Extending the service life of new and existing power plants in the U.S. has become imperative given the escalating cost of construction. According to the IHS Inc. Power Capital Costs Index (PCCI), which tracks the cost of building coal, gas, wind and nuclear power plants, new construction that on average cost $1 billion in 2000 would be $2.31 billion today. Given this reality, it’s understandable why utilities across the country are still operating most of the more than 1,600 coal-fired plants brought online since the late 1940s.

Maintaining conventional coal-fired power plants can be especially challenging given the corrosive environments in which they operate round-the-clock, 365 days per year. Many of these power plants operate in industrial atmospheres with “high” to “very high” corrosivity levels as defined by the International Standard Organization (ISO) Standard 12944-2 Classification of Environments.

Typical of these highly corrosive environments is Cross Generating Station in South Carolina, where the plant’s boilers are not enclosed due to high temperatures and humidity. “It’s as harsh an environment as you can find,” according to coating consultant Dan Anderson. “Cross is in a coastal environment, since it’s within 40 miles from the ocean, so you have this enormous amount of structural steel exposed to ultraviolet (UV) light, along with the environment and humidity. And when you combine the high humidity with sulfur dioxide from the coal, you get a mild sulfuric acid on the metal.”

Ongoing maintenance efforts to extend the service life of existing and new coal-fired power plants exposed to these corrosive conditions includes the use of advanced technology industrial coating systems designed to shield exposed steel and equipment within these service environments.

At the Cross Generating Station, engineers for two new 600-megawatt coal-fired power plants carefully considered the life-cycle cost of protective coatings for structural steel, rather than basing their selection solely on initial price. The engineers with Santee Cooper had a third-party laboratory test products from various manufacturers for quality, ease of application and long-term performance before choosing a three-coat system from Tnemec. “The deciding factor was the success Santee Cooper has had with Tnemec products over the past several years,” reported Anderson. “They have been a customer of Tnemec for more than 20 years.”

Structural steel for the two new coal-fired power plants, known as Units No. 3 and 4, was shop-primed with an advanced technology moisture-cured, polyurethane zinc-rich primer, Tneme-Zinc, which was also used to touch up the steel after it was erected. A waterborne acrylic epoxy, H.B. Tneme-Tufcoat, was applied as the intermediate coat to provide resistance to staining, abrasion, chemicals and moisture. The topcoat was a hydrophobic acrylic polymer, Spra-Saf EN, which was chosen for its long-term corrosion protection and weathering properties.

The same topcoat and intermediate coat were successfully used to recoat structural steel at Cross Generating Station’s existing coal-fired plant known as Unit No. 2, which required nearly 10 years to complete. The field-applied primer at Unit No. 2 was a polyamidoamine epoxy, Chembuild, which offers superior wetting for marginally prepared rusty steel and tightly adhering old coatings. Overall, more than 100,000 gallons of coatings have been used at the Cross Generating Station units over the past two decades.

Long-term performance has also been demonstrated by two-coat polyurethane systems, such as the one used at the Port Neal Power Plant in Sergeant Bluff, Iowa, after a tornado in 1986 damaged more than 550,000 square feet of tanks, silos, conveyers, trusses and other steel structures. After steel at the power plant was blasted, a primer coat of Tneme-Zinc was spray-applied, followed by a topcoat of aliphatic acrylic polyurethane, Endura-Shield, which is highly resistant to corrosive fumes, chemical contact and weathering. Endura-Shield is also specified as a topcoat for especially corrosive power plant environments, such as scrubbers used for desulfurization.

Nearly 25 years later, after numerous inspections, the two-part coating at Port Neal Power Station was reported to be in excellent overall condition. Based in part on the success of projects like Port Neal, the use of this two-part coating system and others like it has become a viable option for providing long-term protection against corrosion in aggressive environments. 

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