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by Jayson L. Helsel, P.E.
KTA-Tator, Inc. (KTA) was contracted to investigate a reported coating failure. The case involved a college sports arena that included an enclosed ice rink within the facility. The existing paint of the interior roof structure was reportedly peeling from the metal surfaces and allowing rust to form on the structural steel. The facility was approximately 12 years old, and the coatings were thought to be original. KTA was asked to provide an independent third party investigation of the coating problems and recommend the best method of preparing the structure for repainting.
The roof structure of the ice rink consisted of free spanning structural steel trusses with cross bracing supporting a galvanized steel roof deck. The coating system originally specified for the steel surfaces was one coat of an alkyd metal primer and two coats of an alkyd enamel finish. There were some claims that the structural steel may have been shop primed. The product technical data sheets recommended that the primer coat be applied at 2 to 2.5 mils and the finish coat at 2.0 mils dry film thickness.
Step 1: The Field Investigation
A site visit was made by a KTA coatings consultant to conduct an overall coating assessment of the painted roof structure, investigate the reported coating problems, and determine the cause of those problems. The coating assessment focused on the roof structural support steel, which included several different members comprising the truss framework that supported the galvanized steel roof deck. All of the surfaces had a white finish coat.
The consultant performed a visual assessment of the coating condition and several field tests including the total coating thickness, the number of coats present and thickness of each, measurement of coating adhesion, and examination of the substrate condition. The condition of the coatings was fair to poor with scattered rusting, mold and dirt present on most surfaces. There was also scattered delamination of the white finish coat from the underlying primer. The white finish coating was somewhat brittle and slightly chalked. The dry film thickness of the coatings typically ranged from 7 to 12 mils. Cross-sectional examination with a Tooke Gage revealed two coating layers consisting of a red/brown primer layer of 3 to 4 mils and a white finish of 7 to 8 mils. Additionally, mill scale could be seen on the steel surfaces.
Adhesion testing was also conducted in accordance with a standard method known as the "Tape Test." This method involves making two intersecting cuts through the coating to the substrate with the smaller angle of the cuts of between 30° and 45°. A pressure sensitive tape is then applied to the X-cut area and rapidly removed. The adhesion is rated in accordance with a published rating scale. The adhesion tests revealed fair adhesion on most surfaces with randomly scattered poor results. The break in the coating system was usually between the primer and white finish coat.
The lower structural members at the base of the support structure exhibited the poorest coating condition with scattered pinpoint rusting of up to 20% of the surface area on bottom horizontal surfaces. There was also significant mold and dirt present over the majority of these surfaces including the vertical portions of members. The coatings on the bottom web of some lower beams running across the width of the ice rink were also in poor condition with the white finish coat delaminating from the red/brown primer in several locations. The exposed primer was generally intact and had good adhesion to the steel, but some heavier rusting and rust scale was present at the delaminated areas. The overall percentage of coating delamination was low, however, these areas were quite noticeable since they were typically over the center of the ice.
The coating on the steel structure above the ice, as described above, was in poorer condition than areas around the perimeter of the rink enclosure. The steel surfaces around the perimeter, which comprised a minority of the total surface area, had significantly less mold, less scattered rusting, and little coating delamination present.
Representative samples of the coatings from failing and non-failing areas were obtained for analysis in the laboratory.
Step 2: Laboratory Investigation
The laboratory investigation consisted of various analyses to determine or confirm the existing coating type and to determine whether any contamination of existing coating surfaces or original substrate surfaces was present. A microscopic examination confirmed the coating thicknesses observed during the field visit. There was also evidence of dirt and a white crystalline material on the backside of some samples that had delaminated (i.e., failing samples). However, it could not be determined whether the surfaces were dirty before or after failure had occurred. The white crystalline material could not be readily identified, but it seemed unlikely to be a salt contaminant, which would be of concern. Nonetheless, its presence warranted testing of surfaces for soluble salts during future coating repairs. The relatively low amount of overall coating delamination suggested that there likely was not any significant contamination of the primer or substrate surfaces prior to coating application.
