Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home
TECHBRIEF
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-12-045
Date: November 2012

 

Federal Highway Administration 100-Year Coating Study

PDF Version (112 KB)

PDF files can be viewed with the Acrobat® Reader®

 

FHWA Publication No.: FHWA-HRT-12-045

FHWA Contact: Paul Virmani, HRDI-60, (202) 493-3052, paul.virmani@dot.gov

This document is a technical summary of the Federal Highway Administration report, Federal Highway Administration 100-Year Coating Study (FHWA-HRT-12-044).

INTRODUCTION

The coatings industry switched from lead-based to zinc-based three-coat systems in the 1970s to protect steel bridges from corrosion after identification of health hazards associated with lead coatings.(1) Studies have shown that these three-coat systems with zinc-rich primer can have a service life up to 30 years, protecting steel from corrosion before a major touch-up is required.(2) Typical cost concerns with zinc-rich systems include the cost of removing mill scale before application of the coating system, the time and space required for shop application, and the logistics of moving heavy steel members to the field after shop application. A good alternative to addressing these cost issues is to extend the service life of the existing coating system on steel before any maintenance is required and/or replace the existing coating system.

The Federal Highway Administration (FHWA) 100-Year Coating Study is an in-house study initiated in August 2009 under the Congressionally mandated high-performance steel program. The objective of this study was to identify and evaluate coating materials that can provide 100 years of virtually maintenance-free service life for steel bridge structures at comparable costs to existing coatings. This TechBrief presents performance evaluation results and major findings for the eight selected coating systems based on experimental data from accelerated laboratory testing (ALT) and outdoor exposure testing.

APPROACH

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 comprised a three-coat system, four two-coat systems, and a one-coat system.

Table 1. Summary of coating systems.

System Number

System ID

Coating Type

Primer

Intermediate

Top

1

Three-coat (control)

Inorganic zinc-rich epoxy (IOZ)

Epoxy (E)

Aliphatic polyurethane (PU)

2

Zinc-rich epoxy primer (ZE)

E

PU

3

Three-coat

Moisture-cured urethane zinc primer (MCU)

E

Fluorourethane (F)

4

Two-coat

ZE

 

PU

5

Inorganic zinc primer (Zn)

 

Polysiloxane (PS)

6

Thermally sprayed zinc primer (TSZ)

 

linear epoxy (LE)

7

Experimental zinc primer (ZnE)

 

LE

8

One-coat

High-ratio one-coat calcium sulfonate alkyd (HRCSA)

Note: One-coat systems contain only one coat of paint that acts as the primer/top coat and do not contain an intermediate coat. Blank cells indicate that the two-coat systems do not contain an intermediate layer.

Test Panels

Two sizes of steel test panels were employed in this study. The small panels (type I) were 4 by 6 by 0.2 inches, and the large panels (type II) were 18 by 18 by 0.2 inches. All test panels were blast cleaned to a Society for Protective Coatings surface preparation standard number 5 white metal blast cleaning condition, and coatings were applied on the cleaned test panels by a professional coating laboratory using airless spray.(3)

Type I Panels

All type I test panels were coated according to manufacturers' dry film thickness (DFT) recommendations. Half of the type I panels (48 out of 96) were scribed diagonally following the instructions specified in ASTM D1654-08.(4) The panels were scribed to study the potential performance of the coating systems with local film damage. The other half of the panels were used to measure physical properties such as gloss, color, pencil scratch hardness, etc. Two additional panels of each coating system were prepared exclusively for two destructive tests only: initial adhesion strength and Fourier transform infrared spectroscopy analysis.

Type II Panels

A new type of test panel design was adopted for this study to closely resemble steel bridge structure elements having bolt/nut assemblies, overlapped joints, angles attachments, and welding joints. Figure 1 and figure 2 show a type II panel. A wide-angle attachment and a fillet welded T-attachment were secured using bolts and nuts, while the V-notch was directly welded onto the surface of the panel. Type II panels were employed in the outdoor exposure testing only. Three type II panels were coated with each of the eight coating systems, and three uncoated test panels were employed as controls.

The test surface of each type II panel was divided into the following three areas of varying DFT values as shown in figure 1:

All DFT areas were scribed using a high-speed Dremel® tool.

The figure shows a typical type II test panel that is 18 by 18 by 0.2 inches. It has a V-notch (welding joint), a fillet welded T-shaped attachment, and a wide-angle attachment. The illustration shows three areas with varying dry film thicknesses (DFTs). Area 1 on the top left shows low DFT, which is 20 percent less than the manufacturer recommended DFT. Area 2 on the top right shows high DFT, which is 20 percent more than the manufacturer recommended DFT. The third area on the bottom left has a nominal DFT value as recommended by the manufacturer. The corresponding dimensions of each component are as follows: the plate dimensions are 18 by 18 inches; the DFT areas are 7 by 8 inches with a 2-inch scribe line in the center; the V-notch is 2 inches tall and has a 3- by 3- by 3/16-inch V-angle; the T-attachment is 2 inches tall, 12 inches long, and 2 inches wide; and the wide-angle attachment is 16 by 3 inches with an angle of 3 by 3 by 3/16 inches.

