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19 TABLE 9 Composite blister index immersion, except that immersion was intermittent. Test sam- Blister Dense Medium Medium Few ples exposed in this environment were immersed in natural Size Dense seawater for approximately 15 minutes followed by 75 min- 1 0.00 1.00 2.00 3.00 utes of exposure to a harsh marine environment. This cycle 2 0.35 1.65 2.60 3.78 was used to simulate the tidal action of natural waters, which 3 0.55 2.10 3.20 4.56 can accelerate the corrosion of structures with their wet-dry 4 0.75 2.50 3.80 5.33 5 0.90 3.00 4.40 6.11 cyclic actions. 6 1.10 3.70 5.00 6.89 This test was conducted in the same tank used for constant 7 1.60 4.60 6.25 7.67 immersion, with test samples placed just above this environ- 8 3.50 6.00 7.50 8.44 ment. An automated timer was used to cycle immersion and 9 4.80 8.00 8.75 9.22 atmospheric exposure in this zone only (constant immersion 10 10.0 10.0 10.0 10.0 samples were continually submerged in natural seawater). The presence of natural seawater in the lower half of this tank created an atmospheric environment similar to the environ- was placed into the test samples. This was performed to create ment that might be expected during naturally occurring peri- a known defect and measure the coating systems' ability to ods of low tide. resist additional corrosion damage at this location. This tech- Similar to the constant immersion samples, cyclic samples nique is commonly used in laboratory testing to accelerate the were periodically (nominally every 3 months) inspected for natural degradation of samples. In addition to linear scribes, deterioration. These samples were inspected for the same some samples also had circular holidays made through the deterioration as constant immersion samples using the test TSMC. These holidays were 1.5 in. (3.81 cm) in diameter and methods described in Table 8, above. were used to further stress the test samples. These relatively large holidays (compared with the linear scribes) were used to RESULTS OF THE LABORATORY TESTS evaluate the "throwing power" of the TSMCs applied (alu- minum and zinc). This large-diameter holiday increases the Sealer Tests anode-to-cathode surface area ratio, thus increasing the sacri- ficial protection requirements of the TSMC. These scribes rep- Sealers are usually specified to seal the pores in TSMCs to resent an order-of-magnitude increase in anode-to-cathode sur- improve coating performance. The U.S. Army Corps of face area of 48 to 0.18 square in. (scribed) to 48 to 1.8 square Engineers thermally sprayed coating guide lists vinyl, coal in. (circular holiday). Use of a larger-diameter holiday will tar epoxy, aluminum-pigmented epoxy mastic, tung-oil phe- emphasize performance differences between TSMCs. Fig- nolic aluminum, vinyl-butyral wash primer, aluminum sili- ure 11 shows examples of the intentional scribe and circular cone, and silicone alkyd. This test program tested three new holidays used (holidays and scribes highlighted). sealers along with two standard sealers and unsealed metal- lized samples (controls). The sealers were tested on ther- Constant Seawater Immersion. The constant seawater mally sprayed aluminum-coated steel panels and zinc-coated immersion test is indicative of a fully immersed environment steel panels. Unsealed metallized samples were also tested for metallized piles. Panels were immersed continuously and served as controls. Table 10 lists the sealers tested. Two except during evaluation periods. low-surface-energy, high-solids sealers were selected for testing because they are different formulations and because Alternate Wet-Dry Seawater Immersion. Alternate wet- they are both recommended by the Virginia DOT, according dry (or cyclic) seawater immersion was similar to constant to one of the applicators interviewed. Scribe (4 x 6 in.) Scribe (4 x 6-in. complex) Circular Holiday (4 x 6 in.) Figure 11. Representative intentional holidays (1 in. = 2.54 cm).
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20 TABLE 10 Sealers tested GENERIC TYPE SUPPLIER Zinc chromate vinyl wash primer (control material) Elite 1380 Epoxy (coal tar epoxy or equivalent) Devoe BarRust 235 Low-surface-energy, high-solids sealer Devoe 167 Pre-prime Low-surface-energy, high-solids sealer Carboline Rustbond Low-viscosity penetrating urethane Xymax Monolock PP The sealer coats were applied using air spray equipment. in this report. Cracking was observed on almost every sam- Application was performed in accordance with the manufac- ple that was evaluated using this test procedure. However, turers' recommendations for mixing and thinning. All systems disbondment was observed on only four of the five sealer were applied with a maximum target DFT of 1 mil (25.4 µm) coats over zinc (Xymax did not show any disbondment). No or as specified by the manufacturer (if less). The samples disbondment was observed on the sealed aluminum TSMC were scribed as described above. U-bend specimens. Adhesion Falling Weight Impact Tests Comparison of tensile adhesion strength with each of the Results from this testing showed that without a sealer coat, five sealers tested showed similar ranges as those observed for all TSMCs had failures at 12.5 ft-lb (16.9 N-m), which was aluminum and zinc TSMC samples applied over a 100-percent the lowest energy application possible with the test apparatus. grit-blasted substrate. However, the primary failure location The results of the drop weight impact tests showed that for the sealed samples varied from substrate to adhesive fail- when a sealer coat was applied all samples had no failure at ures. This differs from the failures of the unsealed TSMCs; 187.5 ft-lb (254.2 N-m) (highest energy application possible for unsealed samples, all primary failures occurred at the with the test apparatus). This was determined by visual obser- substrate. The change in failure location was observed to vation, where marring of the coating occurred, but no pene- be primarily related to sealer although some variations were tration of the TSMC to the steel substrate was recorded. observed between the aluminum and zinc TSMCs. Table 11 These results suggest that in high-impact or mechanical wear shows the primary failure location and average strength for areas, a sealer coat would improve abrasion- and impact- each sealer. resistance performance. The average tensile adhesion strengths fall within the range of values for unsealed 100-percent grit-blasted steel, which indicates that the use of these sealers does not reduce the Sealer Coverage and Penetration overall strength of the coating system. The change in failure location for several of the sealers from substrate (complete) Several methods were investigated to attempt to quantify to adhesive (used to attach aluminum pull stubs) or