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Marine Exposure Tests of Thermal Sprayed Coatings in Japan

Thermal Spray 2003: Advancing the Science & Applying the Technology, (Ed.) C. Moreau and B. Marple, Published by ASM International, Materials Park, Ohio, USA, 2003


Marine Exposure Tests of
Thermal Sprayed Coatings in Japan


S. Kuroda and J. Kawakita
National Institute for Materials Science, Tsukuba, Ibaraki, Japan

M. Takemoto
Aoyama Gakuin University, Tokyo, Japan

Thermal Spray Committee
Japan Association of Corrosion Control, Tokyo, Japan


Abstract


The thermal spray committee of the Japan Association of Corrosion Control (JACC) has been conducting a marine corrosion test of thermal sprayed Zn, Al and Zn-Al coatings since 1985. Twelve kinds of sprayed coating were deposited onto steel pipes by arc- and flame-spraying to varied thickness and subjected to various post-spray treatments. The samples were set vertically into seawater at a port 80 km south from Tokyo. Corrosion performance of these coatings has been inspected annually by recording the appearance and coatings' thickness at sea air-, splash- and tidal-zones. No significant changes were observed for five years exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, Al and Zn-Al coatings still exhibit superb corrosion performance. The test will continue till 2006 to complete the test period of 20 years. In order to place the test into a proper perspective, we also conducted a survey on the corrosion prevention by thermal spray technology in 2001, collecting more than 170 published reports and asking experts to contribute reviews on various aspects of the technology. This paper first describes several topics from the survey report to explain the past and the present situation of thermal spraying for corrosion prevention in Japan. Then, the corrosion performance of sprayed coatings in the exposure test during 15 years will be summarized.

Introduction

Japan is a country surrounded by oceans and hence corrosion prevention of coastal infrastructures such as bridges, pipelines, and highways as well as port facilities are of great importance. As the expected service life of these infrastructures is becoming longer, typically 50 years and sometimes even 100 years, total life cost has become a more widely accepted criterion for selecting the corrosion prevention measures for a structure. Environmental concern and regulations are becoming more important too. Thermal spray technology seems to be attracting more attention than ever, as a viable measure to provide long-term protection with capability to do on-site application and maintenance. Even though the technology has been around for many years and industrial standards such as JIS and ISO have been established, it is not always clear what type of coating should be (or should not be) adopted for certain application and how it performs over such long periods required today.
In the thermal spray committee of the Japan Association of Corrosion Control (JACC) founded in 1983, assessment of corrosion performance of sprayed coatings, based on the total life cost viewpoint, is one of continued activities. In order to evaluate the corrosion performance of thermal sprayed zinc and aluminum coatings, long-term on-site tests of well- fabricated samples are required. However, such tests are limited. Reports of the 12-year field-test by the Japanese Association for Steel pipe Piles [1] and 18-year field test by AWS [2] are well known. The corrosion performance is often differently evaluated in many reports, possibly due to thermal spray technology and test condition. The corrosion resistance of thermal sprayed coatings strongly depends on the spray procedures and technology, including pre- and post-spray treatments. Long term test is of importance since degradation observed at certain time often disappear during further exposure, exhibiting a kind of healing capability of a living organism. These indicate that test pieces prepared under precisely controlled procedures must be continuously evaluated by long term objective observation.
Based on these considerations and also the comments from the JACC members and thermal spray committee [ 3 ], the committee started a long-term marine exposure test of zinc, aluminum, and zinc-aluminum alloy coatings. Twelve kinds of coating were sprayed onto steel pipes at three factories with JIS (Japan Industrial Standards) authorization, and set to a test rig immersed into seawater at a port in Chiba prefecture on May 1985. A report after 10 years exposure has been published [4]. The test has been continued for 16 years at present and is planned to continue for 20 years.

Table 1: Selected applications of thermal sprayed coatings for the marine environment in Japan. A couple of examples in the
urban environment are included for their historical and industrial importance.

