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Determination of epoxy coating wet-adhesive strength using a standardized ASTM/ISO scratch test.

Abstract A new approach for evaluating the wet-adhesive strength of epoxy-based coatings was carried out based on a recently standardized ASTM/ISO scratch test. A linearly increasing scratch normal load was applied during scratch to induce progressively increased delamination stress at the coating and steel substrate interface. Thus, the applied critical load to cause coating debonding can be experimentally determined. To find out the corresponding stress magnitude to incur coating debonding, finite element methods (FEM) modeling was conducted to analyze the stress fields around the scratch tip during scratching. The wet-adhesive strength is then quantitatively determined. Based on the above methodology, investigation on a set of model coaling systems suggests that the critical load for coating delamination is significantly influenced by water exposure time, coating thickness, and substrate surface roughness. By combining the standardized scratch tests and FEM modeling, the proposed approach is found to be effective for quantitative assessment of epoxy coating wet-adhesive strength and for the development of high performance protective coatings for various industrial applications.

Keywords Epoxy coating, Wet-adhesive strength, Debonding, Scratch, Finite element modeling

Introduction

Polymeric coating adhesion to metal substrate is critically important for various industrial applications. (1-8) It is particularly true when the coating systems are subjected to environmental degradation, thermal degradation, and mechanical impact, which can further weaken the coating adhesive strength. (1), (2) One such example is the utilization of fusion-bonded epoxy (FBE) coating to protect steel pipes against corrosion for oil and gas pipeline applications. The integrity of FBE coating adhesion on steel substrate is critically important for preventing corrosion and for extending service life of steel pipes.

Numerous research efforts have been carried out and many methodologies, such as impact tests, bend tests, pull-off tests, blister test, acoustic emission, etc., have been employed to evaluate coating adhesive strengths. (3-8) Assessment of the wet-adhesive strength, i.e., the adhesion performance under wet condition of FBE coatings on steel substrate is crucial for the pipeline coating industry. While quantitative assessment of adhesive strength under dry condition can be easily made by most of the above test methods, their usage for coating adhesive strength measurement under wet environment is not considered reliable due to their difficulties in sample preparation and in data analysis. Thus, these methods are only suitable for pass or fail evaluation of the coating wet adhesion performance. A reliable test method that can provide quantitative assessment of the coating-metal adhesive strength under a wet environment is still lacking.

Although significant research efforts have been performed on scratch of metallic/ceramic coatings, (9-12) few have been focused on the study of scratch behavior of polymeric coatings. A recent development of the polymer scratch test method (ASTM D7027-05 and ISO 19252:2008), (13-15) which has been demonstrated to be effective for quantitative scratch performance evaluation of bulk polymers, (15-18) can also be adopted for polymer coating study. Our previous effort on scratching of acrylic coatings on steel substrate showed that various types of coating damage modes, including coating delamination, (19) were observed (Fig. 1). Since coating wet-adhesive strength is highly sensitive to how the samples are prepared, the proposed scratch test, which requires little or no special sample preparation or handling, is suitable for investigating the wet-adhesive strength of coatings. It is thus anticipated that the above scratch approach will work for quantitative determination of the wet-adhesive strength of FBE coatings on steel substrate.

[FIGURE 1 OMITTED]

Three-dimensional (3-D) finite element methods (FEM) analysis has been adopted to simulate a progressively increasing normal load scratching process and shown to be effective for understanding the scratch-induced damage mechanisms. (20-22) To quantitatively determine the coating adhesive strength, the onset normal load of scratch-induced debonding can be experimentally obtained. By a correlation between the experimentally observed coating delamination and the corresponding stress state and magnitudes obtained by FEM modeling, it becomes possible for quantitative determination of the coating wet-adhesive strengths.

The objective of this study is to establish an effective test method for the quantitatively measurement of wet-adhesive strength of FBE coating on steel substrates. It is anticipated that the testing methodology proposed in this research will assist coating formulators as well as pipeline fabricators to develop high performance coatings that will meet the demands of the oil and gas pipeline industries.

Experimental

Model coating systems

Two commercially available high [T.sub.g] epoxy coating systems were chosen for this study. System I is an experimental bisphenol A epoxy-based coating (Tuboscope), having a dry-film-thickness (DFT) of 50 and 100 [mu]m, applied on both smooth ([R.sub.a] = 3.5 [mu]m) and roughened ([R.sub.a] = 64 [mu]m) steel surfaces. System II is based on 3M Scotchkote 626-140 FBE (dry [T.sub.g] = 140[degrees]C and wet [T.sub.g] = 134[degrees]C) with DFT of 100, 200, and 300 [mu]m, which was prepared by spray coating on a flat, smooth steel substrate preheated at 177[degrees]C. The samples were then post-cured for 10 min at 249[degrees]C, followed by air cooling.

