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The Case of the Paint That Did Not Adhere to a Flake Filled Aluminum Alkyd
By Kenneth A. Trimber, KTA-Tator, Inc.
During the Fall, a contract was issued for the repainting of an elevated water storage tank located in a manufacturing facility in a northern climate. The tank had been previously coated with a red lead primer and flake filled aluminum alkyd finish coats. The facility owner could not tolerate dust emissions from the project, and did not want to remove any of the existing paint due to the presence of the lead. For these reasons, abrasive blast cleaning was not allowed in the facility. Instead, the specification required pressure washing the tank at a minimum of 3,000 psi to remove dirt, chalk, and foreign material. Loose rust, loose paint, and loose mill scale were to be removed by hand tools in accordance with SSPC-SP2 "Hand Tool Cleaning." A full coat of aluminum urethane primer and a full coat of urethane finish were to be applied.
The tank was prepared and primed over a period of approximately 1 week. The finish coat was then applied to the entire tank at one time. Application was accomplished by brush and roller to avoid problems with overspray.
Localized disbonding of the coating in areas of heavy coating thickness occurred soon after project completion and repairs were made. A little over one-year later, patches of peeling paint again became visible. Shortly thereafter, KTA was contracted by the facility owner to conduct an investigation into the cause of the disbonding.
The analysis involved visual observations of the type and amount of failure, adhesion tests in accordance with ASTM D 3359 "Measuring Adhesion by Tape Test, " dry film thickness measurements and the removal of samples for laboratory analysis. Representative photographs of intact and failing areas were also taken.
Field Investigation
A visual examination of the exterior of the tank showed cracked and disbonded coating covering approximately 2 percent of the surface. The detachment was scattered across the legs, riser, and bowl with no apparent pattern. A close examination revealed that the peeling was occurring predominately between the newly applied primer and the original aluminum finish.
The adhesion of the system ranged from sound and intact to very poor. The total thickness of the new system typically ranged from 6.0-8.0 mils with no trends observed between coating thickness and adhesion.
The investigation then concentrated on the surface of the original aluminum alkyd finish. A different tank coated with aluminum alkyd and with a similar painting history was located next the tank in question. The adjacent tank had not been repainted and both the facility owner and contractor indicated that the aluminum alkyd was in a condition similar to the suspect tank at the time of painting. Pressure sensitive tape was applied to the surface of this tank and removed. In each test location, after the tape was removed the surface of the coating appeared clean, while the backside of the tape was covered with aluminum pigmentation. A visual examination for aluminum pigmentation on the backside of disbonded chips from the failing tank was unsuccessful in the field due to the presence of aluminum in the newly applied primer.
Laboratory Testing
Samples of failing and non-failing coating were then removed from the tank for laboratory analysis. The samples included:
Chips of spontaneously disbonded coating. Note that in some cases such chips are not of significant value because 1) the cause of the problem can remain on the paint still attached to the substrate or 2) rain water can wash away the evidence, or 3) rain water, dust, etc. can contaminate the disbonded interface.
Forcibly removed chips in poorly adherent areas. Because of the concerns addressed above, many samples were removed by cutting and peeling the material from the substrate. Such poorly adherent, yet "intact" areas often demonstrate the failure mechanism, even though they have not yet lifted from the surface. As a result, they are typically free of the influence of contamination from external sources. The underlying aluminum finish coat was also carefully swabbed with cotton, and in some cases, scrapings of the aluminum coat were removed.
Scrapings of the newly applied coats and the original system in intact areas. These samples were removed for comparison with failing samples.
The tape from the adjacent tank and samples of its finish coat.
Note that in coating failure analysis work, the preferred samples involve pieces of the substrate itself with the coatings still attached. The advantage is that the paint layers can be removed in the laboratory under controlled conditions. In addition, as indicated earlier, often times the evidence responsible for the problem can remain on the surface of the coating which is attached to the substrate. The sampling of these underlying coat(s) in the field by cutting and scraping can often destroy the evidence, whereas in the laboratory, the analysis can be accomplished on the undisturbed surface. In this case, however, a piece of the painted steel could not be removed.
In the laboratory, the samples were examined microscopically using stereo microscopes with magnification up to 80X. The microscopic work did not contribute any additional information beyond that collected in the field. Samples were then analyzed by infrared spectroscopy (IR). The IR analysis was instrumental in determining the causes. The analysis of the backside of the disbonded urethane chips indicated that a small amount of the aluminum alkyd finish was present. This analysis in combination with the examination of the adjacent tank indicated that a thin oxidized layer of the original alkyd system had not been completely removed from the surface of the tank at the time of painting. The newly applied primer adhered to this weak layer which after months of weathering and freeze/thaw cycling ultimately resulted in disbonding.
Conclusions
It was concluded that although high pressure water may be capable of cleaning the surface, extreme diligence would be required to assure that the "invisible" oxidized layer is completely removed. Water flushing assisted by mechanical scrubbing will more effectively clean the surface. Other possible methods of preparation (aside from abrasive blast cleaning which was not allowed in this case) included wet abrasive blast cleaning using disposable abrasives or sodium bicarbonate, specialized urethane foam abrasive media, or using alkaline cleaners mixed with the water used for high pressure water cleaning. Prior to using any of these methods, however, it was recommended that they be evaluated through the use of test patches in accordance with ASTM D 5064 "Standard Practice for Conducting a Patch Test to Assess Coating Compatibility." In addition to the use of such preparation methods, it was recommended that the future painting of such tanks include a requirement that the cleanliness of the existing aluminum coat be verified prior to coating application. This can be accomplished by rubbing a transparent or semi-transparent pressure-sensitive tape onto the surface, removing it, and inspecting the tape for the presence of aluminum particulate, as well as inspecting the surface for a "cleaner" appearance after removal of the tape. Chalk tests in accordance with ASTM D4214 are also useful in order to verify that chalking has been removed.
While such suggestions may prevent problems with the repainting of other tanks, they do not address the repair of the failing tank. Three repair options were discussed:
Option 1 - Do nothing.
Option 2 - Spot repair only the peeling areas on an annual or semi-annual basis for a few years.
Option 3 - Totally remove and replace the coating.
If nothing was done, it appeared that corrosion of the steel would not occur as the new system applied to the bare steel possessed excellent adhesion. Peeling occurred only when the original system was present, and the original system in those areas continued to provide satisfactory corrosion protection. However, such an option might not be acceptable due to aesthetics. If total removal and replacement was selected, the repair would be complicated substantially by the presence of the lead-based paint. It was determined that it would be difficult, if not impossible, to remove the poorly adherent coating without removing some of the lead-based primer at the same time. Total removal in this case was further complicated by the restrictions on abrasive blast cleaning within the facility.
Because of the above, it was recommended that the owner consider a compromise
approach (Option 2) involving the hand or power tool roughening (sanding)
of disbonded areas only. Repair coatings would be applied only to the
prepared areas without the application of a full finish coat to the entire
tank as it was expected that a full finish coat would only serve to disbond
additional material. If the "patchwork" appearance of this approach
was acceptable aesthetically, it was believed that a crew of painters
could conduct such repairs in less than one week every year or two to
provide a serviceable coating.
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