In order to compare the results of the exposure test with already published data around the world as well as to understand the current status of the technology, JACC conducted a survey on the corrosion prevention by thermal spraying in 2001. In this paper, several topics are featured from the survey report firstly. Then, the results of marine exposure test of thermal sprayed Zn, Al and Zn-Al coatings by JACC after 15 years are described in detail.


Recent survey by the JACC Thermal Spray Committee


In the fiscal year 2001, National Institute for Materials Science (NIMS) sponsored a survey by the thermal spray committee of JACC headed by Prof. M. Takemoto on the corrosion prevention by thermal spray technology. Apart from regular members of the committee, it asked several experts such as researchers at universities and government institutes, and representatives from companies related to the subject matter to join the team and formed a survey committee of 21 members. Its activity mainly consisted of 1) gathering published reports on the subject matter and 2) asking experts to write reviews on the following subjects: spray materials, substrate preparation, spray processes, post-spray treatment, coatings evaluation, field exposure test, and application history. In addition, a valuable report was written by Dr. S. Cramer from US DOE on the large-scale application of thermal sprayed anodes for protection of steel reinforced concrete bridges in the state of Oregon, USA. As the result of the survey, a 160-page report with a CD-ROM that contains data sheets of more than 170 papers was published [5]. Several highlights of the report are described below.

Fig.1: Kanmon bridge constructed in 1973. Its substructure is protected by thermal sprayed Zn with 6 layers of painting.

Application history of thermal sprayed coatings for corrosion prevention in Japan
The earliest application of thermal spray for corrosion prevention of steel structures can be found on the front gate bridge called "Niju-bashi" of the Imperial Palace in Tokyo constructed in 1963. The first long-span suspension bridge constructed in 1973, Kanmon bridge shown in Fig.1, connecting the mainland Honshu with the Kyushu district, was protected by thermal sprayed Zn with 6 layers of top-coat painting. Thermal spray has not been used for other large- scale bridges constructed thereafter except for small areas such as the inspection girder, however, due to high cost. The most recent large-scale application of thermal spray is an urban highway in Fukuoka. Approximately 500,000 m2 of steel structures supporting the highway will be thermal sprayed. According to the public corporation that owns it, longer maintenance cycle is essential as the cost of maintenance and repair is very high in such densely populated urban area with heavy traffic. Selected applications of sprayed coatings for corrosion protection are summarized in the chronological order in Table 1.

Observation of sprayed structures
Reports on a couple of structures after years of service are particularly meaningful. One such example is Suzu pedestrian bridge, located in Yamagata prefecture shown in Fig.2, constructed in 1977 [6]. It is a small pedestrian bridge crossing over a railway and located only 50m away from the seashore. Its specification for corrosion prevention is steel grit blasting, Zn sprayed to a thickness from 100 to 150 µm, and tar epoxy painted with brush twice. It was visited in 2001 after 24 years of service. A striking feature of the bridge is that whereas the surface of its superstructure was in a good condition, its sub- structure was more severely corroding. On the vertical surface in the substructure were found many spots of white rust but the steel substrate was essentially intact. On the horizontal surface of the beam flange, coating delamination and rusting of steel substrate were found. Cl content in the rust collected from the substructure was 10 to 17 mg/cm2 indicating that accumulation of sea salt has occurred because rinsing by rain does not happen there. Also, edges of the flange were more severely corroded because coating thickness tends to be less than required.

Fig.2: Suzu pedestrian bridge after 24 years of service. Notice that the vertical surface of its superstructure is essentially intact, considerable corrosion has taken place in the substructure, though not to the point of reducing the strength of the structure yet.


Haneda guiding lights
Guiding lights for airplanes landing to Haneda airport were constructed in 1993 over a bridge structure in Tokyo Bay as shown in Fig.3. There are two sets of such guiding lights for the B and C runway, which sum up to 917 m in length and are the largest application of thermal sprayed coatings for port facilities to date [7]. Since the steel pipe piles are protected by consumable anodes in seawater, aluminum was sprayed above –1 m from the low water lever (L.W.L.) The Al coating was 300 µm thick and sealed with a zinc rich paint. The bridge structure above was sprayed with Al of 120 µm thickness, which was sealed with epoxy resin (160 g/m2), and painted with polyurethane paint twice (210 g/m2). When inspected after 6 years from construction, corrosion of the steel pipes were well controlled though some biofouling and color change of the coating were recognized. On the bottom side of the bridge were found some exfoliation of paint and rusting from the steel substrate, especially at edges. Similar to the previous example, the lower structure of the bridge undergoes severer corrosion attack due to sea salt particles by splashing beneath and longer wetting time. Therefore, it was recommended to apply thicker coatings to the lower portion and edges. It was also pointed out that how sprayed metallic coatings behave in seawater when cathodic protection is used needs to be clarified.

