Metal pretreatment process - sandblasting

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Sandblasting uses a stream of compressed air or centrifugal force to propel sand particles of a certain size and high kinetic energy at the surface of the metal substrate; the impact and cutting action of these sand particles serve to roughen and clean the metal surface. As the most commonly used and efficient method for surface texturing, sandblasting improves the adhesion of coatings to the substrate surface through the following mechanisms: 1.1 Increasing the bonding area – Sandblasting creates larger protrusions and pits on the substrate surface, thereby significantly increasing the bonding area between the coating and the substrate, and thus enhancing the strength of the bond between them ； 1.2 Tensile stress at the dispersion interface: Sandblasting the substrate results in residual compressive stress. This compressive stress can, on one hand, counteract the tensile stress inherent in the metal, and on the other hand, it helps to make the stress distribution on the substrate surface more uniform. As a result, it is possible to effectively prevent the formation and further expansion of cracks in the coating, thereby improving the coating’s resistance to cracking ； 1.3 Further purification and activation of the substrate metal surface: Over the course of more than 100 years, sandblasting technology has seen significant development, with its applications continuing to expand across various engineering and technical fields. Sandblasting is widely used in the field of substrate roughening due to its low cost and high efficiency, especially for the roughening of large workpieces. Research has shown that sandblasting can significantly improve the bonding strength between coatings, adhesives, and other materials with the substrate. However, the sandblasting technique also has certain drawbacks: during the sandblasting process, sand particles tend to remain on the surface of the substrate, which negatively affects the adhesion between the coating and the substrate. Increasing the pressure and particle size within a certain range can enhance the surface roughness of the substrate, thereby improving the adhesion of the coating; however, this increases the amount of sand particles remaining, so it is necessary to select appropriate pressure and particle size. The sandblasting angle also has a significant impact on the surface roughness of the substrate and the sandblasting residues, so it is necessary to establish an appropriate range. When the sandblasting angle and pressure are fixed, increasing the sandblasting distance within a certain range can improve the adhesion of the coating to the substrate; however, if the distance is too large, the adhesion of the coating decreases instead. This is because by increasing the distance appropriately, the sand particles gain enough acceleration distance and thus possess sufficient kinetic energy upon reaching the surface of the substrate. However, when the sandblasting distance is too large, the sand particles lose kinetic energy due to air resistance during their movement, resulting in less kinetic energy upon impact with the substrate, which in turn leads to a reduction in the roughness of the treated surface. Scholars at home and abroad have conducted extensive research on sandblasting treatment technology, exploring the effects of sand particle properties such as type and size, as well as sandblasting process parameters like distance, pressure, and angle, and the characteristics of the substrate material, on the morphology of the substrate and the strength of the bond between the substrate and the coating. Wei Min et al. used CaCO3, brown corundum, and quartz sand particles for sandblasting of 45 steel and 7A52 armored aluminum alloys, and studied the changes in the surface morphology and roughness of the workpieces after sandblasting. Experiments show that the surface roughness of the substrate is lower after CaCO3 sandblasting, while it is higher after brown corundum sandblasting; the surface roughness after using a mixed abrasive for sandblasting lies between the two. By using different ratios, a continuous range of roughness levels can be achieved. Mellaili et al. investigated the effects of sandblasting pretreatment on the surface roughness and sand particle residue of substrates such as cast iron, aluminum alloy, and high-strength steel. The experimental results show that the particle size of the sand has the greatest impact on roughness. An increase in sandblasting pressure can increase the surface roughness of the substrate to a certain extent, but it also leads to more residual sand particles. Bahbou et al. studied the effect of sandblasting angle on the surface roughness of the substrate and the sandblasting residues. Studies show that when the sandblasting direction is at a 75° angle to the substrate surface, more sand particles remain on the surface of the substrate. Wang studied the effect of sandblasting intensity on the morphology and properties of the micro-arc oxidation coating on iron surfaces, and observed the influence of sandblasting time on the surface roughness and sandblasting residues of iron. The experiments showed that when the sandblasting time was 15 seconds, there were only a small number of ridge-like rough structures on the iron surface, and no residues from sandblasting were detected ； When the time was increased to 30 s, numerous ridge-like rough structures and grooves appeared on the titanium surface, and the amount of sand particles remaining embedded in those ridge-like structures gradually increased. As time continued to increase, the rough structures hardly changed, while the remaining particles kept increasing. Liao Menghao used sand particles of different sizes to perform sandblasting on various substrates such as steel, titanium, and aluminum, studied the sandblasting contamination on metal surfaces, and explored the mechanism of sand particle retention. Experiments showed that the size of the residual particles is much smaller than the particle size of the sand used for sandblasting, indicating that the fragmentation of the sand particles upon impact with the substrate is the main reason for their residue. Li Qinfeng studied the factors affecting sandblasting efficiency, pointing out that sandblasting pressure is directly proportional to the cleaning efficiency; in principle, the higher the pressure, the better the cleaning efficiency. However, increased pressure can cause the sand particles to become damaged and lead to more residue remaining after sandblasting. Therefore, a pressure of around 0.7 MPa is the most suitable, with values generally kept between 0.55 and 0.75 MPa. Therefore, when using sandblasting as a pre-treatment method for substrates, it is necessary to select appropriate sand particles, sandblasting angle, sandblasting time, and sandblasting pressure based on the hardness of the substrate, in order to minimize the impact of sandblasting residues on the adhesion strength of the coating. In recent years, some scholars have also used small-grained sand for sandblasting, followed by treatment of the substrate surface using an acid bath. This method not only removes a small amount of sandblasting residue from the substrate surface but also increases its surface roughness and creates a uniform micro-nano hierarchical structure, thereby achieving an appropriate surface roughness and sufficient adhesion strength for the coating. Polishing, engraving, pickling, passivation, and sandblasting of metal surfaces can all be used to clean the surface of the substrate and increase its roughness. However, each of these preprocessing methods has its own shortcomings; using them alone does not yield perfect results. A combination of different preprocessing methods might be an effective solution. Although increasing the surface roughness of the substrate through roughening treatment can enhance the adhesion strength of the coating, the relationship between roughness and coating adhesion strength is not always positive. When the roughness is too high, especially when the rough surface structure is uneven, it leads to an uneven distribution of the coating thickness, which in turn results in lower adhesion ； It also causes stress cracking in the coating, reducing its ability to shield against corrosive agents. Furthermore, excessive roughness can result in the coating thickness at the wave peaks being below the specified requirements, leading to premature failure of the coating. Deep and narrow wave valleys, on the other hand, tend to trap bubbles, causing bulging in the coating and reducing its protective properties. On the other hand, depending on the spraying method, the pattern by which the surface roughness of the substrate affects the adhesion of the coating also varies. Taking arc spraying and plasma spraying as examples: within a certain range, the greater the roughness, the lower the adhesion of the plasma-sprayed coating, whereas the bonding strength of the arc-sprayed coating increases. Therefore, it is necessary to determine the optimal roughness for different design requirements, different coating systems, and different spraying methods based on experiments.