Sample water immersion conditioning

The coating panels were exposed to hot water at 80[degrees]C for System I and 95[degrees]C for System II, respectively, using a custom-built water immersion set-up. Thermal couples were installed for temperature reading and control. Foam insulation layers were incorporated to help maintain the designated temperatures for a specified period of time from 1 day to 2 weeks to probe the coating residual adhesive strength.

Determination of adhesive strength via scratch test

After the prespecified time of water immersion at elevated temperatures, the specimens were removed from the hot water tank and rinsed with tap water and wiped dry before scratch testing. The scratch tests were performed immediately using a commercially available scratch test apparatus (Surface Machine Systems, Model G4, www.surfacemachines.com). A linearly increasing normal load scratch test, ranging from 1 to 95 N, was conducted on most samples. The scratch velocity and length were set at 100 mm/s and 100 mm, respectively. A 2-mm diameter spherical tungsten carbide tip was chosen for the scratch test.

Determination of critical load of coating debonding

To experimentally detect the critical load of coating debonding, three different methods were chosen: (1) direct visual observation, (2) exposure of the scratched samples to oxidative liquid, and (3) laser confocal scanning microscopy (LSCM) examination of surface profile changes. For a linearly increasing normal load scratch test, once the onset of the scratch-induced debonding is detected based on the above methods, the critical load value for coating delamination can be easily determined by measuring the distance of onset delamination from either the start or the end of the scratch point.

Similar to the scratching of transparent coatings, (19) the delamination onset of thin opaque coatings can be readily detected by visual observation. Upon coating debonding, the light can penetrate through the thin opaque coating and scatter more light on the delaminated surface. The very first location of such a visual change in appearance is the onset of coating delamination. Figure 2 shows the onset delamination phenomena for Systems I and II after 1 day of water immersion. The yellow circles indicate the location of onset of sample color change, i.e., coating delamination.

[FIGURE 2 OMITTED]

For thicker opaque coatings, their delamination phenomenon cannot be directly observed by the naked eye. Alterative methods have to be employed. One approach is to expose the scratched samples to oxidative liquid by which the delaminated region will be oxidized easily. With an open cut on a more severely scratched coaling region, the sample was then immersed in DI water containing 5 vol.% chlorine bleach for 6 h. If coating debonding is present along the scratch path, the steel surface will be directly exposed to the oxidative liquid and rust readily. Subsequently, the rust can be observed after peeling off the coating layer. Figure 3 shows the rusted scratch region of the sample shown in Fig. 2b. Here, the onset of coating delamination can be indentified as the location of the very first rusted spot.

[FIGURE 3 OMITTED]

KEYENCE VK-9700K LSCM was also used to observe the subtle surface profile change by scratch-induced delamination. As illustrated in Fig. 4a, the lowest point (valley) of the scratched profile is always below the original coating surface if there is no debonding. However, if delamination occurs, the debonded coating layer will bulge and induce a surface profile change as shown in Fig. 4b, which can be nondestructively observed using LSCM. Actually, the bulging phenomenon has been observed upon the coating debonding for both thin opaque coatings (Fig. 2) and transparent coatings. (18)

[FIGURE 4 OMITTED]

Determination of coating wet-adhesive strength via FEM

To determine the corresponding stresses induced by the progressively increasing normal load scratch test, a 3-D FEM simulation approach was used here similar to Jiang et al.'s work. (22) ABAQUS/Explicit [R] 6.4.5 is employed to simulate the scratch process of a polymeric coaling. (23) As shown in Fig. 5, (22) the mesh is assumed to be symmetrical about the 2-axis. The typical size of an eight-node tri-linear brick element along the scratch path is 0.05 mm x 0.05 mm x 0.05 mm and is chosen to give converging simulation results and assure numerical accuracy. The spherical stainless steel tip is assumed to be a rigid surface. The perfect bonding between the coating layer and substrate exists before the occurrence of debonding. The elastic-pure-plastic material type is adopted for the steel substrate, and the epoxy coaling is described by piecewise linear elastic-plastic stress-strain curves. The normal load applied on the indenter is linearly increased from 1 to 95 N during the scratch process. The adaptive remeshing methodology provided in ABAQUS is performed to preserve mesh quality during the scratch simulation.

[FIGURE 5 OMITTED]

Although the scratch always induces multiaxial stress components at the interface, the Mode I stress component (i.e., [[sigma].sub.33]) of the debonding stress is always dominant and is an order of magnitude larger than other stress components (Fig. 6). Thus, Mode I debonding stress is expected to be the dominant damage mode at the interface.

