Photo By Denna Jones
How long does it take for a premature coating failure to occur? The likelihood of failure is high shortly after application: from days to perhaps a year. In this month’s Case from the F-Files, a coating delaminated shortly after installation of the coated assemblies. Repairs were made, and the units went into service. Can failure after six years be considered premature?
A company was contracted to supply to large inlet filter assemblies for installation at a natural gas-fired power plant in the Midwest. The assemblies were constructed mostly of carbon steel. The coating system specified for application to the filter assemblies consisted of abrasive blast cleaning to SSPC-SP 6, “Commercial Blast Cleaning,” and application of one coat of a three-part epoxy zinc-rich primer and a polysiloxane top-coat. The product data sheet for the epoxy zinc-rich product stated that it had a recommended typical dry film thickness of three mils but also that the material could be applied in one coat up to five mils. The product data sheet warned that at thicknesses above five mils, the product was prone to mechanical damage.
The epoxy-modified polysiloxane topcoat was in two components and had volume solids of 90%. It had a recommended dry film thickness per coat of 3–7 mils. The epoxy zinc-rich primer was a recommended primer for the topcoat, although the product data sheet warned that a mist coat might be necessary before application of the full coat when using the epoxy zinc-rich primer.
Shortly after installation of the filter assemblies, a few instances of coating delamination were noted on the floor of one of the assemblies. The coating delamination was repaired by applying an additional coat of the topcoat over the areas of delamination. No additional coating failures were noted until nearly 6 years later. The failures were found in the spring after a particularly cold winter. At that time, a large quantity of paint chips was caught in the one of the assembly’s trash screens. Subsequently, large sheets of coating were found delaminating in that unit’s plenum. Similar failures were found on the exterior of two of the other assembly units.
The field visit was made to the site of the natural gas-fired power plants shortly after the failures were discovered. The filter assemblies were first visually examined. Both the interior and the exterior of the filter assemblies were coated with a gray glossy paint. The paint in the majority of the areas was in good condition.
There were various areas of coating delamination on two of the filter assemblies. The most severe area of delamination was the plenum area of one of the units. In that area, there were various spots of coating delamination on the floor, on the walls, and on the I-beams supporting the ceiling (Fig. 1). Although the coating delamination was widespread, the total percentage of coating delamination in the plenum was no greater than 5 percent. Additionally, there was some coating delamination on the floor on the interior right-hand side of another unit. A close examination of the coating failure in these areas revealed that there had been previous delaminations that had apparently been repaired. The repair coating was a slightly different color and gloss than the rest of the paint, and a relatively distinct edge could be seen between the area where the coating had delaminated and the surrounding areas that had remained intact.
|Fig. 1: Exterior surfaces experienced delamination especially on horizontal surfaces. Photos courtesy of KTA-Tator, Inc.|
There was also some coating delamination on the exterior surfaces of both units that had coating delamination on the interior. On one unit, the coating delamination occurred both on the plenum section and the right-hand side of the unit. On the other unit, the coating delamination was limited to the right-hand section.
In all cases, when the coatings delaminated, the plane of failure occurred as a cohesive break within the zinc-rich primer. There appeared to be a thin film of zinc-rich primer on the back of the delaminated paint chips and a relatively thick coat of zinc-rich primer on the surface of the steel. The remaining zinc-rich primer was adequately protecting the surface; no rust was visible in any area.
In one area on the floor of a plenum, the coating was delaminating in an unusual blister pattern. In this area, there was a pattern of raised blisters with a diameter up to ½ inch. In some cases, the blister caps were broken, exposing the underlying zinc-rich primer. When the blister caps were forcibly removed with a utility knife, a white spot of what appeared to be zinc corrosion product was visible on the uncovered zinc-rich primer. This pattern of delamination was unique to this particular area.
In several areas in the plenum, the top-coat was detached from the surface yet still attached to the surrounding topcoat, creating large, irregularly shaped blisters. When the surface of the topcoat in those areas was probed with a utility knife, coating could be removed easily. In most cases, when the surrounding intact coating was probed with a utility knife, the surrounding coating was also found to have poor adhesion. In all cases, the plane of delamination was within the zinc-rich coating, leaving zinc on the back of the delaminated coating chips and on the surface of the steel.
The areas of delamination on the floor on the interior right-hand side of the unit that had obvious coating repair areas were probed with a utility knife. Where coating had previously delaminated and been recoated, the new repair coating adhered well to the originally applied zinc-rich primer. In the surrounding areas where the originally applied topcoat was left intact, it had extremely poor adhesion to the zinc-rich primer and could be removed easily with a utility knife.
The adhesion of the coating was assessed in accordance with ASTM D3359, Method A, Measuring Adhesion by Tape Test (Fig. 2). The adhesion of the coating system varied considerably. Near areas of delamination, adhesion was often found to be poor (1A-0A). In other areas, adhesion varied between 1A (poor) and 4A (good).
|Fig. 2: The adhesion of the coating was variable, even adjacent to large sections of coating delamination|
The dry film thickness of the coating system was measured in several areas in both units. In areas where the topcoat delaminated, exposing the primer, the primer that remained ranged in thickness between 1.7 and 5.2 mils and averaged approximately 4 mils. The thickness of the total system ranged from between 8.3 and 13.5 mils. There was no noticeable correlation between dry film thickness and delamination.
The laboratory investigation consisted of visual and microscopic examination, solvent resistance testing, and infrared spectroscopy analysis.