intra-coat sealer coverage and penetration. Successes and limitations (between sealer and TSMC) failures suggests that some ben- were encountered with each method, all of which are dis- efit is gained by using a sealer coat. In these cases, the adhe- cussed below. sion bond between the TSMC and substrate is greater than the reported value. Chemical Indicator Solutions. Sealer coverage and pene- Figure 12 shows the results of the U-bend adhesion tests tration can be determined by using chemical solutions that on the 100-percent grit-blasted sealed zinc panels. The results react with specific metal alloys to indicate the presence of of the bend tests on the abrasive mix variations are given later these metal alloys. The chemical solutions cannot be used TABLE 11 Tensile adhesion results for sealed TSMC samples Aluminum TSMC Zinc TSMC Sealer Strength, psi (MPa) Location Strength, psi (MPa) Location Elite 1380 1,575 (10.9) Substrate 1,126 (7.8) Substrate Devoe BarRust 235 1,507 (10.4) Adhesive 1,214 (8.4) Substrate Devoe 167 Pre-prime 2,167 (14.9) Adhesive 1,698 (11.7) Adhesive Carboline Rustbond 1,527 (10.5) Substrate 1,303 (9.0) Adhesive Xymax Monolock PP 1,330 (9.2) Intra-coat 1,276 (8.8) Intra-coat
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21 3.5 3 2.5 DISBONDMENT (mm) 2 1.5 NONE OBSERVED 1 0.5 0 ELITE 1380 DEVOE BAR DEVOE PRE-PRIME CARBOLINE XYMAX RUST 235 167 RUSTBOND MONOLOCK PP SEALER Figure 12. Average U-bend disbondment results for grit-blasted sealed zinc TSMC panels. to determine the thickness of the sealer. Reaction of the was easily observed. The reaction caused by the sodium chemical solution with the TSMC (aluminum or zinc) would hydroxide solution on the aluminum samples often obscured indicate inadequate or incomplete coverage by the sealer. coated areas. Chemical indicator solutions are easily employed Two chemical solutions were used to verify coverage of the and can be implemented in field evaluations, but the reaction aluminum and zinc TSMCs. These were sodium hydroxide of the sodium hydroxide solution on the aluminum TSMC (NaOH) and saturated copper sulfate (CuSO4) solutions. may prevent an accurate measurement of exposed metal. Sodium hydroxide is used to detect the presence of aluminum The data presented in Table 12 suggest that most sealers as evidenced by a foaming reaction. Copper sulfate is used to provided near complete coverage of the aluminum and zinc detect the presence of zinc, which changes color to black TSMCs, with the possible exception of Devoe 167 Pre- when copper sulfate is applied. prime and Carboline Rustbond on the aluminum TSMC. The These indicator solutions were applied to representative lower values are the result of the thin sealer coat (about 1 mil samples for each coating to determine if complete coverage [25.4 µm]) and a rougher aluminum TSMC surface, which, of of the TSMC was achieved. Solutions were applied using course, is the same situation encountered in the field. brush and roller techniques, and the samples were visually monitored for the above-described reactions. The percentage Metallographic Evaluation. Visual metallography was tested of the surface area not covered by the sealer was determined as a method to determine sealer thickness, coverage, and pen- on the basis of these chemical reactions. Table 12 shows a list etration on cut sections of test samples. Evaluation of these of the sealers and the estimated percentage of the surface area samples was difficult because the thin, lightly tinted sealers covered for each TSMC. were not easily discernable from the TSMC. A second eval- For zinc TSMC, area estimation was more readily per- uation of freshly cut samples was conducted with a thin black formed than with the aluminum TSMC samples. This was pri- coating applied on top of the sealer coats. This was used as a marily because the chemical reaction--a change in color-- contrasting color to distinguish between the TSMC and sealer TABLE 12 Percent surface area covered by sealers as shown by chemical indicator solutions Sealer Surface Area, % Surface Area, % Aluminum Zinc Elite 1380 99.0 95.0 Devoe BarRust 235 98.0 100 Devoe 167 Pre-prime 45.0 100 Carboline Rustbond 75.0 97.0 Xymax Monolock PP 99.5 100
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22 coats. This also was used to prevent the sealer coat from dis- appearing into the background as can often occur when per- forming metallography on thin coatings. Despite these efforts, the sealer coats could still not be readily viewed using visual microscopic techniques. Optically Stimulated Electron Emission Evaluation. Opti- cally stimulated electron emission (OSEE) was tested to quan- tify the coverage of the sealer coats over aluminum and zinc TSMCs. This technique uses photon emission technology to measure coating quality. An ultraviolet light source is focused on a specific area of a conductive, coated substrate. The reflec- tion and absorption of photons is measured as a current value (OSEE value) to determine coating quality. Figure 13 shows a picture of this test apparatus. OSEE is a comparative technique, which can be used to determine the general coverage of material or substrate. Con- Figure 13. OSEE test apparatus. ductive coatings (such as TSMCs) will have a higher value, while tinted coatings will have a lower value. Voids, thin spots, or other coating anomalies will allow for increased pho- graph. In general, the addition of a sealer reduced the OSEE ton transmission and thus result in values closer to an unsealed values by an order of magnitude. The average differences TSMC. This technique returns a dimensionless value, which is between the samples were minimal, and overlap of the con- used for comparison between sealer coats. The relative rank- fidence intervals suggests that these sealer coats have similar ing of sealers using this method was compared with other test electron emission characteristics. Compared with the indica- methods to determine if OSEE provides an accurate ranking of tor solution results, similar electron emission characteristics the sealers and which sealer provides optimal coverage. might be explained by the relatively high percentage of sur- Figure 14 shows the average OSEE measured values for face area coverage (demonstrated by this visual technique). the sealers applied over aluminum and zinc TSMCs. The While the OSEE method can detect whether a sealer is pres- 95-percent confidence interval for the data was also calcu- ent, further work is needed to determine if the method can be lated and found to be very close to the averages shown in the used to provide an estimation of sealer quality. 1,000 ZINC ALUMINUM AVERAGE OSEE VALUES 100 10 1 NONE ELITE BAR RUST PRE-PRIME CARBOLINE XYMAX SEALER Figure 14. Average OSEE measured values for the sealers applied over aluminum and zinc TSMCs.