[FIGURE 6 OMITTED]

If the adhesive strength is higher than the cohesive strength of the coating, the in-layer coating cracking may occur. (19), (21) Figure 7 shows the stress magnitude variations of both debonding stress and the in-layer tensile stress as a function of the increasing scratch normal load. The occurrence of either in-layer cracking or coating delamination depends on many factors, such as the coating ductility, Young's modulus, yield stress, coating thickness, cohesive strength, and adhesive strength.

[FIGURE 7 OMITTED]

Once the debonding stress exceeds the interfacial adhesive strength, the coating debonding will take place. With the known critical load for coating debonding from the scratch experiment, FEM modeling can then be utilized to determine the corresponding wet-adhesive strength of the coatings.

Results and discussion

Scratch test

Scratched coating samples of System II having a DFT of 100 [mu]m after an immersion time of up to 5 days are shown in Fig. 8. The results suggest that the proposed scratch methodology for determining the onset of coating debonding gives good reproducibility and follows the expected trend of coating adhesion degradation due to environmental exposure. In this case, the onset of delamination can be visually observed. Under a higher normal load, the in-layer coating cracking and buckling damage are also observed, which is consistent with our previous studies. (19)

[FIGURE 8 OMITTED]

Substrate surface roughness effect

For System I with a smooth steel substrate surface ([R.sub.a] = 3.5 [mu]m), the scratched samples are shown in Fig. 9. After 3 days of water immersion, the coating wet-adhesive strength is weakened and the scratch-induced coating debonding can be detected using LSCM surface profile analysis. Meanwhile, for the rougher steel substrate ([R.sub.a] = 64 [mu]m) counterpart, no coating debonding was observed even after 21 days of hot water immersion. Only in-layer cracking was found. The degradation of wet-adhesive strength is known to be partially due to the deposit of water at the metal/coating interface. (24) The absence of detectable debonding for the coating system that contains rough steel surface implies that the effect of substrate roughness is important for maintaining good coating wet-adhesive strength. This study also suggests that the proposed scratch test is an effective method for evaluating steel surface treatment effect on coating wet-adhesive strength.

[FIGURE 9 OMITTED]

Coating thickness effect

To investigate the coating thickness effect, System II with coating thicknesses of 100, 200, and 300 [mu]m on a smooth steel substrate was immersed in water at 95[degrees]C for up to 2 weeks. The samples were then scratched and evaluated by the three detection methods mentioned above. The onset locations for debonding are marked for direct visual comparison, as shown in Figs. 10, 11, and 12.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

The onset location and critical normal load values for all the System II samples are reported in Table 1. It is evident that interfacial degradation is more severe for thin coatings and for longer hot water exposure time. For the sample with 100 [mu]m in coating thickness, the coating becomes extremely weak after 2 weeks of water immersion. Cohesive coating damage occurs so readily upon scratching that only in-layer coating damage is observed. One can also find that the STDV of the test results is considered acceptable (<9%), which suggests that the proposed coating scratch test can generate reliable, reproducible results for wet-adhesive strength evaluation.
Table 1: Onset location and critical normal load for debonding of
FBE coating

Immersion time (days)       DFT= 100 [mu]m
                       Location (mm)      Load (N)

0                      X                  X
1                      81.7 [+ or -] 0.6  77.8
2                      51.7 [+ or -] 3.1  49.6
3                      46.5 [+ or -] 1.4  44.7
4                      34.7 [+ or -] 2.1  33.7
7                      25.6 [+ or -] 2.3  25.1
14                     O                  O

Immersion time (days)      DFT = 200 [mu]m
                       Location (mm)      Load (N)

0                      X                  X
1                      X                  X
2                      79.0 [+ or -] 1.4  75.3
3                      69.5 [+ or -] 0.7  66.3
4                      63.5 [+ or -] 2.1  60.7
7                      61.5 [+ or -] 2.1  58.8
14                     56.5 [+ or -] 0.7  54.1

Immersion time (days)       DFT = 300 [mu]m
                       Location (mm)      Load (N)

0                      X                  X
1                      X                  X
2                      82.5 [+ or -] 0.7  78.6
3                      80.8 [+ or -] 1.8  76.9
4                      79.0 [+ or -] 2.8  75.3
7                      77.5 [+ or -] 3.6  73.9
14                     69.5 [+ or -] 0.7  66.3

X: no damage and O: in-layer cracking occurs first


It is interesting to note that the size of the delamination area and its onset location are quite different for coating samples with different thicknesses. A thicker coating slows the adhesion degradation rate early on. This is because a longer time is needed for sufficient water to diffuse to the coating/steel interface to cause damage. Furthermore, a thicker coating lowers the stress magnitude generated at the interface under the same applied scratch normal load level. Consequently, a higher scratching load is needed to achieve similar stress level at the interface to induce debonding for thicker coatings. Similar effects on immersion time and coating thickness have also been observed for System I.

