Effect of steel shot size on workpiece surface roughness

　　1) Increase the actual bonding area between the coating and the workpiece surface, which is beneficial to improving the coating bonding strength. Statistics on the size of steel shot and the ratio of actual surface area to theoretical surface area.

　　2) The coating will generate large internal stress during the curing process. The existence of surface roughness can effectively eliminate stress concentration in the layer and prevent the coating from cracking.

　　3) The existence of surface roughness can support the quality of part of the paint and help eliminate sagging. This effect is particularly obvious for vertically painted surfaces. Surface roughness is caused by the combined effects of impact, cutting and erosion when steel shot and steel grit particles contact the surface at a very high speed. Therefore, the size, hardness, particle shape, hardness of the workpiece itself, compressed air The pressure, the distance between the nozzle and the workpiece surface, and the angle between the nozzle and the normal direction of the workpiece surface determine the size and contour shape of the surface roughness. Surface roughness, like surface cleanliness, must also be determined by corrosion engineers based on coating requirements. The main basis for determining the surface roughness value is the type of coating and the thickness of the final coating. In the absence of the conditions or technical information required to establish the surface roughness value, the surface roughness value can be temporarily determined to be between 25% and 30% of the final coating thickness. For example, if the final coating thickness requirement is 150µm, then the surface roughness value can be determined within the range of 38-45µm. Another method is to determine the surface roughness value based on the thickness of the primer. The surface roughness value cannot be greater than the thickness of the primer, except for those who apply the second coat of paint immediately after spraying the primer. If the surface roughness value is low, it is not conducive to improving coating adhesion; if the surface roughness value is high, the coating thickness specified in the coating design may not be able to cover all the peaks in the surface roughness. Pitting corrosion will occur, causing premature failure of the coating. Maintaining the consistency of the abrasive particle size throughout the shot blasting process is a necessary prerequisite to ensure consistent surface roughness. Abrasives with different uses must not be mixed or substituted. For example, a steel structure parts manufacturing company undertook a steel structure parts manufacturing project. Some of the steel structure parts in this project needed to be painted, and some needed to roughen the joint surfaces of high-strength bolt connections to improve the joint surfaces. friction factor between them. The abrasives used for roughening are not only large in particle size, but also have high hardness. Since the steel shot required for surface treatment before painting could not keep up for a while, the company used the abrasives used for roughening. Shortly after the workpiece was transported to the construction site, the problem was exposed. The dense pitting corrosion was shocking, so it had to be re-polished and painted with paint. The root cause of this problem is that the surface roughness caused by the size and high hardness of the steel shot cannot be effectively covered by the coating specified in the design, and corrosion is prone to occur.