Suggestion of a New Repair Technique for Steel Structures by Low-Pressure Cold Spray and Laser Cleaning

Abstract

The effectiveness of a corrosion repair technique consisting of laser cleaning and cold spraying was investigated. The effect of laser pulse frequency on the removal of surface corrosion on steel specimens was analyzed. Subsequently, a zinc coating was cold-sprayed on specimens cleaned of surface corrosion using conventional disc grinder and laser methods. Furthermore, salt spray tests were conducted to compare the corrosion protection performance of the zinc coating on these specimens. The results showed that laser cleaning can effectively remove surface corrosion and that a laser pulse frequency of 15 kHz is more effective than that of 40 kHz for removing the surface oxide layer. A comparison of cold-sprayed zinc coatings on laser-cleaned and non-treated specimens indicated that surface oxidation during laser treatment may negatively affect zinc deposition efficiency. The zinc coating on the laser-cleaned specimen was more than twice as thick as that on the conventionally cleaned specimen, and the coating–specimen interface maintained good adhesion after a 168 h salt spray test. Although no corrosion was observed in both steel specimens after the salt spray test, cracking of the remaining corroded areas on the substrate and delamination of the coating occurred in the conventionally cleaned specimen.

 

This Paper was Originally Published in Japanese in J. Jpn. Thermal Spray Soc. 59 (2022) 137–143.

1. Introduction

Corrosion is one of the essential issues for structures near the sea, and many studies have been conducted to understand the corrosion process and protect structures.^1–4) Large-scale steel structures near the sea are coated with heavy-duty anticorrosion coatings⁵⁾ with high corrosion resistance, and epoxy resin coatings are the mainstream. However, one of the problems with utilizing heavy-duty anticorrosion coating for repairs is the long waiting time due to the need for multiple overcoating.^5,6) The coating process consists of four steps: surface preparation, primer paint, intermediate paint, and top paint. In this process, the next paint operation can be applied after the undercoat has dried. In addition, the surface preparation also requires time due to the use of a paint stripper for the old paint removal.

Removing most of the old paint is generally performed through the cleaning process with machine tools or sand blasting method.⁶⁾ This cleaning removes rust and corroded paints on the substrate surface, a necessary process affecting the paint’s adhesion and durability. The cleaning process is classified into four categories, depending on the method and degree of removal. For large structures, mechanical grinding using sandblasting (1st category) or disk grinders (2nd category) is commonly used. However, these cleaning processes often result in significant emissions of industrial waste such as dust, and it is necessary to take precautions such as covering the work area with dust-preventing mesh.⁷⁾ The current repair process using heavy-duty anticorrosion coating, including such temporary work, incurs significant time and economic costs, and efforts to improve the efficiency of the repair process are needed. Efficiency in the repair process can reduce costs and improve workers’ safety, mainly when working on dangerous, high-elevation structures such as coastal steel towers.

Against this background, laser cleaning, a non-contact surface preparation method, has been attracting attention for its ability to improve the efficiency of the cleaning process.^8,9) This method irradiates a laser beam onto the oxide film and old paint to remove them by ablation. Compared to conventional cleaning work, this method generates less industrial waste and can simplify the temporary construction process. Another advantage of laser cleaning is that it can work on complex structures as long as the laser is focused on the area. In contrast, conventional methods are difficult to apply hollows and complex, intricate structures. Furthermore, by adjusting the pulse frequency of the laser, the energy density per unit pulse can be changed, which allows for the appropriate treatment of the substrate material. To date, Ogami et al.⁹⁾ have studied the applicability of laser cleaning by evaluating the effect of laser pulse frequency on rust and salt removal performance on steel structures.

In addition, Shimozato et al.^10,11) have reported an attempt to replace the corrosion protection process by conventional coating methods with a high-speed coating technique using a low-pressure cold spray (CS) method. In this method, metal particles are accelerated to subsonic or supersonic speeds by compressed gas and thus impact the substrate in a solid state. The low-pressure CS method, which allows on-site repair and no need to wait for the undercoat to dry, is expected to shorten the repair process. The combination of laser surface preparation and CS coating can significantly shorten the conventional coating process.

