The difficulty in removing the oxide scale from the workshop primer and heat-affected zone. How can the sandblasting room prevent the residue from causing premature failure of the coating?
Publish Time: 2025-09-16
In the fabrication process of large steel structures, ship sections, or pressure vessels, after welding, the workpieces enter the sandblasting room for surface pretreatment. This is crucial for the long-term service life of the coating system. However, while the welding process itself enhances structural strength, it also presents further challenges for surface preparation. The HAZs around the welds and residual shop primer become challenging areas for sandblasting. If not thoroughly treated, these areas can become the starting point for subsequent coating failure, allowing rust to spread from microscopic interfaces and ultimately destroy the entire corrosion protection system.Shop primer is a pre-applied weldability protective coating applied to steel plates before cutting and assembly. Its primary function is to prevent initial rust during transportation and storage. However, it is not the final anti-corrosion layer and must be completely removed before the final coating is applied. The problem is that this primer is designed with weather resistance and adhesion in mind. Especially under the high temperatures of welding, some areas may carbonize or form a semi-crosslinked structure with the substrate, becoming extremely stubborn. Traditional sandblasting often only removes the loose surface layer, while primer residue embedded in the edges or reattached by melting is difficult to completely remove. While these residues may appear clean on the surface, they are incompatible with subsequent coatings, forming a weak interface. Once moisture and electrolytes penetrate, they create electrochemical corrosion pathways between the coating and the substrate.The heat-affected zone (HAZ) presents an even more complex problem. The localized high temperatures during welding cause changes in the metal's crystal lattice, simultaneously forming a dense oxide scale on the surface that is much harder than the base metal. Conventional abrasives are easily deflected or shattered upon impact, making it difficult to effectively break up this oxide layer. Even if the surface oxide scale is partially removed, suboxides or metal salts may still be lurking in microcracks or pores. These substances are hygroscopic and continuously release ions in a humid environment, accelerating localized corrosion. Furthermore, the HAZ's microstructure exhibits stress concentrations and uneven surface energy distribution. Even after sandblasting, its surface activity and cleanliness remain lower than those of normal areas, hindering coating wetting and adhesion.Sandblasting rooms relying solely on standard process parameters struggle to address these specialized areas. An effective solution begins with understanding the root cause of the problem and precisely adjusting the process. First, before sandblasting, thorough visual inspection and nondestructive testing are required to identify the weld distribution, heat-affected zone extent, and primer coverage, allowing for the development of differentiated treatment strategies. For stubborn primer areas, higher-hardness or sharper abrasives can be used to enhance cutting power. For dense scale areas, the spray angle should be optimized to avoid vertical impact and rebound, and an oblique injection method should be employed to improve stripping efficiency.Automated sandblasting systems offer advantages in this complex treatment. The robotic arm, with a programmable path, performs multi-angle, multi-pass scanning blasting along the weld trajectory, ensuring that every point receives sufficient impact. Sensors monitor surface conditions in real time, providing feedback and adjusting pressure and speed, enabling intelligent response by applying more blasts to challenging areas. Compared to the randomness of manual operations, automated systems ensure consistent and thorough treatment.Furthermore, post-blasting cleaning verification is crucial. Visual inspection alone is insufficient to identify microscopic residues. Profilometers are needed to measure anchor mark depth, solvent scrubbing to detect soluble salt content, and high-power magnification or endoscopes are required to inspect hidden areas such as weld roots. Only after confirming that the surface meets the specified cleanliness and roughness standards can the next coating process be initiated.Ultimately, the key to preventing premature coating failure lies not in simply increasing the intensity of sandblasting but in a systematic understanding of material behavior, process mechanisms, and equipment capabilities. The sandblasting room is not just a place for cleaning; it is also a gateway for quality control. Only when every weld edge is precisely treated and every heat-affected zone is restored to a reactive metal surface can the coating truly achieve a strong bond, "growing from the substrate," silently safeguarding the integrity and safety of the structure through the test of time and the environment.
