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Maintaining Corrugated Roofs: Preventing White Rust

ManagerPlus is pleased to kick off our new guest blog series with this excellent piece on white rust prevention by Corrugated Metals, Inc. Check back regularly for more expert content covering a broad range of industries.

White rust

+Corrugated roofing and siding is popular with homeowners and businesses alike, thanks to beneficial attributes such as long life, easy installation, and low maintenance requirements.  Most corrugated products are made from galvanized steel.  Galvanization is the process of applying a thin layer of zinc on the surface of the metal to protect it from rust (oxidization) and corrosion.

Galvanized steel products are designed to last for many years in roofing and siding applications.  However, it is very important to know that packaged bundles of corrugated products must be stored indoors, in a dry environment. Improper storage of stacked sheets (i.e. outdoors, or in environments where moisture can form between sheets) exposes galvanized steel to white zinc corrosion, or “white rust”. White rust is the result of electrolytic action which takes place between the steel sheets when water exists without oxygen.

White rust’s corrosion usually appears as a white deposit on one or more areas of the galvanized metal.  If seen wet, it will feel waxy in texture, when dried it can feel brittle and hard.  If this is spotted early enough, it can be treated and prevented.  The good thing is that the corrosion of the zinc is only on the surface level and hasn’t penetrated to the metal.  However, if left unattended, the zinc will be further eroded and will leave the metal unprotected and vulnerable to oxidization.  Of course, it can be unsightly as well, so tackling the issue when it is first is noticed is important.

Here are a few tips that can help prevent white rust and what you can do if it develops on your roof, siding or other places that utilize corrugated metal:

  1. Most importantly, be certain that the materials are stored in a way that will provide enough air to allow the zinc coating to dry and protect.  Also, be certain that in the storage area the zinc coating doesn’t come into contact with the various elements that could begin the corrosion process for the zinc.  This is the most critical part of maintaining rust-free corrugated metal.  Too many suppliers and wholesalers fail to properly store the corrugated metal.
  2. Remove any item that may be causing destructive corrosion in areas already damaged.  This could be strapping or other items used in storage.  If the white rust has begun while the product is on the building, make sure that it isn’t in contact with things like copper pipes or lead, etc.
  3. Pour white vinegar over any areas that display white rust.  Allow the vinegar to sit for five minutes, then rinse and wash the area with warm water.  For light rusted areas, this should remove the stain.  If so, apply a coat of zinc-rich water or perhaps a solvent-based rust resistant primer.
  4. If the vinegar doesn’t remove the white rust, scrub off the rest of the stain with sandpaper until the white coating of the rust has been removed.  Do this for all areas of the surface that may be exhibiting white rust.
  5. Rinse the sanded area with warm water, dry with a clean cloth rag.
  6. Apply a layer of zinc-rich primer to the sanded areas.  Wait until dry, brush on a coat of galvanized iron primer.  Paint the entire roof or siding.  Let dry.
  7. You may then finish the process by painting on a layer of exterior paint for metal.
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