The combination of laser and CS has been studied in basic research, focusing on a simultaneous process in which CS is performed immediately after laser treatment.^12–16) According to Bray et al., utilizing laser cleaning to heat the substrate directly before cold spraying resulted in successful coating of titanium particles below the melting point on various substrates. They reported that the deposition efficiency was higher than that of CS alone, and a dense oxide-free film was obtained.¹²⁾ Barton et al. successfully deposited AISI steel, which is commonly used in various steel structures, on a similar substrate with higher deposition efficiency than the conventional CS method by heating the substrate immediately before deposition using laser cleaning, and also reported the effect of surface temperature on the deposition mechanism.¹³⁾ As described above, it has been reported that laser effectively improves deposition efficiency and coating quality. In contrast, it is necessary to select appropriate laser conditions to avoid issues such as oxidation of the treated surface.^13,14)

Although the combination of laser cleaning and CS has the potential to serve as an alternative to current repair technology using heavy-duty anticorrosion coating, studies have yet to be reported on the actual application of that combination. Therefore, the present study aims to investigate the applicability of the combination mentioned above for a new repair method to shorten the repair process of steel structures near the sea. First, the degree of removal of the corroded part of the steel structure was evaluated for different laser pulse frequencies to examine appropriate laser processing conditions. Next, the effect of laser cleaning on a metal deposition by the low-pressure CS method was evaluated. The powder used was zinc, commonly used as a corrosion-protective coating material due to its coating properties, corrosion resistance, and cost-effectiveness.^10,11,17,18) Furthermore, the proposed method’s applicability was examined by comparing corrosion resistance by salt spray test of zinc-coated corroded specimens treated with conventional and laser cleaning methods.

2. Experimental Procedure

2.1 Surface preparation using laser cleaning

Specimens (approximately 40 mm × 33 mm × 8 mm) were taken from corroded parts of steel structures near the sea, and laser treatment was performed at different pulse frequencies. Specimens NL, L15, and L40 were prepared; NL is the specimen without laser treatment, L15 is the specimen treated with laser at 15 kHz pulse frequency, and L40 is the specimen treated with laser at 40 kHz pulse frequency. Nd: YAG laser system (cleanLASER: CL150) was used for laser treatment. Figure 1 shows a view of the whole device and laser gun. The system consists of the central unit, the gun, and the fiber cable connecting them. The laser beam emitted from the gun is irradiated onto the object for processing. The average output power of the device is 150 W, and the laser wavelength is 1064 nm. A lens with a focal length of 150 mm was used, and the scan frequency was set to 107 Hz. The processing was performed manually using a laser gun, and the number of irradiations for each pulse frequency condition was 21 times. The number of irradiation was determined through a visual inspection where the rust and old coatings on the surface of the specimens were removed, and no visible changes to the appearance occurred. Using X-ray diffraction equipment (Maxima XRD-7000 manufactured by Shimadzu Corporation), each specimen’s surface composition was identified, and the effect of laser pulse frequency on corrosion removal was evaluated. A Cu tube was used as the X-ray source.

Fig. 1

Photographs of (a) laser cleaning machine and (b) laser gun.

