Protective Coatings on Welded Steel Storage Tanks: Is It Time to Repair or Replace?
The decision to repair or replace a protective coating on welded steel storage tanks depends on several factors, including the condition of the existing coating, the extent of damage or deterioration, the remaining service life of the storage tank, budget considerations, and environmental considerations.
Before diving into guidelines that may help you to make an informed decision on coating repair or replacement, it’s important to briefly discuss average lifespans for protective coatings on welded steel storage tanks.
Average Lifespan for Protective Coatings on Welded Steel Storage Tanks
Generally, a high-quality protective coating on a well-maintained welded steel storage tank can sometimes have a lifespan of 20 to 40 years or even longer. However, premature coating failure can occur due to various factors, and ongoing inspections and maintenance are crucial to ensuring the longevity of the coating.
Factors that affect the lifespan of a protective coating include:
Type of Coating: The type of protective coating used significantly influences its lifespan. Standard coatings for water storage tanks include epoxy, polyurethane, zinc-rich, and various corrosion-resistant paints. High-performance coatings, like 100% solids, tend to have longer lifespans, lasting up to 20-30 years. Some manufacturers of high-quality coating systems include Carboline, PPG, Sherwin Williams, and Tnemec, to name a few.
Surface Preparation and Application: Proper surface preparation ensures the coating adheres effectively to the substrate and provides the intended level of protection against corrosion, environmental degradation, and other factors.
Proper coating application is critical to its longevity. If the coating is misapplied or has insufficient thickness, it might not provide adequate protection, leading to premature failure. Advancements in coating systems, pushed by technology, have resulted in more restrictive acceptable application conditions.
When repairing or recoating, owners need educated third-party inspectors to attend before, during, and after coating to ensure specifications are followed and good surface prep and application practices prevail.
Environmental Conditions: The environmental conditions in which the tank is located play a significant role in lifespan. Factors such as temperature fluctuations, humidity, exposure to sunlight, and the presence of corrosive drivers (e.g., salt and industrial pollutants) can accelerate coating degradation.
Water Chemistry: The composition of the stored water can also impact the coating’s lifespan. Water with high levels of certain minerals or contaminants can contribute to premature coating deterioration. Moisture and chemicals in the water, like chlorine, can increase the likelihood of corrosion.
Maintenance: Regular maintenance and inspections are essential to identifying any coating defects or areas of deterioration early. Timely repairs can extend the lifespan of the coating.
Corrosion Protection: Some tanks may have additional corrosion protection (CP) measures, such as sacrificial anodes or impressed current cathodic protection systems. CP helps extend the life of protective coatings by reducing the underlying steel's corrosion rate.
This photo shows a protective coating that has disbonded from the substrate, most likely due to inadequate surface preparation. In this case, full replacement of the coating is necessary to achieve long-term protection of the asset.
Guideline to Deciding Whether to Repair or Replace a Protective Coating
Inspection and Assessment
Regular inspections are necessary to evaluate the condition of the existing coating. Inspections might involve visual assessments, adhesion testing, non-destructive testing, or material sampling to determine the extent of underlying steel corrosion or degradation. ASTM D610, Standard Test Method for Evaluating the Degree of Rusting on Painted Steel Surfaces, can help an inspector and owner visually determine the amount of corrosion on the exterior and interior of tanks. The American Water Works Association (AWWA) recommends that water storage tanks be inspected every three to five years; however, state regulations might require more frequent inspections.
If the damage is localized and limited to certain areas, such as roof plates or isolated areas on the tank shell, repair might be a cost-effective option. However, if the damage is widespread, a complete replacement of the protective coating might be a better long-term solution.
Cost Analysis
Compare the costs of repairing the existing coating versus a complete replacement. Sometimes, ongoing repairs can add up and become more expensive than a full coating replacement, especially if the storage tank requires frequent attention. Owners and engineers can review recent bid data on public works online bidding platforms like PlanetBids to develop capital budget estimates.
Operational Impacts
The protective coating on this steel tank has degraded due to environmental conditions and the primer is now exposed.
Complete coating replacement projects usually involve significant downtime for the infrastructure facility. Considering the impact downtime can have on water supply reliability and system operation might assist in the decision-making process. Evaluate the risks associated with a failing protective coating exposing the underlying steel to corrosion, possibly leading to structural integrity issues if not addressed quickly. The structural repair will require significantly more downtime than a coating replacement project.
Long-term Planning
Consider the overall long-term maintenance strategy for the infrastructure. If coating replacement aligns better with a comprehensive maintenance plan, it might be the preferred choice. As an owner, choosing an approach that complies with relevant regulations and standards for water infrastructure maintenance and protection is critical.
It’s essential to engage with engineers, coating specialists, and professionals with experience in water infrastructure to get their recommendations based on the specific conditions and life-cycle requirements of the infrastructure. The decision to repair or replace a protective coating on water storage tanks should be based on a comprehensive evaluation of factors such as the condition of the coating, the extent of damage, cost, remaining service life, environmental impact, and overall long-term goals. It’s often a multi-faceted decision that requires a balance of technical, financial, and operational considerations.
At V&A, infrastructure preservation is at the core of our mission. Protective coatings offer the first line of defense in extending and preserving infrastructure service life. Our coatings experts assist water and wastewater clients nationwide select the proper coating or lining system to mitigate damage caused by corrosive environments. We welcome your coatings and lining questions in the comment section below.
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