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How to Select Industrial Coatings for Steel Water Tank Exteriors

A steel water tank after coating.

Over the course of my career, I’ve had the opportunity to protect countless steel water tank exteriors—from rolling overcoat systems on tanks in the woods and painting the Pepsi logo on a water tower, to stenciling a high school mascot on a water tank near the football stadium and applying a three-component coating system on a tower near a southern coastline.

Depending on the specific circumstances at hand, there’s a surprising level of variability in how the application process is performed and which types of industrial coating systems are best suited for the job. For instance, a fluorourethane or polysiloxane coating may be chosen for a water tank that has significant sun exposure and public visibility, while an overcoat system may be all that is needed for a water tank in the woods that has little exposure to the elements and moderate shade.

Generally speaking, traditional coating systems in these types of situations will come equipped with an inorganic zinc primer, epoxy intermediate, and aliphatic polyurethane, siloxane or fluorourethane topcoat. Siloxane and fluorourethane coatings are ideal for gloss and color retention, but can be somewhat expensive. Aliphatic polyurethanes are also no slouch in that regard and perform very well in coastal, salt-laden environments. In addition, they tend to perform well when there is the potential for industrial fumes being present—especially high-build urethanes containing polyester or other similar materials. Corrosion-inhibitive epoxies as a primer and intermediate are not uncommon as well and, although they generally don’t provide the same corrosion protection as an inorganic or organic zinc-rich primer, they are still a solid choice for steel water tanks.

Moisture-cured products can be beneficial when a water tank isn’t able to be drained and condensation issues may be present, or in a high-humidity environment where a traditional system won’t work until cured. This can be a zinc-rich, moisture-cured urethane primer with two coats of a moisture-cured urethane topcoat of similar coating technologies.

In some cases, it may even be as simple as a four-component alkyd coating system, with a red oxide or aluminum primer for added corrosion protection. Although typically considered to be a cost-effective system, it will perform exceptionally well in an environment that’s not particularly corrosive.

In terms of cost-effectiveness, an overcoat system may be sufficient for extending service life in the short term. This is not an equal substitute for commercial blasting to bare substrate and starting from scratch, given the service life of coatings are considered proportional to the surface prep involved. However, an overcoat system is a great choice for a quick, relatively cheap job that will look great and continue to provide protection. This typically involves SSPC-SP 1 for Solvent Cleaning, SSPC-SP 2 for Hand Tool Cleaning or SSPC-SP 3 for Power Tool Cleaning—often by means of pressure washing, abrading and applying the coating. Once the surface is prepped, it’s most often coated with a relatively surface-tolerant coating (such as an epoxy, or even a water-based or alkyd coating) and then sealed with a UV-resistant topcoat that’s compatible with the tie-coat.

Because this topic can certainly be a lot to digest, the American Water Works Association (AWWA) offers a variety of helpful resources, including the AWWA D102-17 guide for coating steel water-storage tanks. This guide covers everything from pre-construction primer options, to different types of interior and exterior coating systems. For the purposes of this blog post, Section 4.3 for Exterior Coating Systems offers seven coating system options with mil thicknesses, generally ranging from 5.5 to 6.5 mils.

Learn More About Carboline’s Potable Water Tank Linings & Coatings

Download our water storage tank brochure for optimal protection, inside and out.

How to Select the Best Lining for Concrete in Wastewater Applications

An aromatic polyurethane is applied to a concrete clarifier

When it comes to linings for wastewater applications, concrete substrate has always been trickier than its steel cousin. This is largely because there are so many factors to consider—from concrete repair, laitance removal and new concrete cure times, to outgassing and hydrostatic pressure.

By comparison, steel surfaces are fairly straightforward. Prior to coating, you must prep the surface to a near-white or white degree of surface cleanliness, use the proper anchor profile for coating application, follow the NACE SP0178 weld designations and maintain acceptable chloride levels.

On the other hand, concrete has a more complicated relationship with linings—even though it’s the most widely used construction material in the world. That’s because it is porous (which can lead to moisture absorption and outgassing, among other things), prone to cracks, and much more vulnerable to chemical attacks than steel in the event the lining becomes compromised.

