What Can Happen When the Service Environment Isn’t Characterized or the Wrong Coating is Selected
|Fig. 1: This tank lining showed insufficient resistance to the acidic tank contents. Photos courtesy of KTA-Tator, Inc.|
Typically when a premature coating failure is encountered, it is assumed that either there was a mistake made during the surface preparation and/or the application of the coating, or that there was something seriously wrong with the formulation of the coating. In fact, in the majority of cases, one or more of these circumstances is exactly what happened. Sometimes, though, the surface preparation and the application are top-notch, and the coating is a quality product that was formulated and manufactured to exacting standards, and yet the coating still fails prematurely. In these cases one has to look elsewhere to determine the cause of the coating failure and ask the question, “Was this the right coating for the job?” There are literally thousands of different coating products, each appropriate for some type of service environment. When a coating system is specified for a project, much effort is taken to assure that the surface preparation is adequate and the thickness of the coating is appropriate. Yet, in some cases, not enough thought is given to whether the coating system itself is appropriate for the service environment to which it will be exposed and the surface to which it will be applied, and both must be considered when a coating system is chosen.
Given the vast number of coating products available and the multitude of different service environments to which they can be exposed, there are probably hundreds of different ways that a coating can fail if the wrong coating is selected for a particular environment. These failure mechanisms include, but are not limited to, rust-through, peeling, chemical degradation, chalking and cracking.
|Fig. 2: Peeling resulted after application of this white coating, which was incompatible with the factory-applied gray coat.|
Several years ago, a large warehouse was newly constructed, and the warehouse floor was to consist of coated poured concrete. The original specification called for an epoxy floor coating to be applied, but the engineer asked for an alternate system based on cost. Ultimately, an equipment enamel was chosen to coat the floor. The floor was well-prepared using a portable centrifugal blast unit and the coating was applied to the recommended thickness, yet one month later the applied coating began to soften and deteriorate. The equipment enamel was based on alkyd resin, which deteriorated due to the presence of moisture and alkalinity in the concrete in a process known as saponification. This failure was described in further detail in a previous F-Files article, “Saponification: From Paint to the Grave.” (JPCL, May, 2014).
This alkyd coating, while perfectly suitable for its intended purpose of coating metal surfaces of equipment, was clearly the wrong product for this concrete substrate. Similar problems could arise if the same alkyd coating was applied to galvanized steel surfaces (even if those surfaces were on equipment), because zinc corrosion products are alkaline and will similarly deteriorate the alkyd paint in the presence of moisture.
Other coating failures can occur if a coating is not well-suited for the substrate. For example, coatings for wood can be problematic, especially on exterior exposure environments. Wood expands and contracts cyclically upon exposure to moisture. Coatings that are applied to wood must have sufficient flexibility to resist cracking when the substrate underneath is expanding and contracting. Applying a hard brittle coating to wood in this exposure environment will ultimately lead to premature cracking of the coating.
Surfaces that have been previously coated should also be considered, including when items are supplied with the shop primer or the OEM coat of the wrong type or color, or when surfaces are being recoated. In any case, the newly applied coating must be compatible with the existing coating. Peeling is the most common type of coating failure that can occur when previously applied coatings are overcoated. This problem can be avoided by choosing a compatible coating that is known to achieve sufficient adhesion to the previous coat or sometimes by roughening the surface of the previously applied coat. Roughening the surface may also be necessary when the recoat window has been exceeded.
Another type of failure can occur when a relatively hard, brittle coating is applied over a soft, flexible coating. One such example occurred at an iconic midwestern Major League Baseball stadium. Structural steel columns at the stadium had previously been coated with a soft black bitumen coating. As part of a renovation project, the columns were painted with a white epoxy topcoat. Although this coating had been used successfully on many tons of structural steel, it quickly cracked over the bitumen coating. Solvents from the epoxy had softened the bitumen coating, and when the epoxy cured, it developed cohesive stresses. Because the bitumen coating underneath the epoxy was soft enough to move slightly under the stress, the epoxy coating cracked, creating an alligator skin pattern also known as “alligatoring.”
|Fig. 3: The coating used on this metal roof had poor UV resistance and degraded significantly.|
Service Environment Consideration
A more common mistake associated with choosing the wrong coating system is the failure to properly characterize the service environment to which the coating will be exposed, or simply choosing the wrong coating system for a known environment. There are several environmental factors that affect a coating’s ability to perform. These factors include, but are not limited to, temperature extremes, pH, solar radiation (sunlight), chemicals, solvents, immersion or splash and physical damage.
