Keith Holmes, of Dunedin, asks :-

Recently on inspecting a used sheet of corrugated roofing I noticed patches of a white powdery substance on the under side, mainly where the metal was nailed to timber. In some places rust was showing through the white powder. What are the factors which cause the sheeting to deteriorate, how can it be slowed, and can it be treated?

Scientists at the Building Research Association of New Zealand, responded.

"Roofing iron" or "galvanised iron" should be called "galvanised steel" because it is steel coated with zinc metal or zinc-aluminium alloy. The coating layer is generally applied by immersing the sheet steel into a molten bath of the coating metal.

This surface coating separates the steel from moisture and oxygen in the atmosphere, i.e. it acts as a physical barrier. In addition, zinc can protect steel by sacrificing itself when exposed to aggressive environments (it corrodes while the steel does not). These effects usually increase the life span of roofing steel.

When in contact with timber, coated roofing steel will deteriorate (or be corroded) slowly due to the presence of moisture and/or acidic substances released from the timber. This process will be accelerated when the timber is wet and/or is treated with copper based wood preservatives. The white powdery material found on the coated roofing steel results from corrosion of the zinc coating and is relatively complex in its chemical composition. It, in general, comprises zinc oxide, zinc hydroxide and basic zinc carbonate, but other components might be expected under more complex service conditions, e.g. in marine or industrial environments.

Once this protective coating layer has been consumed, the underlying iron-rich substrate will be directly exposed to the atmosphere causing the steel to rust forming a wide variety of iron oxides and oxyhydroxides. These red rust layers grow fast and provide virtually no protection to the underlying steel.

Zinc-rich and iron-rich corrosion products found on roofing steels can be cleaned by immersing them into specially prepared chemical solutions, which is what we do in the laboratory when undertaking corrosion tests. This process is not applicable when dealing with large-sized roof sections that are still attached to roof framing. Red rust can be removed by mechanical means, using sandpaper, wire brushes, or abrasive blasting. The cleaned areas must then be protected by some form of coating, usually a coating containing 95 per cent metallic zinc when cured.

Some chemical treatments (or rust converters) claim to be able to “stabilise” the rust, and enable it to be painted over. In chemical terms, the rust is already as “stable” as it can be (it is the lowest-energy form of iron in many atmospheres – that’s why steel “wants” to rust) so rather than relying on a purely chemical reaction to slow the physically weak rust , it is best removed, and coated, as described above. See the Consumer Home and Garden article from issue 54, (Nov/Dec 1999) for more details.

To extend the planned lifetime of roofing steel in service, thicker zinc coatings can be specified or a modified coating composition employed. Aluminium can be incorporated into the molten zinc bath to produce various zinc-aluminium alloy coatings on steels. The most popular one is 55 per cent aluminium-zinc alloy coating (available under the trade names of Zincalume or Galvalume). Roofing steels with this coating system could last two to four times as long as those coated with pure zinc in a wide range of environments, especially near the sea. Even better performance can be achieved by applying a protective paint coating over the metallic coating. Paint of a suitable formulation, with good maintenance, will provide long-lasting additional protection and delay degradation of the underlying metallic coating.