This month’s Case from the F-Files describes the problem of bubbles, pinholes, and blisters in a polyurethane finish coat applied to new structural steel members at a coal-fired power generation plant. Many of the pinholes and bubbles were so small that they were difficult to detect with the unaided eye. Many of the largest blisters on the webs of structural members were very flat and shallow and also difficult to detect by eye. These conditions became more difficult to see overtime as thin layers of dirt from normal plant operating processes formed on the surface of the polyurethane finish coat. This case file illustrates that interacting variables, rather than a single cause, can combine to cause a failure, and gives a coating condition assessment.
The specification required that the structural steel be blast cleaned in the shop in accordance with SSPC-SP 6/NACE No. 3, Commercial Blast Cleaning. Following blast cleaning, a two-coat system, consisting of a moisture cured urethane (MCU) zinc-rich primer and an aliphatic polyurethane finish, was shop-applied. The MCU primer was specified to be applied at a dry film thickness (DFT) of 2.5 to 3.5 mils, and the polyurethane finish was to be applied at a DFT of 4.0 to 5.0 mils. The total two-coat DFT was to be 6.5 to 8.5 mils.
|Fig. 1: Sections of newly-coated steel members at a coal-fired power plant displayed blistering and other signs of coating failure. Photos courtesy of James D. Machen, KTA-Tator, Inc.
Field touchup work was specified to be SSPC-SP 2, Hand Tool Cleaning, and/or SSPC-SP 3, Power Tool Cleaning, followed by the application of a coat of surface-tolerant epoxy mastic (4.0 to 6.0 mils’ DFT) and a finish coat of polyurethane (4.0 to 5.0 mils’ DFT).
The steel was delivered to the project site for sequenced erection. In mid-summer, near the completion of the project, blistering and peeling were observed. At that time, the shop contractor mobilized a field team to make repairs. Repairs were reported to have been performed using low-pressure water cleaning (4,000–5,000 psi), in conjunction with hand and power tool cleaning, to identify and remove defective areas, which were then touchup repaired.
In the spring of the next year, additional coating defects were discovered and field touchup was again performed. However, the same problems reportedly continued to appear. As a result of the continuing problems, an independent investigation of the coating problem was undertaken.
|Fig. 2: Close-up of typical concentrations of small, fine blisters in the polyureathane finish coat
The tests and inspections performed during the field investigation were those typically associated with failure investigations, and included the following.
- A visual assessment was performed to determine the degree and distribution of coating defects (in this instance bubbles, pinholes, blisters, and peeling).
- Total coating thickness was measured using a Type 2 electronic film thickness gage operated according to ASTM D7091, Nondestructive Measurement of Thickness of Nonmagnetic Coatings on a Ferrous Base.
- The number of coatings present and the thickness of each were determined using a destructive coating thickness gage as described in ASTM D4138, Standard Test Methods for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive Means. An integral portable microscope (50X) was used to observe a cross-section of the applied coating. The number of coating layers and thickness of each were measured. Further, evidence of intercoat contamination, voids, underlying rust, mill scale, and pinholes was recorded.
- Adhesion testing was conducted using Method A (X-Cut) of ASTM D3359, Measuring Adhesion by Tape Test. Method A involves cutting an “X” through the coating to the substrate using a razor knife. Pressure sensitive tape is placed over the X-cut, then rapidly removed. The amount of coating detached by the tape is rated in accordance with the ASTM rating scale. Ratings of 4A and 5A are considered to represent good adhesion, 2A to 3A represent fair adhesion, while 0A and 1A represent poor adhesion.
- The coating system was removed in small areas, and the substrate was examined for under-film corrosion or mill scale. Active under-film corrosion may be associated with the coating failure and may also contribute to a shortened life of the system.
Coating samples at both failing and non-failing areas were removed for laboratory analysis, and digital images of the typical field coating conditions were obtained.