Visual and Microscopic Examination
A visual and microscopic examination of the samples was conducted using a digital microscope with magnification to 200X. The samples consisted of two to three layers of gray coating. The topcoat generally ranged from four to seven mils thick. The underlying coat(s) generally ranged from 0.3 to 0.8 mils thick.
Solvent resistance testing was performed using a modification of ASTM D5402, Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs. The method was modified to accommodate the small sample size. A cotton tip swab was used in place of a piece of cheesecloth. The swab was saturated with methyl ethyl ketone (MEK). The samples were tested by rubbing the tested area with the swab for a total of 50 double rubs. The test was performed in duplicate on the backside of the sample chips where a thin layer of the epoxy zinc-rich primer was present. All samples of the epoxy zinc-rich primer were found to have extremely poor solvent resistance as the coating could be totally removed with 50 double rubs, and softening and color transfer were noticeable after as few as 13 double rubs.
Infrared spectroscopic analysis was performed on two of the samples and a control sample of an epoxy zinc-rich primer that had been drawn down and allowed to cure thoroughly. The samples were found to match the control sample of the polyamide epoxy zinc-rich primer except for a noticeable difference in an intense polyamide and silicate band relative to other epoxy bands. The polyamide and silicate bands in conjunction with large bound water bands suggested an excess of non-reacted polyamine.
The field investigation and the laboratory analysis revealed that the cause of the delamination of the coating system from both the interior and the exterior surfaces of the filter assemblies at the power plant was the presence of a relatively weak polyamine-rich surface layer on the shop-applied, zinc-rich primer (Fig. 3). The weak layer fractured when the polysiloxane topcoat developed stresses during cold weather.
|Fig. 3: Delamination of the topcoat often occurred between the filters|
The plane of failure of the coating system was consistent in all areas and was within the zinc-rich primer. The split in the zinc-rich primer occurred very near to the top surface. Laboratory microscopic examination revealed that the zinc-rich primer that remained on the back surface of the delaminated coating chips ranged in thickness from 0.3 to 0.9 mils. There was no indication that the zinc-rich primer was applied in multiple coats, and, therefore, it was thought that the separation was cohesive within the zinc-rich primer, not adhesive between two coats of zinc-rich primer.
The zinc-rich primer on the back of the coating chips had extremely poor solvent resistance. In the laboratory, rubbing the back of the coating with a Q-tip soaked with methyl ethyl ketone completely removed the zinc-rich primer from the back of the delaminated coating chips. A Q-tip soaked with water had no effect on the zinc-rich primer. The affected coat was an epoxy zinc-rich primer. Properly cured epoxy coatings have good to excellent solvent resistance. A Q-tip soaked in methyl ethyl ketone should have had little effect on the primer if it had cured properly.
Additional evidence of the improper cure of the zinc-rich primer was found in the infrared analysis of the coating on the back of the delaminated coating chips. The infrared spectra were compared to a properly mixed sample of primer, and the samples removed from the filter units were found to have an excess of polyamine at the surface. This excess was an indication that either the coating was improperly mixed with an excess of the polyamine-containing components, or that unreacted polyamine exuded to the surface during the curing process. Either problem could have caused a poorly cured, weak layer to form on the surface.
There are several ways that mixing problems can cause the formation of uncured polyamine. The primer was a three-component coating, with two liquid components and one zinc powder component. The two liquid components must be thoroughly mixed together before the addition of the zinc powder. If the zinc powder is added to one of the liquid components, and the second liquid component is added later, the cure of the coating is hindered, because the intermixing of the liquid components is reduced. This could cause the polyamine components to come to the surface during the curing process, creating a weak layer on the top surface. Additionally, the polyamine-rich layer could be created simply by adding too much of the polyamine-containing component.
In most cases, the coating was thicker than the specified three mils. The actual thickness of the primer was estimated by taking the average thickness of the zinc-rich primer found on the back of the removed coating chips of 0.5 mils and adding that to the thickness of the zinc-rich primer that remained on the surface of the steel. It is thought that the actual dry film thickness of the zinc-rich primer varied from a little greater than 2 mils up to approximately 6 mils. Regardless, poor adhesion was found within the entire range of dry film thickness, including in areas where the thickness was only a little greater than 2 mils, so it is unlikely that excess dry film thickness of the zinc-rich primer was a significant contributing factor to the coating problems.
The coating delamination problem became evident after the coating had been in service for several years. It is likely that the coating delamination problem was aggravated by recent winter cold weather. The particular polysiloxane coating was relatively brittle and developed significant tension in cold weather. In this case, it is thought that the tension developed by the polysiloxane topcoat eventually became greater than the cohesive strength of the primer, causing the zinc-rich primer to split and the coating system to delaminate.
In this instance, the early delamination problem was repaired by reapplication of the topcoat. The underlying cause was not determined at that time, and the repair was considered sufficient. Yet the foundation for future failure was present at the time of installation but was unknown. There is no way of knowing how long it would have remained hidden if not for the cold weather. The local weather was considerably colder than had been experienced in the previous six years, but was not necessarily atypical for that area. Since the cold weather that precipitated the delamination could reasonably been expected sometime during the lifespan of the coating, and the expected life of the coating was far greater than six years, the coating failure was certainly considered to be premature.
Rick Huntley is the Technical Manager of Consulting Services and a Senior Coatings Consultant for KTA where he has been employed for over 20 years. He is a NACE Certified Coatings Inspector Level 3 (Peer Review) and an SSPC Certified Protective Coatings Specialist. In his current position, Rick is responsible for managing a staff of coating consultants which involves technical review of proposals/reports and project management. He also performs consulting activities that include coating failure analysis, coating condition assessments, coating system recommendations and specification preparation review, and expert witness testimony.