In addition to these variables, there is a wide range of environmental aggressiveness that can impact concrete exposure and corrosivity in wastewater applications. This is why it’s critical to select the right lining when it comes to protecting concrete in wastewater—whether it’s a municipal septic holding tank, a chlorine contact basin or a clarifier.

The Basics of Concrete Surface Prep

Picking a lining that is compatible with your wastewater service—whether it be municipal or industrial—is of the utmost importance. As a starting point, let’s discuss four basic factors that should be considered when prepping your concrete surface, per the NACE No. 6/SSPC- SP 13 joint standard:

  • Concrete Surface Profile (CSP) Standards: The CSP standards range from 1-9, with CSP 1 being virtually smooth and generally done by mild acid etching, while CSP 9 has heavy scarification typically induced by mechanical means. Many thick film linings will require a CSP 3-5, or even a CSP 6 or higher. Thin film coatings, on the other hand, tend to more commonly be CSP 2-4.

  • Laitance: In simple terms, laitance is defined as the weak, brittle layer of cement and fine aggregates that rise to a concrete surface when too much water is added. It is greatly influenced by the type and quantity of admixtures, the degree of working, and the amount of water in the concrete. Laitance is generally formed from new concrete, or even after old concrete has been resurfaced or repaired. Prior to coating a concrete surface, laitance must first be removed.

  • Moisture Content: Moisture vapor transmission (MVT) is the passage of water vapor through a substance, such as concrete. Because this can often cause blisters and other issues to occur when applying a coating over the concrete surface, it’s important to check for moisture throughout the process. Both the ASTM D4263 plastic sheet standard test method and ASTM F2170 standard test method for checking relative humidity with in-situ probes are good ways to measure moisture content in the concrete.

  • Void, Bug Hole & Honeycomb Filling: These issues can usually be found in the surface of new and old/repaired concrete. Over time, they tend to lead to significant outgassing problems with coatings due to the air void that tries to escape as the film cures—especially when concrete temperatures rise as a result of the sun or other factors. Voids, bug holes and honeycombs can typically be filled by cementitious epoxy-based repair mortars or similar products.

In addition to these four variables, the NACE No. 6/SSPC-SP 13 joint standard outlines many more factors that come into play during the concrete surface prep process—such as watching for running and/or static cracks and hydrostatic pressure.

Coating Types for Various Exposure Levels

The lining itself will depend heavily on the type of wastewater service at hand. Coal tar and coal tar epoxy both have long histories of service in mild and moderate exposures across different concrete systems—from sludge handling and water treatment, to preliminary treatment such as settling and aeration tanks. It used to be the case that coal tar served as a cheap, all-purpose coating for most wastewater applications. However, more coating technologies have been integrated into the industry over time—especially as environmental aggressiveness increases.

Many moderate, and some severe, exposures will be a good fit for aromatic urethane. It adheres well to properly prepped concrete, has a quick return-to-service time, and can offer surprising resistance to solutions like chlorine and sodium hypochlorite that even novolac epoxy coatings have trouble matching. When we bump that up to the severe exposure category, you’ll start to see novolac epoxy coatings and sometimes even vinyl esters being used.

Although the overwhelming majority of severe sludge handling and water treatment can be held at bay with 100% solids novolac epoxy coatings, it’s always best practice to reach out to a coatings manufacturer with a chemical analysis list in order to determine which lining will work best.

Want to Explore Coatings for Wastewater Asset Protection?

Check out Carboline’s new wastewater asset protection guide, which offers an interactive, step-by-step look through the wastewater collection and treatment process. It outlines each asset, along with recommendations for coating and lining options for each substrate within the structure.

How to Select Industrial Coatings for Wastewater Collection Systems

An applicator in a manhole preparing to coat the structure.

Serving a vital function in the local community, wastewater collection systems are designed with underground pipes and maintenance structures used to collect and convey wastewater away from homes and businesses to be treated at a water resource recovery facility. During this process, wet wells temporarily store the wastewater, lift stations pump the wastewater to a discharge point at a higher elevation, and then the wastewater is sent through a gravity line to a treatment facility.

For a moment, let’s imagine that this wastewater collection system didn’t function properly and all of our municipal waste had nowhere to go. It’s not a pretty picture, but that’s exactly why it’s so important to protect these wastewater collection systems from degradation using the right industrial coatings.

