How to prevent the oxide film surface red frost?

[Alcoa.com] The surface of the workpiece after oxidation is sometimes attached with a layer of red precipitate. This is caused by the excessive use of the oxidizing solution and the excessive accumulation of iron trioxide. This less crystalline iron oxide is Concentrated alkali solution has a very small solubility, so it will precipitate on the surface of the workpiece during oxidation. This phenomenon can be eliminated and prevented by the following measures. (1) Strictly control the operating temperature of the blue solution. The working temperature required for the blue solution is based on the carbon content of the steel parts or the alloy composition. Parts with higher carbon content have a lower initial temperature and a lower end temperature when the solution is blue, and the opposite is true for articles with low carbon content or alloy steel parts. When different types of steel parts are blued at the same time, the time must be staggered. The former tank can be used to dispose of parts with high carbon content. When the treatment reaches the end point, the temperature of the solution also goes up, just for the low carbon content. The temperature at which the workpiece begins to enter the slot. This avoids the appearance of red frosting. (2) Adopt secondary blue process. The specific approach is: the parts first in the working temperature of the solution formulation in the blue treatment for about 15min, and then move it to the solution slightly higher than the normal working temperature continues to send blue 40~60min, by the front tank solution temperature control At 130-135°C, the temperature of the second bath solution is controlled at 140-150°C (different with the carbon content of steel parts). The workpiece forms crystal nuclei in the front groove, and then continues to grow in the second groove to form a sheet, thereby obtaining a dense and thick oxide film with high corrosion resistance, which can greatly reduce the red frosting appear. (3) Increase the washing step of the product during the blue process. After processing for 5 minutes in the blue trough, the parts are taken out of the trough and rushed under the tap. At this time, the red frost on the surface of the workpiece has just formed, is thin, easy to wash off, and then continue to send When it is blue, there will be less red frost. (4) Add potassium ferrocyanide to the bluing solution. Because potassium ferrocyanide can complex with high-valent iron ions to form Prussian blue precipitate, thereby reducing the iron ion content in the solution and reducing the appearance of red precipitates on the surface of the parts, this method is slightly expensive and slightly toxic. (5) Strengthen the maintenance of blue solution. Strengthening the maintenance of blue solution is one of the more effective methods to prevent the appearance of red frost. It is a method for permanent cure. The red frost on the surface of the product is to a certain extent aggravated by the increase of high iron ions in the blue solution. For this reason, when a certain amount of high-valent iron ions accumulate in the blue solution, it needs to be cleaned. The specific method is as follows: First, add 1/4 of the original volume of hot water to the blue solution, dilute it, and then heat it to boiling (this time The boiling temperature of the solution is about 120°C). After standing, a red precipitate is sucked out from the bottom of the tank (the supernatant liquid can be returned to the blue tank after precipitation), and the supernatant is concentrated or supplemented to make it suitable for the process. The mass concentration required for the formulation, followed by bluening, will not appear on the surface of the film in red. This method needs to be carried out frequently. (6) Removal of sodium cyanide solution. The red frost formed on the surface of the oxide film can be soaked and removed in the sodium cyanide solution. After this solution is processed, the red frost on the surface of the part can be completely eliminated. It should be noted that afterwards, the cleaning should be strengthened. (7) Remove by hand finger rubbing. This method is suitable for large parts, after the blue hair and oil immersion with a soft flannel gently wipe the order in the surface of the workpiece, the red frost will be removed with the velvet wherever they go.

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