Steel coatings – when looks are not enough
The Insight
So you’ve calculated your design actions, sized your steel members, and detailed your connections. All done right? Not quite.
The selection of protective coatings for steel elements to defend against atmospheric corrosion is often given little thought yet it has a major effect on durability.
While it can be tempting to let others, such as the architect, choose finishes, consideration needs to be given to more than the aesthetics. Selecting the right system requires consideration of various other factors including environment, use, and ease of future access for repairs and maintenance.
The Impact
Protective coatings commonly consist of paint systems or galvanizing.
For hot dip galvanised elements, the thickness of the coating achieved is dependent on the thickness, composition and surface condition of the steel. In areas of medium corrosivity, galvanising can give a life to first maintenance in the order of 50 years. In areas of very high corrosivity, this reduces to around 10 years.
Many light gauge cold-formed steel members come with a standard zinc coating. This coating is not equivalent to hot dip galvanising and, if no further protection is applied, can leave members with relatively minimal protection.
When paint systems are used, the type, number and thickness of coats are specified. This should be considered as an overall system using paints that are compatible with each other. Various systems are set out in AS/NZS 2312.1 Table 6.3. The protection provided by these systems varies widely from 2 years to more than 25 years life to first maintenance in areas of medium corrosivity.
Repair of damage is also an important consideration. Site welds in particular are often overlooked. These require removal of existing coatings prior to welding, and a repair methodology compatible with the selected coating system.
Choosing the right protective coating is a critical step in the design process. A failure to correctly select the protective coating can greatly increase the ongoing repair and maintenance costs and greatly reduce the economic life of your steel structure.