After hard oxidation of aluminum alloy, a layer of red precipitate sometimes adheres to the surface, which is caused by too much ferric oxide after the oxidation solution has been used for too long. This less crystalline iron oxide has little solubility in concentrated alkali solution, so it will precipitate on the surface of the workpiece during oxidation. The following measures can be taken to eliminate and prevent this phenomenon.

Strictly control the working temperature of aluminum alloy hard oxidation bluing solution. The working temperature required for bluing solution is based on the carbon content of steel parts or whether there is alloy composition.

For the parts with high carbon content, the initial temperature and end temperature of the solution are lower when bluing, while for the parts with low carbon content or alloy steel, the opposite is true.

When different kinds of steel products are blued at the same time, the time should be staggered. The products with high carbon content can be treated in the first tank first. When the treatment reaches the end point, the temperature of the solution also rises, which is just the temperature required for the products with low carbon content to enter the tank at the beginning. This can avoid the appearance of red hanging frost.

Adopt secondary bluing process. The specific method is as follows: aluminum alloy hard oxidation shall be blued in the solution formula with low working temperature for about 15min.

Then move it to the solution slightly higher than the normal working temperature and continue bluing for 40 ~ 60min. The temperature of the first tank solution is controlled at 130 ~ 135 ℃, and the temperature of the second tank solution is controlled at 140 ~ 150 ℃ (it varies with the carbon content of steel parts).

Then, the thick frosting can be formed in the first frosting groove, and then the corrosion resistance of the second frosting film can be reduced.

The flushing step of aluminum alloy hard oxidation is added in the bluing process. After the parts are treated in the bluing tank for 5min, take them out of the tank and rush them under the tap. At this time, because the red hanging frost adhered to the surface of the parts has just formed, it is thin and easy to wash off. Then, when the bluing continues, there will be less red hanging frost again.

Potassium ferrocyanide is added to the hard oxidation bluing solution of aluminum alloy. Because potassium ferrocyanide can complex with high valence iron ions to form Prussian blue precipitation, so as to reduce the content of iron ions in the solution and reduce the appearance of red sediment on the surface of the workpiece, but this method is slightly expensive and toxic.

Strengthen the maintenance of aluminum alloy hard oxidation bluing solution. Strengthening the maintenance of Bluing solution is one of the most effective methods to prevent red frost. It is a permanent solution. To a certain extent, the red frost on the surface of parts increases with the increase of high valent iron ions in bluing solution. Therefore, it needs to be cleaned after a certain amount of high valent iron ions are accumulated in bluing solution.

The specific methods are as follows:

First, add 1 / 4 of the original volume of hot water into the bluing solution to dilute it, and then heat it to boiling (at this time, the boiling point temperature of the solution is about 120 ℃). After standing, suck out the red sediment at the bottom of the tank and put it in the basin (after precipitation, the supernatant can return to the bluing tank).

Concentrate the supernatant or add materials to make it meet the mass concentration required by the process formula, and then bluish, and there will be no red frost on the surface of the film. This method needs to be carried out frequently.

Aluminum alloy hard oxide is removed from sodium cyanide solution. The red hanging frost formed on the surface of the oxide film can be removed by soaking in sodium cyanide solution. After treatment with this solution, the red hanging frost on the surface of the parts can be completely removed. It should be noted that cleaning should be strengthened afterwards.

Manual finger wipe removal. This method is applicable to large parts. After bluing and soaking in oil, gently wipe the surface of the parts with a piece of soft flannelette in order, and the red hanging frost will be removed with the flannelette.