Guaranteed Design

Steel House Frames have In-built Durability. Corrosion Protection is Built In.

Protective coatings of zinc and aluminium alloy defend the steel against corrosion. New Zealand Steel Galvanised and ZINCALUME® coated steel coil, when used in framing components of above floor, steel frame cavity wall construction will meet a durability of 50 years, subject to provisions in the New Zealand Steel durability statement.

A natural phenomenon known as sacrificial protection prevents corrosion of any steel exposed at cut edges and penetrations.

The protective coatings on steel are tough, continuous, of uniform thickness, tightly adherent, and strongly resistant to corrosion. They are applied to both sides of the steel by the hot-dip process. In this process the steel strip is passed through a bath of molten metal, the amount of coating applied being closely controlled. The coating mass must meet the requirements of the relevant Australian Standard (AS 1397), or its equivalent.

New Zealand Steel provides a 50 year warranty for its GalvSteel® and ZINCALUME® coated steel coil when used correctly in steel house frames. Steel framing is the only framing material for which any kind of structural durability warranty is offered.

Forming and fabrication does not impair the coatings. Because the protective coatings are tough, and their bending properties understood, the forming processes involved in shaping the building frame components do not impair the effectiveness of the protective coatings. The coatings are also tough and resist damage during fabrication and handling on site.

Some steel framing systems are partially or completely joined by welding. The high temperatures reached in localised areas during the welding process remove a small amount of the metallic coating. The fabricator restores the corrosion-resistance of the affected areas by applying a suitable zinc-rich paint (commonly called "cold galvanizing") as required by AS 4600 Cold-formed sections code.

Sacrificial Protection. Frame sections are usually produced from narrow coils of steel strip slit from wider coils. Service holes are punched into the studs during manufacture to enable plumbers and electricians to install piping and wiring. During fabrication of the frame a variety of joining methods are used that usually penetrate the steel. At the slit edges of the plates and studs, at fixing points where the steel is penetrated and where components have been punched or cut, the steel base is exposed, but will not rust. It is protected against corrosion by sacrificial protection.

Sacrificial protection refers to the process initiated when dissimilar metals are in contact in the presence of moisture. The more active metal, in this case the zinc or zinc/ aluminium coating, will sacrifice itself in preference to the steel, thus protecting the steel base and its structural properties.

The effect is well known. It is evident at the ends of sheets of corrugated steel roofing where they have been cut to length, or at the cut edges of steel rainwater guttering. These articles do not corrode away from cut edges even though they are exposed to more severe conditions than house frames. It is worth noting also that galvanized steel strips with cut edges are commonly used as bracing on timber frames.

Good Building Practice. In certain conditions Galvanised or ZINCALUME® coatings can be corroded by galvanic action through contact with dissimilar metals. In particular, contact with copper or brass in the presence of moisture will lead to the rapid removal of the coating from galvanized or ZINCALUME®, followed in due course by corrosion of the underlying steel. Plumbers must therefore take care to isolate copper piping from steel framing by inserting specialized plastic plumbing grommets into the pre-punched service holes in the studs.

For the same reasons CCA treated timbers (which contain soluble copper-based chemicals) used in some external claddings must not come into contact with steel frames. They must be isolated by an approved building membrane.

Masonry wall ties must be installed in a way that prevents moisture traveling along the tie to the inner surface of masonry or frame.

When installing elevated steel sub-floor systems, it is important to follow the manufacturers' instructions. The thickness of the galvanizing depends on the coating process, and for certain exposure conditions some manufacturers may recommend additional protective measures.

It should be noted that for all steel framing there can be some unusually corrosive conditions where heavier coatings than normal may be required. NASH or a steel supplier is able to provide advice on this.

Durability Comparison Steel and Timber

Steel Stud	Timber Stud
Coated to protect from rusting which is a surface condition	Needs full thickness treatment to prevent deterioration
Rust requires air + dampness	Deterioration may occur in visibly dry timber
No moisture absorption	Will absorb moisture
Requires thermal break and hence remains above dew point	Does not require thermal break and may fall below dew point
Fix leaks and corrosion stops once location dries	Fix leaks and deterioration continues

References:

Environmental Categories; New Zealand Steel, Auckland, September 2003
AS/NZS 2312: 2002 Guide to the Protection of Iron and Steel Against Exterior Atmosphere Corrosion; Standards New Zealand, Wellington.
E2/AS1:July 2005, Acceptable Solution for External Moisture, Department of Housing and Building. Wellington.
The Thermal Insulation Performance of Light-Weight Steel Framed, External Wall Elements: HERA, Manukau City, 1993, HERA Report R4-72.
NZS 4218: 2004, Energy Efficiency-Housing and Small Building Envelope; Standards New Zealand, Wellington.
E3/AS1: Oct 2004., Acceptable Solution for Internal Moisture, Department of Housing and Building , Wellington.
Zinc Coated Steels in Buildings; BBA, Watford, England, 1983 BBA Report No.20.
AS 1397:2001, Steel Sheet and Strip - Hot-Dipped Zinc Coated or Aluminium/Zinc Coated; Standards Australia, Sydney.
Recommendations Regarding the Use of Synthetic Building Wraps on Light Steel Framed Buildings; NASH New Zealand , Manukau City, 2002.