The seven-tank process, also known as the seven-stage pretreatment process used for surface preparation primarily in metal finishing. It helps to prepare metal surfaces for subsequent coating or finishing processes such as painting, powder coating, or plating. This process involves several sequential steps to clean, treat, and protect the metal surface. It ensures optimal adhesion and durability of the final finish. Here’s a detailed explanation of each stage in the seven-tank process:
01. Degreasing/Pre-Cleaning:
Purpose: The first stage involves removing dirt, grease, oil, and other contaminants from the metal surface. This step ensures that the subsequent cleaning and treatment stages are more effective.
Method: The metal parts are immersed in a degreasing solution or sprayed with a degreasing agent to dissolve and remove any surface contaminants. This may involve using alkaline cleaners, solvent-based degreasers, or aqueous cleaning solutions.
02. Rinsing:
Purpose: After degreasing, the metal parts are rinsed to remove any remaining traces of the degreasing solution and loosened contaminants.
Method: The metal parts are rinsed with water in a rinse tank or spray booth to wash away the degreaser and contaminants. Multiple rinsing stages may be used to ensure thorough cleaning.
03. Derusting:
Purpose: In this stage, the metal surface is chemically treated to remove rust and improve adhesion.
Method: The metal parts are immersed in an acidic or alkaline solution to etch the surface and remove any oxide layers or surface impurities. This creates a roughened surface with increased surface area, which enhances the bonding of subsequent coatings or finishes.
04. Rinsing:
Purpose: Similar to the previous rinsing stage, this step is essential for removing any residual etching solution and preparing the metal surface for the next treatment.
Method: The metal parts are rinsed with water to wash away the etching solution and neutralize the surface. Thorough rinsing is critical to prevent contamination of subsequent process stages.
05. Phosphating:
Purpose: In this stage, a conversion coating is applied to the metal surface to further enhance adhesion, corrosion resistance, and paint adhesion.
Method: The metal parts are immersed in a phosphating solution containing phosphoric acid, metal phosphates, and other additives. The solution chemically reacts with the metal surface to form a thin crystalline phosphate layer, which acts as a protective barrier against corrosion and improves paint adhesion.
06. Rinsing:
Purpose: Once again, rinsing is necessary to remove any excess phosphating solution and neutralize the surface before proceeding to the final stage.
Method: The metal parts are thoroughly rinsed with water to wash away the phosphating solution and ensure that the surface is clean and ready for the next treatment.
07. Passivation/Sealing:
Purpose: The final stage involves applying a protective coating or sealant to the metal surface to enhance corrosion resistance further and prepare the surface for subsequent coating or finishing processes.
Method: The metal parts may be immersed in a passivation solution or sprayed with a sealant to create a protective barrier on the surface. This step helps to prevent oxidation and corrosion of the metal surface during storage and handling.
After completing the seven-tank process, the metal parts are typically dried thoroughly before proceeding to the next stage of the manufacturing process, such as painting, powder coating, plating, or assembly. This comprehensive surface preparation method ensures that metal surfaces are clean, treated, and protected, resulting in superior adhesion, durability, and quality of the final finish.
