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Reducing Steel Surface Prep Time by Eliminating Aggregate Blasting

WEDNESDAY, SEPTEMBER 20, 2017

By Lou Frank, Coatings For Industry, Inc.


Every industry has a standard practice that has outlived its usefulness. Though born out of necessity, certain processes or techniques continue to be used, despite being rendered obsolete by technology. This can be especially true in the industrial maintenance and marine coatings business where taking risks can be very costly.

Perhaps the outdated standard practice, in the case of steel coatings, is the use of aggregate blasting — once called sandblasting — to prepare structures for re-coating. For decades, re-coating a steel structure meant sandblasting the surface to take it down to white metal or near-white metal. This was necessary to remove all of the corrosion so the new coating could be effectively applied.

Blasting can be a difficult and expensive process, especially on large structures such as bridges and water tanks. It requires tenting, to contain the aggregate during blasting, and environmental remediation afterwards. Both of these practices add time, cost and logistical headaches to the job.

For decades, re-coating a steel structure meant sandblasting the surface to take it down to white metal or near-white metal. But that can be a difficult and expensive process, especially on large structures such as bridges and water tanks.

In some cases, blasting may now be unnecessary. By using AI-impregnated, moisture-cured coatings, contractors and owners can drastically reduce the return-to-service time and cost associated with surface preparation, and they can achieve results that are at least as good or better with regard to long-term corrosion resistance.

How Al-Impregnated, Moisture-Cured Coatings Work

Though they’ve been on the market for some time, moisture-cured, aluminum-flake, rust-preventive coatings are relatively new to many contractors. With these coatings, contractors can limit their surface preparation to the removal of loose rust, scale and debris from a dry surface. Then, simply applying two to three coats of 2-3 mils dry-film thickness (DFT) will give the steel the long-term corrosion protection required.

Moisture-cured, aluminum-impregnated, rust-preventive coatings work by bonding existing corrosion to itself, to the substrate steel and to the aluminum-flake barrier, eliminating any trapped moisture. Made with a combination of a resin and solvents, the coating penetrates the corrosion on the surface, binding it together, and in the process, scavenging for and consuming existent moisture that may be present in the corrosion. This helps eliminate one of the key conditions necessary for corrosion to continue. The result is a barrier coating with a hard, uniform surface that resists the elements and, if desired, can accept a variety of topcoats.

Moisture-cured, aluminum-impregnated, rust-preventive coatings work by bonding existing corrosion to itself, to the substrate steel and to the aluminum-flake barrier, eliminating any trapped moisture.

Some moisture-cured, rust-preventive coatings, such as U-104 from Coatings for Industry, have an added layer of protection from the elements: the aluminum-flake pigment. In addition to giving the coating its color, the aluminum flakes have another important job. Once the coating is dry, the flakes lie flat on top of each other within the coating. This creates a multi-layer, “labyrinth” seal that makes it difficult for moisture to navigate its way to the steel.

The combination of the moisture curing and the corrosion bonding in this multi-layered barrier coating gives this family of coatings excellent corrosion resistance, even if the surface has not been prepared by blasting.

Aggregate Blasting: Impractical, Imperfect

Commonly used barrier coatings, such as epoxy, work very differently in contrast to moisture-cured, rust-preventive coatings. When applied to a steel surface with any corrosion remaining, epoxies will bind the corrosion but may trap any moisture that is still present. As a result, corrosion will continue after coating, with bubbling often starting soon after the re-coating job has been completed.

That is why aggregate blasting is so often specified as a standard process in preparing surfaces. Moisture is ever-present in existing rust, so all of it must be removed. The only way to be sure of its complete removal is to bring the steel surface down to white metal, and then, of course, keep it dry until the primer coat is applied.

In some cases, however, blasting can be an imperfect or impractical approach. Large structures are extremely difficult to tent and blast, and they often have hard-to-reach areas where fully effective blasting is all but impossible.

Even when the steel surface is prepared effectively through blasting, ambient moisture can restart the corrosion process almost instantly. Even after a seemingly perfect application of a standard barrier coating, the structure may begin to show signs of corrosion.

Left (two panels), ASTM Salt Fog Test: After 2500 hours, moisture-cured, aluminum-impregnated, rust-preventive coating applied over pre-rusted panels, L panel scribed, no surface prep. Right (one panel), Salt Fog Test, 14,000 hours: Moisture-cured, aluminum-impregnated, rust-preventive coating applied over blasted panel. No rust crept under the scribe.

Proven Performance

Moisture-cured coatings like CFI’s U-104 have been shown in industry salt fog tests to effectively resist corrosion for up to 14,000 hours (more than 6 times longer that the relevant ASTM test) without corrosion creeping under the coating at the scribe.

More important, the coatings have proven their value in real-world applications, including the Homestead High-Level Bridge in Pittsburgh. The bridge was re-coated with U-104 under a urethane topcoat, and after 20 years of exposure to the weather — including lots of road salt each winter — it had less than 2-percent corrosion.

Microscopic cross-section shows how a moisture-cured, aluminum-impregnated, rust-preventive coating penetrates the existing rust, bonds it to the good steel beneath, and essentially halts further corrosion.

Conclusion

For contractors, using moisture-cured, aluminum-flake, rust-preventive coatings isn’t the perfect primer coat for all applications, but it can help yield results superior to many commonly specified coatings. Further, they may eliminate the need for blasting, a difficult, expensive and messy part of the surface-preparation process.

For more information about U-104, contact CFI.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

ABOUT THE AUTHOR
Lou Frank, Coatings For Industry, Inc.

Lou Frank is a recognized expert in the coatings industry, notably as the founder of CoatingsPro magazine, an industry-leading publication now owned by NACE International. Today he leads CFI’s business development efforts. Frank has a business degree from the University of Maryland and is a NACE coatings inspector.

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