Troubled Water


Minimizing the Effects of Trapped Water and Condensation on Metal Coil

Painted aluminum or steel coil can suffer water damage prior to fabrication or installation. But with proper routine handling and storage, metal roofing and gutter contractors can avoid most problems associated with improperly stored coil or fabricated product.

Coils and even bundled sheets can attract moisture and get wet in two ways. One way is when water gets inside the coil while the product is in transit or when it is exposed to the elements at a jobsite. The second way is when the coil is below the dew point of the local atmosphere and condensation occurs. Similar to the condensation of moisture onto a cold windowpane, condensation can occur when coils are shipped from one climate to another. If the temperature of a metal coil is lower than the dew point of the surrounding air, moisture from the surrounding air will condense onto the surface of the coil. This is a condition known as sweating.

The mass of a metal coil is capable of condensing large quantities of water under such conditions. Water can penetrate the laps either by capillary action or by penetration of high-humidity air into the coil laps and subsequent condensing of water on the surfaces between laps. It is important to store coils of coated steel and aluminum products in controlled environments so that the dew point of the ambient air always is below the temperature of the coils that are being stored.


Other Causes of Condensation

The material could also be shipped in winter and then placed in a warehouse that is warmer than the metal and where the humidity is not at a controlled low level. Under these conditions, the moisture simply condenses onto the steel’s surface as the cold coil causes the local air temperature to drop.

Condensation also can occur if the coil temperature and the temperature inside a customer’s storage facility are about the same when the coil arrives at a plant, but the warehouse is not temperature controlled. Cooling overnight might allow condensation to occur. Once moisture condenses on a metal coil, it takes a long time for it to dry because there is little or no air movement between wraps in a coil.

Because there is no way to completely prevent problems once the material gets wet, it is important to apply best practices during all steps of the process. The most typical cause of condensation or water damage is leaving coil on a jobsite—even for short periods of time. The longer the metal remains in coil form, the greater the chance that the paint will become compromised and fail. If the moisture remains trapped in the coil, the environment becomes extremely corrosive to either galvanized or painted material.

Once water penetrates the coil, it may soften the paint and destroy its adhesion to the base metal. When this adhesion is compromised, the paint will begin peeling, exposing the base metal. In the peeled area, moisture can spread more easily under the exposed edges of the paint film, causing even more damage. As with galvanized material, severe corrosion can occur in a short time in the presence of trapped water. This could happen in as few as a couple of days, depending on temperature and humidity. The same effects can happen to the coils on skids or on a portable rollformer.

The impact of trapped water or condensation often can be plainly observed. The coil-coated paint would feel soft and the adhesion of the coating would not be consistent across the width of the strip. Paint may come off on one edge when a contractor attempts to run the material. Condensation may cause the rollers in the rollformer to slip or spin on the paint. This friction can result in the paint coming off the material in a tearing manner at the point of contact.

Trapped water can cause the same result. If the adhesion between the metal or primer has been broken due to prolonged exposure to trapped water, the paint will peel and tear beyond the work area of the rolls. Another condition associated with the corrosion of unpainted coil is storage stain, or white rust. The continual wetness prevents the formation of a protective passive film on the edges and surface of the material. The result would be a discolored material that is virtually impossible to return to its original metallic appearance.


Preventing Condensation

Storage corrosion can be prevented by four best practices. First, the manufacturer must use proper techniques when shipping to the contractor. Second, the coil must be stored properly at the customer location. Third, the customer must use proper shipping techniques to get the material to the jobsite. And fourth, the coil should have minimal storage time at the site.

At the manufacturer location, the coils should be wrapped with waterproof paper and plastic designed for this application. The plastic should not be wrapped too tightly because is important that the coil be provided with air circulation. The shipper should protect the coil during shipment to the customer’s plant. Typically, coil should be shipped in covered, watertight containers. If it is necessary to use an uncovered conveyance, the coil should be completely wrapped with a tarp and the floor of the truck should be sealed and waterproofed to prohibit water intrusion if it rains while in shipment.

Once the coil is delivered to the customer, it should be stored indoors in a climate-controlled warehouse. The customer should use the material promptly. Whenever possible, coil should not remain in storage for extended periods of time. Material stored outdoors should be placed on wooden skids at least 6 inches off the ground and away from high traffic and flood areas. The material should be slightly elevated on one end to prevent standing water. The skidded material should be loosely wrapped with waterproof tarps to prevent it from getting wet or exposed to moisture while still allowing proper ventilation. Plastic-coated metal sheets should not be exposed to direct sunlight for prolonged periods of time. Direct sunlight can bake the protective plastic to the sheet metal.

Finally, a quality coil or sheet manufacturer will encourage its customers to inspect material when it is delivered and to contact the company immediately if water is present on the material. A manufacturer can be expected to respond more positively to a contractor who reports condensation or water problems upon receipt of the material either by telephone or on the bill of lading versus one who waits two days or more to file a report.

Gene Johnson has been the quality and environmental manager at Perth Amboy, N.J.–based Englert for the past 15 years. He is responsible for the integrity of a broad range of roofing, gutter, and environmental products, and has been in the coil-coating industry since 1968.

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