In stainless steel sheet metal processing, the tarnish produced on the weld after welding is a direct result of high-temperature oxidation. It is primarily composed of metal oxides such as iron, chromium, and nickel. This not only affects the appearance but can also reduce the material's corrosion resistance. Therefore, effective tarnish removal is crucial for ensuring the quality of stainless steel sheet metal processing. It requires a combination of physical and chemical methods, with appropriate processes tailored to the material and degree of oxidation.
Mechanical polishing is a basic method for removing tarnish and is suitable for lighter tarnish or localized treatments. The grit of the sandpaper or grinding wheel should be selected according to the degree of oxidation. Begin with a coarse-grit tool to quickly remove the surface oxide layer, then gradually transition to a fine-grit tool for fine grinding until the weld surface is flush with the parent material and free of visible scratches. Grinding should be done along the metal grain to avoid lateral friction that increases surface roughness. This method is simple to use, but requires controlled application. Excessive polishing can damage the stainless steel's passivation film and even induce new oxidation.
Chemical pickling removes tarnish through the dissolving action of an acid solution and is an effective method for treating large or complex welds. A common pickling solution is a proportional mixture of strong acids such as nitric acid and hydrofluoric acid. Nitric acid dissolves iron oxides, while hydrofluoric acid targets chromium oxides. During pickling, the workpiece should be immersed in the pickling solution or evenly applied to the weld with a soft brush. The reaction time should be controlled to avoid excessive corrosion. After pickling, the workpiece must be thoroughly rinsed with plenty of clean water and neutralized (such as with limewater) to remove residual acid and prevent rust. This method can evenly remove tarnish, but strict precautions must be taken to prevent acid splashes from injuring operators or corroding equipment.
Pickling passivation paste, a derivative of chemical pickling, is suitable for removing tarnish from localized or delicate areas. Based on a pickling solution, this paste contains a thickener and corrosion inhibitor. It can be applied directly to the weld, where it chemically dissolves the tarnish. During application, the application thickness (approximately 1-2 mm) and the reaction time (usually 15-30 minutes) should be controlled. Once the tarnish has completely dissolved, rinse with clean water. The advantage of pickling and passivation paste is its flexibility and the lack of large equipment. However, be aware that the paste's ingredients may cause excessive corrosion on certain stainless steel materials, so a small-scale test should be conducted before use.
Sandblasting involves applying a high-speed jet of micro-glass beads or steel grit to the weld surface to remove oxidized discoloration. This method is suitable for workpieces with complex shapes or those difficult to mechanically polish. It can evenly remove the oxidized layer and create a certain surface roughness, enhancing coating adhesion. After sandblasting, cleaning is required to remove residual sand particles to prevent them from interfering with subsequent processes. The efficiency of sandblasting depends on the media type, spray pressure, and workpiece material, and parameters should be adjusted accordingly.
Electrolytic polishing utilizes electrochemical principles to remove oxidized discoloration and smooth the weld surface through anodic dissolution. During operation, the workpiece, acting as the anode, is immersed in an electrolyte. Direct current is applied, preferentially dissolving the raised areas of the weld surface, resulting in a smooth surface. This method can simultaneously remove oxidized discoloration and fine burrs, improving surface finish. However, the equipment cost is relatively high, making it suitable for applications with stringent surface quality requirements.
Passivation is a key step after removing tarnish. It aims to form a dense passive film on the stainless steel surface, enhancing corrosion resistance. Common passivating agents include oxidants such as sodium nitrate and sodium dichromate, or highly concentrated nitric acid solutions. During the process, the workpiece must be immersed in the passivating solution, with controlled time and temperature to form a uniform passivation film. After passivation, the workpiece must be rinsed with clean water and dried to prevent corrosion caused by residual passivation.
Post-weld tarnish removal in stainless steel sheet metal processing requires a combination of mechanical grinding, chemical pickling, sandblasting, electrolytic polishing, and passivation. In practice, the selection of a single or combined process should be based on the workpiece material, degree of oxidation, shape complexity, and cost requirements. Strict adherence to operating procedures ensures that the weld surface quality and corrosion resistance meet standards.
