Preventing Wet Storage Staining of Galvalume Sheet

Preventing Wet Storage Staining of Galvalume� Sheet

Unpainted and prepainted Galvalume sheet products are well known for providing a long, trouble-free life when used for roofing and siding panels and other building components. However, there are certain precautions which should be observed in the handling and shipping of Galvalume sheet products. One of the consequences that can result when these precautions are not followed is wet storage stain. This Technical Bulletin will explain storage stain, how it can be prevented, and how to remove it if it occurs.

What Is It?:

Storage stain is a dark grey to black stain that can occur on coils and tightly packed stacks of sheets or panels of Galvalume sheet. In its very early stages, it can appear as a white stain similar to the storage stain that can form on galvanized steel. Although storage stain is usually superficial, it is unattractive and can progress quickly to a more severe state if the cause of the stain is not eliminated. When it is severe, there can be a substantial loss of coating material and subsequent reduction of service life. The storage stain itself will not worsen once a panel is in place.

The cause is water or moisture. Water can get into an unwrapped steel coil or lift of cut-length sheets by exposure to rain or high humidity. Even though the coil laps, cut sheets or roll formed panels are tightly packed, moisture can enter the closed surfaces by capillary action. Water often gets on the sheet by condensation. When cold steel is brought in from outside to a warmer building, the moisture in the warm air condenses on the colder steel.

Galvalume has excellent durability in the atmosphere because of the protective, air-formed oxide that forms on the surface. However, the situation is different inside coils or in bundles of closely nested formed panels, because there is no free access to air. If water or moisture is present, a faster type of corrosion occurs due to the lack of an inhibiting oxide film. Under these conditions, storage stain on Galvalume sheet can occur in as little as 24-48 hours.

Even prepainted Galvalume sheet is not immune to storage stain. Roll forming prepainted sheets into building panels can result in microfracturing of the paint. These very fine microcracks are of no consequence on a building and in no way interfere with durability, but they can permit access of moisture to the metal surface. Inside a bundle of painted panels, the same accelerated corrosion can occur as with bare Galvalume sheet.

Galvalume is a registered trademark of BIEC International, Inc.

Preventing Wet Storage Stain:

Prevention of wet storage stain is the responsibility of the mills, shippers, fabricators and erectors. Any letdown in the chain from mill to final erection can cause rapid corrosion if moisture is present.

Bethlehem's Role:

Galvalume sheet is treated in the mill with a thin film of an inhibitive chromate-containing coating as a first defense against storage stain. Oils containing inhibitors are also applied when ordered which can be an additional barrier to moisture penetration of the coil laps. Although chromate films and inhibitive oils both are effective in delaying the onset of storage stain if the sheets become wet, it should be recognized that the protection afforded by these treatments is only temporary.

At the mill, proper packaging in a dry area using a water resistant paper is most important. The method of shipment is also considered. For example, when water shipment is to be used, special metal side shields are applied to minimize tearing of the paper by the ship's handling and storage equipment. The paper wrapping is folded into the eye of the coil and then a paper insert is placed into the eye to cover the folds. This helps to prevent water or moisture getting under the paper wrapper.

Shippers:

When shrouding is ordered, Bethlehem supplies asphalt-laminated shroud paper and the trucker assumes the responsibility of placing the paper over the coils or lifts of sheets to protect the material from any water that could get through or under his tarps. Trucks are not permitted to leave our plant with any sheet products unless the trucker has tarpaulins in place.

Fabricators:

Customer assistance when ordering Galvalume sheet for shipment and upon receipt plays a vital role in preventing wet storage stain. The following steps should be followed:

Order wrapping paper if the material is to be shipped in open railcars or if the shipment involves more than one mode of transportation.
Check the tarpaulin on open trucks at the unloading station to see that it is in place on arrival.
Inspect the coils or lifts when they are received and note on the delivery receipt any exceptions, such as damage, corrosion or wet material. This procedure will ensure that claim settlements will be facilitated if the carrier is to be held responsible.
Remove the wrapping paper if the storage area is heated and dry. If the material is wet, the sheets should be wiped dry. Wet coils should be scheduled into production as soon as possible.
Do not store sheets near areas with fluctuating temperatures such as in open doorways or in high humidity areas.

When the Galvalume sheets are roll formed into roofing or siding panels and bundled for shipment to the jobsite, these precautions should be taken:

Paper-wrap bundles if the panels are not scheduled for erection on the day of delivery. To correctly wrap a bundle, the bottom paper sheet is put in place first and the top laps are covered smoothly with the top covering sheet with the folds underneath the bundle. If folded improperly, the laps on top can create a catch for water and actually encourage accumulation of water in transit. The sketches below show the right and wrong ways to wrap a bundle.
Do not use plastic material for covering. Non-breathing materials should not be used to shroud bundles because they tend to trap moisture.
Use a tarpaulin on open trucks.

Erectors:

Erectors, as the final links in the chain to prevent storage staining of Galvalume sheet, should do the following:

Inspect the bundles on arrival at the building site and note on the delivery receipt any exceptions such as damage, corrosion or wet material.
Store the bundles on racks at least one foot above ground level. Do not use uncured lumber.
Under-roof storage is recommended when possible. If the bundles must be stored in the open on bare ground, then a plastic ground cover should be used under the bundles to minimize condensation on the sheets from moisture in the soil.
Elevate one end of the bundle to allow any moisture to run off rather than puddle on the top of the bundle or between nested panels. Water resistant paper will not keep out puddled moisture beyond its rated moisture vapor transmission time.

Stain Removal:

Storage stain on Galvalume sheet is mostly hydrated aluminum oxide which can be very difficult to remove. In mild cases, a solvent such as mineral spirits applied with a soft rag has been known to effectively remove the stain. This method is also used to remove stain from prepainted Galvalume sheet without damaging the paint.

For more advanced cases on unpainted Galvalume sheet, it is impossible to remove the stain without also affecting the good coating under and around the stained area. The amount of damage to the coating during removal will depend on the method used. In more severe stages, storage stain can be removed from bare Galvalume sheet with a mild household cleanser such as Bon Ami* or Clorox Soft Scrub* and a wet sponge or rag. Industrial products such as Oakite 84M* may also be used, but are more aggressive to the coating. In any case, the Galvalume panel should be thoroughly rinsed with water after the stains have been removed.

Harsh alkaline cleaning solutions and high pressure sprays should be avoided, as these have been known to dramatically alter the corrosion resistance and appearance of the Galvalume coating. Steel wool should not be used for two reasons: It is too abrasive and it can leave behind iron fines which will rust and cause a cosmetic staining problem.

Regardless of what method is used to remove storage stain, remember that the coating has been affected by the stain and cleaning. These areas will have a different appearance than the surrounding coating. And, depending on the severity of the stain, may have a shorter lifetime.

* Bon Ami is a registered trademark of Faultless Starch Company.

* Clorox Soft Scrub is a registered trademark of Clorox Company.

* Oakite 84M is a registered trademark of Oakite Chemical.

Galvalume Technical Bulletins