Issues Impacting Bridge Painting: an Overview

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Issues Impacting Bridge Painting: an Overview
FHWA/RD/94/098 –August 1995

Chapter 6. Task E - Accelerated Testing

Table of Contents

Background
Objectives and Approach
Materials and Procedures
Results and Discussion
Conclusions and Recommendations

BACKGROUND

Corrosion detrimentally effects bridges, reducing section thickness and resulting in a weakened bridge structure that compromises safety. Corrosion control is normally achieved by applying a coating system to the steel bridge. Unfortunately, coating systems do not completely protect and have limited durability. Moisture vapor penetration, water absorption, stress from thermal gradients across the coating, and ultraviolet radiation from sunlight can deteriorate the coating and cause it to fail prematurely. Furthermore, deicing chemicals and salts from the atmosphere can penetrate the coating and allow corrosion to occur at the steel substrate underneath the seemingly intact coating, causing premature blistering and delamination.

Types of Coating Failure

Coatings on bridges can fail in many ways. In general, coating failure is defined as the premature deterioration of the coating system under normal service conditions with subsequent corrosion of the structure's steel substrate. Coating failure may occur because of inadequate substrate preparation, poor adhesion of the coating, improper coating application, formulation problems with the coating, or unsuitable coating selection for a given environment. Additional factors, such as non-flat bridge geometries with hard-to-coat areas and exterior forces such as abrasion of the surface and bridge movement from vehicle traffic also increase the chance of premature coating failure. TABLE 11 lists the main factors influencing coating failure as well as the common types of coating failure that occur on coated steel bridges. Three types of coating failure that occur frequently are highlighted below:

Undercutting: Moisture penetration to the steel substrate and buildup of corrosion products underneath a coating can cause disbonding and failure of a coating. Osmosis is an important factor, especially when coatings are subjected to salt exposure, water immersion, frequent condensation, or high-humidity environments. A coating must have strong adhesion to the substrate to be resistant to undercutting. Organic barrier coatings tend to be less resistant to undercutting than inorganic zinc and thermally sprayed metals. The reason is that adhesion of organic coatings is primarily physical, whereas the adhesion of inorganic zinc and thermally sprayed metals is a result of chemical bonds as well. This combined physical and chemical bond tends to be more durable and more resistant to undercutting.
Cracking: Cracking, or breaks in the coating extending from the surface through to the substrate, are caused by stresses in the coating film and between the coating and steel substrate that exceed the strength of the coating. Cracking results in further water penetration and corrosion of the steel substrate. Cracking is caused by polymer chain breakage due to aging and weathering of the paint system as well as premature failure due to exterior forces such as bridge movement between overlapping joints.
Holidays: Holidays are bare or thin areas of the coated surface where reduced barrier protection can lead to a concentration of the corrosive environment at the steel substrate and accelerate corrosion. Holidays and reduced coating thickness are most often found in areas that are difficult to coat and are caused by inadequate coating application. Bridge geometries, such as edges, corners, welds, overlapping joints, and bolted faying surfaces, represent areas where uniform coating application is difficult and non-uniform. Failure of the coating in a critical area, such as a nut-and-bolt or overlapping joints, can lead to rapid corrosion, such as pitting and steel loss that compromises the structural integrity of the bridge. Bridge geometry and design plays an important role in reducing coating durability. However, it is often overlooked in accelerated and long-term field testing procedures because of the difficulty reproducing similar effects in laboratory tests.

Coating Properties

A protective coating's function is to prevent corrosive service environments, (e.g., salt-air atmosphere or deicing chemicals) from contacting the underlying steel substrate and initiating corrosion. To accomplish this function, a coating must have several properties essential to maintaining a proper barrier to the environment. Some of the more important properties are: water permeability resistance, weathering resistance, sunlight resistance, ease of application, good adhesion, and abrasion resistance.

As noted, an important property of a coating is its resistance to water penetration. Two related properties are coating dielectric strength and coating resistance to ionic movement. Water can penetrate a coating either as a liquid or as a vapor. Water penetration decreases the dielectric strength of a coating, decreasing its resistivity and making the coating less insulative. Water penetration can also cause chemical breakdown of the coating, allowing increased ionic movement to the substrate, and further decreasing the useful life of the coating. Moisture also transports oxygen that is necessary for corrosion to occur. Since corrosion is an electrochemical process, ionic and oxygen transport towards the steel substrate increases the chance of corrosion being initiated. Once corrosion has begun, the corrosion products formed can cause undercutting and loss of adhesion of the coating. Water penetration may swell the coating and produce stresses that eventually lift the coating from the substrate. Although water containing naturally-occurring salts or deicing chemicals penetrate coatings at a slightly slower rate than pure water, their presence increases the likelihood of coating deterioration and substrate corrosion, since they can accumulate underneath the coating, cause delamination by blistering, or accelerate corrosion of the substrate.

Although immersion of a coating in salt water alone is a severe environment, periods of wetness and dryness and frequent changes in temperature can also cause mechanical damage to a coating. Surprisingly, only small amounts of atmospheric salts, such as ammonium sulfate or sodium chloride, need be present to cause coating degradation because wet/dry cycling tends to enhance ionic movement and concentrate salt in the coating and at the substrate, leading to accelerated corrosion rates.