Fig.3: Haneda guiding lights in Tokyo Bay. Steel pipe piles as well as the bridge like structure are protected with thermal sprayed aluminum coatings. The pipe piles are catholically protected in the submerged zone too.

Fukuoka Urban Expressway
Fukuoka is the largest city in the Kyushu district, populated by about 2 million people during daytime. Fukuoka urban expressway route No.5 is going to be 18 km long, and will be constructed in 4 terms. During the first term, most part of its steel substructure such as girders and piers of more than 200,000 m2 have been thermal sprayed. Though this is not in a marine environment, it is worth mentioning due to its scale and importance for the thermal spray industry in Japan. Based on the life cycle cost estimation over the expected service period of 100 years, thermal sprayed coating was considered to be advantageous over painting. Two types of protection technology have been competing in this construction. One is spraying of 85Zn-15Al alloy finished with colored inorganic sealer. It has been notified that organic painting over a sprayed metal coating is disadvantageous because the deterioration of paint determines the cycle and hence the cost of maintenance. They adopted a newly developed inorganic sealer mixed with color pigment [8], which is expected to seal and color the sprayed coating at the same time. The other is using a special surface reformer for surface preparation, spraying Zn and Al wires simultaneously to form the so-called pseudo-alloy coating, followed by sealing with colored sealer [ 9]. The reformer is a mixture of room temperature curable resin and ceramic particles and it is claimed to provide a proper adhesion with lighter surface treatment of steel substrate such as disc sanding or pickling before spraying it. Since this application has been attracting great attention of the owners and constructors of infrastructures, performance of these coatings may influence the future trend of corrosion prevention technology for steel structures in Japan significantly.
Marine exposure tests of protected steel pipe piles by a government project (1973-1987) [10, 11]
A large-scale marine exposure test of steel pipe piles protected by various measures was conducted by the Public Works Institute, which then belonged to the Ministry of Construction (is now an independent administrative institution), and the Japanese Association of Steel Pipe Piles from 1973 to 1987. Two test sites called Chiba and Ajigaura were chosen. As shown in Fig.4, Chiba site was located inside Tokyo Bay where waves are quiet but there is contamination of seawater. Ajigaura site is located at the sea coast of Ibaraki Prefecture facing the Pacific Ocean where waves are rough but the seawater is clean. Steel pipes were 508 mm diameter and 44 m length at Chiba and 609 mm diameter and 16 m length at Ajigaura, which were driven into the seabed. 24 pipes were set at the Chiba site and 18 pipes were set at the Ajigaura site. Total of 39 steel pipes were given protection coverings of 6 m in length at the splash-tidal zones. Protective measures included metal coverings, thermal sprayed coatings, organic linings and inorganic linings. Table 2 lists the coverings used at Chiba. Among these, thermal sprayed coatings all performed very poorly at both sites and ranked as the worst. In the lower tidal zone to the submerged zone, sprayed coatings were mostly consumed after 4 years and general corrosion of the steel substrate became predominant thereafter. Based on the results, guideline for corrosion prevention of steel structures in the splash and tidal zones was published [12], which had no mention of thermal sprayed coatings because of the poor result in the test. Also the results lead to a general view that thermal sprayed Zn and Al coatings dissolve in seawater in a short time and hence should not be used. In this test, however, the pipes were only partially covered in the splash and tidal zones with sprayed Zn and Al coatings and hence it is most likely that these coatings were consumed as sacrificial anodes for the bare steel pipes beneath the tidal zone to the seabed. By gathering reports in the past for the recent survey, we found that sprayed Al maintains its protection capability over 10 years in seawater whereas Zn dissolves rather quickly [2, 13, 14, 15, 16, 17, 18, 19].

