Preventing corrosion on structural steel is essential to the overall integrity and aesthetics of the structure. Whether the structural steel supports a bridge, commercial building or plant, asset owners must be able to count on the infrastructure for the long haul. Corrosion is a risk to that infrastructure.
Asset owners and their corrosion engineer or project manager should evaluate the best ways to implement corrosion protection for the steel supporting the asset. Here we’ll explain the role service environment plays in corrosion protection, then describe design methods and coating systems that will ensure the structural steel is protected from corrosion for decades to come.
Understanding structural steel’s exposure
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 salt water 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 (using a resource such as SSPC’s Environmental Zones) and protect at that level, but not over.
Steel selection and design considerations for corrosion prevention
Before a coatings team applies a single component, asset owners can implement corrosion prevention methods with informed steel and design choices. Coatings alone are not effective at protecting structural steel from all forms of corrosion. For instance, coatings are effective at controlling uniform corrosion but are less effective at controlling localized attacks such as pitting. Use the following advice on steel selection and design considerations to set up a coating system for success.
The quality of the steel itself can have a bearing on corrosion prevention. High-alloy steel is naturally more corrosion-resistant than low-alloy steel (though it should still have protective coatings applied) and is more expensive. If asset owners choose a more affordable low-alloy steel, it will likely need a more comprehensive coating system to effectively fend off corrosion.
Corrosion control is just one part of the steel selection process. Asset owners should balance those needs with the steel member’s end use, its initial cost and its future maintenance costs.
The design of structural steel can also prevent corrosion. These design factors won’t play a part in coating or re-coating a structure that’s already been constructed, but are important to know because failing to follow them leads to an increased risk of corrosion.
For a new structure, keep these design considerations in mind early in the process.
- Reduce exposure to the atmosphere. Any areas where exposure to the atmosphere can be limited (especially when the environment is particularly corrosive) will aid the overall corrosion prevention system.
- Stay away from dissimilar metals. Galvanic corrosion (one of many types of corrosion) is possible when two or more dissimilar metals are used in a structural steel system. Be conscious of metal choices to prevent this type of corrosion.
- Prevent water from building up. Water traps are intrinsically corrosion-prone because moisture accelerates corrosion. They are even more problematic if the environment contains dirt and debris because when trapped, they tend to retain moisture. Ensure no areas cause unnecessary water build up or stagnation.
- Avoid surface irregularities. These include crevices, sharp edges and inaccessible areas, are difficult to coat and inspect — and they are also at a high risk for corrosion. Not all irregularities can be avoided. For those that remain, pay careful attention to them while coating.
Protective coatings to defend from corrosion
Coatings — the first line of defense — 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 SP 5 white metal blast or SP 10 near-white blast. 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 salt water 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 color 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 labor with the elimination of a coat. It also offers better color 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 odor, 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.
A balanced corrosion prevention system
Corrosion prevention for structural steel isn’t about just checking off one box — it’s about an entire system that will protect the substrate for many years. A good system balances service environment, design and coating systems to obtain the desired performance and service life-cycle at the least cost.
For the best industry advice on corrosion prevention and protection methods for structural steel, consider purchasing Standards and Best Practices for Surface Preparation of Steel Substrates. Protecting structural steel is essential to maintaining the integrity of an asset, and this SSPC resource will share valuable industry knowledge for the next coating project.