Coating Evaluation

The performance and durability of bridge coatings are often evaluated by either an accelerated test, such as a salt-spray fog, or by long-term atmospheric exposure to a particular environment. Condition assessment of bridge coatings is accomplished using ASTM standard methods such as those listed in TABLE 12 to determine the degree of rusting, cracking, blistering, gloss retention, and adhesion to the substrate. Most of the ASTM standard methods require visual inspection of the coating to determine coating degradation and are subjective. In addition, ASTM B-117, the standard salt-spray test, does not correlate well with long-term field exposure. For instance, waterborne coating systems exhibit poor performance using ASTM B-117, but show improved performance in field environments. ASTM D-1014, the long-term field exposure test procedure for coatings, is the best method to judge how well a coating will last in a particular environment. However, it can take years to acquire meaningful data. Current accelerated tests lack credibility as predictors of field service performance.

Electrochemical techniques have also been used to evaluate bridge coatings. Direct-current electrochemical measurements such as linear polarization and potentiodynamic polarization have been attempted on various coating systems, however, the complexity of the coating/steel substrate does not allow accurate interpretation of the data. Electrochemical impedance spectroscopy (EIS) measurements, on the other hand, can resolve these complexities and provide a fast, quantitative method of assessing coating properties related to water penetration and identify the early onset of corrosion of the steel before it is seen visually. When used in combination with accelerated tests and long-term field testing, EIS can be a powerful technique for comparing a coating's ability to reduce the corrosion rate of the substrate. In addition, EIS measurements taken on existing bridge coatings in the field and as part of a total bridge management program can provide ongoing data on the condition of the coating. Such information would allow bridge owners to schedule overcoating before severe corrosion of the structure has occurred and ultimately would reduce overcoating costs and increase the life of the coating system.

EIS is capable of probing the electrochemical interface of a coated metal and providing quantitative information on the influence of corrosive environments affecting protective coatings and metallic substrates. For example, an EIS scan of a well-coated sample gives a high electrical resistance (R_c), and low capacitance (C_c), due to the dielectric property of the protective coating. The steel substrate properties are not measurable and corrosion is not occurring under the coating because the substrate has not been exposed to corrosive elements such as salt and oxygen. If the coating is subjected to an electrolyte by immersion, accelerated salt spray, or atmospheric exposure, the electrolyte can eventually permeate the coating, reducing R_c and increasing C_c, The change in these parameters is related to water penetration into the coating and indicates that the coating is degrading. Furthermore, electrolyte and oxygen may penetrate to the steel substrate and initiate corrosion of the steel substrate. At this point, the polarization resistance (R_p) and capacitance (C_p) properties related to the steel corrosion rate will become measurable. EIS parameters such as R_p, C_p, R_c, and C_c, can be plotted versus time to determine how well they correlate with the rate of deterioration and the onset of corrosion for each coating.

Another important EIS parameter that has been used to correlate steel corrosion is the frequency at maximum phase angle (w_max). The frequency at maximum phase angle is given by the equation:

This equation is independent of area because the area dependence of R_p and C_p and R_p and R_s, the electrolyte resistance, cancel out each other. The area-independence of w_max is desirable property since the active, corroding steel area under a coating is usually not known. Research by G.T. Ruck et al. (9) has shown that for bare steel pipe in soil, w_max stayed relatively constant with time when the corrosion rate was low and the pipe was protected. However, w_max decreased two orders of magnitude when the pipe was not protected and the corrosion rate increased. The same decreasing trend of w_max can be used to determine the onset of corrosion underneath a coating. Further details on the EIS technique and the important parameters measured are given in Appendix A.

OBJECTIVES AND APPROACH

The objectives of this task were: (1) to identify potential accelerated test methods that improve the evaluation of bridge coating performance and (2) to use these methods to evaluate the corrosion resistance and weathering performance of several coatings.

Three accelerated test procedures were identified and used to evaluate the performance and durability of five bridge coatings: (1) a cyclic salt-spray exposure test (2) a test combining cyclic salt-spray exposure, a freeze/thaw cycle, and carbon-arc, ultraviolet-light exposure, and (3) an electrolyte immersion test. Four of the five coatings studied were thermally sprayed zinc, aluminum, and zinc/aluminum, and a three-part VOC-compliant coating system. Test panels were also cut from a bridge section removed from service that contained a naturally weathered, red-lead primer/alkyd topcoat. In addition to flat test panels, non-flat panels were constructed to simulate a welded section and a bolted faying surface configuration commonly found on bridges. EIS was performed periodically on the test panels in the immersion experiments and before, during, and after the two cyclic tests to quantify coating resistance and capacitance (R_c and C_c), polarization resistance and capacitance (R_p and C_p), and the frequency at maximum phase angle (w_max). Consumption rates of the metallic coatings and corrosion rates of the steel substrate were calculated from R_c and R_p. Visual and photographic inspection of the test panels were also performed before and after testing.

MATERIALS AND PROCEDURES

Test-Panel Preparation

Experimenters prepared 99 coated test panels for exposure in the three accelerated tests. Of these test specimens, 94 wer 10.0- by 15.0- by 0.64-cm (4- by 6- by 0.25-in), ASTM A36, hot-rolled, carbon steel plates that were either grit-blasted with a G-40 grit to a white-metal finish (SP-5), or were left with as-received millscale. The remaining five test panels were cut from a coated bridge section that had been taken out of service. Of the test panels, 52 contained either of 2 modifications: a welded steel support or 2 right-angle brackets welded to the panel and attached to each other with 2 nuts and bolts (as shown in FIGURE 63). The non-flat geometries were used in the two cyclic tests to simulate welded sections and bolted, faying configurations (surfaces that are difficult to coat properly). TABLE 13 summarizes the test specimens used in the experiments. The flat panels and flat panels with the welded steel supports were purchased from KTA-Tator Company. Additional KTA-Tator-supplied flat panels were modified to construct the welded and bolted right-angle bracket configurations.

