CORROSION CONTROL: Coating, Surface Preparation Technology Allow In-service Painting of Water Tank

Nov. 1, 2000
When the skies burst open unexpectedly and rain pours down on a fresh coat of paint, the only song most painting contractors are singing in the rain is the blues, as their day's work is washed away.

When the skies burst open unexpectedly and rain pours down on a fresh coat of paint, the only song most painting contractors are singing in the rain is the blues, as their day's work is washed away. Condensation— common on water tank exteriors below the water line— can cause similar headaches.

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Kessinger Service Industries (KSI) got around this problem by using a three-coat, moisture cure urethane coating system when performing a repaint on a 10 million gallon water tank in the Louisville, Ky., suburb of Middletown. Because atmospheric and surface moisture actually completes the chemical reaction in moisture-cure polymers, coatings of this type adhere well to damp surfaces. As a result, the system could be applied with excellent results while the tank, one of the largest in the Midwest, remained in service during painting.

Repaint Challenge

Mike Dudukovich, KSI general manager, has been involved in hundreds of projects in which the exteriors of potable water tanks were coated. When he learned his company had won the contract to paint the 25-year-old Middletown tank, he was both pleased with the challenge and concerned about it, especially because the tank serves as a major hub in a service network that supports approximately 300,000 residents.

The project called for the removal of existing lead-based coatings and repainting during a 60-day period in which the tank would have to remain 70 percent full. Draining the tank would cause severe system strain, leaving some service network components under unusually high pressure and others dealing with low pressure. But when the project was first sent out for bid, the tank was to be removed from service. As a result, the initial specification called for a two-part epoxy coating.

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"We later became concerned about that because this tank is so crucial to the people here," said Yolanda Jackson, a mechanical engineer for the Louisville Water Company, the municipally owned entity that provides potable water for the Louisville metropolitan area. "The population has grown tremendously in this area. How would we meet demand, especially at this time of year when usage peaks? What about fire protection? And to complicate matters further, we've had a major drought here."

Knowing that a two-part epoxy could be subject to blushing, poor adhesion and improper cure due to the "sweating" that occurs when the water tank is in service, Dudukovich and Jackson scheduled a meeting with Sherwin-Williams Industrial and Marine representative Eric Sowers and Corrosion Specification Specialist Tony Ippoliti to discuss moisture-cure urethanes as an option. Based on what they learned, they specified a three-coat moisture-cure urethane system that would start with a prime coat of CorothaneRegistered I MIO-Aluminum at 2 to 3 mils DFT, followed by an intermediate coat and topcoat of CorothaneRegistered I Aliphatic, also at 2 to 3 mils DFT per coat.

Moisture-cure Chemistry

A brief review of the chemistry of moisture-cure urethanes reveals why the Sherwin-Williams system would allow KSI to keep the tank in full service over the course of the project.

The backbone of all polyurethane coatings is isocyanate groups. These groups react with any compound containing reactive hydrogen. With moisture-cure polyurethanes, that reactive hydrogen compound is water. In the first stage of the reaction, the water and the isocyanate groups in the coating produce an amine and carbon dioxide. In the second stage, the amine reacts with other isocyanate groups to form a urea, until all available isocyanates are consumed, while the carbon dioxide leaves the paint film through evaporation.

Sherwin-Williams' moisture-cure urethane coatings contain Micaceous Iron Oxide (MIO), a metallic pigment that forms a platelet-type structure in paint that aligns in closely packed layers. During application, pigments in the coating align themselves more or less parallel to the substrate. This creates a barrier-coat effect that reinforces and strengthens the paint film, so subsequent coats adhere better while moisture and contaminants are prevented from penetrating to the substrate.

Because atmospheric and surface moisture completes the chemical reaction in moisture-cure polymers, coatings of this type adhere well to damp surfaces, penetrating even into pores and tight crevices to form strong chemical bonds. Since the moisture is consumed in the process, the risk of blisters caused by water trapped under the coating film is greatly reduced.

The result is a durable coating providing enhanced resistance to chemicals, abrasion and impact that can be applied in poor weather conditions and on marginally prepared surfaces.

Smooth Application

Application of the Corothane coatings was performed almost completely with rollers due to concerns about overspray spreading to a heavily traveled street nearby and an adjacent new car dealership. About 900 gallons of paint was used on the 112-foot-tall cylindrical tank.

But first was the issue of removal of lead-based paint. Sandblasting was dismissed as an option due to the high winds often present and the impracticality of constructing and maintaining functional containment. Instead, surface preparation was subcontracted out to Broadbent Inc., an Essington, Pa., company that markets the Broadbent Hydro-Crawler, or the "bug" as it became known at the site.

The bug resembles a miniature army tank, with two tracks each comprised of 52 powerful, six-inch wide magnets that hold the unit fast to the side of a metal structure. Inside the unit is a neoprene gasket containing a rotating nozzle capable of applying up to 40,000 psi of water pressure supplied by a Jet Edge pumping unit. An operator with a remote unit and a joystick directs the bug as it tracks back and forth across the face of the water tank, leaving a bare-metal surface ready for painting. The 500-lb. unit, measuring about three-feet square, leaves a white-metal surface up to 19 inches wide on each pass.

Nearly all of the water used for blasting is contained by the unit and vacuumed into a holding tank. Water that escapes the unit is gathered in secondary containment setups of plastic sheeting and berms on the ground. This water is also vacuumed to the holding tanks.

Water-friendly Project

Below the water line— some 84 to 90 feet off the ground— condensation and the tank exterior were frequent companions. The amount of condensation fluctuated with weather conditions. The temperature of the steel itself varied little from the 58

The project moved remarkably quickly, especially considering that KSI painters were using the moisture-cure system for the first time.

"Using other systems, we would have been getting about 35 hours in a week under these conditions," Dudukovich said. "We've been able to work 70 hours. These moisture-cure coatings have opened up a window we've never been able to work through before."

Larry Luther, KSI project manager, notes that circumstances that would typically cause long shutdowns on other jobs were mere inconveniences on this one.

"We had one day that a storm came up out of nowhere and 15 minutes after we had stopped painting, rain was all over us," Luther said. "We came back and looked at the paint in that area after the rain had stopped and you couldn't tell the difference. The inspector looked at it and he couldn't tell the difference either."

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