Table 2: Various protection coverings tested at the Chiba site in the marine exposure test of steel pipe piles carried out by Public Works Research Institute and Japanese Association of Steel Pipe Piles from 1973 to 1987.

Marine exposure test of thermal sprayed Zn, Al and Zn-Al coatings by the JACC thermal spray committee:
A 15-year interim report


Samples for the test

Carbon steel pipes (JIS G3452, SGP-B100A, 114.3 mm dia. x 2,000 mm x 4.5 mm t); both ends were sealed with steel plates (JIS G3101, SS400) by shield metal arc welding (SMAW), were pneumatically blasted with alumina grit (fused alumina, WA #20-40, JIS H9300 (1977) "Recommended practice for zinc spraying on iron and steel", and JIS H9301 (1977) "Recommended practice for aluminum spraying on iron and steel"). Steel grit (mixture of G70 and G100 at 2:1 in mass) was also used for some aluminum spraying. Surface was finished as Sa21/2 of ISO850101. Thermal spray was applied within 1 hour after blasting. In order to investigate the effect of spraying processes on the corrosion performance, both the flame- (oxygen-acetylene flame) and DC arc-spraying of wire were utilized. Wires of 3.1 mm diameter were mostly used, however, 2.0 mm wire was used for Al arc spraying. Aluminum wire of 99.7 purity (JIS H2102), 99.9 purity zinc (JIS H2107), and 87mass%Zn-13%Al alloy on the market were used. Coating thickness was controlled at 175+-25 µm, according to such specifications as BS2569, AWSC2.2, JISH8300, and JISH8301 (1971). For aluminum, however, thick coating of 400 µm was also fabricated because it is specified by JISH8301, AS7 and often used for special cases. Arc and flame spraying were achieved by spray equipment from Metallisation Co. and an 11E of Sulzer Metco, respectively.

Table 3: Thermal spray and post-spray painting specifications.

Table 3 lists the specifications for thermal spraying, sealing and painting. Specimens No.1 to No.5 were tested at as- sprayed conditions. No.6 to No.9 were sealed with a cold- setting epoxy resin (Shell Co. Epicote 828 diluted with a thinner at 100:50). Sealant was applied to coating twice with a brush within 24 hours after spraying. Heavy duty painting was applied to specimen No.10 to No.12, according to the specification by the Honshu Shikoku Bridge Authority, by a specialized company. Specimens No.10 and 12 were painted with epoxy resin type paint (PE) for 100 µm thickness 3 times with a brush and then urethane resin type paint (UP) of 100 µm thickness. Washprimer was applied to No.10, while epoxy primer with calcium plumbate was applied to No.12. For No.11, a special urethane resin (MITSERON) was air sprayed to 2.6 to 3.5 mm thickness after 8 days from thermal spraying.

Fig.4: Appearance of the test rig and the location of the test site.

Spare samples were also fabricated for structure check, flattening test (JISG3452), and final comparison with the field exposed samples after the exposure test. The result of the flattening test has been already reported [4].

Table 4: Items of observation.

Location of the test site.
The field test has been conducted at a coastal site in Chikura-town in Chiba prefecture faced to the Pacific Ocean (N:34° 56 'and E:139 °58 '). Figure 4 shows the test rig supporting the sprayed pipes. Pipes were immersed vertically into seawater. The rig is situated behind a concrete wall of the fishery port, and not directly exposed to the wave of the outer ocean, but attacked and washed by high waves during typhoon and winter seasons. The highest and lowest temperatures at the test site are 30 and 2℃, respectively. Average solar radiation is 11 to 12 MJ/m2day, and annual rainfall is reported as 1800 to 2000 mm. The composition of sea salt particles is as follows: NaCl: 1.6-2.9 g/dm2day, SO4: 0.03 to 0.04 g/dm2day and NO3: 0.033 to 0.048 g/dm2day.