The structural steel consisted primarily of vertical and horizontal I-beam members. Both intact and fractured (peeling) blisters were observed. Blisters were observed on virtually all members inspected. Some of the blisters appeared to be fractured as a result of someone physically scraping the areas, while others appeared to have cracked and fractured on their own. Blistering ranged in size from concentrations of very fine blisters (approximately 1/64 to 1/128 of an inch in diameter) up to single blisters with diameters of approximately 2 to 3 inches. Both irregularly shaped and circular blisters were observed. The fine concentrations of blisters were located primarily on beam flanges and in the corner areas where the webs and flanges meet. Larger shallow blisters were generally located on the webs of the I-beams. The fine blisters and larger shallow blisters on the webs were more difficult to see, oftentimes becoming visible only when viewed at the proper angle with sunlight hitting the surface after the film of surface dirt and grime was removed.
|Fig. 3: Blisters formed in the polyurethane finish coat on a flange
Upon scoring around the perimeter of the larger blisters or areas of concentrated fine blisters with a razor knife, the full blister area could be removed. Upon removal, a portion of the zinc-rich primer remained on the steel surface, and a portion remained attached to the backside of the removed blister (cohesive break within the zinc primer). Both faces of the split primer films contained a visible white powder-like residue.
Areas that had been repaired by field touch-up were visible across the structure. Blisters were still visible in some touch-up areas. It was not apparent if the blisters had reoccurred in the touch-up area or if some of the blisters were not completely removed and touch-up material was applied over them.
The results of the total system thickness measurements from various locations on the structural steel are summarized below.
- Minimum DFT (mils): 6.3
- Maximum DFT (mils): 15.7
- DFT Average (mils): 13.2
Destructive film thickness measurements most often identified two distinct layers of paint on the steel. In some instances where touch-up repairs had been made, additional coats were apparent, and three to five individual layers were evident. When two coats were present, the first coat was a metallic gray/green and ranged from 4 to 10 mils; the second coat was dark green and ranged from 3 to 7 mils.
|Fig. 4: Blisters in the polyurethane finish coat on a lateral brace web
Adhesion of the coating system in and immediately around blistered areas was rated poor (0A to 1A rating); however, adhesion of the coating system in blister-free areas was rated fair to good (3A to 4A rating). The adhesion test process consistently forced a break within or at the surface of the zinc-rich primer layer.
The substrate was examined at destructive film thickness measurement areas and sample acquisition areas. Because a thin layer of zinc-rich primer remained adherent to the steel surface, a thorough inspection of the underlying substrate was not possible. However, under 50X power illuminated magnification of the destructive coating thickness gage, a roughened bright metal substrate was sometimes visible. This evidence suggests that the steel surface had been abrasive blast cleaned.
The laboratory investigation consisted of visual and microscopic examination, infrared spectroscopy and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS). The test methods and results are described below.
|Fig. 5: Formation of whitish-colored zinc salts on the surface of the zinc-rich primer, beneath areas where the blistered finish coat was removed
Microscopic examination of the samples was conducted using a digital microscope with magnification to 200X. The samples had between two and five coating layers. Coating layer thickness measurements, obtained by laboratory microscopic methods, are in Table 1.
Coating Layer Thickness Measurements
|Coating Layers and Thickness (mils)
(Non-Failing Repair Area)
(Non-Failing Repair Area)
Infrared spectroscopic analysis revealed the following.
- The spectrum obtained of the green top-coat was consistent with a urethane. Water (moisture) and crystalline silica were also indicated.
- The spectrum obtained of the gray primer was most consistent with a zinc urethane. No distinct characteristic bands are associated with zinc coatings although the baseline noise appearance was consistent with a zinc coating (confirmed by elemental analysis).
SEM-EDS analysis revealed that the white powdery substance on the gray surface of the primer was primarily zinc. Other elements detected included magnesium, aluminum, and silicon.
Conclusions/ Coatings Condition Assessment
The field investigation and laboratory analysis identified multiple variables that contributed to the blistering coating problems on the structural steel.
|Fig. 6: Close-up of zinc salt formation on the zinc-rich primer surface, beneath the removed blistered finish coat
The zinc-rich primer used on the project was a MCU material. MCUs react with moisture (atmospheric humidity or other moisture source) to cure. During the curing reaction with moisture, carbon dioxide gas (CO2) is liberated as a reaction product. The CO2 gas escapes from the coating film in a process commonly referred to as “out-gassing.” When a lot of moisture is available, MCUs cure at an accelerated rate, and CO2 formation and out-gassing increase. When an additional paint layer is applied while the MCU is still out-gassing, the release of CO2 from the MCU can be inhibited. The gas must now pass out of the MCU and through the newly applied layer. Depending on the state of drying and curing of the newly applied layer, some CO2 gas may escape, and some may become trapped in the new film. The CO2 that escapes produces pinholes or craters when the topcoat has begun to gel, while CO2 that is trapped creates sufficient pressure to form bubbles through the cross-section and at the surface of the new film.