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23 Visual Evaluation. Visual evaluations were performed with and the 12 months of exposure time does not afford enough the unaided eye and under 5x magnification to determine over- time for a meaningful comparison of sealer materials. The all sealer quality and coverage. During this inspection, the plan is to keep all of the corrosion test panels in testing long coatings were observed for apparent voids, cracking, check- enough for significant differences to be observed. ing, blistering, color, and uniformity. Variations in general appearance and coating quality were recorded. Visual evaluation techniques were not well suited for Abrasive Mix Effects determining sealer coverage, especially when lightly tinted Adhesion Tests sealers were used. Here, the sealer coat was difficult to dis- tinguish from the rough TSMC surface. Although in most Figure 15 presents the surface profile measurements made cases the presence of the sealer coat could be verified, com- using Testex replica tape on the samples prepared by CSI plete coverage was difficult to determine. Coatings using different abrasive mixes (see Table 4). Figure 15 presents the average of the profile data and the confidence interval. The confidence interval was estimated using the Stu- Corrosion Tests Comparing Sealers dent t distribution, where the confidence interval is given by the following equation: The results from constant immersion testing after 12 months suggest that the presence of a sealer coat is beneficial for t × StdDev reducing corrosion and blistering. The overall corrosion rat- Confidence Interval = Mean ± n ing for unsealed zinc TSMC was 8, and the rating for unsealed aluminum TSMC was 9. Overall corrosion ratings for sealed where aluminum and zinc TSMCs were 9 to10. Composite blister ratings for unsealed zinc and aluminum TSMCs were 6.9 and Mean = statistical mean of the data points, 10, respectively. Composite blister ratings for the sealed StdDev = Standard Deviation of the data points, TSMCs remained at 10 for both TSMCs except for Carboline t = the Student's t variable at a 95-percent Rustbond, for which the rating was 7.5. Zinc TSMC showed confidence and 3 degrees of freedom, and increased cutback with all but the Elite 1380 sealer. The n = number of data points. amount of cutback ranged from about 0.03 in. (0.76 mm) to 0.12 in. (3 mm). Aluminum TSMC was not observed to have Other graphs in this report showing a confidence interval the same cutback issues and performed well both sealed and were generated in the same fashion. The results show that the unsealed. For zinc TSMC, the optimum sealer for immersion 100-percent grit and 67-percent grit mixtures produced deeper service appears to be a conversion coating. Note that chro- profiles than did the 100-percent shot or 70-percent shot mix- mate conversion coatings use hexavalent chromium, which ture. The average profile produced on the 33-/67-percent is regulated as a hazardous waste product. Its use may not be shot/grit mix is deeper than the profile produced on the possible in all areas. Alternative conversion coatings are a 100-percent grit, but there is overlap in the data. suggested alternative, but their performance in these envi- Figure 16 provides the tensile adhesion data for zinc and ronments is untested. Aluminum appears unaffected by the aluminum TSMCs on the panels prepared with different abra- use of a sealer. sive mixes. The results show clearly that there is an increase Following 12 months of exposure in a cyclic immersion in adhesion strength going from the shot blast profile to environment, none of the sealed samples experienced any 100-percent grit blast profile. The results also show that the significant deterioration. Corrosion ratings for both zinc and abrasive mixtures result in decreased adhesion strength, but aluminum TSMCs were an average of 9 (out of a possible 10) they do not show whether this will reduce the effectiveness or higher for samples (including unsealed controls), and of the coating. composite blister ratings were greater than 9.5 for all sealers The U-bend adhesion test specimens showed cracking on (the control zinc average was below 5.5). Similarly, none of almost every sample that was evaluated using this test pro- the samples experienced any cutback from the intentional cedure. However, disbondment was only observed on spe- scribe. In total, the results suggest that the presence of a cific samples. Disbondment occurred on aluminum and zinc sealer coat may be beneficial over zinc TSMC by reducing TSMCs prepared with 100-percent shot and zinc TSMC pre- blistering in cyclic immersion service. The presence of a pared with grit/shot mixtures. No disbondment was observed sealer coat did not appear to be beneficial or detrimental on aluminum or zinc TSMCs applied to 100-percent grit- when applied over aluminum TSMC. prepared coupons. Figure 17 shows a plot of average dis- The results after 12 months of exposure indicate that the bondment length for each surface preparation method. This performance of aluminum TSMC may not be improved by figure demonstrates that zinc TSMC is more susceptible to using a sealer, but that the performance of zinc TSMC might disbondment on surfaces prepared with less angular abra- be improved. Differences were relatively small, however, sives than 100-percent grit.
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24 4 UPPER CONF. LIMIT LOWER CONF. LIMIT AVERAGE 3.5 PROFILE, MILS 3 2.5 2 100% SHOT 100% GRIT 33% SHOT/67% GRIT 70% SHOT/30% GRIT MEDIA Figure 15. Testex replica tape profile ranges (panels prepared by CSI). 2,000 ZINC ALUMINUM 1,800 1,600 UPPER CONF. LIMIT LOWER CONF. LIMIT 1,400 AVERAGE ADHESION, PSI 1,200 1,000 800 600 400 200 0 100% 100% Grit 33% Shot/ 70% Shot/ 100% 100% Grit 33% Shot/ 70% Shot/ Shot 67% Grit 30% Grit Shot 67% Grit 30% Grit SURFACE PREPARATION Figure 16. Tensile adhesion of zinc TSMC versus abrasive mix (panels prepared by CSI, 1 psi = 6.89 KPa).
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25 25 ZINC ALUMINUM 20 DISBONDMENT (mm) 15 NONE DETECTED 10 5 0 100% SHOT 100% GRIT 33% SHOT/67% GRIT 70% SHOT/30% GRIT SURFACE PREPARAT ION Figure 17. Average U-bend disbondment results for unsealed TSMC panels. Profile Measurements RQ, and Figure 19 shows the values of RPC on the A36 steel panels prepared by CSI Coatings. Similar measurements were As described in the section on procedures, surface profile performed on the A36 and Grade 50 samples prepared at the characteristics were measured with a profilometer for the val- Ocean City lab. The values of RA, RY, RQ, and RZ for the A36 ues of RA, RY, RZ and RQ (see Table 7 for the definitions of and Grade 50 samples on the CSI-prepared panels all have these values). Figure 18 shows the values of RA, RY, RZ and significant overlaps in the confidence bands. No distinction 4 UP P E R CONF. LIM IT RA LOW E R CONF. LIM IT A V E RA GE 3 RA, RY, RZ and RQ, mils RQ 2 1 RY RZ 0 GRIT GRIT GRIT GRIT SHOT SHOT SHOT SHOT 33/67 70/30 33/67 70/30 33/67 70/30 33/67 70/30 Figure 18. Average values and 95-percent confidence ranges for RA, RY, RZ and RQ profilometer data on A36 panels prepared with different abrasive mixes (1 mil = 25.4 µm).
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26 130 250 CSI OC - A36 120 200 RPC, peaks per inch RPC, peaks per inch 110 150 100 100 90 UPPER CONF. LIMIT UPPER CONF. LIMIT 50 LOWER CONF. LIMIT 80 LOWER CONF. LIMIT AVERAGE OC - A50 AVERAGE 0 70 GRIT GRIT GRIT SHOT SHOT SHOT 33/67 70/30 33/67 70/30 33/67 70/30 SHOT GRIT 33/67 70/30 Figure 19. Average values and 95-percent confidence ranges for RPC profilometer data on A36 panels prepared Figure 21. RPC versus abrasive mix and applicator with different abrasive mixes (1 in. = 2.54 cm). (1 in. = 2.54 cm). can be made between the abrasive mixes except that the shot- sive equipment, abrasive source, profilometer instrument used, blasted panels have significantly lower values than the grit- and steel, so which variable(s) are responsible for the differ- blasted panels. The panels prepared by the Ocean City labo- ences in RPC and RQ cannot be ascertained from the existing ratory exhibited the same characteristics. data. Comparison of the adhesion strength values along with For all profilometer data except peak count, definite their corresponding confidence intervals indicates that the increases in the RA, RY, RQ and RZ values are seen for abra- adhesion strengths are higher for the panels with higher RPC sive containing more angular grit. Figure 20 shows the graphs values and lower RQ values. This holds true for the grit-blasted of RQ versus abrasive mix and applicator, and Figure 21 shows aluminum and 70-/30-percent shot/grit and 33-/67-percent the similar graphs of RPC. Peak count on the panels prepared shot/grit zinc TSMC panels, but not the grit-blasted zinc or at CSI, as seen in Figure 21, showed significant overlap shot-blasted panels. Figure 22 shows the tensile adhesion val- ues versus abrasive mix and applicator for A36 steel. between the abrasive mixes used, and no definite distinction The importance of peak count on coating performance has between mixes could be made. been emphasized by several authors (3336). Generally, a Interestingly, the values of RPC are generally higher and higher peak count results in higher adhesion (as found in this the values of RQ are lower on the panels prepared at Ocean study) as long as the coating can wet the prepared surface. The City than are the values of RPC and RQ on panels prepared by increase in adhesion strength occurs because the peaks and CSI. Average RPC for the coated grit-blasted panels prepared valleys cause the disbondment forces to change from tension at CSI is 113 peaks/in., and the average peak counts for the to shear. However, if the coating bridges the valleys rather panels prepared at Ocean City are 173 and 176 peaks/in. for A36 and Grade 50 steel, respectively. Variables were abra- 3,500 Zn OC Al OC UPPER CONF. LIMIT 1.2 3,000 LOWER CONF. LIMIT UPPER CONF. LIMIT AVERAGE 1 LOWER CONF. LIMIT 2,500 AVERAGE Zn CSI Al CSI Adhesion, psi 0.8 2,000 1,500 RQ 0.6 1,000 0.4 500 0.2 A36 STEEL CSI APPL. OC - A36 OC - A50 0 33/67 70/30 33/67 70/30 33/67 70/30 33/67 70/30 Grit Grit Grit Grit Shot Shot Shot Shot 0 GRIT GRIT GRIT SHOT SHOT SHOT 33/67 70/30 33/67 70/30 33/67 70/30 Figure 22. Tensile adhesion for aluminum and zinc Figure 20. Values of RQ versus abrasive mix and TSMCs on A36 steel versus abrasive mix and applicator applicator. (1 psi = 6.89 KPa).
