturally attractive and resistant to corrosion. Unlike most other metals, it takes care of itself. When exposed to dry or moist air it combines with oxygen to form a tough, transparent, protective oxide coating. in the presence of moisture, aluminum will not rust as will steel but under certain conditions aluminum will stain. Such stains are usually found objectionable for esthetic reasons; they have no significant effect on strength but may cause processing problems where additional surface finishing or fabrication is to be formed. Virtually all staining problems occur during shipping, handling, or storage. Some of the incorrect practices that will result in water staining and ways to minimize or avoid the staining are discussed in the following text.
Causes of Water Stain
Aluminum and other metals can stain when water is trapped between wraps or sheets of other mating surfaces. If there is no air flow to remove the water, prolonged contact between the water and the metal causes a reaction between the two which results in water stain. There are two main sources of water staining:
Obvious sources are rain, snow and water leaks. Water coming in contact with the surface of the aluminum from sources such as a leaking tarp on a truck or a leaking roof in a storage area has the potential for causing water stain.
This is the more complex of the two sources and therefore harder to understand and control. Air normally contains moisture in the form of invisible water vapor. Relative humidity, expressed as a percentage, provides a measure of the amount of water vapor actually in the air compared to the total amount of water vapor that the air can hold. As the temperature of air increases, its capacity to hold water vapor increases.
Dew point is the temperature at which water vapor from the air begins to condense and is affected by the relative humidity and temperature of the air. The dew point can be determined from a chart (see Table 1), based on simple measurements of air temperature and relative humidity.
Water vapor condenses on the surface of a metal if the temperature of the metal drops below the dew point of the surrounding air. A familiar example of condensation is the fogging of one's eyeglasses upon entering a warm room after being in the cold outdoors. Another common example is condensation on a cold glass. As mentioned before, when the temperature of aluminum drops below the dew point of air, water comes out of the air and deposits on the surface of the aluminum. The temperature of the aluminum can drop below the dew point of the air under the following circumstances:
When storing metal, it is not recommended to leave warehouse doors open especially during the spring and fall months, when there are extreme differences in temperature between day and night. During the night, cold air enters and starts cooling the metal. During the day, if the temperature and humidity of the air increases rapidly, the dew point rises quickly. But, the temperature of the aluminum increases at a much slower rate; this sets up the condition where water begins to condense on the surface of the aluminum.
Obviously this process is not limited to fall and spring months, but may occur whenever there are large enough fluctuations in temperature and humidity.
Loading metal removed from a cool or cold storage area into a warm trailer or railroad car, on a humid day, can result in condensation on the aluminum. This can occur during any season of the year; however, for some areas it may be more pronounced in summer.
During Unloading - Moving Cold Metal Into Warm Storage:
Condensation at the unloading point is more likely to occur during the cooler months. The following example may help to clarify this point:
Metal at 16°C (60°F) in a warehouse is loaded into a trailer and shipped. The metal is in transit for about two days. The outside temperature is -1°C (30°F). Within a two-day transit period, the temperature of the metal gradually decreases to -1°C (30°F). When the aluminum reaches its destination, it is unloaded and moved directly into a warehouse where the temperature is 16°C (60°F) and the relative humidity is 50%. From Table 1, the dew point of the air is 5°C (41°F). Since the temperature of the aluminum is now lower than the dew point of the air, conditions are ideal for water to condense on the surfaces such as the edges of stacked sheets or coils. The condensed moisture can enter between the sheets and wraps by capillary action, and may produce water stain.
Table 1: Dew Point Calculator
Air Temperature in Degrees Celsius
Table 1: Dew Point Calculator
Air Temperature in Degrees Fahrenheit
Example: Read the air temperature in the left hand column and the humidity at the top of the achart above. If the temperature of the storage area is 13°C (55°F) and the relative humidity is 60%, the intersection of the two shows the dew point of the area to be 4°C (40°F). If the metal coming in is below 4°C (40°F), water will condense on the metal.
Packaged aluminum coils, flat sheet, and plate are wrapped with various papers, plastic films, or laminates that provide reasonable protection against moisture penetration during shipment and storage. However, such packaging may not be air-tight and changes in temperature or humidity may still result in condensation on the aluminum surfaces. Packaged aluminum, even if protected with "waterproof" materials, should not be stored outdoors.
Wrapping meterials should be left intact (and repaired if torn) until the aluminum is used. If moisture is condensing on the packaging, the moisture should be removed but not the packaging. If the packaging is soaked or if the aluminum is wet, then the procedures mentioned under prevention should be followed.
