2021 Volume 62 Issue 7 Pages 1001-1008
A four-year atmospheric exposure test was performed on steel with a Zn–30 mass%Al thermal-sprayed coating using an accelerated atmospheric exposure test, and then the atmospheric corrosion properties were evaluated. X-ray diffraction results showed that the corrosion products formed on the coating owing to the accelerated atmospheric exposure test were the same as those formed in a typical atmospheric exposure test. The corrosion weight loss of the thermally sprayed coatings in the accelerated atmospheric exposure test was promoted by approximately 1.5 to 2.0 times compared to the atmospheric exposure test.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 68 (2019) 187–193.
Fig. 12 Corrosion weight loss of (a) Zn–Al sprayed, (b) Hot-dip-Zinc coated steel, after 4 years using accelerated atmospheric exposure test.
Thermally sprayed coating for corrosion control is widely used as a surface modification coating to protect the surface of a steel structure from the atmospheric environment, especially for large steel structures that require long-term durability and objects having a complex shape, such as a pole-transformer. This is because it has various superior properties, including a wide range of materials for substrates and thermally sprayed coatings as well as their combinations, fast film formation rate, easy on-site application, and small environmental load as it is a dry process.1,2) A flame thermally-sprayed and an arc thermally-sprayed coatings have been frequently used for film formation processes of a thermally-sprayed coating for corrosion control, and Zn, Al, and their alloy, which can be a sacrificial anode, have been used for coating material.3–5)
Atmospheric exposure tests are effective for evaluating the long-term corrosion resistance of thermally-sprayed coatings in atmospheric environments. Long-term practical tests have been performed globally in various locations, and the superiority of the thermally sprayed steel has been confirmed.5–21) In studies conducted outside Japan, high long-term corrosion resistance was confirmed for Al, Zn–Al, and Al–Mg thermally-sprayed coatings with high Al content.18,19) In Japan, the high corrosion resistances of Zn, Zn–30 mass%Al, and Al thermally-sprayed coatings are shown in the long-term atmospheric exposure test conducted in a coastal area by Katayama et al.21) In addition, in the indoor accelerated exposure tests, in the case of salt spray tests (neutral salt spray test: JIS H8502), the formation of blisters and rust was not observed on the surface of the thermally-sprayed coating, even when the exposure time exceeded 10,000 h, and clear results have not been obtained.22–25) When combined cyclic corrosion tests (cyclic neutral salt spray test: JIS H8502) were applied, results similar to those observed under the actual environment, that is, the corrosion resistance according to film thickness was higher than the results obtained in salt spray tests; however, there are many issues need to be examined regarding the characteristics of the corrosion promotion.26) As aforementioned, the definite life span of thermally-sprayed coatings is not currently revealed, although their high corrosion resistance is shown.
In this study, the corrosion characteristics of thermally-sprayed Zn–30 mass%Al coatings were evaluated using an accelerated atmospheric exposure test. Here, the accelerated atmospheric exposure test is an accelerated exposure test in which artificial seawater is applied to the coating surface for general atmospheric exposure tests. In particular, the atmospheric corrosivity of thermally-sprayed steel under the accelerated atmospheric exposure test for four years at the exposure test site of the Department of Engineering, University of the Ryukyus, was evaluated by comparing it with a general atmospheric exposure test using corrosion weight loss and surface as well as cross-section coating analyses.
Test specimens were prepared using a thermally-sprayed steel sheet (150 mm × 70 mm × 2.3 mm), whose surface had been roughly formed by abrasive blasting using an arc thermal spraying method with pressurized air. The composition of the thermally sprayed coating was Zn–30 mass%Al; the film thickness was approximately 120 µm, and sealing treatment on the coating was not performed. In addition, hot-dip galvanized steel with a film thickness of approximately 100 µm was used as the comparison material. The back and edge faces of both specimens were painted for protection with anti-corrosion paint to ensure that the resulting exposed specimen was 59 mm × 115 mm. In addition, a line-shaped cut was applied to a part of the surface of the thermally-sprayed and hot-dip galvanized coatings.