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27 than wetting the surface, the adhesion could be worse with The zinc TSMC in constant immersion tests had a corrosion a high-peak density than with a low-peak density. Data in rating of 8 for the grit and shot/grit mixtures (no shot abrasive the literature show that finer abrasive size (shot or grit) pro- was tested for this coating). The blister ratings were 6.9 for duces higher peak counts (33), but optimum peak counts for grit, 10 for 70-/30-percent shot/grit, and 3 for 33-/67-percent TSMCs have not been found in the literature. The labora- shot/grit after 12 months. There was no cutback observed for tory data for this study seem to suggest that a peak density the grit-blasted panel, 0.19 in. (4.8 mm) for the 70-/30-percent of about 175 peaks/in. increases adhesion; however, the data shot/grit mixture, and 0.38 in. (9.7 mm) for the 33-/67-percent are far from consistent, and the confidence bands in both the shot/grit mixture. RPC data and adhesion data are much larger for the samples These tests show that at 12 months of exposure, the alu- prepared in Ocean City. RPC might have value as a field mea- minum TSMC is insensitive to the abrasive mixes used in surement for predicting TSMC adhesion performance. Addi- these tests. On the other hand, the zinc TSMC performs better tional work is required to develop this concept and determine when a 100-percent grit or high-content grit mixture is used. optimum values of RPC for TSMCs. All thermal spray guides and specifications call for the sur- face profile to be "angular," but do not define acceptable angularity limits or methods of measuring angularity. Angu- Effects of Application Parameters on larity is defined not only by the number of peaks per unit area Metallurgical Characteristics and Performance but also by the rapidity, or sharpness, of how peaks and val- Metallography leys change shape. Part of this measurement can be obtained through the use of surface profilometers. RPC and RQ values Most TSMC guides and specifications for wire-arc spray have shown promise in this study as indicators of good call for the distance between the tip of the gun to be within "angularity." Work reported recently by the U.S Army Corps 6 to 8 in. (15 to 20 cm) and the angle of the gun to the work sur- of Engineers indicates that this profilometer data may not be face to be 90 degrees (optimum) and 45 degrees (maximum). adequate (37). Angularity can be quantified using scanning The extremes of distance and deposition angle (45 degrees) are electron microscopy, but this is hardly a field-friendly method. situations that are expected to be encountered when coating It might be that the best measure of angularity is indirect-- difficult-to-reach surfaces such as the inside flange surfaces characterizing the abrasive used. A chart that compares the of H-piles. In order to test whether the extremes of these roundness of abrasive grains can be found in a 1994 article ranges are detrimental to TSMC performance, tests were con- by Hansink (38). According to the results presented in the ducted to evaluate porosity, oxide content, adhesion, and cor- U.S. Army Corps of Engineers study mentioned above, very rosion performance. Table 13 shows the application parame- angular, angular, and subangular shapes produced similar ters tested. adhesion strengths (37). Table 14 shows the results of the metallographic exami- nation. Some studies (10, 39, 40) imply that greater spray Corrosion Tests Comparing Abrasive Mixes distances and more acute angles of incidence between the metal spray and surface lead to more porosity and oxides in The aluminum TSMC in cyclic immersion tests appeared the coating. In our studies, the pore size in the zinc and zinc/ to be tolerant of the use of various shot/grit mixture ratios in aluminum TSMC samples was slightly smaller at the 12-in. the constant immersion test. The corrosion and blister ratings (30.5-cm) distance than at the 8-in. (20.3-cm) distance. The were 10 for all abrasive mixtures tested, and no cutback was pore size for the zinc TSMC was larger at the 45-degree observed. In the constant immersion tests, the corrosion and application angle than at the 90-degree application angle. blister ratings were 9, and there was no cutback for all the The porosity distribution was a larger percentage of the coat- abrasive mixes tested. The aluminum TSMC appears to be ing volume at the 8-in. (20.3-cm) distance. The degree of insensitive to the abrasive mixture, at least up to 12 months interconnection between pores was also larger at the 8-in. of exposure. (20.3-cm) distance. The zinc TSMC in cyclic immersion tests had a corrosion rating of 9 for all abrasive mixes tested. Blistering of the zinc TABLE 13 Test protocol for application parameter study TSMC was observed for all shot/grit mixtures, and visual cut- back was observed for all mixtures except the 70-/30-percent Application Wire Deposition Alloy Standoff Rate Diameter Angle shot/grit. The 100-percent shot mixture was the most sus- Aluminum 8 in. 45° ceptible to attack, with a composite blister rating of 2 and an 20 lbs/hr 1/8 in. (Al)* 12 in. 90° average cutback of 1 in. (25.4 cm). The 70-/30-percent shot/ 8 in. 45° Zinc (Zn)* 80 lbs/hr 1/8 in. grit and 100-percent grit mixtures were the best performers, 12 in. 90° having composite blister ratings of 4.6 and 5.3, respectively, 8 in. 45° 85 Zn15 Al 60 lbs/hr 1/8 in. 12 in. 90° and average cutback measurements of 0 and 0.25 in. (6.5 mm), respectively. * Commercial purity wire; 1 cm = 2.54 in., 453.5 g = 1 lb.
