Before performing an application in a wastewater environment, a coatings contractor must fully understand everything from concrete composition to the different coating types needed to withstand some of the common forms of corrosion found in these environments.
Concrete Requires a Coating
As acid can be prevalent in wastewater streams, it is essential to remember that concrete comprises almost entirely alkaline materials. Studies have shown that even the smallest percentages of sulfuric acid in short-term exposure can cause long-term catastrophic issues leading to concrete replacement. The sulfuric acid, formed during the biogenic sulfide corrosion process commonly seen in wastewater environments, attacks the calcium hydroxide within the Portland cement, causing mass loss and a drastic reduction in compressive strength. These losses contribute to premature structure failure and, eventually, replacement.
Bobby Tate, a project manager/industrial superintendent for Commercial Painting Inc., offered this advice to asset owners: “If you defer coating now, down the road, it will cost you more.”
Inspection Helps to Identify Premature Coating Failure
A crucial part of surface preparation prior to coatings is the substrate’s initial inspection. When asked about assessing concrete, Gordon Hancock, owner of Three Rivers Services near Birmingham, Ala., offered, “One of the things I look for includes asking how old the concrete is.”
Several simple tests will help to identify contaminants that may cause a premature failure of a coating system. A water bead test can quickly identify the presence of greases and oils that wreak havoc on coating adhesion. Less effective with new concrete, pH paper can also be used to determine the presence of pollutants. It is necessary to identify and address honeycombs, bug holes, fins, and protrusions before applying a coating system. The thickness of the final coating system will deter-mine the level of treatment for these phenomena.
There are three methods to test for moisture vapor transmission rate (MVTR) in concrete: the plastic sheet test, the calcium chloride test, and the relative in situ probe test. Most manufacturers will recommend performing two of these tests to under-stand the concrete’s moisture content accurately. Moisture-resistant coatings, such as cementitious urethanes, are another effective remedy for MVTR.
Concrete Surfaces Are to Be Prepared Before Coating
“Dry abrasive blasting is preferred. However, vapor abrasive blasting (using garnet as the media) has become a ‘go-to’ method, especially for concrete surfaces,” offered Tate when asked about preferred surface preparation methods. These methods, plus others, are outlined in NACE No. 6/Society for Protective Coatings (SSPC) Surface Preparation (SP) 13, “Surface Preparation of Concrete.” This all-inclusive standard covers all aspects of preparing concrete surfaces for a coating. Laitance, a brittle layer at the surface of concrete, must be removed. Coating manufacturers will outline a surface profile in conjunction with International Concrete Repair Institute (ICRI) 310.2R, “Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair.” This standard provides comparators commonly referred to as the concrete surface profile (CSP), allowing contractors to have a visual standard for the profile.
Coating Types and When to Use Them
When it comes to industrial coatings for water/wastewater projects, there is no one-size-fits-all approach. Coatings used in these environments have become more robust as the waste stream has become more corrosive over time. For that reason, thick-film coatings have become the standard in these more aggressive exposures.
Tate said that he has seen the coatings used in this market become much more technical over the past decade. He added, “With these coating changes, a required skill is being able to adjust and think on the fly.” Similarly, Hancock stated, “Coating technologies are changing and becoming more sophisticated. Many of these coating technologies have become directed toward more effectively protecting the owner’s assets. With this shift, the requirements for surface preparation and installation have greatly increased and, in some cases, become more stringent.”
There are a variety of coating options that may work with a water/wastewater asset:
Epoxies: The most widely used thick-film coatings for wastewater exposure are epoxies. This broad category can have multiple combinations and formulation types, with different degrees of flexibility, chemical resistance, corrosion resistance, and other performance characteristics. Epoxies are typically the easiest to apply because they allow for a greater degree of flexibility in application.
Many of the thick-film, high-performance epoxies used for wastewater exposure exhibit low permeability, excellent adhesion, and good overall chemical resistance. One negative is that these materials have minimal elongation.
Polyurethanes: In addition to offering higher elongation properties than epoxies, aromatic polyurethane lining materials provide some additional features. Applied at even greater thicknesses and sometimes in lower temperatures, they exhibit a speedy cure and return to service.
Many of the formulations offer excellent abrasion resistance along with crack-bridging capabilities. However, aromatic polyurethane linings have a low moisture tolerance and often require a primer.
Polyurea: The last thick-film lining type discussed here is polyurea. Similar to polyurethanes, these linings have high elongation properties and even speedier cure-to-service properties. In addition, they are applied at very high film thicknesses. Some negative attributes include their necessary application by special equipment as well as lower adhesion values due to the rapid gel times present in some formulations.
Overall, there are several pros and cons to each type of thick-film coating. A contractor must be aware of these various coating types because each has its merit in certain situations.
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Source: Originally appeared in CoatingsPro Magazine, written by Brian Cheshire, Market Manager, Water/Wastewater, and Jack Walker, Content Strategist for Carboline.