COP:durability; crevice-corrosion

4.12 Crevice Corrosion 

Roofing materials exposed to the air react with the atmosphere to form a relatively stable surface. Exposing metals to water in the absence air causes the formation of unstable surface films.

Crevice corrosion occurs in crevices and confined spaces. Crevices are often created because of overlapping flashing or sheets of cladding, or between the sheets of close stacked materials.

Design details that trap moisture, dirt, and debris should be avoided.

Corrosion can appear even with a chemically neutral electrolyte. An example of this type of corrosion is the corrosion on metals underneath paint coatings and “white rust” — the wet storage stain on closely nested zinc coated roofing sheets. Other metals, such as aluminium/zinc coated and non-ferrous metals, can suffer similar damage.

 

 

Capillary action can cause white rust to occur throughout a length of cladding

 

 

White rust on galvanised sheets causes bulky white deposits to form that can quickly lead to red rust

If end-lapping of roof sheets cannot be avoided, both ends of the lap must be continuously sealed to ensure that neither condensation run-off from the under-surface nor rainwater run-off enters the lap.

Capillary action can cause water to be drawn into closely stacked sheets, resulting in crevice corrosion or wet storage stain on both metallic coated and non-ferrous materials. On metallic-coated steel sheets, the passivation coating gives some temporary protection against this process, as do organic coatings, but longevity cannot be guaranteed for the duration of this protection.  On non-ferrous, metals, wet storage stain can commence very rapidly.

Wet packs of sheets should be separated to allow surfaces to dry before substantial storage.

If wet storage stain appears on unpainted surfaces, the degree of erosion of the metallic surface may be slight despite the bulky appearance of the deposits. However, when left unchecked it, can quite quickly lead to substantial degradation.  If required, measurements can be taken of the thickness of the material or the metallic coating to determine the extent of erosion.

Even if the damage is superficial, the white deposits must be removed to allow exposure to the air to allow the normal formation of stable surface films.  Use a stiff bristle brush; wire brushes are not recommended as they will remove more of the protective coating.