Effect of Repainting Process on Atmospheric Corrosion Behavior of Perforated Panel

Steel structures are often painted to protect against corrosion. Repainting is one of the most important factors in maintaining the corrosion protection function of the coating. Various factors affect the life of the coating film, such as the surface preparation, the type of coating, and the number of coats. Surface preparation is important for the life of the coating film. However, appropriate surface treatment is difficult due to the complex shape of perforated panels, and it has been confirmed that corrosion progresses from the machined area. Therefore, appropriate pretreatment of the machined area is important for corrosion prevention. In this study, we investigated the effects of the repainting process on the atmospheric corrosion behavior of perforated panels. To reproduce the repainting process, a number of pretreatments were performed, such as salt spraying, blasting, and zinc phosphate treatment. In the salt spray test after pretreatment and painting, the corrosion progressed in cases with no zinc phosphate treatment and those left untreated for 48 h before painting. In addition, the coating film on the processed area was confirmed to be thin. These results suggested that appropriate pretreatment and sufficient thickness of the coating on the machined area would affect the occurrence of corrosion.


Introduction
Steel structures are often painted to protect against corrosion, and repainting is How to cite this paper: Kuratani long time, the number of times repainting will be necessary during the life cycle could be reduced, and thus reduce maintenance costs [1] [2] [3] [4]. A number of factors affect the life of the coating film, including the surface preparation, the type of coating, and the number of coats, and it has been demonstrated that surface preparation is important for prolonging the life of the coating film [5]. However, in real-world application, the entire repaint process is rarely completed in a single factory, and so it may be unclear whether appropriate treatments were carried out. Although dependent on the weather, temperature, and humidity, it is usually recommended to paint within 4 hours after pretreatment [6]. Thus, if the transportation time of the material between factories is excessive, the pretreatment would become ineffectual. In the field of architecture, perforated panels, which are often used for the exterior walls of apartment buildings as well as corporate and social infrastructure facilities, are also painted for protection against corrosion. In addition to their decorative purpose, perforated panels have advantages, such as providing shade from the sun, reducing noise, and saving energy [7] [8] [9]. However, corrosion is known to progress in the machined area due to the residual stress of the punching process, and rust often becomes apparent from the edge of the machined area due to deterioration of the coating film [1] [3] [10] [11] [12] [13]. Machined parts are often not flat but complex in shape, and it is difficult to obtain a sufficient coating thickness. Therefore, in the painting of steel structures, especially in the repainting process, it is important to perform appropriate pretreatment before painting and to obtain a sufficient coating thickness on the machined parts.
In this study, we investigated the effects of the repainting process on the corrosion behavior of perforated panels.

Specimens
The specimens were perforated Steel Plate Cold Commercial (SPCC) sheets with a diameter of 20 mm, which were pretreated by salt spraying, blasting, zinc phosphate treatment, and painting to reproduce the repainting process ( Figure 1).
The salt-sprayed specimens were used as rust specimens and the as-received specimens were used as new specimens. The unprocessed general area was referred to as the flat area, the perforated section as the whole area, and the perforated corner as the edge area. The painting thickness of the flat part was set to 60 mm using a thickness meter (DUALSCOPE MP0R; Kett Scientific Laboratory, Tokyo, Japan), and the time until painting was set to 4 or 48 hours. Specimens were sprayed with salt water (5% NaCl aqueous solution) for 1600 hours at a temperature of 35˚C ± 1˚C, and white alumina grids with grain sizes of 850 -1180 mm (hereafter referred to as #20) and 600 -850 mm (hereafter referred to as #24) blended at a ratio of #20: #24 = 1:3 were blasted at 0.4 MPa.   shows the preparation conditions for the specimens. The specimens were evaluated by visual inspection, examination of cross-sections, and scanning electron microscopy (SEM) (S-4300; Hitachi, Ibaraki, Japan) with energy dispersive X-ray spectrometry (EDX) (EX-220; Horiba, Kyoto, Japan), and the painting thickness was measured on the cross-sections.

Salt Spray Test
The salt spray test was carried out as described in Section 2.1 for 2 weeks, 1 month, and 2 months with a cycle of 5 days of salt spraying followed by 9 days of drying at room temperature with reference to environmental acceleration expe-    The total thickness of the three layers was used as the painting thickness, and the thickness of each specimen was measured. Table 2 shows the film thickness of each part of the specimen. The painting thickness of the flat area was >60 mm as measured with the thickness meter, and it was thought that the thickness meter displayed a value smaller than the actual thickness. On the other hand, the hole and edge areas were thinner than 60 mm, less than half that of the flat area. The thinner film was thought to be associated with a greater water penetration rate, and the corrosion would therefore tend to progress from the hole and edge areas. These observations confirmed the usefulness of zinc phosphate treatment and it was suggested that the rust was removed by the process of zinc phosphate treatment. As an example, Figure 4 shows Observation of cross-sections of the holes confirmed that the corrosion progressed under the painted film. As corrosion progressed from the machined part, it was thought that the rust in the machined part was not sufficiently removed by blasting treatment. Figure 5 shows the results of SEM and EDX anal-    progression of corrosion when repainting perforated steel sheets. In addition, it was inferred that the film thickness affected the timing of corrosion appearance because the film thickness at the machined area was thin and corrosion was observed in this area.

Conclusions
The effects of repainting on the corrosion behavior of perforated panels were investigated in each process of repainting, and the following findings were obtained: 1) Appropriate pretreatment, especially zinc phosphate treatment and coating within 4 hours, was effective in inhibiting corrosion.
2) The thickness of the painting film on the machined area affected the timing of the appearance of corrosion.
In the future, we would like to study the appropriate pretreatment method and new painting method to obtain enough painting thickness.