When surface quality is critical, aluminum flat sheets and plate are additionally interleaved with special paper, foam sheets or other materials to protect the surfaces against abrasion. Even though some interleaving papers contain an additive to inhibit water staining, the normal precautions during transit, handling, and storage should be followed.
There is normally a time limit on carrier claims. Therefore, incoming inspection becomes a vital part of the receiving process. Without incoming inspection the receiver cannot know what the condition of the metal is and how susceptible it may be to water staining. Without this knowledge the necessary steps to prevent the staining cannot be taken:
- If any physical or water damage is found on the skids, packaging or the metal, this should be noted on the receiving papers.
- Water vapor will condense on the surface of the metal if its temperature is lower than the dew point of the air in the storage area. It is important, therefore, to check the temperature of a few coils in every shipment and compare that temperature to the dew point of the storage area. (See Prevention Section under Receiving)
The temperature of the aluminum can be easily measured by using a probe thermometer, or by cutting a 6 inch by 6 inch flap in the wrapping paper and placing a contact, dial-type thermometer on the surface of the aluminum. After recording the temperature, it is important to reseal the package by replacing and taping the flap. If not repaired, condensate could run down the inside of the wrap and result in water stain.
There are three methods of preventing water stain:
- Prevent water in any form from coming into contact with the metal.
- If water is already in contact with the metal, remove it.
- Consider precautionary measures to minimize water staining.
Prevent water in any form from coming into contact with the metal
To prevent the water from getting between wraps, sheets or other mating surfaces, it is critical to prevent the temperature of the metal from getting below the dew point of the surrounding air.
Storage:Temperature and humidity variations within the warehouse should be kept to a minimum by keeping outside doors closed.
It is also advisable to have forced circulation heaters in the storage area to maintain the temperature of the metal above the dew point of the ambient storage environment. Without fans to distribute heat, storing aluminum near walls, doors, windows, and other cooler areas may provide the conditions for condensation. Neither packaged nor bare aluminum should be stored outdoors unless water stain is not detrimental for the end use.
Shipments: It is important to:
- Minimize in-transit times.
- Use insulated or heated trailers in the winter whenever possible.
- Check tarps on open-top trucks to make sure there are no holes. The carrier should re-tarp the truck after each intermediate delivery.
Receiving: If during the incoming inspection process the temperature of the metal is found to be below the dew point of the air in the storage area, the following steps should be taken:
- Do not move cold metal into a warm storage area (remember the fogging of the eyeglasses). Allow the metal to warm up slowly. This can be done by placing the unopened package in a cooler area, free from drafts.
- Check the condition of the metal every few hours to make sure no water has condensed on the surface.
If water is already in contact with the metal, remove it.
Steps should be taken to remove this water before it stains the aluminum.
Unfortuantely, there is no easy way to remove the water. Contacting the supplier may be helpful. The supplier is generally familiar with prevention techniques and can provide assistance. Some possible procedures that might be recommended are:
- If at all possible, process the aluminum immediately and dry it as it is being used. This is the only sure way of getting rid of the water.
- If in coil form, unskid and unpackage the aluminum, lay it on its side and use fans to blow air around it - hot air if possible.
- Consider precautionary measures to minimize water staining. If water stained metal is a major recurring problem, a way of minimizing the problem would be to coat the aluminum with a water stain preventative.
Consider precautionary measures to minimize water staining.
If water stained metal is a major recurring problem, a way of minimizing the problem would be to coat the aluminum with a water stain preventative.
Most aluminum suppliers have such preventatives but hesitate to use them without prior knowledge of a customer's operations to ensure that there are no compatibility problems. Some aluminum suppliers may not be able to coat all items.
Aluminum Water Stain Prevention
When you receive metal:
- Check for wetness.
- Is the metal wet? Is the wrapping paper puckered up or wet?
- If it is wet, note it on all copies of the receiving papers.
- Inform the Purchasing Department or Quality Control immediately.
- Check to see if the metal feels cold.
If it does:
- Tell your supervisor immediately.
- Leave the metal in a cool indoor area away from drafts to allow it to warm up slowly. (If this is not done, and the metal is put in a heated warehouse immediately, it may sweat and get water stained.)
- After the metal is reasonably warm (about a day later), move it to the warehouse.
When you move metal between areas:
Check to see if the temperature in the area the metal will be taken to is higher than the temperature in the area the metal is coming from. If the difference is more than 11°C (20°F):
- Only move as much metal as will be used immediately.
- Tell your supervisor.
- Leave the remainder of the metal where it is until ready for use.
Almetals Company hopes this information has provided a better understanding of water stain, its causes, and some of the steps that can be taken to prevent it.
Some of the recommended procedures may require additional efforts, but the value of the aluminum and assured production schedules may justify the suggested precautions.