Coating Type and Application

Five coatings systems were evaluated in this task: thermally sprayed (TS) zinc, aluminum, and 85/15 Zn/Al, a VOC-compliant coating system; and a weathered, red-lead primer/alkyd topcoat taken from a bridge section that had been removed from service. The VOC-compliant coating system consisted of a three-component zinc-filled, epoxy primer [(VOC, 302 g/L (2.52 lb/gal)]; a two- component, high-build, modified aluminum epoxy mastic intermediate coat [VOC, 89 g/L (0.74 lb/gal)]; and a high-gloss, high-solids, aliphatic polyurethane topcoat [VOC, 288 g/L (2.4 lb/gal)]. The bridge section containing the red-lead primer/alkyd topcoat was obtained from the Illinois Department of Transportation (IDOT). Further information on the thermally sprayed metal coatings are given in the previous task. Application of the coatings was done following either manufacturing recommendations or standard practices. Prior to coating, the test panels were ultrasonically cleaned in trichloromethane to remove any grease or dirt on the surface. After coating, thickness measurements were performed using either an Elcometer thickness gauge or Positector-2000 thickness gauge. Wet thickness was also measured on the VOC-compliant coating as recommended by the manufacturer.

Linear polarization, potentiodynamic polarization, and EIS were made on ASTM A36 carbon-steel rods that were left bare or thermally sprayed with zinc, aluminum or Zn/Al. Initial consumption rates of the metallic coatings and initial corrosion rates of the steel substrate were obtained for comparison with values obtained from the three accelerated exposure tests. Any change in the metallic coating consumption rate or steel corrosion rate would mean that the metallic coating has become ineffective and that significant corrosion of the steel is occurring.

The carbon-steel rods were grit-blasted to a white-metal finish (SSPC-10) or left with original millscale. The electrolyte used was 0.40 percent ammonium sulfate and 0.05 percent sodium chloride. Two thermally sprayed coating thicknesses were coated onto the steel rods [75 µm (0.003 in) and 150 µm (0.006 in). The 75-µm thickness represents a coating of minimal through-porosity and the 150-µm thickness a flow-through porous coating. The experiments were performed in triplicate and average values of R_p, C_p, R_c, C_c, and w_max were obtained. Coating consumption rates and steel corrosion rates were calculated from R_c and R_p. The carbon-steel rods were thermally sprayed by ASB Industries, Incorporated, Barberton, Ohio.

The linear polarization and potentiodynamic polarization experiments were carried out using the Model 342C Soft corr^TM corrosion measurement system. The system uses and EG&G Princeton Applied Research Model 273 potentiostat driven by software in an IBM computer. The EIS measurements were performed using the EG&G Princeton Applied Research Electrochemical Impedance System. It consists of an EG&G Model 273 potentiostat, a Schlumberger 1255 Frequency Response Analyzer, and Model 388 software used to control the system and acquire the data. The EIS parameters were calculated from the impedance spectrum using the EQUIVCRT software program from EG&G. A brief overview describing the electrochemical techniques of EIS, linear polarization, and potentiodynamic polarization is given in Appendix A.

Electrolyte Preparation

The electrolyte used in the three accelerated tests contained 0.05-percent sodium chloride and 0.40-percent ammonium sulfate, two salts commonly found in industrial, atmospheric environments. The particular concentrations of the salts have been used in the Mebon Prohesion salt-spray test and are recommended by the Q-Panel Company, Cleveland, Ohio. Reagent-grade sodium chloride and ammonium sulfate were used to make up the electrolyte for the immersion experiments and the salt solution for the cyclic tests.

Accelerated Test Procedure Identification and Modification

Immersion Experiment

An experiment involving continuous immersion in an electrolyte was chosen for accelerated testing because it provided a severe environment that accelerates water penetration into the coating, a significant failure mechanism that occurs on bridge coatings.

Immersion Test Procedures

To quantify the characteristics of the coating and steel as the coating deteriorates in an electrolyte-saturated environment, 14 flat, coated steel specimens were exposed to an electrolyte containing 0.05-percent chloride and 0.40-percent ammonium sulfate. For each coating and surface condition, two steel panels were used. In addition to the laboratory-prepared samples, two test specimens were fabricated from a weathered bridge section obtained from IDOT. TABLE 14 lists the number of immersion cells by coating type and surface condition.

The immersion cells consisted of a 20-cm (8-in) long, 8-cm (3-in) diameter, translucent polyvinyl chloride (PVC) pipe to which an 8-cm (3-in) PVC flange was attached. The coated test panels were placed between the flange and tightened with four bolts, as seen in FIGURE 64. A rubber gasket between the upper flange and the coated test panel prevented electrolyte leakage from occurring. A PVC cap was placed on top of the cell to prevent evaporation of the electrolyte. Periodic EIS measurements were performed using a graphite anode and a saturated calomel reference electrode (SCE) placed at the top of the cell.

Cyclic Test I

The cyclic (Prohesion) salt-spray test was chosen for evaluation because wet/dry cycling has been shown to correlate with outdoor weathering exposure more realistically then the standard salt-fog test, ASTM B117. The failure mechanism associated with wet/dry cycling is similar to water/electrolyte immersion in that osmotic pressure causes water and other ions, such as chloride and sulfate, to diffuse and concentrate in the coating near the steel substrate. Thus, it was thought to be a good alternative test to the immersion experiments. It was decided that a cyclic test with only the salt spray would be run to determine whether the cyclic salt spray alone is a good method for determining coating performance and durability.