2.2 Zinc deposition by low-pressure CS method

As in the section 2.1, corrosion specimens (approximately 30 mm × 26 mm × 9 mm) taken from steel structures were laser-treated, and the effect of the laser treatment on zinc deposition by the low-pressure CS was evaluated. To precisely compare the effects of laser treatment on the surface properties of the specimens, they were polished with an abrasive paper of grit 80 until the characteristic metallic luster of steel was fully exposed, and the surface oxide film and old coating were removed. Specimens PNL, PL15, and PL40 were prepared; PNL is the specimen without laser treatment, PL15 is the specimen treated with laser at 15 kHz pulse frequency, and PL40 is the specimen treated with laser at 40 kHz pulse frequency. Table 1 lists the presence or absence of surface polishing and the pulse laser frequency conditions for the specimens in sections 2.1 and 2.2. Like section 2.1, XRD analysis was performed on the prepared specimens to evaluate and compare the surface composition. Additionally, the surface roughness of the specimens was measured using a laser microscope (KEYENCE: VK-X100) to evaluate the effect of laser treatment on the substrate.

Table 1 Surface treatment and laser pulse frequency parameter conditions.

After the above measurements and evaluations, zinc coating was deposited by low-pressure CS on each specimen. The experimental setup utilized a low-pressure CS system (OCPS: DYMET403j), compressed air as the working gas, and zinc powder (Nippon Paint anticorrosion coating: LS-4H) with a particle size of approximately 3.6 to 5.0 µm as the material powder. Figure 2 shows SEM images of the zinc powder used. The CS conditions were set as a powder feeding rate of approximately 0.1 g/s, a nozzle-substrate distance of 15 mm, a traverse speed of 30 mm/s, a pass number of 5, a pitch distance of 3 mm, a gas temperature of 460°C, and gas pressure of 0.4 MPa. In the coating experiment, the specimen mass before and after the cold spray was measured, and the deposition efficiency of the zinc particles was calculated by dividing the mass change by the amount of zinc powder supplied. Based on the results of sections 2.1 and 2.2, we investigated the appropriate laser pulse frequency conditions for cold spray coating.

Fig. 2

Zinc particle SEM image.

2.3 Salt spray test

Specimens (approximately 150 mm × 70 mm × 10 mm) were cut from the corroded parts of steel structures near the sea, and specimens LC and CC were prepared; LC is the specimen treated with laser cleaning, and CC is the specimen treated with conventional mechanical cleaning. The second category of conventional mechanical cleaning was performed using an industrial disk grinder (BOSCH: GWS7-100E), assuming an environment such as working on large structures at high elevations where blasting cannot be applied due to the intricate shapes of structural components and the high cost for protective covering and temporary work. The old coating was removed entirely, and corroded areas except those that were firmly adhered to the specimens were removed through visual inspection. Zinc was deposited under the conditions described in section 2.2, and the coating cross-section was observed using a scanning electron microscope (SEM) (SU-70, Hitachi High-Technologies Corporation). For comparison, observation was also conducted on a specimen NC coated with conventional paint and no surface treatment.

Next, salt spray tests were conducted on the specimens mentioned above with cold-sprayed zinc coatings. First, the surfaces not exposed to the spray were covered with adhesive tape (DCM: cloth tape) to protect against corrosion. These specimens were set in the salt spray test system (Luyi Test Equipment (Shanghai) Co., Ltd.: RK-90) at an angle of 15–20 degrees relative to the salt spray nozzle and subjected to the corrosion resistance test. The test was conducted in compliance with JIS Z 2371 and employed a neutral salt spray test. The salt solution was prepared by JIS K 8150, using sodium chloride and deionized water with an electrical conductivity of 20 µS/cm or less, adjusted to a concentration of 50 ± 5 g/L and a pH range of 5.0–8.0. Other conditions were as follows: the spray chamber temperature was 35 ± 2°C, the water temperature in the air saturation chamber was 47 ± 2°C, the compressed air pressure was 70–170 kPa, the average volume amount of spray solution on a horizontal plane area of approximately 80 cm² was 1.5 ± 0.5 mL/h, and the test duration was 168 hours (7 days). After the test, the surface of each specimen was rinsed with water and gently scrubbed with a sponge to remove salt from the surface. Afterward, the cross-section of each test specimen was examined by SEM (Scanning Electron Microscopy) and energy-dispersive X-ray spectroscopy to compare and evaluate the corrosion state of the specimens. Additionally, XRD analysis was performed to identify the surface composition materials.

