Water, oxygen and salts
Everyone is familiar with the principle of corrosion in its most advanced state in examples like rusty old nails. But with modern, long-life pre-finished steels, the corrosion process is prevented for decades.
Corrosion is a chemical process generally promoted by the presence of water, oxygen and salts such as road salts and airborne marine salt in the environment. The barrier properties of coatings prevent access of these agents to the substrate and can delay the onset of corrosion for many years. Because of the variations in atmospheric environments, corrosion rate data cannot be generalised.
When building designers seek to specify a particular pre-finished steel product with guaranteed performance, the location of the building is the most important factor in the equation. For example, coastal sites can be the most aggressive and will be far more demanding of the product than urban or rural sites.
In order to assist the specification process, Tata Steel defines the following broad classifications of building location and individual pre-finished steel products will perform better or worse in each of these different locations:
- Rural – unpolluted, inland areas;
- Urban – low-pollution, inland areas;
- Industrial – high-pollution areas;
- Coastal (10 to 1 km) – with a significant amount of salt-water in the air;
- Coastal (<1 km) – this close to the sea, the airborne salt will significantly affect corrosion and only the best coatings are suitable.
Tata Steel defines a minimum ‘Period to Re-paint Decision’ (PRD) as the earliest time the building owner should consider re-painting.
In practice, the Confidex® guarantee for Colorcoat HPS200® Ultra and Colorcoat Prisma® only differentiates between Coastal (<1 km) and Non-Coastal environments but the definitions provided help to identify the optimum life of the pre-finished steel in all environments. The Colorcoat® durability figures provided by Tata Steel have a unique basis. They are the result of 50 years of product testing and actual weathering experience in many environments.