2.2 Accelerated atmospheric exposure testThe accelerated atmospheric exposure test was performed on an exposure rack, whose exposed area was set up at a horizontal angle of 35° southward in the exposure test field at the Department of Engineering, University of the Ryukyus. The exposure test was conducted at four standards: one, two, three, and four years. In this study, the accelerated atmospheric exposure test was the atmospheric exposure test in which one cycle was assumed to be 24 h, and artificial seawater whose salt composition was close to that of natural seawater was applied to the surface of the specimens at each cycle. Artificial seawater was used for the metal corrosion test in accordance with ASTM D1141. By manually applying artificial seawater with a brush on the surface of the specimens every 24 h, the influence of chloride on the corrosion was promoted compared to the general atmospheric exposure test. The applied amount of artificial seawater was 2.3 × 10−3 L. In addition, a general atmospheric exposure test was conducted simultaneously.
2.3 Surface and cross-sectional analysis 2.3.1 Surface appearance and film thickness measurementSurface analysis was carried out with appearance observation and X-ray diffraction to identify the corrosion products using RINT ULTRA (Rigaku Corporation). The thicknesses of the coatings before and after the exposure tests were measured using a digital microscope VHZ100 (KEYENCE Corporation). Specimens for cross-section analysis were cut into 20 mm × 20 mm pieces, vertically buried in epoxy resin, and mirror-polished. The thickness was measured at 20 locations in total with 1 mm equal intervals, and the average value of these measurements was assumed to be the mean film thickness.
2.3.2 Corrosion weight loss measurementThe chemical removal method of the corrosion products of the Zn–Al thermally sprayed and the hot-dip galvanized coatings were based on JIS Z 2371, and the corrosion weight loss was measured after immersing the specimens in a 20% aqueous solution of chromic acid for 10 s at 80°C to clean them. This was repeated four times for one specimen to remove the corrosion products, and then the corrosion weight loss of each coating was calculated.
2.3.3 Scanning electron microscope images and EPMA analysisTo analyze the structure and constituent elements of the coating before and after the exposure test, the cross-section was examined through observation using a scanning electron microscope (SEM), and element distribution was analyzed with an electron probe micro-analyzer (EPMA) using JAX-8230 (JEOL Ltd.). The acceleration voltage was 15 kV, and the magnification was 300 times. The distributions of Zn (Lα), Al (Kα), O (Kα), Cl (Kα), and S (Kα) were examined.
Figure 1 shows the surface appearance of thermally-sprayed Zn–Al specimens exposed in the accelerated atmospheric exposure test and atmospheric exposure test for one, two, three, and four years. On the surface of the Zn–Al thermally-sprayed coating under the atmospheric exposure test, it was observed that white granular corrosion products were dotted on the central part of the specimen after one year of exposure period. Furthermore, the corrosion products increased and spread over the entire area of the specimen after two years. The White granular corrosion products after three years of exposure test were not significantly different from those after two years; however, the whole surface changed to white after four years. After a one-year exposure period in the accelerated atmospheric exposure test, the white granular corrosion products were dotted throughout the surface and occurred more than the atmospheric exposure test. After two years, the number of white granular corrosion products increased. The white granular corrosion products after three years of exposure test were not significantly different from those after two years; however, the whole surface changed to white after four years, and the granular corrosion products became inconspicuous. In all the specimens, it was observed that the corrosion that started from the cut part did not progress. Figure 2 shows the surface appearance of hot-dip galvanized specimens exposed in the accelerated atmospheric exposure tests and atmospheric exposure tests for one, two, three, and four years. On the surface of the hot-dip galvanized coating under the atmospheric exposure test, white minute corrosion products occurred after one year of exposure. Although the corrosion products were dotted on the Zn–Al thermally-sprayed coating, they were distributed over the whole coating on the hot-dip galvanized coating. After two years of exposure, the white minute corrosion products were distributed over the entire coating surface, and the surfaces of the specimens were whitened. As the exposure period increased, the number of white corrosion products increased. After one year of exposure in the accelerated atmospheric exposure test, the white minute corrosion products were distributed over the entire surface and occurred more than in the atmospheric exposure test. After two or more years of exposure, the white minute corrosion products increased with increasing exposure time. Note that the progression of corrosion starting from the cut part was not observed in any specimen. White minute corrosion products covered the surface of the cut part, and the occurrence of red rust was not observed.
Surface appearance of thermal sprayed Zn–Al specimens exposed using accelerated atmospheric exposure test for 4 years.
Surface appearance of Hot-dip-Zinc coated steel test specimens exposed using accelerated atmospheric exposure test for 4 years.
