Federal Highway Administration 100-Year Coating Study

CHAPTER 2. EXPERIMENTAL PROCEDURES

2.1 SELECTION OF COATING SYSTEMS

Table 1 summarizes the eight coating systems employed in this study. Two three-coat systems were used as controls, and the remaining coating systems were test systems composed of a three-coat system, four two-coat systems, and a one-coat system. The acronyms used for all the coating systems are listed in the table. The coating systems will be identified using these acronyms throughout the report.

Table 1. Summary of coating systems.

2.2 PREPARATION OF TEST PANELS

All of the coating systems were applied onto steel substrates prepared according to the Society for Protective Coatings (SSPC) surface preparation standard number 5 (white metal blast) condition.⁽¹⁰⁾ Subsequently, individual coating systems were applied on the cleaned test panels using an airless spray method by a professional coating laboratory.

Conventional test panels (type I) used for previous FHWA in-house coating studies were 4 by 6 inches. For this study, a new type of test panel (type II) was designed in addition to the conventional test panels to closely simulate detailing of steel bridge members. Each of the type II panels was 18 by 18 inches and contained "V"-shaped and inverted "T"-shaped welding joints, an overlap joint, an angle attachment, and five bolt-nut assemblies, as shown in figure 1 and figure 2. In this report, all type II base plates and individual components were separately spray coated with the primer before panel assembly. After the primer was allowed to dry for 24 h, the components were assembled and coated with an intermediate coat and/or top coat. Introduction of complex geometry to the panel design was employed to understand how coatings applied over the crevices and interfaces created by the attachments, nuts, and bolts of the type II panel perform compared to those applied on the small type I flat panels.

Figure 1. Illustration. Type II test panel.

Figure 2. Photo. Images of type II test panels.

All type I test panels were coated according to manufacturers’ dry film thickness (DFT) recommendations. Each type II panel consisted of three coated areas with the following varying DFT values:

• Nominal DFT (N): Target DFT recommended by the manufacturers.

• Low DFT (L): DFT 20 percent less than the target DFT.

• High DFT (H): DFT 20 percent higher than the target DFT.

Test results from these three DFT areas revealed how much DFT of a particular coating system influenced coating performance. The physical locations of these test areas on the surface of the type II test panel are shown in figure 1.

A total of 100 type I and 27 type II test panels were prepared for ALT and outdoor exposure testing. Table 2 and table 3 list the exposure condition and number for type I and II panels, respectively. Outdoor exposure tests were performed in the backyard of TFHRC in McLean, VA, and at the Golden Gate Bridge (GGB) in San Francisco, CA. The TFHRC exposure testing consisted of natural weathering (NW) and natural weathering with daily salt spray (NWS).

For type I test panels, each coating system was sprayed on 12 panels, where 5 panels were used for ALT (three scribed and two unscribed), 5 panels were used for NW (one scribed and one unscribed) and NWS (two scribed and one unscribed), and 2 panels were used for physical tests of adhesion strength and Fourier transform infrared spectroscopy (FTIR). In addition, two uncoated type I steel panels were deployed on each of the TFHRC outdoor exposure racks.

For type II panels, three panels per coating system were used, one each for the NW and NWS racks at TFHRC and the third one on the GGB rack. Also, an uncoated type II panel was exposed on each of three exposure racks. The overall test matrix of type II panels is provided in table 3.

Table 2. Number of exposure conditions for type I test panels.

Table 3. Number of exposure conditions for type II test panels.

After the test panels were delivered to TFHRC, their as-received condition was documented, and six type I panels (three for ALT and three for outdoor testing) for each coating system were scribed following the instructions specified in ASTM D1654-08, "Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments."⁽¹¹⁾ A 2-inch-long scribe was made diagonally on each panel using a mechanical scriber shown in figure 3.

Figure 3. Photo. Scribing tool.