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28 TABLE 14 Effects of application parameters on TSMC metallurgy ALLOY PARAMETER POROSITY OXIDES SUBSTRATE 45/90 degrees Max size Distribution Geometry Degree of Max size Geometry PHASES INTERFACE 8 in. or 12 in. in. % interconnection % in. Trapped Debond spacing Grit Zinc 45 deg 8 in. 0.004 20 irregular moderate ND N/A N/A 1 ND minor 0.002 20 irregular moderate 5 0.002 irregular 1 ND major 90 deg 8 in. 0.001 5 rounded minor ND N/A N/A 1 ND moderate 0.002 <5 irregular moderate <5 0.002 irregular 1 ND moderate 45 deg 12 in. 0.001 <5 irregular minor ND N/A N/A 1 ND minor 0.003 6 rounded minor <5 0.002 irregular 1 ND major 90 deg 12 in. 0.001 <5 rounded minor ND N/A N/A 1 ND minor 0.0003 <5 rounded minor ND N/A N/A 1 ND minor Aluminum 45 deg 8 in. 0.002 7 rounded minor ND N/A N/A 1 ND minor 0.002 5 rounded moderate <5 0.007 elongated 1 ND minor 90 deg 8 in. 0.001 5 rounded minor ND N/A N/A 1 ND minor 0.001 <5 rounded minor 30 0.006 irregular 1 ND minor 45 deg 12 in. 0.001 <5 rounded minor 10 0.005 irregular 1 ND moderate 0.001 <5 irregular minor 5 0.002 irregular 1 ND minor 90 deg 12 in. 0.001 <5 irregular minor 30 0.007 irregular 1 ND moderate 0.003 7 rounded minor ND N/A N/A 1 ND minor Zn/Al 45 deg 8 in. 0.001 <5 irregular minor <5 0.007 irregular 2 ND moderate 0.003 5 irregular moderate ND N/A N/A 2 ND major 90 deg 8 in. 0.003 6 irregular moderate ND N/A N/A 2 ND moderate 0.002 <5 rounded minor ND N/A N/A 2 ND major 45 deg 12 in. 0.001 <5 rounded minor ND N/A N/A 2 ND major 0.001 <5 rounded minor <5 0.007 elongated 2 ND minor 90 deg 12 in. 0.001 <5 rounded minor ND N/A N/A 2 ND minor 0.002 <5 rounded minor ND N/A N/A 2 ND minor NOTE: 1 in. = 2.54 cm, ND = not detected, N/A = not applicable, deg = degrees, Zn/Al = zinc/aluminum. Adhesion cation angle and gun-to-surface distance within the parame- ters tested. Figure 23 shows the average tensile adhesion strength Recent research by the U.S. Army Corps of Engineers values measured for each of the application parameters. concluded that distance affected the variation in porosity and The adhesion of aluminum is seen to decrease at the 12-in. oxide levels of zinc and zinc/aluminum, but none of the param- (30.5-cm) distance from the value at 8 in. (20.3 cm), but is eters affected the variation in oxide and porosity of alu- not affected by the angle (45 or 90 degrees). The adhesion minum (39). That report recommended the following opti- of zinc and zinc/aluminum appear to be unaffected by appli- mum angles and distances: Angle 2,500 TSMC (degrees) Distance UPPER CONF. LIMIT LOWER CONF. LIMIT Zinc 90 6 in. (15.2 cm) 2,000 AVERAGE Aluminum 90 611 in. (15.227.9 cm) ADHESION, PSI Zinc/Aluminum 90 610 in. (15.225.4 cm) 1,500 1,000 Corrosion Tests Comparing Application Parameters 500 ZINC ALUMINUM ZINC/ALUM. The aluminum TSMC in constant immersion displayed a 0 corrosion rating and composite blister rating of 10 for all of the 45- 90- 45- 90- 45- 90- 45- 90- 45- 90- 45- 90- application parameter variables. No cutback was observed on deg, deg, deg, deg, deg, deg, deg, deg, deg, deg, deg, deg, 8-in 8-in 12-in 12-in 8-in 8-in 12-in 12-in 8-in 8-in 12-in 12-in any of the test panels in this test. In the cyclic immersion APPLICATION PARAMETER tests, the aluminum had an overall corrosion rating of 9, a blister rating of 10, and displayed no measurable cutback for Figure 23. Average adhesion strengths and confidence all of the application parameters. limits for the application parameters tested (1 psi = The zinc coating in the constant immersion tests displayed 6.89 KPa). corrosion ratings of 9 at 45 degrees, 8 in. (20.3 cm); 8 at
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29 90 degrees, 8 in. (20.3 cm); 7 at 45 degrees, 12 in. (30.5 cm); 3.5 and 8.5 at 90 degrees, 12 in. (30.5 cm). The composite blister A36 rating was 10 for all application parameters except 45 degrees, 3 A50 12 in. (30.5 cm), where it was 6.5. The cutback was 0.19 in. 2.5 PROFILE, MILS (4.8 mm) at both 45 degrees, 8 in. (20.3 cm), and at 90 degrees, 12 in. (30.5 cm), and it was 0.38 in. (9.7 mm) at 45 degrees, 2 12 in. (30.5 cm). The zinc TSMC in the cyclic immersion tests displayed an overall corrosion rating of 10 for all of 1.5 the application parameters. The blister rating was 10 for the 1 90-degree angle at both spacings, but fell to 4.6 at 45 degrees, 8 in. (20.3 cm), and 5.2 at 45 degrees, 12 in. (30.5 cm). No 0.5 measurable cutback was observed at any of the application parameters. 0 The zinc/aluminum TSMC in the constant immersion tests Shot Grit 70/30 33/67 had an overall corrosion rating of 10 for all application param- SURFACE PREPARATION METHOD eters tested, and no cutback was observed. In the cyclic Figure 24. Surface profile of A36 and A50 steel immersion tests, the zinc/aluminum coating displayed an over- all corrosion rating of 10 for all parameters. The composite samples using Testex tape (1 mil = 25.4 µm). blister rating was 5.2 at 45 degrees, 8 in. (20.3 cm); 4.6 at 90 degrees, 8 in. (20.3 cm), and 45 degrees, 12 in. (30.5 cm), and 3.9 at 90 degrees, 12 in. (30.5 cm). There was no mea- Corrosion Tests Comparing Steel Hardness surable cutback at any of the application parameters. The results at 12 months of exposure time suggest that the In the constant immersion tests, the overall corrosion ratings aluminum and zinc/aluminum TSMCs are insensitive to the of both alloys were similar, differing by less that 1 unit for all application parameters tested, but that the zinc TSMC is sen- of the abrasive mixes used. The blister ratings were 6.8 for sitive to shallow angles from gun to work surface. A36 steel, 8.7 for Grade 50 steel over a 100-percent shot/ prepared surface, and 10 for both steel alloys on 100-percent grit-prepared surfaces. For a 70-/30-percent shot/grit-prepared Effect of Carbon Steel Hardness surface, the blister ratings were 10 for A36 steel and 8.4 for Grade 50 steel. Cutback was less than 0.05 in. (1.3 mm) for The effects of small differences in hardness caused by dif- all panels except the Grade 50 on a 100-percent grit surface, ferent steel alloys were addressed by measuring the surface where it was 0.11 in. (2.8 mm). profile and adhesion of TSMCs on ASTM A36 and ASTM In the cyclic immersion tests, the overall corrosion ratings A572 Grade 50 steel panels abrasive blasted in the same man- were within 1 unit for both alloys and abrasive mixes, except ner. The tensile strength of ASTM A36 steel can range from 58 to 80 ksi (400 to 550 MPa), and the tensile strength of the 70-/30-percent shot/grit mixture, where the A36 steel had ASTM A572 Grade 50 steel is specified as 65 ksi (450 MPa). Because this means that there can be overlap in the hardness between the two materials, we measured the hardness of the 4,000 samples and found a Rockwell B (RB) hardness of 90.8 for UPPER CONF. LIMIT 3,500 the A36 material and 75 for the Grade 50 material. Figure 24 LOWER CONF. LIMIT shows the surface profiles of the A36 and Grade 50 steel sam- 3,000 AVERAGE ADHESION, PSI ples as measured with Testex tape. The profiles appear to be 2,500 the same for both alloys. 2,000 1,500 Adhesion 1,000 Figure 25 shows the adhesion strength of aluminum and 500 zinc TSMCs on grit-blasted A36 and Grade 50 steel panels. 0 The adhesion of the zinc TSMC was essentially the same on ZINC ZINC ALUMINUM ALUMINUM the A36 and Grade 50 panels. There is a considerable varia- A36 A50 A36 A50 tion in the aluminum adhesion strength; however, this wide COATING - STEEL ALLOY variation was observed on both aluminum and zinc TSMC panels. No discernable difference was observed in the adhe- Figure 25. Tensile adhesion of zinc and aluminum sion of the zinc or aluminum TSMCs on A36 and Grade 50 TSMCs on A36 and A50 grit-blasted steel panels samples prepared by grit, shot, or the mixtures tested. (1 psi = 6.89 KPa).