XRD spectra of Zn–Al thermally-sprayed and hot-dip galvanized coatings using accelerated atmospheric exposure tests and atmospheric exposure tests after four years are shown in Fig. 3. The results of the as-sprayed Zn–Al and as-hot-dip galvanized coatings before atmospheric exposure tests are shown for comparison. From the as-sprayed Zn–Al coating, diffraction peaks of Zn and Al were detected, indicating that Zn–Al compounds were not formed as the constituent elements of the thermally sprayed coating, and that a pseudo alloy constituted the thermally-sprayed coating. After the exposure test, diffraction peaks of Zn, Al, and Zn6Al2(OH)16CO3·4H2O (basic ZnAl carbonate) were observed in both the atmospheric exposure and accelerated atmospheric exposure tests.5) Zn, ZnO (zinc oxide), and 2ZnCO3·3Zn(OH)2·H2O (basic zinc carbonate) were confirmed in as-hot-dip galvanized coating. After the exposure test, in addition to the same diffraction peaks observed in the initial materials, Zn5(OH)8Cl2·H2O (basic zinc chloride) was detected as a corrosion product.
XRD spectrum of (a) Zn–Al sprayed, (b) Hot-dip-Zinc coated steel, after 4 years using accelerated atmospheric exposure tests.
As indicated above, the same diffraction peaks of the corrosion products were detected in both the general atmospheric exposure and accelerated atmospheric exposure tests. It is perceived that identical corrosion products with the general atmospheric exposure test were generated, even when artificial seawater was applied.
3.3 Film thickness measurement of thermally-sprayed coatingFigure 4 shows the SEM images of the cross-sectional specimens of the as-sprayed Zn–Al (a) and as-hot-dip galvanized coatings (b). The surface of the thermally sprayed coating was considerably rough, and many defects, such as voids, existed inside. Zn and Al formed a coating in a layered form as a pseudo alloy. The surface of the hot-dip galvanized coating was considerably smooth, and a dense coating with a few defects, such as voids inside the coating, is formed. Figure 5 shows the film thickness of thermally sprayed Zn–Al (a) and hot-dip galvanized coatings (b) after four years of accelerated atmospheric exposure and atmospheric corrosion exposure tests. The average film thickness of the as-sprayed Zn–Al coating was approximately 120 µm; however, the standard deviation of the thickness was large, because the coating surface was significantly rough. One year after the exposure tests, the film thickness was at the same level in both tests, mean film thickness was approximately 106 µm, and influence of artificial seawater on the corrosion promotion was not observed. After four years of the exposure tests, the thickness of the thermally-sprayed coating in the atmospheric exposure test was at the same level with that after one year of the exposure; however, the mean film thickness in the accelerated atmospheric exposure test decreased to approximately 96 µm. Mean film thickness of as-hot-dip galvanized coatings was approximately 99 µm, and the standard deviation was smaller than that of the thermally-sprayed coating, as the coating surface was smooth. The average film thickness after one year of the atmospheric exposure test was similar to that of the as-hot-dip galvanized coating, and no difference was observed. The corrosion was promoted after one year of the accelerated atmospheric exposure test with the artificial seawater application, and the mean film thickness decreased to approximately 79 µm. The thickness of the galvanized coating after four years of exposure in the atmospheric exposure test was at the same level as that after one year of exposure in the accelerated atmospheric exposure test. The thickness was approximately 52 µm after four years of accelerated atmospheric exposure test; thus, it decreased to approximately half of the film thickness of the as-hot-dip galvanized coating, showing a significant effect of artificial seawater. The aforementioned results were caused by the surface profile of each coating, and the corrosion products tended to deposit because the surface of the thermally sprayed coating was rough. However, because the surface of the galvanized coating was smoother than that of the thermally sprayed coating, its corrosion products slid down without deposition. Consequently, the anticorrosive effect of the protective coating decreased, and that the original hot-dip galvanized coating decreased.23)
SEM image for as-sprayed Zn–Al (a) and as-Hot-dip-Zinc coated steel (b).
Film thickness of (a) Zn–Al sprayed, (b) Hot-dip-Zinc coated steel, after 4 years using accelerated atmospheric exposure test.
