Corrosion Protection
AS/NZS4600 requires that cold formed sections must be adequately protected from corrosion attack.
The available protection options range from basic painting systems through to heavy galvanised coatings (like that used by BlueScope Steel for GALVASPAN® steel). Before specifying what type of purlin, you need to consider:
- the structure, climatic or other local conditions
- maintenance provision, and
- the forming process used (in cases where the coating is applied before forming).
The zinc coating and quality controlled galvanising process used by BlueScope Steel to make GALVASPAN® steel ensures a high standard of corrosion protection.
Coating Classes
GALVASPAN® steel is available with two standards of corrosion protection. These are:
- 350g/m2 zinc coating weight.
- 450g/m2 zinc coating weight.
Both 300mm and 350mm purlin sizes are usually produced with 450g/m2 zinc coating.
The zinc coating weight is the total weight of zinc on the base steel, and is usually referred to by the coating class, ie. 350g/m2 of zinc is class Z350. The respective coating classes represent approximate coating thicknesses of:
- Z350 – 24μm
- Z450 – 31μm
of zinc on each side of the strip.
| Base Metal Thickness | Coating Class | Total Coated Thickness |
|---|---|---|
| 1.5 | Z350 | 1.548 |
| 1.6 | Z350 | 1.648 |
| 1.9 | Z350 | 1.948 |
| 2.0 | Z350 | 2.048 |
| 2.4 | Z350 | 2.448 |
| 2.5 | Z350 | 2.548 |
| 1.5 | Z450 | 1.562 |
| 1.6 | Z450 | 1.662 |
| 1.9 | Z450 | 1.962 |
| 2.0 | Z450 | 2.062 |
| 2.4 | Z450 | 2.462 |
| 2.5 | Z450 | 2.562 |
| 3.2 | Z450 | 3.262 |
The Galvanising Story
The galvanised finish on GALVASPAN® steel is applied by BlueScope Steel on a continuous galvanising line as part of the steel production process. (The base steels used are produced to AS1397.)
In conventional hot-dip galvanising, the clean steel surface is wetted by the molten zinc, producing zinc/iron alloy layers. This alloying is controlled to ensure that the ductility of the coating matches that of the base steel.
Consequently, the molecular bond ensures that the zinc coating on GALVASPAN® steel:
- does not flake or peel during cold forming
- is highly resistant to damage during handling, transport and erection, and
- maintains its bright, attractive appearance.
In addition, the sacrificial action of the unique cathodic properties of GALVASPAN® steel, ensure that any cut edges, holes and minor coating damage are protected from subsequent corrosion.
In comparison, hot rolled painted purlins do not offer this inbuilt corrosion protection. They are prone to coating damage during handling and erection, which in turn may lead to corrosion and higher maintenance costs over time. For this reason, painted purlins have been largely superseded by galvanised purlins.
Compatibility With Other Steel Products
The zinc coating on purlins made from GALVASPAN® steel is fully compatible with the zinc and aluminium/zinc coatings used on roof and wall sheeting made from other quality BlueScope Steel products. Not stainless steel.
If minor damage occurs to the purlin coating, the base steel is protected by its own surrounding coating.
Specifying for Corrosive Environments
Z450 purlins should be specified for environments requiring additional corrosion protection. Contact us, for assistance in detailing purlin specifications for severe exposure environments.
A more complete guide to coating class recommendations is also provided in our Technical Bulletin: Download Technical Bulletin TB 17 - Selection Guide for Galvanized Steel Purlin Products (290 KB)