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30 a rating of 9.6, and the Grade 50 steel had a rating of 8.3. The Galvanic Behavior composite blister ratings were all within 1 unit for each steel alloy and abrasive mix tested. The shot-prepared panels had To determine the overall ability of aluminum and zinc to pro- the lowest blister ratings, 7.7 (Grade 50) and 7.9 (A36). The vide sacrificial protection to Grades A36 and A588 steels, test- other panels had blister ratings between 9 and 10. The cut- ing was conducted to measure current flow versus time in sev- backs for the shot-prepared surface were 0.06 in. (1.5 mm) eral aqueous environments. Sacrificial protection is provided for A36 and 0.36 in. (9.1 mm) for Grade 50. The cutbacks by the TSMC to the steel substrate onto which it is applied. for the grit-prepared surfaces were 0 for A36 and 0.02 in. Intimate contact between the TSMC and the steel occurs dur- (0.51 mm) for Grade 50. The cutbacks for the 70-/30-percent ing the application process, providing an electrical pathway for shot/grit-prepared surfaces were 0.2 in. (0.51 mm) for A36 sacrificial protection, but it is impossible to measure the pro- and 0.03 in. (0.76 mm) for Grade 50. The cutbacks for the tection current being provided to the substrate. However, 33-/67-percent shot/grit-prepared surfaces were 0 for A36 by simulating such an exposure using temporary connections between a TSMC sample and a bare steel panel, a measure of and 0.02 in. (0.51 mm) for Grade 50. the protective current provided by the TSMC was performed. The corrosion test data indicate that, with the exception of A panel coated with aluminum TSMC and a panel coated cutback at defects in cyclic immersion on shot-prepared sur- with zinc TSMC, each with equal areas exposed in an elec- faces, the substrate hardness does not affect the performance trochemical test apparatus, were used for testing. In tests, the of the TSMC. coated panel was coupled to freshly blasted Grade 36 and Grade 588 steel panels through individual wire connections. Provisions were made to periodically monitor current flow HSLA Steel versus Carbon Steel between the TSMC panel and bare panels to determine if sac- rificial protection was being provided. Visual evaluations HSLA steels, such as ASTM A588, are designed to have were also conducted to verify the mitigation of steel substrate better strength and atmospheric corrosion resistance than corrosion mitigated and the presence of TSMC corrosion. conventional carbon steel. The corrosion resistance of HSLA Figure 26 shows a sketch of this test setup. steels is due to small quantities of alloying elements such as By their very nature, aluminum and zinc TSMCs are not chromium, nickel, manganese, copper, and molybdenum in uniform and contain voids, pores, and other areas where elec- the steel. Weathering steels, a type of HSLA steel, are some- trical contact to the substrate and/or surrounding aluminum or times used as steel pilings because of their strength and cor- zinc particles may be compromised. As such, the electrical rosion resistance. The low alloy content of the steel might current measurements are subject to variation, and time-based make it slightly nobler than carbon steel (41). This testing activation of the TSMC may occur unlike the behavior of an was designed to determine what effect, if any, that has on the aluminum or zinc anode connected to a steel structure. How- performance of TSMCs allied to the HSLA steel. ever, these measurements can be indicative of the presence NOTE: A = ammeter, V = voltmeter, SCE = saturated calomel electrode. Figure 26. Electrochemical sacrificial protection test cell (1 in. = 2.54 cm).
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31 and relative level of sacrificial protection being provided. ginally reduced for A588 steel compared with A36 steel, Figures 27 through 30 show plots of current versus time for suggesting that this more corrosion-resistant alloy requires aluminum and zinc TSMCs to A36 and A588 steel, using less sacrificial protection. electrolytes of 25, 500, 1,000, and 5,000 ohm-cm. Current flow is also shown to be inversely proportional to Sacrificial current flow monitoring has demonstrated that solution resistivity (i.e., low resistivity = higher current flow). after 2 to 5 days in tests, a measurable sacrificial current is This is as expected, because a higher resistivity electrolyte is generated by both TSMC materials to protect the A36 and typically considered less corrosive (without the presence of A588 bare steel panels. Current flow to these samples is mar- other factors). 3.00E-03 25 ohm-cm 500 ohm-cm 1,000 ohm-cm 2.00E-03 5,000 ohm-cm Current (amperes) 1.00E-03 0.00E+00 -1.00E-03 -2.00E-03 0 2 4 6 8 10 12 Elapsed Time (days) Figure 27. Galvanic current, zinc TSMC versus A36 steel. 3.00E-03 25 ohm-cm 500 ohm-cm 2.00E-03 1,000 ohm-cm 5,000 ohm-cm Current (amperes) 1.00E-03 0.00E+00 -1.00E-03 -2.00E-03 0 2 4 6 8 10 12 Elapsed Time (days) Figure 28. Galvanic current, zinc TSMC versus A588 steel.