Exposure to temperature extremes can cause a coating system to deteriorate quickly. In chemical plants and other manufacturing facilities, the normal operating temperature range of the equipment used in the process is typically determined before a coating system is selected for that equipment. There have been many instances where either the temperature range was not well understood during the design phase, or upset conditions occurred where the temperatures rose above expected operating temperatures. Coating systems must be planned to withstand the highest temperatures to which they may be exposed. It is not uncommon for an industrial coating to have a temperature limitation of 250 F or 300 F. Once the temperature increases above this level, the resin in the coating will begin to char as chemical bonds in the coating begin to break. Eventually the coating deteriorates, usually turning dark brown, and coating failures such as cracking and peeling often follow.
Failure to choose a coating that adequately resists exposure to sunlight on exterior substrates can lead to premature failure of the coating. This type of failure is commonly found even with consumer paints. Different types of coatings have varying resistances to solar radiation. Latex paints that are commonly purchased by homeowners in paint stores or home remodeling stores generally come in interior and exterior versions. The exterior latexes are generally based on acrylic resin technology and have good resistance to the ultraviolet (UV) spectrum in sunlight while the interior latexes are based on polyvinyl acetate (PVA) and have poor resistance to both UV light and water. The UV light will degrade this resin system and the interior paint will chalk severely if exposed.
Epoxies have notoriously poor resistance to the UV spectrum in sunlight and chalk readily upon exterior exposure. The aesthetic issues associated with the chalking may not be a problem when the structure is not in an area where the public can see it, but sometimes the chalking is so severe that the coating loses thickness in a process known as erosion. The coating chalks and rain washes the chalk away, exposing more coating. The newly exposed coating chalks and the deterioration continues until most or all of the coating is eroded away. The coating is then no longer able to protect the substrate from corrosion.
|Fig. 4: This failure was caused by an epoxy coating that was not intended for immersion service.|
When a PVA block filler is used on exterior concrete block, the block filler will deteriorate in areas where moisture is prevalent. This is a common occurrence especially in areas with significant amounts of rainfall. The PVA block filler is well suited for interior applications but is prone to premature failure on exterior exposures.
Sometimes the severity of the environment is not understood well enough or is underestimated. This can happen in coastal areas where metal structures are frequently exposed to airborne salt mist, regardless of whether or not they are located directly on the coastline. Coating systems that are designed to protect inland metal structures are often inadequate to protect the metal near saltwater coasts. For example, an alkyd system may be sufficient to protect steel in inland areas as long as the steel is not exposed to other corrosion-producing elements. On the seacoast, application of the same alkyd coating as the primary barrier coat will likely lead to premature rusting and rapid deterioration of the steel.
Coatings that are exposed to immersion in water or a water solution can fail catastrophically if the wrong coating is selected for that application. If the coating is generically unsuitable for constant immersion, the failure can be almost 100 percent. Even if the coating has a resin type that is generally considered to be compatible with immersion service, it is important to choose a coating that is formulated specifically for immersion. For example, epoxy coatings are generally suitable for immersion, but there are specific products that are formulated with appropriate solvents and pigments to perform well in immersion. An epoxy coating not specifically formulated for immersion may absorb and transfer too much water to perform adequately and rusting and/or delamination may occur.
Condensation can cause similar failures as well. This has occurred on tank exteriors where groundwater was being transferred to the tanks almost constantly. During the summer months the temperature of the groundwater was consistently less than the exterior air temperature resulting in continuous condensation on the exterior of the tank. The exterior coating was in constant contact with this water, similar to the immersion service on the inside of the tank. The exterior coating blistered and delaminated as if it had been put directly into immersion service.
The list of failures that can occur if the wrong coating system is selected and installed is almost endless. Care should be taken to choose a coating system that will perform in the most severe scenario of the intended service environment and to clearly convey that information to the coating manufacturer so that appropriate recommendations can be made. There are plenty of opportunities for something to go wrong on a coatings project without allowing coating selection to be among them. Spend adequate time evaluating the substrate and service environment. After all, you don’t want to be the one to say, “Oops, wrong paint.”
About the Author
Rick Huntley is the technical manager of consulting services and a senior coatings consultant for KTA-Tator, Inc. He is a NACE-certified Coating Inspector Level 3 (Peer Review) and an SSPC-certified Protective Coatings Specialist. He is a primary instructor for various KTA training courses and holds a Bachelor of Science degree in chemical engineering from Washington State University.