Cyclic Test I Procedure

To quantify the characteristics of the coating and steel as the coating deteriorates in a wet/dry cyclic environment, 41 flat and non-flat, steel panels coated with 4 of the 5 coatings were exposed to a cyclic salt fog. The four coatings were the VOC-compliant coating and the thermally sprayed zinc, aluminum, and zinc/aluminum. Two or three coated panels for each coating and surface condition were tested for statistical significance. TABLE 15 lists the number of panels for cyclic test I by coating, surface condition, and geometry. The salt solution used was an electrolyte consisting of 0.05-percent sodium chloride and 0.40-percent ammonium sulfate.

TABLE 16 gives a summary of the cyclic experimental procedure. Before testing, the test panels were photographed and initial EIS measurements run on the flat panels by placing them in an immersion cell filled with an electrolyte of 0.40-percent ammonium sulfate and 0.05-percent sodium chloride. The samples were then placed in the salt-fog chamber and were exposed to a modified cyclic Prohesion salt-spray cycle for 28 d (672 h). The cycle consisted of 1.5 h salt spray, followed by 1-h dry cycle at 35°C. This differs slightly from the Prohesion cycle as given by the Q-Panel Company, where the salt spray is for 1 h. It was observed in initial tests with the Prohesion cycle that it took approximately 15 to 20 minutes for the chamber to fill completely with salt fog. The slightly longer spray time allowed the test samples to encounter a more evenly distributed salt spray for a full hour. After 14 d, the flat samples were taken out and EIS measurements were performed. After 28 d, a final EIS measurement was made on the flat panels. All 41 test panels were photographed to show any changes in the coating, and ASTM Standard 610, Evaluating the Degree of Rusting on Painted Surfaces, was performed.

The salt-fog chamber used in the tests was a Q-Fog Corrosion Chamber SF/MP450 manufactured by the Q-Panel Company, Cleveland, Ohio. The test panels were placed in the salt-fog chamber on plastic ledges that are attached to the wall of the chamber, as recommended by the Q-Panel Company for the cyclic test. In addition, flow rate and air pressure were periodically adjusted to maintain an even distribution of salt spray in the chamber.

Cyclic Test II

The cyclic salt-spray, freeze/thaw, and ultraviolet-exposure test was chosen for the same reasons as the cyclic salt-spray test; however, it has the additional benefit of providing thermal stressing due to the freeze/thaw cycle and elevated temperature achieved in the ultraviolet exposure. The ultraviolet exposure also subjects the coatings to possible chemical breakdown of the constituents in the organic coatings. In addition, any water that penetrated the coating in the salt-spray portion of the cycle would freeze in the freeze/thaw cycle and expand in volume and could cause cracking and loss of adhesion. Also, the higher temperatures in the ultraviolet exposure might drive off water from the coating. This continual cycle of water penetration, expansion due to freezing, and evaporation at elevated temperature represents an accelerated cycle of a northern-type exposure of rain or snow, freezing conditions, followed by drying conditions with sunlight and heating of the bridge surface.

Cyclic Test II Procedure

To quantify the characteristics of the coating and steel as the coating deteriorates, 44 flat and non-flat, steel panels coated with the 5 coatings were exposed to a cyclic salt-fog, freeze/thaw cycle, and carbon-arc-generated ultraviolet radiation. The five coatings were the VOC-compliant coating, thermally sprayed zinc, aluminum, and zinc/aluminum, and the weathered red-lead/alkyd topcoat. Two or three coated panels for each coating and surface condition were tested. TABLE 15 lists the number of panels for cyclic test II by coating, surface condition, and geometry.

TABLE 16 gives a summary of the cyclic experimental procedure. Before testing, the test panels were photographed and initial EIS measurements run on the 18 flat panels. The samples were then placed in the salt-fog chamber and were exposed to the modified, cyclic, Prohesion salt-spray cycle for 7 d (168 h). The on/off cycle time was the same as for cyclic test I. After 7 d, the test panels were taken out and placed in a freezer. The initial temperature was slightly above 0°C (32°F). The final temperature of -23°C (-10°F) was reached in about 2 h, and held for 24 h. After freezing, the samples were immediately taken out of the freezer and placed in the ultraviolet (UV) weatherometer. The UV exposure period was 8 h on, 4 h off, for 7 d. The temperature in the UV weatherometer was initially at ambient temperature, but rose to about 60°C (120°F) in about 1 to 2 h. After seven days in the UV-weatherometer, the flat panels were taken out and EIS measurements performed. The cycle was repeated for a total exposure of 30 d (720 h). Afterwards, the test panels were taken out and a final EIS measurement was made on the flat panels. All 44 test panels were photographed to show any changes in the coating, and ASTM Standard 1014, Evaluating the Degree of Rusting on Painted Surfaces, was performed.

The salt-spray experiments were conducted in the Q-Fog Salt-Fog Chamber. The UV-weathering cycle was done in an Atlas Model XW-W weatherometer that uses Sunshine Carbon-Arc Lamps to provide the ultraviolet radiation. The freezer used for the freeze/thaw cycle was a True Manufacturing Company Model T-23F, [0.57-m³ (20-ft³)] upright freezer. It was purchased from Pierce Food Service Equipment Company, Countryside, Illinois.

RESULTS AND DISCUSSION

Immersion Experiments

Immersion experiments were performed on eight, thermally sprayed (TS) test panels, four test panels coated with a VOC-compliant coating, and two test panels cut from a weathered bridge section containing a lead (Pb) primer and alkyd topcoat. The TS-coated test panels were immersed for 174 d, two each of the VOC-compliant coatings were immersed for 92 and 98 d, respectively, and the two weathered bridge sections were immersed for 72 d. FIGURES 65a, 65b, and 65c show representative EIS Nyquist plots for the TS-Zinc, Pb/Alkyd, and VOC-compliant coatings as a function of immersion time.