3. Results and Discussion

3.1 Effect of laser pulse frequency on corroded area removal

Figure 3 shows the surface appearance of NL, L15, and L40. Red rust is visible on the paint of the NL specimen, indicating the progression of corrosion. In L15 and L40, which were laser-treated on corroded specimens, the red rust and most of the old coatings were successfully removed. Meanwhile, black discoloration was observed on the entire surface of the specimens due to oxidation caused by heating during the laser treatment.

Fig. 3

Specimen appearance of (a) NL, (b) L15, and (c) L40.

Figure 4 shows the results of the XRD analysis of the surface of each specimen using a Cu tube as the X-ray source. In the specimen NL without laser treatment, the red rust Fe₂O₃ peak was detected, in addition to peaks derived from paint components such as titanium oxide, magnesium oxide, and silicon oxide. In contrast, these peaks were not detected in the laser-treated specimens L15 and L40, indicating that laser cleaning is an effective surface preparation method for removing red rust and old paint. However, peaks originating from FeO and Fe₃O₄ were detected in the specimens after laser treatment. These are likely a mixture of oxides in corroded areas that could not be removed by laser treatment and newly generated oxides by laser heating. Comparing the peaks of L15 and L40, clearer peaks of Fe₃O₄ spectra were detected in L40 than in L15, suggesting that more Fe₃O₄ remained on the surface. The lower intensity of oxide peaks in L15 can be attributed to the enhanced removal of the oxides, as the lower pulse frequency possesses a greater energy density per unit pulse.

Fig. 4

XRD patterns of the NL, L15, and L40 specimens.

3.2 Evaluation of the influence of laser cleaning on zinc coating properties

Figure 5 shows the arithmetic mean roughness (Ra values) of the substrates of PNL, PL15, and PL40. Error bars indicate standard deviations. The surface roughness of the laser-treated specimen was larger than that of the PNL specimen, which was only surface polished, indicating that the laser treatment resulted in surface roughening. In addition, when compared to L40, the surface roughness and standard deviation of L15 were relatively larger. This is probably due to the larger energy density per unit pulse of L15, resulting in a larger roughening effect, and the longer laser pulse intervals cause larger surface roughness variability. However, at both pulse frequencies, the effect on Ra was small, less than five µm. and damage to the substrate is considered to be negligible.

Fig. 5

Arithmetic average surface roughness (Ra) of the NL, L15, and L40 specimens.

Figure 6 shows the zinc deposition efficiency for each specimen. The deposition efficiency of the PNL specimen without laser treatment was approximately 38%, while that of PL15 and PL40 specimens with laser treatment was relatively low by about 10%. The results are discussed in conjunction with the XRD analysis results of the substrate before coating deposition shown in Fig. 7. The XRD analysis results show that only Fe was detected in PNL, while Fe, FeO, and Fe₃O₄ were detected in PL15 and PL40. FeO and Fe₃O₄ are produced by high-temperature oxidation of the substrate surface due to laser irradiation, and the formation of these surface oxides is considered to be the cause of the decrease in deposition efficiency. Ichikawa and Ogawa¹⁹⁾ reported the results of CoNiCrAlY particle deposition using the CS method on a mirror-polished nickel alloy substrate, and the same kind of substrate oxidized at high temperature to form an oxide film. The deposition efficiency of the mirror-polished substrate was 55%, while that of the high-temperature oxidized substrate significantly decreased to 0.03%. Although quantitative discussion is not possible because of the significant difference in materials and high-temperature oxidation conditions between the above report and this experiment, it is possible that the high-temperature oxide film generated by the laser treatment hindered the adhesion between the particles and substrate, resulting in the decrease in deposition efficiency. From these results, it is essential to consider laser processing conditions that strike a balance between corrosion removal and high-temperature oxidation. In the comparison between L15 and L40, the difference in deposition efficiency was approximately 2%, and no significant effect of pulse frequency on deposition efficiency was observed.