All test areas of type II test panels were scribed. A mechanical scribing tool used for type I panels was not suited for the large-size test panels. As a result, the test areas were scribed manually using a high-speed Dremel^® tool with a scribing bit, as shown in figure 4. C-clamps were used to support a metallic guide along which the scribing was done.

Figure 4. Photo. Dremel^® scribing tool.

Initial trial scribes were made on dummy test panels to optimize the speed of rotation of the Dremel^® bit and applied pressure. The pattern (depth and width) of the scribe made on the dummy test panels was examined using a handheld optical microscope. From this trial, operational parameters were finalized to obtain the ASTM D1654-08 specified width and depth of the scribe, and actual scribes were made on all of the type II panels.⁽¹¹⁾

2.3 TEST CONDITIONS

ALT

Table 4 summarizes the test conditions for ALT and the total number of cycles. Each ALT cycle was carried out for 360 h, and a total of 20 cycles were planned. Upon completion of every cycle, the panels were examined for their performance. The test panels were also evaluated at the termination of ALT.

Detailed description of each 360-h test cycle is as follows:

• Freeze: 24 h.

• Temperature: -10 °F.

• UV/condensation: 168 h (7 days).

• Test cycle: 4-h UV/4-h condensation cycle.

• UV lamp: UVA-340 nm (wavelength recommended for UV tests).

• UV temperature: 140 °F.

• Condensation temperature: 104 °F.

• Prohesion (cyclic salt-fog, ASTM G85-11, "Standard Practice for Modified Salt Spray (Fog) Testing"): 168 h (7 days).⁽¹²⁾

• Test cycle: 1 h wet/1 h dry.

• Wet cycle: A Harrison mixture of 0.35 weight (wt) percent ammonium sulfate and 0.5 wt percent sodium chloride was used. Fog was introduced at ambient temperature.

• Dry cycle: Air was preheated to 95 °F and was then purged to the test chamber.

Table 4. ALT of type I panels.

Figure 5 and figure 6 show a salt fog chamber and a weathering tester, respectively. A 16-h salt-fog accumulation test was conducted before each cyclic salt-fog test to check the atomizing and fog quantity as well as the pH of the collected solution. Typical collection volume of the accumulation test was 0.034–0.068 oz/h, with uniform accumulation in all collection vessels placed at the four corners of the salt-fog chamber and at the center a few feet away from the spray nozzle. Pump speed, throw pressure, and flow rate were adjusted to optimize the collection volume.

Figure 5. Photo. Salt-fog chamber.

Figure 6. Photo. UV weathering tester.

Outdoor Exposure Testing at TFHRC

Type I and II test panels were deployed on two wooden racks inclined at 30 degrees facing south in the backyard at TFHRC in McLean, VA, as shown in figure 7 and figure 8.

Figure 7. Photo. NWS rack at TFHRC.

Figure 8. Photo. NW rack at TFHRC.

In the beginning of the study, NWS test panels received salt spray manually once a day, 5 days per week. Later, an automated daily salt spray system was designed and installed on the NWS rack as shown in figure 9 using the following components:

• 15 wt percent aqueous sodium chloride solution.

• Storage tank for the salt solution (30 gal).

• Electromechanical pump operating in combination with a timer that switches on for a specific time period at a specific time of the day.

• Tubing/piping required for salt solution to flow through to spray onto the deployed panels and a wooden rack.

• Shower heads to spray salt solution onto the deployed panels.

Figure 9. Photo. Automatic salt spray system for NWS at TFHRC.

The automated salt spray system provided benefits such as eliminating the need to manually spraying the panels while uniformly spraying all the panels. As a result, daily salt spray could be carried out on weekends and holidays without human intervention. The timer turned the pump on for 15 s of salt spray at 10 a.m. each day. After a week of salt spray, it was clear that the 15 wt percent solution was too severe. Therefore, the salt solution was changed to the Harrison mixture (0.35 wt percent ammonium sulfate and 0.5 wt percent sodium chloride), which was the same solution employed in ALT. Digital pictures of test panels on the racks were recorded every month, and test panels were evaluated for coating performance after 6 months and again at 10 months at the project termination.