Thermally Sprayed Coatings

FIGURE 66a shows a plot of the high-frequency section of the EIS scan for a TS-zinc-coated test panel illustrating the increase in coating resistance as given by the increasing semicircle. The straight line going off of the scale is actually a second semicircle at the lower frequencies due to the steel corrosion rate. The second semi-circle was absent on the EIS scans for the TS-Al- and TS-Zn/Al coated test panels, indicating that steel corrosion was not yet measurable on these coatings.

FIGURES 66a and 66b show the consumption rate of the thermally sprayed coatings; the corrosion rate of the steel underneath the TS-zinc coating; and w_max, the frequency at maximum phase angle versus immersion time. The TS coatings initially showed a high coating consumption rate of 102 to 635 µm/yr [(4 to 25 mil/yr (mpy)], which decreased with time to approximately 25 µm/yr (1 mpy). The sacrificial coatings are actively corroding and protecting the steel substrate. Assuming a constant consumption rate, the 300-µm (4-mil) thick coatings would be completely consumed in 12 years of electrolyte immersion. The immersion test has accelerated the coating consumption rate threefold, given an estimated service life of 40 years for a 300-µm thermally sprayed coating thickness.

In addition, corrosion under the thermally sprayed coatings was measurable immediately upon immersion in the electrolyte. The EIS measurements indicated that electrolyte penetrated through the TS-zinc coating to the steel substrate. Initially, the corrosion rate of the steel substrate was 128 µm/yr (5 mpy), but decreased to 25 µm/yr (1 mpy) after 20 d. Though it appears that the TS-zinc coating is sufficiently protecting the steel substrate. Corrosion of the steel under the TS-Zn/Al coatings was also not observed. The EIS scan for the TS-Al coating, however, showed an additional diffusional element due to ionic movement in the coating, most likely towards the steel substrate, and suggesting that corrosion of the steel may become measurable in a short time.

The frequency at the maximum phase angle, w_max, is plotted in FIGURE 66b. The w_max decreased with time and with decreasing consumption rate for the thermally sprayed coatings, amplifying the change from a fivefold decrease in consumption rate to a three to four orders-of-magnitude decrease in w_max. On the other hand, w_max due to the steel under the TS-zinc coating stayed relatively constant, although the corrosion rate decreased from 128 to 25 µm/yr (5 to 1 mpy). As noted earlier, the TS-zinc is protecting the steel, which is reflected in the relatively constant value of w_max. A significant change in w_max would probably not occur until the TS-zinc coating had been consumed or was not sacrificially protecting the steel, causing the steel corrosion rate to increase to higher values.

Pb/Alkyd and VOC-Compliant Coatings

FIGURE 65b shows EIS Nyquist plots for the Pb/alkyd-coatings as a function of immersion time. Initially, one semicircle was observed. After 2 d immersion, two semicircles were measured that continually became smaller with time. The first semicircle is representative of the Pb/alkyd coating and the second is due to the steel substrate. Theoretically, for a weathered coating, one would expect an EIS scan with three semicircles representing the alkyd topcoat, the lead primer, and the steel substrate properties. However, visual observation of the test-panel surface under immersion showed areas of exposed steel where significant corrosion was occurring. This provides evidence that the second semicircle on the EIS scan is due to steel corrosion and not the lead primer. The Pb/alkyd coating properties appear to be lumped together into a single semicircle on the EIS scans and can be represented by one coating resistance.

FIGURE 65c shows EIS Nyquist plots as a function of immersion time for the VOC-compliant coatings. One high-impedance semicircle was obtained, indicative of a highly insulative coating. However, the high initial coating resistance decreased after 29 d, indicating there was an initial uptake of electrolyte into the coating. Further changes in the coating resistance were small, but they still indicated further electrolyte penetration into the coating.

FIGURES 67a and 67b show the coating and polarization resistance, and w_max as a function of immersion time for the Pb/alkyd and VOC-compliant coatings. FIGURE 67a shows coating resistance decreasing for both coatings. This is indicative of a coating absorbing electrolyte and becoming less resistive. However, the VOC-compliant coating is still five orders of magnitude higher in resistance than the Pb/alkyd coating, meaning it is still insulative and providing a barrier for moisture penetration toward the steel substrate. Steel corrosion was not observed under the VOC-compliant coatings. On the other hand, the Pb/alkyd coating gave a relatively low coating resistance that continued to decrease with time, indicating that the coating has degraded. In addition, corrosion of the steel substrate was observed from the EIS measurements and confirmed visually. However, the polarization resistance, R_p, appeared to be high -- three orders of magnitude higher than obtained from experiments done on bare-steel rods of known area in the same electrolyte. A possible reason is that R_p measured on the test panels is due mostly to the exposed steel areas. However, the total area of the test panel, including the area covered by the Pb/alkyd coating, was used in the calculation. This area is larger than the actual corroding steel area and would give an apparently larger R_p than the actual value.

More importantly, the EIS measurements are able to distinguish between the highly resistive VOC-coating that is beginning to degrade and the weathered Pb/alkyd coating with corrosion of the steel occurring underneath. Furthermore, the immersion experiments can accelerate coating degradation in a reasonable length of time. Extrapolation of the VOC-compliant coating resistance to values observed with the weathered Pb/alkyd coating give immersion times of between 350 to 500 d (1 to 1-1/2 yr). In addition, corrosion of the steel underneath the coating would be measurable before this time. Such a timeframe for an accelerated test is reasonable given that long-term field exposure takes 3 to 5 years before meaningful data is obtained.

