Before setting up a corrosion prevention system for structural steel, asset owners should understand how much corrosion the steel will be up against. For example, a saltwater bridge exposed to stagnant moisture and electrolytes faces higher corrosion risk than an internal structural beam in a commercial building. Both need corrosion protection, but at different levels.
Asset owners want the highest protection available but going overboard with an intense corrosion prevention system when only mild corrosion protection is needed will cost extra money and time that could be spent elsewhere. Evaluate the corrosion risk the asset will face and protect at that level, but not over.
Coatings — the first line of defence — play a major role in protecting structural steel from corrosion. Here we’ll outline surface preparation standards, coating systems and application methods for an effective corrosion protection system.
Surface preparation standards for structural steel
The preferred surface preparation standards for structural steel are SSPC-SP 5 (WAB)/NACE WAB-1, “White Metal Wet Abrasive Blast Cleaning” or SSPC-SP 10 (WAB)/NACE No. 2 (WAB), "Near-White Metal Wet Abrasive Blast Cleaning." Hand tool cleaning or a brush blast are always options, but strict standards for this type of surface preparation need to be followed for desired system performance.
Coating options for varying levels of environmental exposure
The best-fit coating system is dependent on the corrosiveness of the environment. Here are the most fitting options for each type of environment.
Highly corrosive environments
For environments with high humidity, a chemical atmosphere or saltwater exposure, a zinc-epoxy-urethane system is the most common choice. Zinc provides cathodic protection for the steel and will sacrifice itself before the substrate. Inorganic zinc primers provide better cathodic protection than organic zinc primers, but organic is more easily applied. The zinc primer is then coated with an epoxy intermediate, then a urethane topcoat for colour retention and gloss.
Polysiloxane is a resin-type, two-component coating also suitable for highly corrosive environments. This option is more expensive but is often used because it saves time and labour with the elimination of a coat. It also offers better colour and gloss performance compared to urethanes and meets emissions regulations in strict environments.
Moderately corrosive environments
A wide variety of epoxy coating systems work well in service environments with moderate corrosion risk. The coating system will still provide corrosion protection (just not as well as a system with a zinc-rich coating) and is easy to apply. Epoxies are also surface tolerant, meaning they can be applied over a tightly adhering rusted surface that couldn’t be blasted down to bare steel (making them a viable choice for re-coating jobs).
Lightly corrosive environments
In interior or controlled environments with little to no chemical or moisture exposure, single-component, water-based acrylics are an appropriate choice. They are low in odour, easy to work with and only require soap and water clean-up. For minimally corrosive environments, this coating system would perform well (compared to a more extensive coating system that would be overdoing it).
The stand-by of an oil-based primer with an oil-based topcoat is available as an option. But this coating system is slower drying, meaning time and VOCs are a concern, and future maintenance issues can arise depending on the exposure.
The role of application methods
Application methods — typically brush, roller or spray — should also be front of mind when deciding on a coating system. Certain coatings perform better when sprayed, but environmental restrictions may not allow field-applied spraying (to prevent overspray). Going in armed with the substrate’s application limitations will prevent asset owners from picking the best-fit coating system, only to find out it can’t be applied in its intended method.
Crossfire are specialist steel remediation contractor, who can bring your steel back to life again. Crossfire have a dedicated team of experts who would be able to inspect, advice and treat your steel to prolong the life of your asset.
Some of our steel remediation projects include:
Published 09/05/2024
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