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32 2.00E-03 25 ohm-cm 500 ohm-cm 1,000 ohm-cm Current (amperes) 1.00E-03 5,000 ohm-cm 0.00E+00 -1.00E-03 -2.00E-03 -3.00E-03 0 2 4 6 8 10 12 Elapsed Time (days) Figure 29. Galvanic current, aluminum TSMC versus A36 steel. In addition to electrical measurements, periodic visual eval- Long-Term Marine Atmospheric Exposure uations were made without disrupting exposure. After approx- Analysis imately 1 week of exposure, samples in seawater and lower- conductivity electrolytes began to show calcareous deposits on In a past Federal Highway Administration project the bare steel surfaces. Calcareous deposits are a by-product of ("Environmentally Acceptable Materials for the Corrosion cathodic protection. The formation of calcareous deposits Protection of Steel Bridges," Contract DTFH61-92-C- occurs at the cathode of a protected substrate (by either sac- 00091), Corrpro evaluated TSMC for corrosion protection rificial or impressed current cathodic protection). The forma- of steel at its Sea Isle City, New Jersey, marine atmos- tion of these deposits suggests that some degree of cathodic pheric exposure facilities. Although this project is com- protection is being provided. pleted, the panels have been continued in testing. The site 3.00E-03 25 ohm-cm 500 ohm-cm 1,000 ohm-cm 2.00E-03 5,000 ohm-cm Current (amperes) 1.00E-03 0.00E+00 -1.00E-03 -2.00E-03 0 2 4 6 8 10 12 Elapsed Time (days) Figure 30. Galvanic current, aluminum TSMC versus A588 steel.
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33 is located approximately 100 yards from the Atlantic of the coating was measured, which is due to corrosion Ocean to the east and bordered by Ludlum's Bay to the products. However, with the exception of the aluminum west. In 1989, several panels of A36 and A588 steel with TSMC samples, no substrate corrosion was observed on aluminum, zinc, and zinc/aluminum (8515 wt%) ther- these samples. Some substrate corrosion (red rust) occurred mally sprayed coatings were exposed at this test site. The on two aluminum TSMC A36 panels (without sealer) at a panels were prepared by grit blasting with G16 steel grit to scribe and at the edges of the welded channels indicating an SSPC-SP-10 finish and 4-mil (102-µm) profile. The possible inadequate galvanic protection ability at exposed TSMCs were applied by the flame spray wire technique. A substrate defects. Localized corrosion (red rust) was also vinyl chloride/vinylidene chloride copolymer was used to found on the back of two aluminum TSMC A588 panels seal half the panels. These samples have been exposed to a (without sealer), which is due to a crevice between the panel harsh, natural marine environment for a period of 13 years. and wood support. The sealers appeared to be eroded on the Evaluation of these samples is beneficial in determining aluminum panels, but they are generally intact on the zinc the long-term atmospheric performance of a TSMC over and zinc/aluminum panels. low-alloy and weathering steel. The conclusion to these tests is that the TSMCs perform The general condition of the TSMC samples did show similarly on both carbon steel and HSLA steel substrates. minimal signs of deterioration. Panels coated with organic coatings (epoxy powder, electrostatic spray polyester, electrostatic spray epoxy powder with polyurethane, and Edge Geometry Effects acrylic topcoats), and also exposed for 13 years, showed severe deterioration. Figure 31 shows a photo of all TSMC Sheared, saw-cut, and flame-cut edges are common and liquid-coating systems exposed as part of this previous occurrences in sheet piles. Edge retention analysis was per- program. formed on cut sections of the complex panels prepared for Thickness measurements were taken on each of the zinc, corrosion testing (see Figure 5). Prior to the TSMC applica- aluminum, and zinc/aluminum panels. Some corrosion of tion, the edges of these samples were altered using a bench-top the TSMC had occurred and was evidenced by the white shop grinder. Some edges were chamfered (approximately corrosion products on the panel surface. A slight thickening 45 degrees), some were rounded (approximately semi- circular), and some were made flat (approximately 90 degrees from panel face). The chamfered and rounded edges simulate the "relief on an edge" used to promote coating adhesion and coverage. The flat edge provides a "sharp" edge, which has historically shown worse performance. These different edges were used to determine if the TSMCs are susceptible to edge retention issues, like liquid systems, and if sharp edges reduce corro- sion protection. Edge retention/characterization was performed by micro- scopic analysis. Sections of untested panels were cut, expos- ing the cross section of one of the four edges. This section was then mounted in an epoxy resin and polished using suc- cessively finer abrasives to evaluate TSMC thickness and microstructure. Visual metallography was used to examine these samples and compare them with flat sections of the same panel, with the following observations. Figure 31. TSMC and paint systems, 13 years marine Aluminum TSMC atmospheric exposure. (Panels shown in Columns 1, 3, and 5 are not sealed. Panels in Columns 1 and 2 have a zinc The use of a relieved edge (i.e., rounded or chamfered) TSMC. Panels in Columns 3 and 4 have a zinc/aluminum appeared to promote adhesion of the TSMC as evidenced TSMC, and panels in Columns 5 and 6 have an aluminum by more uniform coverage. Figure 32 shows aluminum TSMC. Panels in Columns 2, 4, and 6 are sealed. Panels in TSMC applied to a chamfered edge. The aluminum TSMC Columns 7, 8, and 9 are powder-spray coated. Panels in adheres well along the chamfer and provides complete Columns 10 to 14 are coated with organic materials. coverage. However, at the end of the chamfer (performed Panels in Rows A and B have a Grade 36 steel substrate. on approximately 1/2 the width of the edge) no TSMC is Panels in Rows C and D have a Grade A588 substrate.) present.
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34 Figure 33 shows similar results for aluminum TSMC However, at the end of the chamfer a notable thinning was applied to a rounded edge, although to a lesser degree. The observed. The end of the chamfer can create a sharp corner, semi-circular treatment here covers most of the edge width as this is where the relieved and flat edges meet. The and the TSMC appears to adhere more readily to this surface. observed reduction in the zinc TSMC is consistent with the The exception is where the edge and face of the panel meet, observations on the chamfered edge of the aluminum where a reduction in thickness is observed. Figure 34 shows TSMC. the ground flat edge for the aluminum TSMC panels. This Figure 36 shows the rounded edge for the zinc TSMC figure shows that at sharp corners there is notable thinning of panels. This figure shows that the zinc TSMC has no visible the aluminum TSMC. The TSMC was also observed to have thinning or voids when applied on this type of edge. This was thin spots and voids along the flat edge. also the best edge (with respect to TSMC integrity and reten- tion) for aluminum. Figure 37 shows the flat edge for the zinc TSMC panels. Zinc TSMC This figure shows that although the zinc TSMC could be applied consistently, there was a gap between the substrate On a relieved (chamfered) edge, the zinc TSMC was and the thermally sprayed coating. The clean edge with which observed to have a consistent thickness (see Figure 35). this coating disbonded, combined with the lack of visible Aluminum on Chamfer No Aluminum at End of Chamfer Figure 32. Aluminum TSMC photomicrographs, chamfered edge. Aluminum on Rounded Thin/Porous Aluminum at End Figure 33. Aluminum TSMC photomicrographs, rounded edge.