FIGURE 67b shows w_max as a function of immersion time for the Pb/alkyd and VOC-compliant coatings. The w_max for the steel under the Pb/alkyd coating shows values indicative of a significant corrosion rate, i.e., above 25 µm/yr (1 mpy). This confirms that the apparent polarization resistance measured is incorrect due to the area term. On the other hand, w_max for the two coatings shows the same trend even though the coating resistances are much different in value. This suggests that w_max is a useful parameter for determining the onset and extent of steel corrosion underneath a coating, but not necessarily for characterizing the level of coating degradation. Fortunately, however, coating resistance is an excellent parameter to characterize the state of the coating. In addition, the coating capacitance can be used to further quantify water penetration into the coating, however, it was not calculated in this task.

In conclusion, it appears that electrolyte immersion combined with EIS is a good method to accelerate coating degradation and corrosion of the steel substrate underneath. EIS measurements allow determination of the consumption rate of sacrificial coatings, quantification of coating degradation, and determination of the onset and extent of steel corrosion. The electrochemical properties of coating resistance and capacitance, polarization resistance, and w_max are good parameters that can be used to correlate the performance of metallic and organic coatings subjected to an accelerated, corrosive environment. Although immersion times of approximately 1 year are needed to obtain significant coating degradation, the onset of steel corrosion under the coating will occur in a much shorter time.

Cyclic Accelerated Tests

Two accelerated tests consisting of a cyclic (Prohesion) salt-spray test (cyclic test I) and a cyclic (Prohesion) salt-spray, freeze/thaw, and carbon-arc UV-exposure test (cyclic test II) were performed on 85 flat and non-flat test panels coated with TS-zinc, aluminum, and zinc/aluminum, and a VOC-compliant, coating system. Flat test panels were also cut from a weathered bridge section taken out of service and that contained a lead primer/alkyd topcoat coating system. The test panels were subjected to cyclic test I for a total of 672 h, and cyclic test II for 720 h. The salt solution used in the salt spray was an electrolyte of 0.4-percent ammonium sulfate and 0.05-percent sodium chloride.

In general, cyclic test I subjected the test panels to a longer salt-spray exposure (twice as long as cyclic test II), which caused more salt solution to penetrate into the coatings. This was especially detrimental to the sacrificial TS coatings, which are porous, and the weathered Pb/alkyd coating that was already naturally degraded. However, the combination of salt spray, freeze/thaw, and UV exposure represented a more severe test that not only allowed salt penetration from on/off cycling of the salt spray, but also allowed chemical and mechanical breakdown of the coatings due to the freeze/thaw and UV-exposure cycle. A longer exposure period for cyclic test II is recommended because it would allow a higher degree of salt penetration, coating degradation, and possible steel corrosion to occur after several salt spray, freeze/thaw and UV-exposure cycles.

Non-flat Panel Results

FIGURES 68a1, 68a2, 68b1, 68b2, 68c1, 68c2, 68d1, and 68d2 show representative before-and-after photographs of the four coatings on the two non-flat test panels subjected to cyclic tests I and II. TABLE 17 lists the corrosion rating of each coating and test panel on a scale of 1 to 10 according to the ASTM-D610 visual corrosion rating standard, with 10 representing a coating of excellent performance and durability, and 1 indicating a coating of poor performance and durability.

The VOC-compliant and TS-zinc coating performed the best of the four coatings, showing excellent coating performance. The TS-zinc showed excellent durability and the VOC-compliant coating showed better-than-average durability. Both coatings had visual ratings of between 8 and 10, performing well in the accelerated tests with virtually no corrosion occurring on the surface. Some yellow staining was observed on one of the TS-zinc-coated test panels that was due to the consumption of the zinc. However, several of the VOC-compliant coated test panels showed several small areas of steel corrosion on the welded bracket or the nut-and-bolt area. Though these corroded spots were not enough to decrease the rating significantly, they are noted because they probably occurred due to the difficulty in uniformly coating the non-flat area. The coating thicknesses in these areas were thinner compared to the rest of the flat panel area. In addition, one of the VOC-compliant coated test panels subjected to cyclic test II showed cracking along a nut and bolt as well as on the welded bracket. The cracking was caused by the thermal stressing from the freeze/thaw cycle and subsequent UV exposure at elevated temperatures and this decreased the durability rating of the coating. These results highlight the usefulness of using non-flat test panels to rate coating performance because such observations were not seen on the TS-zinc or VOC-compliant coated flat test panels. The corrosion rating for the TS-Zn/Al-coated test panels was between 5 and 6, indicating moderate coating performance and better-than-average durability. Although rust from the steel was not observed, the TS- Zn/Al coating showed visible signs of consumption, with many white "pits" where corrosion of the coating had occurred. The surface of the coating was generally rougher than before testing, and the nut-and-bolt area showed fair amounts of coating consumption. This made the coating aesthetically unpleasant. However, overcoating the TS-Zn/Al with a topcoat or a TS-polymer may solve this problem and would also increase coating performance and durability.

The corrosion rating for TS-aluminum was the worst of the four coatings, with a wide scatter of values between 1.5 and 8.5. Coating performance and durability was moderate to poor. The poor rating was due to significant consumption of the aluminum that caused large amounts of aluminum corrosion deposits on the surface of the coating, as seen in FIGURES 68a1 and 68a2. Steel corrosion was also observed on the nut-and-bolt areas and at the weld and crevice between the two right-angle brackets. As noted for the TS-Zn/Al, an organic or TS-polymer overcoat might solve the problem of coating performance, although it appears once the overcoat is compromised, significant consumption of the TS- aluminum coating would occur, which would further delaminate the overcoat.

