During assembly, a 215mm-thick asbestos pad was used as the bottom layer of the panel. Due to its relatively low hardness, the screw holes became too large. When tightening the bolts (M8, torque of 12 Nm), the asbestos pad compressed and deformed, forming a circular disk-like shape around the bolt hole. It was observed that the greater the tightening force, the more pronounced the deformation, resulting in more severe peeling. This phenomenon can be explained by the fact that when the panel bends, the outer side experiences tensile stress while the inner side undergoes compressive stress. When two adjacent bending directions occur simultaneously, the stresses add up, increasing the likelihood of coating separation. The higher the deformation, the greater the stress, and thus the higher the risk of delamination.

The phosphating film has two main functions: providing rust resistance and enhancing adhesion between the base layer and the coating. However, its brittleness varies depending on its type and thickness. Zinc-based phosphating films, composed of Zn₂Fe(PO₄)₂·4H₂O and Zn₃(PO₄)₂·4H₂O, form a crystalline structure, making them prone to cracking under deformation. If the phosphate layer is too thick or coarse-grained, it may crack and peel off during bending. In practical applications, it is advisable to avoid excessively thick phosphating layers on easily deformable parts. Through testing and observation of the simulated test plates, it was found that the phosphating film was uneven, with the thickest section reaching 13 µm (equivalent to 1339 g/m²) and the thinnest only 3 µm (equivalent to 36 g/m²). After four days of storage, dark rust spots appeared on the surface, resembling the white spots commonly seen on the production line. This indicates that the quality of the phosphating film on the panel was not satisfactory. Further investigation revealed that due to power shortages in winter, the original hot water washing in the phosphating process was replaced with normal temperature washing, and the drying process was changed from forced drying to natural drying. Additionally, the phosphating time was extended from 10 minutes to 25 minutes to prevent rusting.

These changes likely contributed to the poor quality of the phosphating film. It is speculated that the white dots observed during coating delamination may have resulted from corrosive substances present in the cleaning solution, which accelerated the peeling process. Changing the drying method from heated to natural drying could have introduced amorphous water, negatively affecting both the rust resistance and adhesion of the zinc-based phosphating film. The optimal drying temperature for such films is generally between 100°C and 120°C. Therefore, the presence of these white dots may also be linked to the natural drying process. Prolonged phosphating time led to excessive film thickness and increased brittleness, which further exacerbated coating delamination under mechanical stress. To maintain good adhesion, the phosphating time should ideally be kept within 10 minutes. Improper maintenance of the phosphating solution caused uneven crystallization and inconsistent thickness, contributing to the overall degradation of the film quality.

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