TECHBRIEF Federal Highway 

Administration 100-Year

Coating Study

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 threecoat 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. 

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 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov/research/
2

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:

•	 Nominal: Target DFT.

•	 Low: DFT is 20 percent less than target DFT.

•	 High: DFT is 20 percent more than target DFT.

All DFT areas were scribed using a high-speed 

Dremel® tool.

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:

•	 Gloss (ASTM D523-08) and color (ASTM 

D2244-05).(7,8)

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.
3

•	 Pull-off adhesion (ASTM D4541-09e1).(9)

•	 Number of coating defects/holidays (ASTM 

D5162-08).(10)

•	 Rust creepage (ASTM D7087-05a).(11)

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:

•	 Test results from this study indicate that none 

of the selected coating systems, including the 

two three-coat control coatings, will provide 

maintenance-free corrosion protection to steel 

bridge structures for 100 years. 

•	 The two control three-coat systems, IOZ/E/PU 

and ZE/E/PU (systems 1 and 2) and the one-coat

system, HRCSA (system 8), were chosen for 

their good performance records in earlier FHWA 

studies. As expected, they performed well, and 

they were better than the other test coating 

systems in every category (see figure 3). The 

remaining five test coating systems, MCU/E/F, 

ZE/PU, Zn/PS, TSZ/LE, and ZnE/LE, were selected 

to possibly provide superior performance to 

commercially available products in the current 

market. However, they did not deliver desirable 

performance exceeding the three best coating 

systems.

•	 Unexpected premature failure of two of the 

two-coat systems, TSZ/LE and ZnE/LE (systems 

6 and 7), was observed during the study (see 

figure 4). Their performance was the worst 

among the eight coating systems and made a 

negative impact on this study, leading to early 

termination of the ambitious research. Three 

test coating systems, MCU/E/F, ZE/PU, and 

Zn/PS (systems 3, 4, and 5), performed satisfactorily in some categories and poorly in 

others compared to the best performers. None 

of them showed consistently good performance.

•	 It was apparent that cutting-edge coating technology cannot yet deliver super durable coating 

systems, regardless of cost, that can last more 

than 100 years without significant maintenance 

interventions. 

•	 Until future research and development efforts 

produce coating systems with extended service 

life, it should be a goal to use the proven legacy 

coating systems correctly by reducing human 

errors and improper applications. At the same 

time, researchers should strive to develop 

surface-tolerant primers against salt residue,

adhered rusts, and mill scale, which is a simple 

yet reliable in situ test method for surface 

chloride concentration, as well as allowable 

chloride contamination(s) on the blasted steel 

surface. These significant advancements could 

help create more durable steel bridge coatings. 

Figure 1. Type II test panel.

Figure 2. Images of type II test panels.

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

18 inches

2.0 inches

18 inches

1 inch 16 inches 1 inch

3 x 3 x 3/16 Angle

2.0-inch scribe

1 inch12 inches

8 inches

3 inches

2 inches

(Dimensions not to scale) 2 inches 2 inches

Target DFT

Nominal (N)

3 x 3 x 3/16 Angle

(2 inches tall)

20–30 percent 

less DFT area 

(7 x 8 inches) 

Low (L)

20–30 percent 

thicker DFT area 

(7 x 8 inches) 

High (H)

2.0-inch 

scribe

2.0-inch 

scribe

2 inches tall
4

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—The TechBrief is being distributed according to a standard distribution. Direct distribution is being 

made to the Divisions and Resource Center.

Availability—The 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.tfhrc.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 TechBrief only because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides high-quality information to serve 

Government, industry, and the 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.

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. FHWARD-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.

november 2012 FHWA-HRT-12-045

HRDI-60/11-12(600)E

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