In conclusion, the TS-zinc and VOC-compliant coated non-flat test panels gave the best coating performance and durability of the four coatings. The TS-Zn/Al coating gave moderate coating performance and better-than-average durability; the TS-aluminum gave poor coating performance and durability. The usefulness of the non-flat test panels in cyclic accelerated testing was demonstrated and showed cracking of the coating and significant steel corrosion at the nut-and-bolt and welded bracket areas, observations that could not have been made on flat panels alone.

Flat Panel Results

Visual Rating

FIGURES 69a1, 69a2, 69b1, 69b2, 69c1, 69c2, 69d1, and 69d2 show representative before-and-after photographs of the five coatings on the flat test panels subjected to cyclic tests I and II. TABLE 18 lists the corrosion rating of each coating and test panel on a scale of 1 to 10, according to the ASTM-D610 visual corrosion rating standard.

The VOC-compliant and TS-zinc coatings performed the best of the five coatings, showing excellent coating performance and durability. The TS-zinc coating showed some discoloration due to zinc consumption, but steel substrate corrosion was not observed. The VOC-compliant coatings showed no discoloration, cracking, or rust spots on any of the test panels.

The TS-Zn/Al and TS-aluminum coated test panels showed moderate to better-than-average performance and durability. Alternating areas of a white corrosion product and dark discoloration were observed on the TS-Zn/Al coatings. Discoloration and aluminum corrosion pits were observed on the TS-aluminum coatings. However, steel corrosion rust spots were not observed on the test panels.

The Pb/alkyd coated test panels showed poor performance and durability of the coating. Significant steel corrosion was observed on the surface where the steel substrate appeared to have been exposed. However, discoloration of the alkyd topcoat was not observed.

EIS Analysis

FIGURES 70a, 70b, and 70c show the consumption rate of the thermally sprayed coatings, the corrosion rate of the steel underneath the TS-coatings, and w_max, (the frequency at maximum phase angle) for cyclic tests I and II. The TS-zinc coating gave negligible consumption rates throughout cyclic test I, as shown in FIGURE 70a. This corroborates with the excellent rating given by the visual observations and indicates that salt or water has not penetrated to the steel substrate. On the other hand, the TS-aluminum and TS-Zn/Al coatings gave initial coating consumption rates of 127 to 190 µm/yr (5 to 8 mpy), which decreased with time to approximately 25 to 114 µm/yr (1 to 4 mpy). These two sacrificial coatings are actively corroding and protecting the steel substrate. These results are also corroborated by the visual rating that showed significant coating consumption. In addition, steel corrosion rates were observed for the TS-aluminum and TS-Zn/Al after 670 h of exposure. The corrosion rate was less than 25 µm/yr (1 mpy) for the TS-Zn/Al, but was 241 µm/yr (9.5 mpy) for the TS- aluminum. Since steel corrosion was not observed visually on the surface of these coatings, corrosion must be occurring under the TS-aluminum coating. Thus, the EIS measurements were able to characterize the early onset of steel-substrate corrosion under the metallic coating before corrosion was observed visually.

FIGURE 70b shows consumption rates for the three TS coatings subjected to cyclic test II. In this case, the TS-zinc showed fairly high consumption rates before, during, and after the test. The TS-Zn/Al and TS-aluminum consumption rates were much lower than the TS-zinc, but still significant. All three TS coatings are active and are protecting the steel substrate underneath. Steel corrosion rates were not observed for the test panels in cyclic test II, however. As noted earlier, the salt-spray exposure time is half of the cyclic test, or 360 h. Increasing the total exposure time of cyclic test II would provide a longer salt-spray exposure that, when coupled with freeze/thaw cycling and UV-exposure, would accelerate the onset of corrosion under the coatings.

FIGURE 70c shows w_max for the three TS coatings subjected to cyclic tests I and II. In general, the low values of w_max indicate active, corroding metals. The values also agree with the results from the immersion experiments that showed similar consumption rates and values of w_max. As noted earlier, w_max appears to be a sensitive indicator of consumption rate of the TS coatings. In addition, the onset of steel corrosion was observed by EIS in approximately 670 h of accelerated exposure time. Increasing the total exposure time to 1400 h (2 mo) would be sufficient to further quantify steel corrosion underneath the coatings and determine how long the TS coatings can adequately protect the steel substrate.

The coating resistance, polarization resistance, and w_max for the VOC-compliant coating subjected to cyclic tests I and II are given in FIGURES 71a and 71b. The coating resistance was high for all the test panels, but decreased by two to three orders of magnitude for several of the test panels. Electrolyte penetration into the coating was occurring, but there did not appear to be any trend for surface condition or accelerated test method. In addition, a test panel with a mill scale surface subjected to cyclic test II showed measurable steel corrosion underneath the coating, even though corrosion was not observed visually. Thus, it is necessary to measure the onset of steel corrosion underneath organic coatings before it is visually seen. Still, a longer exposure period is needed to further validate steel corrosion and coating degradation. Extrapolation of the coating resistance for two of the test panels showed that a total exposure time of between 1500 to 1900 h (2 to 2 1/2 mo) would be needed to reduce the coating resistance to values simlar to a degraded coating.

EIS was able to reduce the coating resistance to values similar to a degraded coating. Steel corrosion underneath the coating would be initiated as well in that time period.

FIGURE 71b shows w_max for the VOC-compliant coatings subjected to cyclic tests I and II. The w_max due to the coating decreased two orders of magnitude for the test panels subjected to cyclic test I, following the trend of decreasing coating resistance and indicating that the electrolyte is penetrating the coating. The w_max due to steel corrosion on a coated panel with a millscale steel surface was low, indicating that significant corrosion was occurring underneath the coating. A coated test panel with a grit-blasted surface gave a measurable w_max due to steel corrosion after testing, which suggests that the onset of corrosion occurred sometime between 360 and 720 h of exposure time.

