Factors affecting the roughness of the workpiece surface: the size of the steel shot

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

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

　　3) The presence of surface roughness can support the quality of part of the coating, which is beneficial to eliminate the sagging phenomenon. For vertically coated surfaces, the effect is particularly obvious.

　　Surface roughness is caused by the combined effects of impact, cutting and scouring when the steel shot and steel grit particles contact the surface at a very high speed. Therefore, the size and hardness of the steel shot, the particle size, hardness, particle shape, the hardness of the workpiece material itself, the pressure of the compressed air, the distance between the nozzle and the workpiece surface, and the angle between the nozzle and the normal of the workpiece surface determine the size and contour of the surface roughness.

　　Surface roughness, like surface cleanliness, should also be determined by the corrosion engineer according to the coating requirements. The main basis for determining the size of the surface roughness value is the type of coating and the thickness of the final coating.

　　In the absence of the conditions or technical data required to formulate the surface roughness value, the surface roughness value can be temporarily determined between 25%-30% of the final coating thickness. For example, if the final coating thickness requirement is 150µm, 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 primer thickness, except for the second coat of paint applied immediately after the primer is sprayed.

　　If the surface roughness value is too low, it is not conducive to improving the adhesion of the coating; if the surface roughness value is too high, the coating thickness specified in the coating design may not be able to cover all the peaks in the surface roughness, and pitting will occur in a short time, causing the coating to fail prematurely.

　　Maintaining the consistency of the abrasive particle size throughout the shot peening process is a necessary prerequisite for ensuring consistent surface roughness. Abrasives for different purposes must never be mixed or substituted. For example, a steel structure manufacturing company undertook a steel structure manufacturing project. Some of the steel structures in this project needed to be painted, and some needed to roughen the joint surfaces of high-strength bolts to increase the friction coefficient between the joint surfaces. The abrasive used for roughening not only has a large particle size but also has a high hardness. Since the steel shot required for surface treatment before painting could not keep up for a while, the company used the abrasive used for roughening. Soon after the workpiece was transported to the construction site, the problem was exposed. The dense pitting was shocking, so it had to be re-polished and painted again. 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 it is easy to rust.