Chromate passivation process
The chromate passivation process is a systematic surface treatment technology that includes three main steps: pretreatment, passivation, and post-treatment. The control of process parameters in each step directly affects the quality and performance of the passivation film. The purpose of pretreatment is to remove impurities such as oil, scale, and rust from the surface of the metal workpiece to ensure the uniform and continuous formation of the passivation film. Pretreatment usually includes steps such as degreasing, pickling, and water washing. Degreasing can use alkaline degreasing agents or organic solvents to remove surface grease; pickling uses dilute hydrochloric acid or sulfuric acid solution to remove scale and rust; and water washing is used to remove residual degreasing agent and acid to avoid contaminating the passivation solution. For galvanized workpieces, pretreatment also requires the removal of zinc ash and zinc slag on the surface of the galvanized layer to ensure good bonding between the passivation film and the galvanized layer.
Passivation treatment is a core step in the chromate passivation process, requiring strict control of parameters such as the composition, concentration, temperature, pH, and treatment time of the passivation solution. The passivation solution primarily consists of chromates (such as potassium dichromate and sodium chromate), acids (such as sulfuric acid and nitric acid), and other additives. The chromate concentration is typically 20-100g/L. High concentrations can result in a rough passivation film, while low concentrations make it difficult to form a complete film. The acid regulates the pH of the passivation solution, typically between 1 and 3. Acidic conditions promote the passivation reaction, but too low a pH can lead to excessive corrosion of the metal surface and compromise the adhesion of the passivation film.
The treatment temperature has a significant effect on the chromate passivation reaction rate and is generally controlled between room temperature and 60°C. Room temperature passivation is easy to operate and has low energy consumption, but the reaction speed is slow and the processing time is long (usually 1-5 minutes); elevated temperature passivation can speed up the reaction speed and shorten the processing time (30 seconds to 2 minutes), but the energy consumption is high, and too high a temperature may cause the passivation film to become loose. The treatment time needs to be determined according to the concentration, temperature and required film thickness of the passivation solution. If the time is too short, the passivation film will be too thin and incomplete; if the time is too long, the passivation film will fall off.
During the passivation process, the passivation solution requires regular maintenance and adjustment to ensure stable performance. The concentration, pH, and chromium ion content of the passivation solution should be regularly tested. If any parameters deviate from the optimal range, appropriate chemicals should be added promptly for adjustment. Sediment and impurities in the passivation solution should also be regularly removed to prevent them from adhering to the workpiece surface and affecting the quality of the passivation film. For continuous production passivation lines, an overflow system should be used to continuously replenish the passivation solution to maintain a stable composition.
Post-treatment is the final stage of the chromate passivation process and primarily includes washing, drying, and sealing. The purpose of washing is to remove any residual passivation solution from the workpiece surface to prevent the formation of powdery frosting after drying, which can affect the appearance quality and corrosion resistance. Washing is typically done with running water, though deionized water can be used if necessary to ensure that no chromium ions remain on the workpiece surface. Drying removes moisture from the workpiece surface to prevent rust. Drying temperatures typically range from 60-100°C, and drying time varies depending on the size and shape of the workpiece to ensure complete dryness. For demanding applications, sealing treatments such as dip coating with varnish or wax can be performed to further enhance the corrosion resistance and decorative properties of the passivation film.
The chromate passivation process must be rationally selected and optimized based on the metal material and application requirements. For example, for the color passivation of galvanized steel sheets, a higher concentration of chromate solution is typically used, with the treatment temperature controlled at 30-40°C and the treatment time at 1-2 minutes to form a passivation film of moderate thickness and strong corrosion resistance. For the passivation of zinc alloy die-castings, due to their relatively smooth surface, the acidity of the passivation solution and the treatment time need to be appropriately increased to ensure the uniformity and adhesion of the passivation film. Despite environmental issues, the chromate passivation process is still used in some fields with extremely high corrosion resistance requirements. At the same time, researchers are continuously researching and improving the process to reduce its environmental impact.