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35 Thinning Aluminum at Sharp Corner Incomplete Coverage of Flat Edge Figure 34. Aluminum TSMC photomicrographs, flat edge. Zinc on Chamfer Thinning Zinc and End of Chamfer Figure 35. Zinc TSMC photomicrographs, chamfered edge. Zinc on Rounded Edge Zinc at Apex of Rounded Edge Figure 36. Zinc TSMC photomicrographs, rounded edge.
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36 contamination at the interface, suggests that the zinc TSMC Surface Contamination has poor adhesion over this type of edge. This disbondment probably occurred during application. Surface contamination can decrease the performance of a On the basis of the conditions observed for both TSMC coating system, liquid, or TSMC. However, some coatings materials, the optimum edge for coating application would are more tolerant of contamination, and their performance is be achieved by rounding. A chamfered edge can help not as significantly reduced. The U.S. Navy recommends a improve coating retention, but the edges can still be affected chloride contamination limit of 3 µg/cm2, and both higher and by thinning and/or lack of coating adhesion as observed lower limits are found elsewhere for immersion service (42). above. The U.S. Army Corps of Engineers guide for thermal spray coating recommends a level of chloride contamination less than 7 µg/cm2. Experience suggests that performance degra- Corrosion Tests Comparing Edge Geometry dation generally begins at chloride levels above 5 µg/cm2 and significantly affects coating performance at 10 µg/cm2. No differences in corrosion performance caused by edge Some panels were purposefully contaminated using a geometry were observed after 12 months of testing. sodium chloride solution to achieve chloride levels of 5 and 10 µg/cm2 to determine if the aluminum and zinc TSMCs are significantly affected. To contaminate the surface of the test Coating Defects samples, A36 steel panels that had been abrasive blasted using 100-percent steel grit were immersed in a sodium chlo- Several samples (with and without chloride contamina- ride solution made using deionized water. To verify the con- tion) received a 3.8-cm- (1.5-in.-) diameter intentional holi- tamination level, chloride measurements were made using day prior to testing. These were used to test the "throwing the Bresle method. This method uses a latex rubber patch, power" of the TSMC materials. Following 6 months of con- which is adhesively backed for application to a steel sub- stant immersion exposure, minimal deterioration of these strate. During this test, an extraction fluid is injected into the TSMC materials has been observed. Some corrosion of the area exposed to the steel substrate and massaged to dissolve holiday area has occurred on most samples, suggesting that the available chloride ions into solution. The extraction fluid such large defects are not fully protected by the exposed is then removed, and a titration is performed to determine the TSMC surface area. presence of chloride ions. The 5-µg/cm2 panels had actual Similar to the constant immersion samples above, mini- chloride levels of 5 to 7 µg/cm2 and the 10-µg/cm2 panels had mal deterioration of the TSMC on the cyclic immersion actual chloride levels of 9 to 11 µg/cm2. samples has been observed. Some corrosion of the holiday area has occurred on most samples, suggesting that such Adhesion large defects are not fully protected by the exposed TSMC surface area under conditions of cyclic immersion and Figure 38 shows the results of the tensile adhesion tests drying. on the contaminated panels compared with the measure- Zinc on Flat Edge, TSMC and Substrate Zinc on Edge, TSMC Only Figure 37. Zinc TSMC photomicrographs, flat edge.
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37 ments on uncontaminated panels. The data indicate that the level. The ratings at the 5-µg/cm2 level were 8.2 with a holiday adhesion of the zinc TSMC is insensitive to chloride con- and 7.3 without a holiday. The ratings at 10 µg/cm2 were 8 with tamination at the levels tested. On the other hand, the alu- a holiday and 6.7 without a holiday. The composite blister rat- minum TSMC shows a definite decrease in adhesion val- ings were independent of contamination levels, having a value ues at both levels of contamination tested. of 10 across the board. Negligible cutback was observed on all panels. In cyclic immersion tests, the aluminum TSMC had corrosion ratings of 9.3 at 5 µg/cm2 with a holiday, 7.7 at Corrosion Tests Comparing Surface 5 µg/cm2 without a holiday, and 9 at 10 µg/cm2 with or without Contamination a holiday. The composite blister ratings for aluminum TSMC The zinc TSMC in constant immersion tests exhibited a were 10 at 5 µg/cm2 with a holiday, 8.7 at 5 µg/cm2 without a lower corrosion rating at the 10-µg/cm2 chloride level than at holiday, 8.6 at 10 µg/cm2 with a holiday, and 10 at 10 µg/cm2 the 5-µg/cm2 level. The ratings at 5-µg/cm2 of chloride were without a holiday. No significant cutback was observed under 9.6 with a holiday and 9.2 without a holiday. The ratings at any condition in this particular test. 10 µg/cm2 were 8 without a holiday and 7.7 with a holiday. In general, the zinc TSMC appeared to perform better The composite blister ratings were actually higher for the than the aluminum coating with regard to overall corrosion higher level of contamination: the values at 5 µg/cm2 were rating, but worse than the aluminum with respect to blister 6.9 with a holiday and 5.1 without a holiday. At 10 µg/cm2, rating. The performance of the TSMC materials in constant the ratings were 8.7 with a holiday and 8.4 without a holiday. immersion varied depending on the metric being evalu- The corrosion ratings were independent of contamination ated. The presence of holidays in the coating did not appear levels, having a value of 10 across the board. In cyclic to affect performance, with holiday samples often having immersion tests, the zinc TSMC had a corrosion rating of 10 improved performance compared with the scribed samples. across the board. The composite blister ratings were 7.2 at Negligible cutback was observed for all test samples. 5 µg/cm2 with a holiday, 5.2 at 5 µg/cm2 without a holiday, Overall, the aluminum TSMC appears to be more tolerant 4.8 at 10 µg/cm2 with a holiday, and 4.5 at 10 µg/cm2 with- of surface contamination. In general, application of a out a holiday. No significant cutback was observed under any TSMC over a contaminated substrate should be avoided. condition in this particular test. Figures 39 and 40 show typical panels (5 µg/cm2 chloride) The aluminum TSMC in constant immersion tests did not with circular holidays and scribes after 6 months of expo- exhibit a trend in the corrosion ratings with contamination sure to cyclic seawater immersion. 2,500 UPPER CONF. LIMIT 2,000 LOWER CONF. LIMIT AVERAGE ADHESION, PSI 1,500 1,000 500 ALUMINUM ZINC 0 NONE 5 10 NONE 5 10 2 CHLORIDE CONTAMINATION LEVEL, µg/cm Figure 38. Average tensile adhesion and confidence limits for zinc and aluminum TSMCs on chloride-contaminated substrates (1 psi = 6.89 KPa).
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38 Figure 39. Cyclic (alternate) immersion panels with circular holidays after 6 months in test (zinc TSMC on left and aluminum TSMC on right). Figure 40. Cyclic (alternate) immersion panels with scribes after 6 months test (zinc TSMC on left and aluminum TSMC on right).