CONCLUSIONS AND RECOMMENDATIONS

Based on the research performed in this task, it can be concluded that:

The cyclic exposure tests and immersion experiments combined with EIS measurements provide an early indication of coating degradation, degree of water penetration, and the onset of steel corrosion underneath a coating before it is visually seen in a relatively short period of time.
EIS measurements performed on the flat test panels in the cyclic salt-fog test, the cyclic salt-fog, freeze-thaw, and UV-exposure test, and the immersion experiments, allow determination of the consumption rate of sacrificial coatings, quantification of coating degradation, and determination of the onset and extent of steel corrosion, after 1 mo of cyclic exposure and 3 to 6 months of immersion. EIS was able to distinguish between the highly resistive VOC-compliant coatings that were beginning to degrade and the weathered Pb/alkyd coating with corrosion of the steel substrate underneath. Estimates made from the change in coating resistance and frequency at maximum phase angle with exposure period indicate that cyclic tests of 1400 to 1900 h (2 to 3 mo) and immersion experiments of 12 to 18 mo should provide further verification of coating degradation and steel-substrate corrosion. These measurements can then be correlated with EIS measurements performed on similar coatings subjected to long-term, outdoor exposure to further validate prediction of long-term coating degradation and failure with these methods.
The TS-zinc and VOC-compliant coatings showed excellent coating performance and durability on the flat test panels. The TS-Al and TS-Zn/Al coatings gave moderate to better-than-average results, and the naturally weathered Pb/alkyd coating gave poor results.
The TS-zinc and VOC-compliant coatings performed well in the three accelerated tests, with no visible corrosion of the steel substrate. However, the coating resistance for the VOC-compliant coating decreased with exposure time, indicating that electrolyte was penetrating into the coating. Corrosion of the steel was also evident from the EIS measurements, but was not observed visually. The TS-zinc coating showed a significant consumption rate. Steel corrosion was not observed visually or by EIS measurements confirming that the TS-zinc was adequately protecting the steel substrate.
The TS-aluminum and the TS-Zn/Al coatings gave moderate coating performance and durability. Strongly adhered corrosion deposits on the coating surface provided evidence of aluminum consumption for the test panels subjected to 1 mo of cyclic salt-fog exposure. Corrosion of the steel substrate was measurable on both coatings, even though it was not observed visually.
The weathered bridge section with the Pb primer/alkyd topcoat performed the worst of the five coatings. The coating resistance was low and steel corrosion was evident visually and from the EIS measurements. The results provide a baseline for a weathered coating further subjected to accelerated weathering. The changes in the EIS parameters due to accelerated testing can be compared with EIS parameters of coatings still in service to identify their state of coating degradation and possibly to predict future degradation and failure.
The TS-zinc and VOC-compliant coatings gave better-than-average to excellent coating performance and durability on the non-flat panels containing welded and bolted brackets. The TS-Zn/Al gave moderate coating performance and the TS-Al showed poor results.
Modifying the flat test panel to include a nut and bolt and welded bracket allowed coating performance and durability to be simulated on non-flat areas of a bridge, such as overlapping joints and welded supports. Rust spots and cracking were observed on the nut-and-bolt area and on the edges of the welded brackets, whereas the flat sections of the panels were usually corrosion-free. The TS-zinc and VOC-compliant coatings gave the best performance, with visual ratings of 8 to 10 observed on the test panels. Cracking of the coating and evidence of steel-substrate corrosion at the nut-and-bolt area were observed for the VOC-compliant coated test panels subjected to the cyclic salt-fog, freeze-thaw, and UV-exposure test. Thermal stresses appear to have been sufficient to cause coating delamination. The TS-Zn/Al gave moderate coating performance and better-than-average durability. White pits of corrosion products and roughness of the surface due to coating consumption were observed. The TS-aluminum coating gave poor coating performance and durability, with considerable amounts of aluminum corrosion deposit seen and steel corrosion observed due to aluminum consumption on the nut-and-bolt areas of the panel.

Based on the conclusions from this task, the following recommendations for future work are given:

Perform additional experiments using a longer cyclic exposure and immersion period.
Based on estimates from the EIS parameters, a longer cyclic exposure and immersion period would further verify coating degradation and the onset of substrate corrosion. The experiments should also include additional analysis on the coated test panels, such as optical and surface analysis, to provide supplementary evidence of coating degradation. In addition, further refinement of the accelerated tests can be accomplished by performing serial experiments. Test panels will be taken out periodically and analyzed to determine the shortest exposure period needed to determine coating degradation and failure.
Conduct accelerated experiments on additional thermally sprayed and organic coatings.
In addition to the TS-zinc, TS-aluminum, and TS-Zn/Al, a combination of thermally sprayed metal and polymer should be investigated using the accelerated tests. Such a coating system would more typically be used on a bridge, since it combines the sacrificial properties of the metallic coatings with the barrier characteristics of the polymer. In addition, other VOC-compliant organic coatings should also be studied to quantify their coating degradation characteristics and to determine whether the onset of steel corrosion can be predicted.
Conduct EIS measurements on field-exposure test panels and on coated steel bridges to evaluate the viability of field EIS measurements and their correlation with cyclic accelerated tests.
EIS measurements taken on coated test panels subjected to long-term outdoor exposure as well as measurements taken on coating systems presently applied to steel bridges and in various states of degradation, should be performed. Such measurements would allow the investigation of the viability of field EIS measurements and the determination of whether results from cyclic accelerated tests can be correlated with field-obtained EIS parameters such as coating resistance, polarization resistance, and w_max (the frequency at maximum phase angle).