The critical challenge with structural steel is rusting, leading to corrosion and potential failure.

Steel rusts when an electrolyte (e.g. water, soil) and oxygen are both present. The rusting process is accelerated by atmospheric pollutants such as sulphates, chlorides acids or alkalis and certain types of bacterias.

Solid pollutants can be hygroscopic and promote a localised attack (corrosion pitting) at specific points on exposed steel. Paint systems must be resistant to such pollutants and only be applied to a surface free of contamination.

Modern resins provide a high level of protection, but only when careful attention has been given to the importance of surface preparation. The correct coating selection and method of application are the main contributing factors for a successful outcome.

The initial reference point for modern coating specifications should be the ISO 12944 guide which covers the design,preparation and prevention of corrosion of steel structures. This guide can help choose a suitable system for the protection of the steel considering its environment and expected durability

Each coating ‘layer’ in a protective system has a specific function, and the different types are applied in a particular sequence of primer followed by intermediate / build coats in the shop, and finally the finish or top coat either in the shop or on site. 

Before selecting the system to be used, the specifier should consult the relevant standards and guidance information relative to the design, function and maintenance requirements. This is always best done at the earliest stage of the project with the architect ,designer, applicator (contractor) and the coatings manufacturer.

Factors affecting choice

• The expected life of the structure and the feasibility of ongoing maintenance 
• The environment to which the steel will be subjected
• The size and shape of the structure
• The facilities available to the applicator (contractor)
• If the site conditions allow for further treatment after erection
• The money available to provide the best protective coating.

Additional practical considerations

• Type of coating
• Method of application
• Method of surface preparation
• Number of coats and volume solids.

Environment

Protection requirements are minimal inside a dry heated building. Hidden steelwork in such conditions requires the minimum  protection.

Durability

The durability of any system is increased several times over by using abrasive blasting rather than manual surface preparation. Shot blasting is preferred for most painting systems, however, blasting is essential for thermal spray and some types of primer e.g. zinc silicates.

Rust and Scale

Many modern primers based on synthetic resins are not compatible with manually prepared steel surfaces since they have a low tolerance for rust and scale. The removal of Mill scale is particularly important because it is more electropositive than steel and will accelerate the corrosion of the steel.With the phasing out of lead primers which were very tolerant of a poor surface, surface tolerant epoxies have been specially developed and are widely used for maintenance jobs.

UV and chalk degradation

Two-pack epoxies have a poor resistance to UV radiation and are highly susceptible to superficial chalking degradation. Overcoating problems can arise with two-pack epoxies unless they are overcoated before the prior coat is fully cured. This is worthy of consideration when an epoxy is to be partly applied in the paint shop and finished on site.

Fastenings

Fasteners such as nuts and bolts, should also be given consideration in the specification as these can be the starting point for the onset of corrosion. Where steelwork is coated in the fabrication shop and bolted on site, the fasteners may only receive a finish coat unless the specification is written to ensure the film build is applied to the fasteners. The use of galvanised bolts and fasteners should be included where possible. 

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Pronto Paints have been working in structural steel for over 30 years and have an exhaustively researched range of coatings for steel.

Want to book a callback or arrange a coating trial with one of our in-house experts? Drop an email to info@prontopaints.com

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View our structural steel products range here. 

Download the ISO12944 Guide to Structural Steel Coatings here 

 

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How do I select the right coating for my structural steel project?

How do I prepare structural steel for painting with a specialist industrial coating?