Extending the Lives of Municipal Water Tanks - Inside and Out

Nov. 1, 2016
When the purse strings of a municipal government's budget tighten, asset protection projects commonly take a back seat.

By Kevin Morris

When the purse strings of a municipal government’s budget tighten, asset protection projects commonly take a back seat. However, certain projects should remain high on the priority list, especially the municipality’s highly visible water storage tank, which needs to convey a positive community image.

Consider taking a road trip on an unfamiliar highway. It’s time to grab lunch and the landmark at the next exit is a rusting water tank. Are you going to stop for a meal in this town? Probably not, as that dilapidated community “billboard” raises concerns about the town’s water quality. And you wouldn’t be the only one continuing down the road to a different exit, as water quality concerns are at the forefront of citizens’ minds.

For a municipality, it’s difficult or impossible to measure the return on investment for improving the life cycle performance of a water tank. However, the detrimental economic impact resulting from a poorly maintained tank won’t take long to realize. Fortunately, municipal boards can make some important decisions that will help them extend the service lives of these critical assets, as well as reduce the frequency and cost of required maintenance activities.

We’ll review a few key considerations below. By following these recommendations, communities should save funds at the water tank, which may even enable them to address other necessary asset protection projects - like their aging water treatment plants - sooner.

Enhancing Exterior Coating Life

The exterior of a water storage tank is an extremely harsh service environment due to factors such as acid rain, industrial pollutants, salty coastal air, and ultraviolet (UV) light. These elements threaten to degrade the protection provided by high-performance coatings the moment the coatings are cured. When these elements succeed and corrosion begins to form on the exterior of a tank, community residents and passersby will make assumptions about the interior condition of the tank and the quality of the water inside.

To extend corrosion protection and maintenance timelines, while also ensuring a positive image of a high-quality tank and water source, municipalities can look to field-applied fluoropolymer chemistries for their tank exteriors. Such chemistries, which are common on prefinished metal roof panels, have a unique ability to block UV light penetration and prevent film degradation, thereby extending the life cycle of the applied coating film. Industry research, such as Paper No. 4088 presented at CORROSION 2014, “Expected Service Life and Cost Considerations for Maintenance and New Construction Protective Coatings,” indicates a life expectancy improvement for fluoropolymer coatings of approximately 19 percent over traditional exterior coatings in a moderate service environment before 5-10 percent breakdown occurs.

A clean, well-maintained water tank serves as a welcoming community “billboard.” Municipalities can extend the service lives of these critical assets by following some essential best practices when specifying and applying protective coatings.

Enhancing Interior Coating Life

While a water tank’s interior is not visible to the public, it may be even more critical to repair sooner than the exterior due to the constant presence of the electrolyte needed to maintain an active corrosion cell. This corrosion can spread quickly, compromising the integrity of the tank and leaving the owner with a significant increase in repair costs if not addressed in a timely manner. There are two primary factors that limit the life cycle performance of an interior lining: sufficient protection of edges and angles; and holidays (areas of insufficient film build) missed during traditional testing.

Protecting Edges and Angles

Lining failures on tank interiors commonly stem from failures on coated edges and angles due to the lack of a sufficient dry film thickness (DFT) required to provide adequate protection. Traditional coatings have a natural tendency to shrink away from these sharp edges and will retain only about 40 percent of the film build achieved on surrounding flat surfaces. If the coating specification calls for 10 mils of average DFT, the edges and angles may only have 4 mils of average DFT, making them susceptible to early failure.

The development of edge-retentive coatings has overcome this deficiency. As defined by the MIL-PRF 23236C specification, such coatings must retain a minimum of 70 percent of their film build on sharp edges and angles. By these guidelines, a project specified at 10 mils DFT for flat surfaces would require 7 mils DFT in critical edge and angle areas.

A coating’s ability to meet the 70 percent DFT requirement is not a function of its volume of solids but rather a function of rheology - or how the coating flows. For example, epoxies with 100 percent solids that have the ability to self-level in low areas in horizontal applications may still shrink away from the stress areas of edges and angles. Look to ultra-high-solids coatings that meet the MIL-PRF 23236C specification, regardless of their solids volume, to ensure a sufficient DFT.

Choosing the right coating and ensuring good application techniques are especially important due to challenges associated with testing. Industry standards do not allow common DFT gauges to be placed close enough to an edge to determine if the film build is sufficient. Therefore, such areas are not typically identified as non-compliant areas - even when they may be.

Coatings containing fluorescent pigments enhance the contrast of the coating compared to the substrate, enabling inspectors to easily detect any deficiencies.

Seeing What You’ve Been Missing

The last thing a coatings manufacturer wants is to be associated with a problem, and the last thing a paint contractor wants is to miss a spot during application that results in performing rework or first anniversary inspection repairs. Due to inconsistent application techniques, any portion of a cured coating system can have a thinner film build, pinhole, or holiday that may cause early failure. Most specifications attempt to overcome this issue by requiring various coats to have contrasting colors so they provide a visual indicator of improperly coated areas. Such specifications also stipulate using traditional testing methods that are difficult to conduct and inspection tools that are tough to use in hard-to-reach areas, such as (but not limited to) nuts, bolts, flanges, and ladders. All of the effort put into ensuring the tank achieve its required maintenance schedule following a coating application can be reduced or negated by these factors.

To overcome the difficulties encountered with traditional specifications and inspection tools, municipalities can instead specify coatings that contain fluorescent pigments, which enhance the contrast of the coating compared to the underlying substrate to draw attention to insufficient film thickness, pinholes, and holidays. That substrate may be the original surface in a single-coat application or a previous coat in a multi-coat application. Fluorescing pigments provide a visual indication of poorly coated areas while the coating is wet to help applicators minimize areas of nonconformance prior to inspection. After the coating has cured, inspectors can use an eye-safe LED flashlight or larger inspection light to easily detect any deficiencies. This allows the applicator to correct any areas of concern before the water tank is placed back into service.

Freeing Up Resources

Minor enhancements made to the products selected for maintaining water storage tanks can drastically improve the life cycle performance of these critical community assets - and thereby reduce a community’s maintenance expenditures. When a community decides to repaint its tank or construct a new one, it should consider upgrading the materials and processes that have made its water tank prone to excessive maintenance over the years.

Choosing an exterior coating that reduces the degradation caused by environmental factors will help the community improve its image for years to come. Eliminating the deficiencies that cause premature failures on the interior of the tank with edge-retentive products and fluorescent pigments will extend the tank’s maintenance cycle. These enhancements can help a community keep its water tank protected for a longer time, opening up budget money to solve other municipal challenges.

About the Author: Kevin Morris is market segment director, water and wastewater, for Sherwin-Williams Protective & Marine Coatings. He is a NACE Level III Certified Coatings Inspector, a Certified Concrete Coatings Inspector with the Society for Protective Coatings, and an Instructor for the Society for Protective Coatings - Concrete Coatings Basics and Concrete Coatings Inspector Programs. He can be reached at [email protected].