Outdoor Exposure Testing at GGB

The outdoor exposure condition at GGB was considered extremely harsh due to severe fog conditions containing airborne chlorides. Figure 10 shows type II panels deployed on top of the south anchorage house near the south abutment. Test panels were scheduled to be evaluated every 6 months for coating performance.

Figure 10. Photo. Type II panels deployed at GGB.

2.4 COATING CHARACTERIZATION TESTS AND PERFORMANCE EVALUATION TECHNIQUES

A series of characterization tests were conducted on the test panels before, during, and after ALT and outdoor exposure tests. Baseline data of DFT, color, gloss, adhesion, and coating defects were collected for each coating system prior to the scheduled tests. Performance of these coating systems was evaluated in terms of development of surface defects and rust creepage during the tests and soon after the completion of the tests. Changes in color, gloss, and adhesion strength were evaluated at the termination of testing. A digital microscope and a digital camera were also employed to document appearance changes of the test panels throughout the study. In addition, electrochemical impedance spectroscopy (EIS) was used to quantify change of coating barrier properties in terms of coating impedance.

DFT

The mean DFT of a coating system was measured in three spots for each type I panel before ALT and outdoor exposure testing using an electronic thickness gauge according to SSPC paint application specification 2, Measurement of Dry Coating Thickness with Magnetic Gages.⁽¹³⁾ In addition, three spots per each DFT area on the type II panels were obtained to record actual DFTs in each panel.

Gloss

Gloss is defined as the perception of a shiny surface by human eyes. Specular gloss compares the luminous reflectance of a test specimen to that of a standard specimen under the same geometric conditions.⁽¹⁴⁾ Measurements by this test method correlate with visual observations of surface shininess made at roughly the corresponding angles. Measured gloss ratings are obtained by comparing the specular reflectance from the specimen to that from a black glass standard. The measured gloss ratings change as the surface refractive index changes since specular reflectance depends on the surface refractive index of the specimen.

Gloss of all of the coating systems was measured following ASTM D523-08, "Standard Test Method for Specular Gloss."⁽¹⁵⁾ The 60-degree geometry measurements were conducted on the selected unscribed test panels before and after ALT and outdoor exposure tests. Three gloss readings for each type I panel and six readings for each type II panel were recorded. The reported gloss of each coating system per test condition was the mean of the readings obtained from all unscribed test panels.

Color

The color of the coatings was measured using a colorimeter following ASTM D2244-05, "Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates."⁽¹⁶⁾ This technique is based on the calculation from instrumentally measured color coordinates based on daylight illumination, color tolerances, and small color differences between opaque coated panels. The Commission Internationale d'Eclairage (International Commission on Illumination or CIE) lab color system (CIE L*, a*, b*) was used for color measurement. L*, a*, and b* represent three coordinates of the three-dimensional lab color space. These parameters are defined based on the high and low values they represent to identify colors as described as follows:

• L* = 0 represents black, while L* = 100 represents diffuse white.

• Positive values of a* indicate green, while negative values indicate magenta.

• Positive values of b* indicate blue, while negative values indicate yellow.

• The asterisk is used to differentiate the CIE L*, a*, b* system from (L, a, b) parameters of the original Hunter 1948 color space.

Colors were measured for unscribed test panels before and after ALT and outdoor exposure tests. Three color readings were obtained for each type I panel, and six color readings were obtained for each type II panel. Color difference (Δ E) of the test panels before and after the test was calculated using the equation in figure 11.

Figure 11. Equation. Measurement of color.

Where:

The data used in the above equation were the mean of the data obtained from all the unscribed test panels of each coating system.