Figure 6 shows the SEM image and EPMA elemental mapping for the cross-sectional specimen of the as-sprayed Zn–Al coating. Oxygen was detected inside the coating; however, Cl and S were not observed. In addition, a large amount of oxygen was detected at the border between the substrate and coating as well as in the pore region of the coating. This may be owing to oxides formed on the surface of the fine particles of Zn and Al during the thermally sprayed process.5) Figure 7 shows the SEM images and EPMA elemental mappings for the cross-sectional specimen of thermally-sprayed Zn–Al coating using atmospheric exposure test for three years. Layers of oxides of Zn and Al were formed near the outermost layer of the coating, and the distribution of O increased inside the coating compared to the as-sprayed Zn–Al coating. Furthermore, Cl and S were detected in the outermost layer of the coating, and they could not penetrate the coating. It is believed that these elements originated from airborne sea salt in the general atmospheric exposure test. In the SEM image, a large variation in the thickness of the thermally sprayed coating from that of the initial material was not observed. Figure 8 shows the SEM image and EPMA elemental mappings for the thermally sprayed Zn–Al coating using the accelerated atmospheric exposure test for three years. Layers of oxides of Zn and Al were similarly formed near the outermost layer of the coating, and the oxide layers inside the coating increased, compared with those of the atmospheric exposure test. In addition, the locations where Cl penetrated the coating by applying artificial seawater were observed. In the SEM image, a decrease in the film thickness was not observed in the accelerated atmospheric exposure test.
SEM image and EPMA elemental mapping for as-sprayed Zn–Al.
SEM image and EPMA elemental mapping for sprayed Zn–Al after 3 years using atmospheric exposure test.
SEM image and EPMA elemental mapping for sprayed Zn–Al after 3 years using accelerated atmospheric exposure test.
Figure 9 shows the SEM image and EPMA elemental mappings for the as-hot-dip galvanized coating. Oxygen was dotted inside the coating, and Cl and S were not detected. Figure 10 shows the SEM image and EPMA elemental mappings for the hot-dip galvanized coating using an atmospheric exposure test for three years. After three years of the atmospheric exposure test, the surface of the hot-dip galvanized coating became rough, and the coating changed into a two-layer structure, as the corrosion products were formed on the coating layer. The corrosion product layer was the oxide layer of Zn with a thickness of a dozen µm. Cl and S, which were not observed in the initial materials, were found inside the oxide layer, and the invasion into the original hot-dip galvanized coating was not observed. Figure 11 shows the SEM image and EPMA elemental mappings for the hot-dip galvanized coating using an accelerated atmospheric exposure test for three years. Similar to the atmospheric exposure test (Fig. 10), a two-layer structure of the coating and the oxide layer was observed. The dissolution reaction proceeded as a sacrificial anode action of zinc by applying artificial seawater, and a corrosion product of approximately 40 µm was formed. Cl and S were found in the oxide layer of Zn, and the invasion inside the original hot-dip galvanized coating was not observed, similar to the atmospheric exposure test.
SEM image and EPMA elemental mapping for as-Hot-dip-Zinc coated steel.
SEM image and EPMA elemental mapping for Hot-dip-Zinc coated steel after 3 years using atmospheric exposure test.
SEM image and EPMA elemental mapping for Hot-dip-Zinc coated steel after 3 years using accelerated atmospheric exposure test.
As aforementioned, in the thermally sprayed coating of Zn–Al, the formation of the oxide layer and the invasion of Cl and S to the vicinity of the outermost layer were detected in three years of both the atmospheric exposure and the accelerated atmospheric exposure tests; however, these elements had no influence on the corrosion of the coating. Moreover, no significant difference in the thickness of the thermally sprayed coating was observed. Conversely, in the case of the hot-dip galvanized coating, the oxide coating and the original hot-dip galvanized coating consist of a complete two-layer structure, and the original coating found in the initial materials decreased. In particular, in the case of the accelerated atmospheric exposure test, the oxide layers of Zn increased by applying artificial seawater, and there were locations where the original hot-dip galvanized coating layer decreased to less than half the film thickness of the initial coating.
