Corrosion Protection for Treatment Plants

November 1, 2015

Corrosion Protection for Treatment Plants

Ben Franklin told us that “an ounce of prevention is worth a pound of cure,” and as usual he was right. Corrosion in general is an issue that could end up breaking your bank, so you need experts who have done the research and can give you the direction you need. Halff understands the causes of corrosion and how to guard against it. We will focus on water and wastewater treatment plants for this example.

Treatment plants present challenges to engineers and architects designing systems that must function in harsh environments. To protect equipment and materials, the designer must understand the effects of and account for temperature, humidity and chemical activity, both liquid and gas. The primary materials in treatment plants are concrete and steel, both of which can deteriorate quickly if not protected in a harsh environment. The keys are proper design and proactive maintenance.

Corrosion is a Common Challenge

There are a number of means of corrosion and material erosion that may occur in a wastewater treatment plant:
• Hydrogenation may occur when a metal is immersed in non-aerated water or non-oxidizing acid.
• Common chemicals such as chlorine, various acids and alkalis and ferric chloride are corrosive.
• H2S gas can directly attack metallic components (i.e. steel tanks, gratings and walkways, structural members and equipment).
• Anaerobic sulfate-reducing bacteria can attack the protective sulfate coatings on metal and concrete, leaving them vulnerable to corrosion by sulfuric acid resulting from oxidation of H2S.
• Fatigue corrosion occurs in virtually all ductile metals, resulting in cracking or breaking. Corrosive conditions reduce the metals’ ability to withstand the effects of fatigue corrosion. This limit may be reduced substantially.
• Cavitation erosion is normally associated with pump impellers and can occur at any point involving high liquid velocities and sudden, violent reductions of fluid pressure.
• Filiform corrosion may occur on metal surfaces with organic coatings and is induced by pinpoint penetration of moisture at numerous points on the surface.
• Galvanic corrosion occurs from the electrical current created when two or more dissimilar metals are immersed in an electrolyte.

Coatings Can Help

Corrosion can be abated or prevented by specialized coatings, and Halff generally uses two generic coatings: physical barrier coatings, which provide a barrier between the material to be protected and the environment; and sacrificial coatings such as zinc and cadmium, which corrode preferentially and save the primary base metal from attack.

Metallic Coatings: Zinc and cadmium have a higher electromotive force than steel and can be used to cathodically or galvanically protect iron and steel. The coatings are corroded preferentially, preventing attack of the primary metal below the coating.

Non-Metallic Organic Coatings: Organic coatings provide a barrier between the surface and environment. Synthetic resins are commonly used to enhance the coating’s ability to resist acids and alkalis.

Chemical Conversion Coatings: These coatings result from chemical reactions between the coating and the base metal. Two common examples are phosphate coatings and controlled oxidation coatings.

Overcoming Corrosion

Plant operators prefer simple measures that can be implemented and maintained at a moderate expense. Air conditioning is an effective way to protect and prolong the life of equipment sensitive to temperature and humidity; however, air conditioning components are especially susceptible to corrosion. For Building 2 at DWU’s Central Wastewater Treatment Plant, Halff specified an epoxy corrosion-protection coating for highcorrosive environments for rooftop unit cabinets and all components in their air stream. Halff also specified a flexible epoxy polymer e-coat for the aluminum refrigerant and condenser coils, dampers and the galvanized steel hail guards. A spray-applied, UV-resistant polyurethane topcoat was to prevent UV degradation of the e-coat on coated coils.

For electrical devices subject to corrosion, NEMA 4X stainless steel enclosures are sometimes used. Their gasketed, air-tight interiors protect devices and conductors against corrosive environments. For electrical conduit subject to corrosion, PVC-coated metallic and IMC conduit is available, and in extreme conditions fiberglass conduit may be specified. Fiberglass is also suitable for support and suspension system components.

Fiberglass-reinforced polyester may also be used in lieu of steel door and door frame construction. Halff specified such products for use in the Natatorium at the Texas School for the Blind and Visually Impaired.

The Take-Away

Simple practices, such as using galvanized ferrous metals, robust or high-performance coatings and prudent protection of vital components, provide a cost-effective solution easily incorporated into a water or wastewater plant’s operations and maintenance program, eliminating in most cases the need for extreme measures to protect building elements and equipment from corrosion. Halff welcomes the opportunity to help you tackle your problems, and our corrosion protection experts are ready to help. Halff has a multi-disciplined team of professionals working together to help clients achieve long-lasting, cost-effective results. If Halff can assist your team, please call Executive Vice President Jessica Baker, PE, CFM, PMP, at (214) 217-6692.