Coating wet-adhesive strength determination by FEM

After experimentally determining the onset location of coating delamination and the critical debonding load level (Table 1), FEM is performed to obtain the corresponding wet-adhesive strengths under different immersion time periods. Here, only the coating material properties of System II are available, the FEM work focuses mostly on this system. The results with three different thicknesses immersed at 95[degrees]C are shown in Table 2. The calculated adhesive strength values and their trend due to extended water exposure time are shown. The degradation of wet-adhesion strength as a function of water exposure time is quite significant. The thicker coating docs slow the degradation rate in the early stage of immersion due to longer time requirement for water permeation to the interface. With longer immersion time, the water will eventually diffuse further into coaling and reaches the interface. The eventual degradation of interface will occur. The wet-adhesive strength after degradation tends to level off and shows similar values for all the coating samples after long exposure time.
Table 2: Degradation of wet-adhesive strength of System II with
various FBE coating thicknesses

Immersion time  Adhesive        Adhesive         Adhesive
(days)          strength (MPa)  strength (MPa)   strength (MPa)
                DFT= 100 [mu]m  DFT = 200 [mu]m  DFT = 300 [mu]m

0               X               X                X

1                           58                X                X

2                           41               47               50

3                           30               35               38

4                           18               21               23

7                            8               13               14

14                           O               10               11

X: no damage and O: in-layer cracking occurs first


While the wet-adhesive strength of the coating degrades severely upon high temperature water immersion, the mechanical properties of FBE coating itself can deteriorate significantly as well. Upon the scratch test, eventually the coating will fail either adhesively or cohesively, depending on the relative rate of adhesion strength degradation at the coating--steel interface and on the coating itself. For the coating with a thickness of 100 [mu]m, in-layer cohesive failure occurred first after 14 days of hot water immersion. Even though debonding was also observed at a higher scratch normal load, it was no longer feasible to reliably determine the adhesive strength of the coating once coating cohesive failure took place.

This study has clearly demonstrated the usefulness of the scratch tests for quantitative determination of the residual wet-adhesive strengths of coatings after their exposure to hot water. The calculated delamination stress values and their trends shown in Table 2 are quite plausible, suggesting the effectiveness of the proposed scratch test methodology for development of optimized coating formulation and substrate surface treatment for various industrial coating applications. The reason for such a good FEM finding is likely because of the brittle nature of the FBE coatings. Only a small amount of plastic deformation is observed during scratch. This allows for fairly accurate simulation of the scratch process using FEM. However, if the coating layer is ductile, then significant ductile deformation of the coating will take place and make the FEM analysis more complicated, which may result in the assessment of coating delamination stress less accurate. Furthermore, if the degradation at the interface due to water immersion is not uniform, which may be caused by uneven coating thicknesses or nonuniform substrate surface treatment, then the scratch lest results may exhibit significant data scattering. Therefore, care should be taken in sample preparation and in carrying out the proposed tests for the determination of wet-adhesive strength of coatings.

Conclusion

Based on the standardized scratch test method and FEM modeling, a new approach has been established to quantitatively determine the wet-adhesive strength of FBE coating on steel substrate. With the experimentally determined critical scratching load of coating debonding, the wet-adhesive strength can be calculated and determined via FEM. The influences of coating thickness and steel surface roughness on the degradation of coating adhesive strength have also been investigated. A rougher substrate surface leads to greatly enhanced wet-adhesive strength. A thicker coating provides a better resistance against interfacial adhesion degradation and lowers the debonding stress at the interface. This study has demonstrated that the proposed testing protocol is effective for evaluation of epoxy coating wet-adhesive strength and for development of optimized coating systems for industrial applications.

Acknowledgments The authors would like to acknowledge the financial support from the Department of Transportation (DTPH56-06-T-000022). Significant inputs, discussion, and experimental assistance from NOV Tuboscope, 3M, Dow Chemical, and ShawCor in this research endeavor are also greatly appreciated.

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H. Jiang, R, Browning, P. Liu, H.-J. Sue *

Department of Mechanical Engineering, Polymer Technology Center, Texas A&M University, College Station, TX 77843-3123, USA

e-mail: hjsue@tamu.edu

T. A. Chang

PolyLab LLC, Houston, TX 77042, USA

DOI 10.1007/s11998-010-9296-8

[C] ACA and OCCA 2010
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Author:Jiang, Han; Browning, Robert; Liu, Peng; Chang, T.A.; Sue, Hung-Jue
Publication:JCT Research
Date:Mar 1, 2011
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