Table 5: Evaluation criteria for appearance.

Methods of investigation.
Observation and photographic recording together with coating thickness monitoring have been conducted every year since 1986. Changes of coating quality were examined in three zones, i.e., sea-air, splash and tidal zones. Coating thickness was measured on 24 points for each pipe. Before observation, seaweed and moss were removed by sponges and soft brushes and rinsed with seawater. The appearance was observed from sea- and landside, by taking the items as listed in Table 4 into account. Photos were taken from three angles at 120°step.

Results of the field test

Appearance
Change of surface appearance were carefully inspected by two professionals and recorded every year. Based on these data, degradation during 15 years exposure was classified into five grades as listed in Table 5. We understand, of course, that it is not straightforward to establish such judging criteria. For instance, Zn and Al coatings are thought to act as a sacrificial anode, then gradual loss of the coatings is normal but still functioning as long as the rusting of the base steel is prevented. On the other hand, degradation of sealant and rusting of the coating may be regarded as a kind of degradation in the protection capability as a long-term protection system.
Based on the observation up to 15 years, both the degradation of sealant and the rusting from surrounding (not from coating and substrate steel) are not included in the evaluation items since they are not directly related to the corrosion performance of the coatings. For the painted specimens (No.10 to No. 12), however, changes of color, degradation, swelling and cracking of the paint were evaluated. Such evaluation scheme may not be universally applicable to all the samples, but we have opinion that it represents general and qualitative trends.

Table 6: Appearance of each zone after 7, 10, 15 years of exposure.

Table 7: Summary of appearance inspection after 15 years. (H.W.L. stands for high water level.)

Table 6 lists the results of evaluation after 7, 10 and 15 years exposure. Table 7 summarized the appearance of the sprayed coatings. The results are summarized as the following.

Rank A
No.6: Al, wire arc sprayed and sealed No.7: Zn-Al, flame sprayed and sealed
No.11: Al, flame sprayed, primer treated and 3mm thick urethane resin sprayed

Remarks: Sealed Al and Zn-Al coatings are keeping almost perfect corrosion protection even in severe marine environment for 10 years.

Rank AB
No.2: Zn-Al, flame sprayed
No.4: Al, wire arc sprayed
No.5: Al, flame sprayed and sealed, 400 µm thick

Rank B

No.8: Al, flame sprayed and sealed

Rank C

No.10: Zn, flame sprayed, WP+PE+PU painted No.12: Al, wire arc sprayed, 400 µm thick,
CP+PE+PU painted

Rank E

No.1: Zn, flame sprayed
No.3: Al, flame sprayed, 175 µm thick
No.9: Zn, flame sprayed and sealed

Remarks: Red rust from the corrosion of the base steel first appeared after 7 years on the Zn sprayed specimen No.1 and after 6 years on No.9. The amount of rust increased continuously thereafter. This indicates that thermal sprayed zinc coatings are not durable for long term exposure in marine environment. On the Al flame sprayed specimen No.3, red rust started in the splash zone from a scratch made during transportation or setting of the pipes after 3 years of exposure. It has grown to a large bump of 150 mm by 2000 mm and 10 mm height with cracking, about a half of which was gone.

For No.1 and 9 samples, which exhibited heavy rusting, degradation was evaluated based on the ASTM-D610/SSPC- Vis 2 (10 rankings). Table 8 shows the results. The degradation progressed drastically after 9 years for No.1 and 7 years for No.9.

Other points to be noted are the following.

1)On the sample No.4 (Al, arc sprayed), white rust appeared after 7 years, but disappeared at the 8th year. At the 9th year inspection, partial exfoliation of coatings and spotty red rusts were observed, but the red rust was not observed at the 10th year inspection.
2)On the sample No.5 (Al, arc sprayed), white rust appeared after 8 years exposure and spread significantly in the 9th year, but seized to grow thereafter.

These results indicate that rusting once initiated may seize or disappear abruptly, and in some cases the damage may be healed. Such complicated behavior well demonstrates the need for a long-term continued evaluation of corrosion prevention schemes and the study of the related corrosion mechanisms.