Fig. 6

Deposition efficiency of zinc coating on the NL, L15, and L40 substrates.

Fig. 7

XRD patterns of the PNL, PL15, and PL40 specimens.

The results in sections 3.1 and 3.2 show that the L15 specimen has less oxide remaining on the substrate after laser treatment and higher deposition efficiency, although the efficiency difference was not remarkable. Based on these results, a pulse frequency of 15 kHz was more suitable for the substrate preparation in this study, and specimens were prepared for the salt spray test using the same pulse frequency condition.

3.3 Evaluation of the corrosion resistance of zinc coating by salt spray test

Figure 8 shows photographs of the surface appearance of each specimen before and after the CS process. First, the specimen appearance before CS is discussed. The laser-treated specimen LC showed black discoloration on the surface, similar to L15 in section 3.1. In the CC specimen, which was treated by conventional mechanical polishing, red and black rust remained on approximately half of the surface area, although there were areas where the metallic luster of the steel substrate was exposed. The specimen NC without cleaning treatment, had protective paint covering the entire surface, but some of the base paint was exposed. Next, looking at the photographs of the appearance after the CS process, it can be seen that uniform zinc coatings were obtained on the cleaning-treated specimen LC and CC.

Fig. 8

Specimen appearances before and after zinc cold-spraying.

On the other hand, although zinc coating deposition was possible on specimen NC, there were areas where the zinc color was shaded compared to the cleaning-treated specimen, indicating that the base coating was exposed and the zinc coating delaminated. This is due to the weak adhesion between the corroded paint and zinc coating, which resulted in the coating removal by the CS gas pressure during the deposition process. Removal of the corroded paint is considered essential as a pre-treatment for the repair process using the CS method.

Figure 9 shows cross-sectional SEM images of each specimen after the CS process. On the specimens LC and CC, dense zinc coatings were formed on the substrate, with coating thicknesses of approximately 460 µm and 200 µm, respectively. The zinc coating on specimen LC was more than twice as thick as that on specimen CC, which can be attributed to the fact that the laser-treated specimen had less residual oxide on the steel substrate, allowing the impacted zinc particles to adhere to the substrate. The SEM image of specimen CC shows that zinc deposition was possible even with rust on the surface. However, the coating thickness is noticeably thinner than that of LC, indicating that it is essential to remove the oxide film sufficiently before the CS process. The second category of cleaning with a disk grinder used in this experiment is a method that tolerates residual oxide film on the substrate, and completely exposing the steel surface is difficult. Surface preparation using laser cleaning is more suitable for zinc coating deposition by CS.

Fig. 9

SEM images of coating cross-section after zinc cold spraying, (a) LC, (b) CC, (c) NC.

As shown in Fig. 9(c), there were areas where zinc deposition could not be obtained on specimen NC, indicating that direct deposit with CS on the corroded paint is difficult. Based on the above results, corrosion resistance tests by salt spray test were conducted on the specimens LC and CC on which zinc coating could be deposited.