Adhesion Strength

The adhesion strength of the coating systems was determined using two commercially available pull-off adhesion testers following ASTM D4541-09e1, "Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers."⁽¹⁷⁾ A loading fixture, commonly known as a dolly or stub, is affixed to the panel surface by an adhesive. A load provided by the adhesion tester is increasingly applied to the dolly until it is pulled off. The force required to pull the dolly off yields the tensile strength in pounds per square inch (psi). Failure will occur along the weakest plane(s) within the testing system comprised of the dolly, adhesive, individual layers of the coating system, and substrate.

The surface of coated test panels and the base of the dollies were cleaned with detergent water and were lightly roughened with an abrasive pad. The dollies were glued on the test panel surface using a high-strength epoxy adhesive. The cut through the coating around the edge of the dolly was made using a drill press after the complete curing of the adhesive, as seen in figure 12. The initial and final adhesion strengths of coating systems were measured by hydraulic method. Figure 13 shows the hydraulic adhesion tester used in this study. For each coating system, three pull-off adhesion tests were performed on two unscribed virgin type I panels and on each of the tested type I panels. No adhesion strength tests were performed on type II panels.

Figure 12. Photo. Drill press to score a test area around a dolly.

Figure 13. Photo. Hydraulic adhesion tester.

For every panel, the average adhesion strength of three locations was calculated. If the coefficient of variance (CV) of each test panel was more than 20 percent, the test panel and adhesion failure mode were carefully examined to see if the variation was caused by test operation. Repeated tests were performed for quality assurance. If more than 50 percent of a glue failure occurred, the test was also repeated. The reported adhesion strength for each coating system was the mean of the data obtained from tests conducted on all test panels of the coating system. The remaining DFT at the pull-off spots was measured and recorded. The adhesion failure mode of every spot was also documented using digital photographs.

Detection of Coating Defects

The coating defects were identified according to ASTM G62-07, "Standard Test Methods for Holiday Detection in Pipeline Coatings (Method A)."⁽¹⁸⁾ This technique utilizes a low voltage holiday detector to determine the presence of electrically conductive coating defects including holidays (invisible defects with naked eyes) and pinholes, voids, mechanical coating damage, or metal particles protruding through the coating. The reported number of defects after each test cycle was the cumulative number of defects. In addition to using the holiday detector, test panels were visually examined for blisters and rust spots per ASTM D714-02, "Standard Test Method for Evaluating Degree of Blistering of Paints" and ASTM D610-01, "Standard Test Method for Evaluating Degree of Rusting on Painted Steel Surfaces."^(19,20) The reference standards were employed to grade the rust pits and surface blisters on the panels.

Rust Creepage Measurement

The rust creepage at the scribe was measured following ASTM D7087-05a, "Standard Test Method for An Imaging Technique to Measure Rust Creepage at Scribe on Coated Test Panels Subjected to Corrosive Environments."⁽²¹⁾ The rust creepage area from the scribe line on the coating panel was traced using a thin marker and a transparent plastic sheet. The tracing image was scanned and analyzed using imaging software to obtain the creepage areas and the creepage distances. Two traces for each test panel were obtained, and the mean creepage distance was reported as the nominal creepage at the time of measurement for the coating system.

Digital Microscopic Examination

When unusual surface failures were detected by the holiday detector, such panels were examined using a stereomicroscope or a high-power digital microscope. The surface conditions were documented via microphotographs.

Digital Photography

Every test panel was photographed to document surface conditions before initiating the tests as well as after each test cycle for both ALT and outdoor exposure tests.

EIS

The impedance of the coating systems was measured by EIS using an electrochemical instrument equipped with a potentiostat. This technique involves applying a small amplitude alternating current signal into a body of material over a wide range of frequencies and measuring the responding current and its phase angle shift. The output from the EIS instrument is an impedance spectrum of the material, typically ranging from 100 kHz to 0.001 Hz. EIS data are analyzed by the equivalent circuit modeling technique, which can produce appropriate models to evaluate the coating deterioration process and the mechanism of corrosion occurring at the interface between the substrate and the coating. Analysis of the EIS data is not included in this report, and the analysis results will be presented in a separate report in the future.