How Do Wastewater Collection Systems Work?

Wastewater collection systems essentially gather two different types of waste—municipal waste from homes and businesses, and storm water. Municipal waste tends to mainly produce hydrogen sulfide and ammonia, which can be moderately aggressive. On the other hand, waste from storm water run-off collection starts off clean but, as it moves along the surface, picks up sediment and contaminants such as grease, chemicals and other pollutants.

Because of these factors, wastewater collection systems—plus the pipelines that transfer the wastewater and the storage vessels that house it—all take a fair amount of abuse. Not only can wastewater be aggressive to surfaces chemically, but it also has the potential to carry abrasive media along with it—which requires more fortified protection in the form of industrial coatings.

Which Industrial Coatings are Best for Wastewater Collection Systems?

It’s common for 100% solids products to be used throughout wastewater collection systems. If it’s concrete, the surface will typically be repaired with a cementitious mortar or epoxy patching compound, and sealed with a water-based epoxy primer, phenalkamine epoxy or solvent-free epoxy primer. It will then be top coated with either an aromatic urethane—which tends to perform well with concrete, has good abrasion resistance and high film build—or, if it’s chemically severe, a 100% solids epoxy or epoxy novolac. These can also be high build, abrasion resistant (especially for the novolac), and highly chemical resistant.

If an aromatic urethane is selected, it’s crucial to consider which primer can go underneath it. This is because the high cohesive strength of many elastomeric aromatic urethanes is so high that they can be difficult to adhere to other coatings. These can often be applied directly to concrete substrates as well.

However, not all of these wastewater collection systems, tanks and vessels are concrete, so a good reference to follow is the American Water Works Association’s (AWWA) C210-15 Liquid-Epoxy Coatings and Linings for Steel Water Pipe and Fittings. This C210 requirement fits the bill well for wastewater collection systems and provides a good indicator for whether a product will be able to handle this type of service. Keep in mind, though, that this is limited to epoxies. It also is designed for solutions that are meant to be applied directly to steel.

That said, having an appropriate primer, and an AWWA C210-15 compliant industrial coating that’s capable of adhering to it, will still give you a good leg to stand on when it comes to protecting your wastewater collection system—even if it’s a concrete substrate.

Want to Explore Coatings for the Wastewater Collection Process?

Check out Carboline’s new wastewater asset protection guide, which offers an interactive, step-by-step look through the wastewater collection and treatment process. It outlines each asset, along with recommendations for coating and lining options for each substrate within the structure.

CTSP Episode 165 - Coating Ductile Iron

Brian Cheshire joins Jack Walker and Paul Atzemis to discuss coating ductile iron pipe, how it's different from steel, and what standards to use.
  • 0:00 – Intro
  • 1:38 – What makes ductile iron different than steel
  • 2:37 – Ductile iron surface prep requirements
  • 3:30 – Color change when blasting ductile iron pipe and dangers of over-blasting
  • 5:25 – The first three parts of NAPF 500-03 (NAPF 500-03-01 Solvent Cleaning, NAPF 500-03-02 Hand Tool Cleaning, NAPF 500-03-03 Power Tool Cleaning)
  • 7:12 – NAPF 500-03-04 Abrasive Blast Cleaning for Ductile Iron Pipe and NAPF 500-03-05 Abrasive Blast Cleaning for Cast Ductile Iron Fittings
  • 10:27 – Coatings used on ductile iron substrates
  • 12:43 – Lining ductile iron pipe
  • 14:30 – Wrap up
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CTSP Episode 161 - Substrates in Wastewaster Treatment Environments

Brian Cheshire joins Paul Atzemis and Jack Walker to discuss substrates found in a wastewater treatment plant. The trio explores the factors associated with the various structures and what to keep in mind when coating them.

  • - 0:00 – Intro
  • - 2:20 – Substrates in wastewater treatment environments
  • - 2:50 – Concrete in wastewater treatment environments
  • - 4:50 – Steel in wastewater treatment environments
  • - 7:04 – Ductile iron in wastewater treatment environments
  • - 8:54 – Non-ferrous metals in wastewater treatment environments
  • - 9:40 – PVC and various plastics in wastewater treatment environments
  • - 10:53 – Wrap up

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