Step 3: Combining the Field and Laboratory Investigations to Determine the Failure Mechanism
An analysis to determine coating type (by infrared spectroscopy) revealed that the existing coatings were alkyd coatings consistent with the original specification requirements. Additional laboratory analysis (using a scanning electron microscope) was conducted to determine whether any chemical contamination of the existing surfaces existed. These analyses did not result in any notable findings.
The investigation did not identify any specific problems with the coating materials, other coating defects or any obvious contamination of existing surfaces. The coating thickness was observed to exceed the originally specified thickness. In particular, the white finish coat was noted to be two to three times the specified thickness. The current condition of the existing coatings was considered to be generally consistent with the normal deterioration of alkyd coatings in a somewhat aggressive environment such as an ice rink. The environment is considered aggressive due to the relatively higher levels of moisture that should be expected in the facility from the presence of the ice surface. Exposing coatings to higher levels of moisture accelerates rusting or corrosion of surfaces since moisture (along with air) is a key component of the corrosion process, and will eventually permeate most coatings. The presence of mold on the existing coatings and the degree of scattered rusting, particularly heavy in many locations above the ice, supported this general premise. Essentially, the coating system performed as expected given the type of coating and service environment.
Step 4: Determining a Repair Procedure
The relatively high degree of corrosion and marginal coating adhesion in the worst areas warranted the complete removal and replacement of the coatings in those areas. The areas where the coating condition was better, could allow for lesser repair procedures in those areas, such as spot repairs and overcoating of the existing coatings that remained. A spot repair approach attempts to extend the life of the existing coating system rather than simply removing and replacing all existing coatings. However, using multiple approaches to extend the life of the existing coatings was not consistent with the desire for a long lasting coating system. Therefore, the recommended approach for coating repair was total removal and replacement of coatings in all areas.
Surface preparation was recommended to be accomplished by abrasive blast cleaning in accordance with SSPC-SP 6, "Commercial Blast Cleaning." Commercial blast cleaning removes all mill scale (which was found to be present), rust and existing coating from steel surfaces. Only stains or shadows on up to one-third of the cleaned surface are allowed to remain. For a replacement coating system, high performance coatings were recommended due to the aggressive environment described for the ice rink. Coating system options included epoxies, polyurethanes (with epoxy primer), and moisture-cured urethanes.
Epoxy coatings are typically used as primers and/or intermediate coats in industrial environments. These coatings are known for their corrosion resistance and durability, and perform well in moist environments. The recommended epoxy coating system would consist of a polyamide-epoxy rust-inhibitive primer and polyamide-epoxy finish coat. One disadvantage of epoxy coatings is that they chalk with exposure to ultraviolet light and may yellow over time.
Chalking and yellowing could be prevented by using a polyurethane finish coat. Two component polyurethane coatings are considered to have among the best durability and performance characteristics of any coating, including in moist environments. The recommended epoxy/polyurethane system would consist of a polyamide-epoxy rust-inhibitive primer and aliphatic polyurethane finish coat.
The last option was using a moisture-cured urethane (MCU) coating system. MCU coatings are single component polyurethane materials that react with moisture in the atmospheric environment to complete their cure (rather than a second component). These coatings are generally more user-friendly than typical two-component epoxy or polyurethane systems. MCU coatings offer many of the same high performance characteristics as two-component polyurethanes including excellent color and gloss retention. This system would consist of an MCU rust-inhibitive primer and MCU aliphatic finish.
About the Author: Jayson Helsel is a Senior Consultant with KTA. He holds a B.S. in Applied Science (with High Honors) from the U.S. Coast Guard (USCG) Academy and an MS in Chemical Engineering from the University of Michigan. He is a registered Professional Engineer in Ohio, Connecticut, Pennsylvania, Louisiana, Oklahoma, Colorado, Alabama, and Kentucky, and is a NACE Certified Coatings Inspector (#7356). Mr. Helsel has extensive marine and shipboard experience having served 11 years in the Coast Guard, most recently as a Lieutenant Commander in the area of Marine Vessel Inspection. At KTA, Mr. Helsel has managed the field investigations and laboratory analyses of coating failure cases, including those of a major automobile manufacturer and luxury home developer. He has also been involved with coating surveys and inspections of industrial structures.
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