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The Case of the Epoxy That Will Not Stick to Galvanized Steel
By David S. Leyland, KTA-Tator, Inc.
A galvanized pipe rack assembly was erected in a chemical plant located on the Gulf Coast. Approximately one month after the pipe rack assembly was erected, it was painted by a local contractor. The project specifications required the contractor to power wash the galvanized surface using clean potable water and apply a two-coat polyamide epoxy coating system. Within a year after the coating work was completed, the epoxy coating was peeling cleanly from galvanizing. The peeled areas were quickly repainted. Surface contamination was suspected as the cause of the peeling, so when the peeled areas were repaired, the contractor wiped the areas first with mineral solvents and then with clean water. These repairs also began to peel. Shortly thereafter, a coatings consultant was contracted by the facility owner to conduct an investigation that would determine the cause of peeling.
Field Investigation
The field investigation involved an interview with the project foreman from the contracting company, visual observations of the amount and extent of peeling, 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. Also performed at the project site, was a spot test to determine if a chromate conversion coating was present (ASTM B 201, "Testing for Chromate Coatings on Zinc and Cadmium Surfaces"). Representative photographs of intact and failing areas were also taken.
The contractor's project foreman indicated that the galvanized steel was cleaned well by pressure washing prior to painting. There were no visible contaminates or white zinc salts on the surface either before or after pressure washing.
Visual observations made during the field investigation revealed the epoxy paint was peeling cleanly from the galvanized substrate. There were no noticeable zinc salts or other surface contaminates on the back of the peeled paint chips or on the exposed galvanized steel. Areas of peeling paint, which encompassed approximately 2% of the surface area, were randomly located throughout the pipe rack assembly,. Although the paint was peeling in various areas, no blisters were observed. Adhesion testing revealed poor coating adhesion throughout the structure, though adhesion was slightly better on unsheltered upward facing horizontal surfaces. The coating application appeared to be uniform and without significant defects. There was no correlation between the thickness of the epoxy coating and areas of peeling. The thickness of the epoxy coating ranged from 3 to 5 mils.
The spot test which was performed in accordance with ASTM B 201, consisted of applying a drop of lead acetate solution to the surface of the exposed galvanizing and waiting 5 seconds for a black stain to appear. The dark stain will not appear if the surface has been chromate treated. The spot test revealed that the galvanized steel had been post treated, probably with chromium.
Laboratory Investigation
Poorly adhering pieces of epoxy paint were examined and analyzed by the laboratory. The analysis involved washing the back side of some paint chip samples with deionized water and other samples with tetrahydrofuran (THF). The water rinsings were analyzed using ion chromatography to determine if salts were present beneath the coating. Only trace amounts of chlorides and sulfates were found, less than 10 ppm. The THF rinsings were analyzed using infrared spectroscopy to determine if oil or grease residue was present. No oil or grease was detected. The presence of chromium was detected on sample scrapings of galvanizing using scanning electron microscopy equipped with electron dispersive X-ray spectrometer (EDS).
Discussion
The epoxy coating has poor adhesion and is peeling because the galvanized steel received a chromate quenching post treatment. The treatment creates a conversion coating on the surface of the galvanized steel that prevents the formation of zinc oxides and hydroxides during damp storage conditions. Paint adhesion to chromate conversion coatings is poor, because the surface generated has no profile to aid the mechanical bond and has very few active sites for molecular bonding.
A post treatment conversion coating was detected by the spot test method performed during the job site visit. Furthermore, SEM/EDS analysis of samples did detect the presence of chromium on the surface of the galvanized steel. Zinc salts do not form on galvanized steel that has been chromate treated. These facts are consistent with the observations made by the foreman at the time of painting, that there were no white zinc salts on galvanizing. Furthermore, no white zinc corrosion products were observed beneath freshly peeled and poorly adhering epoxy coating.
Poor adhesion and peeling can be caused by other surface contaminates, such as salts (chlorides and sulfates) or by oil and grease. Analytical results indicated neither salts nor oil or grease were present on the back of peeled and poorly adhering paint chips. The contractor claimed that the surfaces were power washed prior to the original epoxy application and wiped with both mineral spirits and with water before the touchup. However, the chromate conversion coating can not be removed by power washing or by wiping with mineral spirits.
Galvanized steel that is scheduled for painting should never be post treated by chromate quenching, because paint has poor adhesion to chromate conversion coatings. The chromate conversion coating can only be removed by abrading or brush-off blast cleaning. A fine abrasive should be used (200 to 500 microns). Alternately, most of the chromate conversion coating could be removed by weathering for at least 6 months.
Every coating contractor should remember one fundamental fact; paint that is applied to galvanized steel, is often not applied directly to zinc, but rather to a layer of various zinc salts. Therefore, the challenge is in getting the paint to adhere well to these salts. It may be possible to enhance paint adhesion by first creating desirable salts by treating the galvanized steel. The salts may be in the form of a post treatment conversion coating or naturally occurring oxides from atmospheric exposure or less stable salts from exposure to environments laden with chlorides or sulfates. The surface of the galvanized steel must be evaluated in order to determine the most appropriate method of surface preparation in order to achieve the best coating adhesion. The American Galvanizers Association in Englewood Colorado offers excellent publications on painting hot dipped galvanized steel.
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