Wednesday, 28 May 2014

Wet Storage Staining of Galvanized and Galvannealed Steel Sheet

Most cold formed steel building products, whether painted or unpainted, are manufactured from a sheet steel material that has some form of metallic coating applied. This metallic coating can be zinc (galvanized), zinc-iron alloy (galvanneal) or a 55% aluminum-zinc alloy (GalvalumeTM). The metallic coating is available in a range of thicknesses to provide the degree of corrosion protection and service life required. One of the concerns expressed by installers relates to the presence of wet storage staining on the products, how this staining impacts the long term performance, and what can be done to remove it. The purpose of this blog post is to address some of these issues, allay some fears, and give guidance on proper storage techniques.

What is Wet Storage Stain?
“Wet storage stain” is a term used in the galvanizing industry to describe the zinc corrosion products that can form on a galvanized steel surface during storage. This staining is also referred to as “white rust”, which is the term generally applicable to all zinc corrosion products. Wet storage stain is voluminous, white, powdery, and bulky and is formed when closely packed galvanized articles are stored under damp and poorly ventilated conditions.

Often the white rusting appears to be quite heavy when, in fact, the amount of zinc corroded is small. This occurs because zinc hydroxide is somewhat voluminous and builds up in areas of wetness. Although wet storage stain can affect the appearance of the galvanized steel articles in some situations, it is generally not harmful in terms of the long-term corrosion performance.

Treatment of Galvanized Steel with Wet Storage Stain 
Galvanized sheet steel affected by wet storage stain can usually be cleaned, but generally cannot be restored to its original high luster appearance. The stain, depending on the severity, irreversibly alters the surface characteristics of the zinc to varying degrees. Nevertheless, there are treatments that are helpful in improving the appearance, depending on the severity of the problem.

Light white rusting:
This is characterized by the formation of a light film of white powdery residue. If left alone it may wash off in service with normal weathering. If it is deemed necessary to remove the white rust it can usually be done with a stiff bristle brush (nylon). If brushing alone is insufficient, rub or brush the surface with a mixture of mineral oil and sawdust. The mild abrasive action may remove the stain, although this treatment is not of much help for advanced wet storage stain.

Moderate white rusting:
If the stain is not too severe, it may be removed by washing with a 10% (by volume) acetic acid solution, followed immediately by a thorough rinsing with water to neutralize the surface. The removal can be assisted by the use of a stiff bristle brush (nylon). The sheets must be dry before restacking. This treatment may remove some of the metallic lustre, even in non-stained areas.

Severe white rusting:
The zinc hydroxide corrosion product will dissolve readily in weak acidic solutions. Ordinary household white vinegar has been found very effective and environmentally benign. Commercial products like CLRTM, widely advertised for scale and stain removal, can also be effective. Alternatively, a solution of 5% (by volume) of phosphoric acid in water, with a wetting agent added, can be brushed onto the sheets. In all cases proper safety precautions are necessary as well as approved disposal of cleaning liquids. After cleaning, the sheets must be immediately well-rinsed to neutralize the surface and then thoroughly dried. This treatment will remove some of the metallic luster, even in non- stained areas.

Dark grey or black staining:
If the stain has progressed to dark grey or black in colour, removal may not be possible. One method of restoring the protective value of the zinc coating, and improving the appearance of storage stain damaged sheets, is to apply a good, colour matched zinc-rich paint. The surface must be thoroughly brushed, rinsed and dried beforehand. After a period of time weathering will largely remove any difference in appearance between the zinc-rich paint and the original galvanized surface.
Note: Any field painting that may be required to cover wet staining is the responsibility of the buyer, not the deck supplier.

To learn more about metallic coatings, white rust and proper onsite storage of galvanized and galvannealed sheet steel, download our Fact Sheet 33: Wet Storage Staining of Galvanized and Galvannealed Steel Sheet

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