Combatting Corrosion

Corrosion. Most people with even a minimal understanding of the word’s destructive meaning know that the problem has become worse in their lifetimes, due to major increases in harmful chemicals and pollutants in the environment.

For the water and wastewater industry, corrosion is a problem of such magnitude that the Water Information Network-a coalition of local elected officials, drinking water and wastewater service providers, state environmental and health administrators, engineers and environmentalists dedicated to preserving and protecting America’s drinking water and wastewater infrastructure-estimates that over the next 20 years it will take $1 trillion to combat.


Increasing Corrosion at Treatment Facilities
There are numerous factors contributing to the breakdown of concrete in wastewater treatment facilities, according to Lake Barrett, director of water/wastewater operations at Tnemec Company, Inc., a leading manufacturer of high-performance coatings for specialized architectural and industrial markets based in Kansas City, MO. These include “increased levels of hydrogen sulfide, a variety of corrosive materials in the wastewater streams, constant abrasion from various media within the waste stream, variations in pH levels of the waste stream, utilization of new coagulants and disinfectants as well as carbonation and cavitation of the concrete,” he said.

Hydrogen sulfide (H2S) generation in municipal waste water treatment plants has always been present, according to Randy Nixon, senior consultant with Corrosion Probe, Inc., Centerbrook, CT. Up until the late 1970s, H2S levels stayed generally below 10 parts-per-million (ppm). During periods of low rain or high temperatures, however, levels could rise to 30-50 ppm, accelerating corrosion of concrete and causing depth losses of up to 3/4″ of concrete a year.

Traditional thin film epoxy and urethane coatings, as shown on this digester, can no longer hold up against the harsh environments now found in most wastewater treatment facilities.

According to Mr. Nixon, following the passage of the Clean Water Act in 1980, industrial pretreatment was mandated to reduce or eliminate the presence of heavy metals from waste water discharges. These metals had been killing anaerobic bacteria found on concrete pipe and tank surfaces, and therefore dissolved sulfide concentrations were maintained at relatively low levels, hence reducing gaseous H2S levels also. Because gaseous H2S is equally as responsible for odors as it is for corrosion of concrete in wastewater treatment systems, both odor and corrosion problems have increased as a result of industrial pretreatment of heavy metals, Mr. Nixon explained.

The building of larger regional treatment plants over the last two decades has resulted in longer travel distances and detention time for wastewater. The pumping of wastewater through force mains means that pipes run full and slime layers cover the entire circumference of the pipes. Both factors have increased the sulfide production, subsequently increasing H2S concentrations in aerated headspaces within wastewater systems.

Mr. Barrett explained that in the collection systems for large domestic plants today, it is not uncommon to measure H2S concentrations in headspaces as high as several hundred ppm. This general trend toward higher H2S concentrations has promoted much higher concrete corrosion rates in domestic treatment plants (especially larger regional plants) than witnessed in the past. The corrosion rate of concrete has been as high as 1/2″ to 3/4″ per year, depending on concrete quality.

Mr. Barrett added that sulfuric acid formation also is more constant due to the higher and more constant H2S presence. As the pH levels have dropped below 2.0, the concentrations of sulfuric acid have risen from one to three percent to well above seven percent in some municipalities.


Decreased Effectiveness of Older Coatings and Linings Technology
According to Mr. Barrett, protecting the concrete in wastewater treatment facilities from H2S and associated sulfuric acid attack has traditionally required the use of various protective coating and lining materials. Prior to the late 1980s, relatively thin film coatings based on coal tar epoxy and amine cured epoxy formulations provided effective corrosion protection of concrete for up to 10-12 years.

However, Mr. Barrett contends as a result of government-mandated industrial pretreatment, construction of large regional treatment plants with longer wastewater detention times, recent air quality regulations and other factors, concrete structures in wastewater treatment systems are typically exposed to more corrosive conditions today than in the past.

In recent years, these same coating materials have failed prematurely (within six months to two years) and severely. These failures were caused by both insufficient acid resistance and inadequate resistance to permeation by wastewater gases. Some thicker film coatings and linings (up to 1/8″ thick) based on polyester, epoxy and polyurethane resins have also failed under these relatively new and more adverse exposure conditions related to higher sulfide generation.

Another factor increasing failure of protective coatings are new methods of odor control, such as covered headspaces. Enclosed headspaces created by covers such as over clarifiers combined with higher concentrations of H2S have led to an increase in the volume of gases to which the coatings are exposed on a routine basis.

These failures, which severely affect many of the more than 15,000 wastewater systems in the country, are now starting to be addressed by cutting-edge coatings manufacturers such as Tnemec, said Mr. Barrett.

“Our extensive research and development processes and new products such as our Series 434 ChemBloc H2S are not only solving problems in wastewater collection and treatment systems, but are also setting new standards for the coatings industry,” he added.


Recommendations for The Future
For all coatings currently being developed for wastewater treatment systems-especially where gaseous H2S concentrations frequently average 25 ppm or higher and attack region surface pH values are below 3.0-implementing rigorous new testing methods is critical, said Mr. Nixon.

Coatings should measure up to standards higher than ever before in the areas of: improved sulfuric acid resistance (note: some municipalities are regularly testing products with a 10% sulfuric acid solution), lower permeability, increased film thickness, resistance to bacterial action, successful field performance history and optimum adhesion.

Mr. Nixon believes that coatings manufacturers can rise to these new challenges.

“With companies like Tnemec committed to developing coatings specifically designed to combat these new corrosion challenges, the industry can look forward to more solutions to these hurdles,” he said.


Tnemec: A Company To Watch
Headquartered in Kansas City, MO Tnemec has distribution facilities in Atlanta, Boston, Chicago, Dallas, Denver, Houston, Indianapolis, New Orleans, Phoenix, Seattle, St. Paul, St. Louis and Benicia and Compton, CA and manufacturing facilities in Kansas City and Baltimore. The company also has technical representatives in the U.S., Canada, the Czech Republic, the Dominican Republic, Taiwan and Puerto Rico.

Tnemec (cement spelled backwards, in tribute to the company’s origin) is one of largest privately held companies in the U.S. specializing in industrial and architectural coatings for new construction and maintenance. The company’s product line currently provides corrosion protection for a wide range of facilities and structures. Industries such as food and beverage processing, water and wastewater processing, power generation, pulp and paper, pharmaceutical and biotech, industrial process manufacturing and water tanks are beneficiaries of Tnemec’s array of protective coating products.

One of the first companies to have ANSI/NSF Std. 61 certification for coatings used in direct contact with drinking water, Tnemec has also become a top provider of interior and exterior coatings for water storage tanks.

Tnemec was listed as a “Company To Watch” in Coatings World’s 2001 Top Companies report, our annual profile of the top companies in the global market based on annual sales of paint, coatings, adhesives and sealants.