Thickness change
The results of thickness measurements in the sea-air, splash and tidal zones are summarized as followings. The actual data will be included in the final report.
1) No.1 and No.9 samples exhibited significant change in the coatings' thickness. The thickness in the splash zone of sample No.1 on the sea and land sides as well as that of No.9 in the splash zone exceeded 500 µm, more than doubled the original coating thickness. It exceeded the range of the thickness meter at some locations.
2) Effects of sealing was noticeable for No.2 and No.7 Zn-Al alloy coatings. No.2 sample without sealing exhibited a moderate thickness increase in the sea-air and splash zones.

Table 8: Change of red rusting on No.1 and 9 specimens.


Such difference could not be detected between other pairs such as No.1 and No. 9 (Zn by flame spray), No.3 and 8 (Al by flame spray), and No.4 and 6 (Al by arc spray).
3)There observed no clear difference in thickness change between spraying methods, i.e., flame and arc spraying, as for the case between No.3 (flame sprayed) and No.4 (arc sprayed).
4)Effects of initial coating thickness could be detected in the
splash zone of No.4 (175 µm) and No.5 (400 µm). Here the both were Al arc sprayed coatings without sealing. The thickness variation during the first 5 years in the splash zone for No.4 was very little whereas that for No.5 varied as much as ±50 µm.

Conclusions

A survey about corrosion prevention by thermal spraying was carried out recently in Japan to better understand the current status of the technology. In this paper, major application history of thermal sprayed coatings in Japan and present situation of some of these structures were reported. Application of thermal sprayed coatings to steel structures was initially regarded as an undercoat for painting. Over the years, corrosion prevention schemes to more effectively utilize the potential of sprayed Zn and Al coatings have become more prevalent. Even though industrial standards are based on the results of many field exposure tests in the past, real structures are posing somewhat complicated situations, such as accumulation of sea salt in the substructure and premature degradation from edges.

In the latter part, 15-year marine field test of thermal sprayed steel pipes was summarized. Some of the aluminum and zinc- aluminum coatings still maintain their superb corrosion protection under such severe conditions after 15 years. The test was initially planned for 5 years but most coatings were in a good condition after 5 years. After 10 years, significant difference became apparent and the test period has been extended for 20 years. By comparing the results of the test with various tests carried out in the past around the world, Zn and Al coatings have performed generally as expected. Al coating has provided corrosion protection to the steel pipes in general. When it was damaged in the splash zone, however, red rust grew out because cathodic protection by the Al coating is not effective in the splash zone. The Zn-13%Al coatings with and without sealing have performed rather surprisingly well, the reason of which should be investigated in detail in the future.

Acknowledgement

Kansai Paint Corp. is gratefully acknowledged for the use of the test site. We are also grateful to the people of the Kotto- town, for their generous understanding and help through these years. This project has been supported by the members of the thermal spray committee of JACC in the past as well as the JACC personnel. The current members of the committee are listed below.
M. Takemoto (Aoyama Gakuin University), G. Ueno (KANMETA ENGINEERING Co., LTD), K. Ishikawa, Y. Ichiyanagi (TOKYO METALLIKON Co., LTD), R. Kobayashi (Dai-ichi High Frequency Co., LTD), T. Ojiro (MITSUI MINING & SMELTING Co., LTD), K. Uehara (ASAHI Co.), K. Takei (JAPAN LEAD ZINC DEVELOPMENT ASSOCIATION), M. Ohkuma (CHIYODA CORPORATION), S. Kuroda, J. Kawakita (National Institute for Materials Science), T. Katoh (Soken Tecnix Co., LTD), H. Saitoh (Japan Association of Corrosion Control).
The additional members who participated for the survey are listed below to gratefully acknowledge their contribution.
S. Uematsu (National Maritime Research Institute), M. Abe (Port and Airport Research Institute), M. Magome (Osaka Sangyo University), K. Katawaki (Japan Association of Structure Painting Contractors), S. Aoki (SANKO ELECTRONIC LABORATORY CO., LTD), K. Sonoya (IHI Co., LTD), Y. Harada (TOCALE Co., LTD).

References

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