Figure 10 shows photographs of the appearance of each specimen after the corrosion test. Although red rust, seen on corroded steel substrates, was not observed on specimens LC and CC, zinc-derived white rust appeared to cover the entire surface of the specimens. Figure 11 shows each specimen’s cross-sectional SEM images and EDX analysis results. The figure shows a representative cross-section of the coating in the area where corrosion was severe. Many white rusts were observed on the surface, and corrosion products were also observed in the SEM images. However, no effect on the steel substrate was observed, and the zinc coating produced by the CS method functions as an anti-corrosion coating. When focusing on the zinc coating-substrate interface of the SEM image, specimen LC maintains good adhesion at the interface, while gaps are observed in specimen CC. EDX analysis shows that the O intensity is small around the gaps due to the cracking and shedding of rust remaining on the substrate after cleaning. Therefore, from the viewpoint of adhesion strength of the anticorrosion coating, it is essential to sufficiently remove the corrosion on the substrate before the CS process. Next, focusing on the vicinity of the zinc coating, a black contrast layer can be seen in the area indicated by the white line in the SEM image for both specimens. The EDX analysis shows that O, Cl, and Zn elements were detected in the area, indicating the formation of these compounds. The XRD analysis results of the surface of each specimen are shown in Fig. 12. From the analysis peaks, Zn, 4Zn(OH)₂·ZnCl₂ and Zn(OH)₂ were detected in both specimens, and 4Zn(OH)₂·ZnCl₂ is considered to be distributed in the black contrast layer. 4Zn(OH)₂·ZnCl₂ is known to have insulating properties, and this compound covers the surface and suppresses the oxidation-reduction reaction inside the coating and substrate, contributing to the corrosion protection of the substrate.^20,21) However, in specimen CC, the layer mentioned above is partially in contact with the substrate surface, and further expansion is impossible. Therefore, thickening the zinc coating and forming more of this layer will improve corrosion protection performance.

Fig. 10

Specimen appearance before and after salt spray test.

Fig. 11

SEM images and EDX mapping results of specimen cross-section after salt spray test.

Fig. 12

Specimen XRD patterns after the salt spray test.

This study investigated a repair method for the corrosion of steel structures using a combination of laser cleaning and CS. The results showed that using a laser system specialized for cleaning can efficiently remove corrosion with minimal damage to the substrate. The results of corrosion resistance tests revealed that the zinc coating deposited by CS after laser cleaning functioned effectively as an anti-corrosion coating. Another advantage of this method is that when corrosion or deterioration of the zinc coating occurs, the corroded zinc coating can be removed by laser cleaning and repaired by the CS method.

4. Conclusion

This study investigated the applicability of the combination of the laser cleaning and cold spray (CS) method to shorten the repair process for corrosion of steel structures near the sea. The effect of laser pulse frequency on corrosion removal and zinc coating deposition was investigated, and the appropriate conditions for the combination with CS were studied. In addition, the effect of the laser cleaning method on zinc coating deposition and corrosion resistance was evaluated compared with the specimens treated by conventional mechanical cleaning. The findings are summarized as follows:

1. (1)    The laser cleaning effectively removed the corrosion and old coating, and the cleaning at a laser pulse frequency of 15 kHz showed a higher oxide removal rate than at 40 kHz.
2. (2)    For specimens with corrosion removed by a surface polishing, the zinc deposition efficiency was approximately 38% without laser treatment, while the deposition efficiency for specimens after laser treatment was approximately 10% lower. Probably, the oxide generated on the substrate surface as a result of high-temperature oxidation during laser treatment hindered the adhesion of zinc particles.
3. (3)    Compared to the conventionally cleaned specimens, the laser-cleaned specimens had a zinc coating that was more than twice as thick, and the coating-substrate interface showed good adhesion even after 168 hours of salt spray testing. The conventionally cleaned specimens exhibited cracking and shedding of corrosion on the substrate.
4. (4)    No corrosion of the steel substrates was observed after the salt spray test for both the conventional and laser-cleaned specimens. The formation of 4Zn(OH)₂·ZnCl₂ was observed on the surface of the zinc coating. This compound covers the surface and suppresses oxidation-reduction reactions inside the coating and on the substrate, thus contributing to the protection of the substrate from corrosion.
5. (5)    Considering the above results, combining surface treatment using laser cleaning and corrosion-resistant coatings using the cold spray method can significantly shorten the conventional process and time required, and the corrosion-resistant properties of laser-treated specimens are equivalent to those of specimens treated by conventional mechanical cleaning. This method can be expected to be promising as a new repair technique for steel structures. Furthermore, another feature of the method is that it allows for corrosion removal of the zinc coating using laser cleaning and subsequent repair using the cold spray method in case of corrosion or deterioration of the zinc coating.

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