Figure 1. Illustration. Type II test panel.

 

This figure shows two views of a type II test panel, which is rectangular. The wide-angle attachment and the T-attachment were secured using nuts and bolts, while the V-notch was welded onto the surface of the panel. All individual components, the attachments, and nuts were spray coated before assembly.

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

Test Conditions

Both ALT and outdoor exposure testing were performed to evaluate performance of the coating systems. For ALT, 10 accelerated test cycles (each test cycle was 360 h) using 40 type I panels were conducted for 3,600 h. This method is similar to ASTM D5894-10, with the addition of a freeze cycle for 24 h.(5,6) Outdoor exposure testing was carried out with eight coated and one uncoated type II panels for 6 months at the Golden Gate Bridge in San Francisco, CA. Another outdoor exposure testing was conducted for 10 months using 44 type I and 18 type II panels in the backyard of FHWA’s Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA. Among them, four type I and two type II panels were bare steel without coating, and half of the TFHRC exposure panels were salt sprayed once a day.

Performance Evaluation

The following parameters were used to evaluate coating performance:

All coating systems were evaluated for color, gloss, rust creepage, and holidays every 360 h in ALT and every 6 months in outdoor exposure conditions. At the termination of the study, all of the above as well as reduction of adhesion strength were evaluated.

CONCLUSIONS

Based on the study, the following conclusions were made:

This figure shows a test panel coated with IOZ/E/PU showing no deterioration after 3,600 h of accelerated laboratory testing (ALT). The coating system performed well in every category.

Figure 3. Photo. IOZ/E/PU coating system after 3,600 h in ALT.

 

This figure shows a test panel coated with a poorly performed ZnE/LE coating system after 3,600 h of accelerated laboratory testing (ALT). The top coat exhibited extensive peeling.

Figure 4. Photo. ZnE/LE coating system after 3,600 h in ALT.

REFERENCES

  1. Kogler, B. (2008). "Managing the Infrastructure: The Role of Cost Knowledge," Journal of Protective Coatings and Linings, 25, 22–31.

  2. Kline, E.S. (2008). "Steel Bridges: Corrosion Protection for 100 Years," Journal of Protective Coatings and Linings, 25, 20–31.

  3. Society for Protective Coatings. (2007). Joint Surface Preparation Standard SSPC-SP5/NACE No.1: White Metal Blast Cleaning, Pittsburgh, PA.

  4. ASTM D1654-08. (2008). "Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments," ASTM Book of Standards Volume 06.01, ASTM International, West Conshohocken, PA.

  5. ASTM D5894-10. (2010). "Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal, (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet)," ASTM Book of Standards Volume 06.01, ASTM International, West Conshohocken, PA.

  6. Chong, S.L., Jacoby, M., Boone, J., and Lum, H. (1995). Comparison of Laboratory Testing Methods for Bridge Coatings, Report No. FHWA-RD-94-112, Federal Highway Administration, Washington, DC.

  7. ASTM D523-08. (2008). "Standard Test Method for Specular Gloss," ASTM Book of Standards Volume 06.01, ASTM International, West Conshohocken, PA.

  8. ASTM D2244-05. (2005). "Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates," ASTM Book of Standards Volume 06.01, ASTM International, West Conshohocken, PA.

  9. ASTM D4541-09e1. (2009). "Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers," ASTM Book of Standards Volume 06.02, ASTM International, West Conshohocken, PA.

  10. ASTM D5162-08. (2008). "Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coating on Metallic Substrates," ASTM Book of Standards Volume 06.02, ASTM International, West Conshohocken, PA.

  11. ASTM D7087-05a. (2005). "Standard Test Method for An Imaging Technique to Measure Rust Creepage at Scribe on Coated Test Panels Subjected to Corrosive Environments," ASTM Book of Standards Volume 06.01, ASTM International, West Conshohocken, PA.

Researchers—This study was performed by the FHWA Coatings and Corrosion Laboratory with support of an onsite contractor, SES Group and Associates, Chesapeake City, MD, 21915.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from FHWA Product Distribution Center by e-mail to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.gov/research/.

Key Words—One-coat, Two-coat, Three-coat, Steel bridge coatings, Corrosion protection, Accelerated laboratory testing, Outdoor exposure, Coating performance evaluation.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objectives of the document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides high-quality information to serve the Government, industry, and public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

 

ResearchFHWA
FHWA
United States Department of Transportation - Federal Highway Administration