3.5 Corrosion weight loss of thermally-sprayed coatingFigure 12 shows the corrosion weight loss of (a) Zn–Al thermally sprayed coating, (b) hot-dip galvanized coating using accelerated atmospheric exposure tests and atmospheric exposure tests for four years. For the thermally-sprayed coating of Zn–Al (a), the corrosion weight losses were 15.1 and 29.4 g/m2 without and with artificial seawater application, respectively, after one year of exposure period. This implies that it increased approximately by 2.0 times by applying artificial seawater. After two years of exposure test, the corrosion weight losses increased compared to those after one year; the corrosion weight losses were 37.4 and 60.0 g/m2 without and with artificial seawater application, respectively; hence, it increased approximately by 1.6 times using artificial seawater application. In addition, both corrosion weight losses were approximately 2.0 times in comparison with those after one year. After four years of exposure test, the corrosion weight losses were 94.5 and 140.1 g/m2 without and with artificial seawater application, respectively. Thus, the amount with artificial seawater application was approximately 1.5 times larger than that without artificial seawater application. Therefore, the accelerated atmospheric exposure test shows that the corrosion of the thermally-sprayed Zn–Al coating is promoted approximately from 1.5 to 2.0 times in comparison with the general atmospheric exposure test. However, the effect of artificial seawater application gradually decreased as the exposure period increased. For the Hot-dip galvanized coating (b), the corrosion weight losses were 47.0 and 59.7 g/m2 without and with artificial seawater application, respectively, after one year of exposure period. In comparison with the thermally sprayed Zn–Al coating for one year, it increased from approximately 1.5 to 2.0 times. In addition, the difference in the corrosion weight loss by artificial seawater application was smaller than that of the thermally sprayed Zn–Al, and it is considered that the corrosion acceleration effect of the artificial seawater was smaller than that of the thermally sprayed Zn–Al. The corrosion weight losses without artificial seawater application were 70.7 and 120.4 g/m2 after two and four years of exposure period, respectively. In this case, because the corrosion rate was rapid that the corrosion products could not be completely removed from the specimens after two years and four years of exposure with the artificial seawater application, the calculation of the corrosion weight loss with gravimetry was not possible. Thus, for the specimens after two years and four years of exposure with the artificial seawater application, the corrosion weight loss of the hot-dip galvanized coating was calculated using the wear loss of the film thickness in Fig. 5. The corrosion weight loss after one year of exposure with the artificial seawater application was 59.7 g/m2, which was 1.3 times higher than that without artificial seawater application. The corrosion weight loss rapidly increased as the exposure period extended, i.e., 244.0 g/m2 after two years and 335.5 g/m2 after four years of exposure. Those values were approximately 3.5 and 2.8 times more than those of without artificial seawater application, respectively; thus, the corrosion of the Hot-dip galvanized coating was promoted by applying artificial seawater. The plated coating has the characteristic that the surface of the coating was smooth and dense. Therefore, it is considered that the corrosion weight loss increased because the corrosion products were unable to function as protective coatings as they easily dropped.
Corrosion weight loss of (a) Zn–Al sprayed, (b) Hot-dip-Zinc coated steel, after 4 years using accelerated atmospheric exposure test.
Thus, an effect to accelerate corrosion by applying artificial seawater was confirmed. In both exposure tests after four years, as for the corrosion weight loss, the thermally sprayed Zn–Al coating showed higher corrosion resistance than the galvanized hot-dip galvanized coating.
3.6 SEM images and elemental mapping for scratch on substrateFigure 13 shows the SEM image and elemental mappings for scratches on thermally sprayed Zn–Al using accelerated atmospheric exposure tests for three years. The scratches reached the steel substrate. Zn eluted along the surface of the cut part more preferentially than Al and formed an oxide of Zn, indicating the sacrificial protection action of Zn. In the cut part of the hot-dip galvanized coating after three years of the accelerated atmospheric exposure test (Fig. 14), Cl invaded inside the coating, and the formed Zn oxide completely covered the cut part. In addition, in the specimens of both tests, corrosion progress from the edge of the specimen with the anticorrosion paint was not observed.
SEM image and EPMA elemental mapping for scratch on sprayed Zn–Al after 3 years using accelerated atmospheric exposure test.
SEM image and EPMA elemental mapping for scratch on Hot-dip-Zinc coated steel after 3 years using accelerated atmospheric exposure test.
A four-year atmospheric exposure test was conducted for steel with Zn–30 mass%Al thermally sprayed and hot-dip galvanized steels using an accelerated atmospheric exposure test. Atmospheric corrosion was evaluated by surface analysis, measurement of film thickness, EPMA surface analysis, and by measuring the weight loss of corrosion. The results of this study are presented below.
The authors would like to thank DAIHEN Corporation for preparing the specimens used in this study.