Laboratory microscopic examination of the paint samples consistently revealed that pinholes and bubbles were present in the green topcoat layer applied over the MCU primer. This evidence indicates that the MCU zinc-rich primer was top-coated with the polyurethane before the primer had sufficiently cured.
Infrared spectroscopic analysis of the green polyurethane finish coat identified bound moisture within the film. In order for moisture to become bound within this layer, the moisture would have had to have been present on the MCU zinc-rich primer layer over which the polyurethane finish was applied. This evidence indicates that the surface of the MCU zinc-rich primer where defects occurred (i.e., bubbling, pinholes) was damp when the polyurethane was applied.
Field thickness measurements and laboratory microscopic measurements revealed that the MCU zinc-rich primer was often applied above the specified DFT range of 2.5 to 3.5 mils. Destructive thickness measurements and laboratory microscopic measurements indicated DFTs of up to 7 mils and 9.9 mils respectively. Excessive primer thickness prolongs the dry and cure time of the primer; as a result, the CO2 out-gassing is also prolonged, serving to increase the likelihood of pinholes and bubbling.
The polyurethane finish coat was also applied above the specified DFT range of 4.0 to 5.0 mils, with measurements up to 8.7 mils in some instances. These thicker films could slow the escape of the CO2 or trap it, possibly contributing to increased bubble and pinhole formation.
The white powdery residue on the backside of the detached blister area and on the substrate was identified as zinc oxide in the laboratory. Zinc oxide (“white rust”) is produced as the zinc dust in the primer oxidizes. This finding indicates that the MCU zinc-rich primer layer was performing as designed: providing galvanic/sacrificial corrosion protection to the carbon steel substrate. Moisture (rain, condensing moisture) was gaining access to the MCU zinc-rich primer through the voids (i.e., pinholes, fractured bubbles) in the polyurethane finish coat. The moisture served as the electrolyte, allowing the MCU zinc-rich primer to oxidize. Moisture condensing on the steel was likely contaminated with sulfides from the coal-fired power generating station. Water-soluble salts such as sulfides, in combination with moisture, increased the corrosivity of the exposure environment.
The defective areas (i.e., bubbles, pinholes) were identified and removed by high-pressure water cleaning. Industry experience has shown that water pressures in excess of 4,000 psi are usually effective for revealing and removing defective coatings. However, because each individual project is unique, some experimentation is needed to arrive at the optimal cleaning pressure. It was ultimately determined that the best removal method involved the use of a zero-degree, rotating tip on the pressure washer gun, with careful observation to maintain the equipment manufacturer’s gun-to-work-piece distance and dwell times. In areas where pressure washing was not entirely effective, supplemental mechanical cleaning with power tools (i.e., power sanding) was used. Once the defective coating was completely removed, any coating that remained was probed with a dull putty knife as described in SSPC-SP 2 and SSPC-SP 3, Hand Tool and Power Tool Cleaning, respectively. Remaining coating that passed the dull putty knife test criteria was considered “tightly adherent” for touchup repairs. The periphery of touchup areas was feather-edged to provide a smooth transition from the repair area to surrounding intact coatings.
Once surface preparation was accomplished, touchup proceeded using the field touchup system, consisting of a coat of epoxy mastic followed by a matching green polyurethane finish coat.
The Journal Of Protective Coatings & Linings ©2013 Technology Publishing Company
Jim Machen is a Senior Coatings Consultant with KTA where he has been employed for over 20 years. He is a NACE Certified Coatings Inspector Level 3 (Peer Review), an SSPC Certified Protective Coatings Specialist, and is a Level II Inspector in accordance with in accordance with ASTM D4537. In his current position, Jim performs consulting activities (i.e., coating failure analysis, coating system recommendations, specification preparation, and major project management) for a variety of clients in the transportation, water and waste, power generation, chemical processing, and marine industries.
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