Process control to prevent passivation film detachment
Author: Marisa
Process control to prevent passivation film detachment
1. Control of the ratio of activator to chromic anhydride
The aging or improper composition of the passivation solution, such as excessive sulfate or chloride ions, rapid film formation, film thickness, and difficulty in controlling time. Short time, poor color; Longer time, thicker film, easy to peel off. So in low chromium passivation, the influence of sulfate ions is the most severe. Its content should not exceed 0.8g/L. Under normal circumstances, the ratio of activator to chromic anhydride is roughly SO2-4/Cr03=
1.0-4.0, Cl -/Cr03=1.5-1.0, insufficient activator, slow film formation rate, unclear film, prone to white mist; Too much activator leads to a fast film formation rate, a decrease in film color, and even the inability to obtain a colored passivation film. Therefore, it is best to use nitric acid to adjust the pH value, rather than sulfuric acid or hydrochloric acid.
But the nitric acid content should not be too high, because as the nitric acid content increases, the dissolution of the membrane accelerates, causing the membrane to become thinner, the bonding is not firm, and it is easy to fall off. Low nitric acid content, poor gloss of the film, not shiny. In normal operation, sulfuric acid cannot be directly added, but rather sulfate is added. If there is too much sulfuric acid, add a small amount of carbonate lock or hydrated lime, and then adjust the pH value to around 1.5 with nitric acid.
At the same time, if there is too much zinc in the passivation solution, the solution will age and the passivation film is prone to detachment or fogging. Should be regenerated or renewed, and no more chromic acid should be added to prevent an increase in chromic acid concentration, which would lose the meaning of low chromium passivation.
2. Control of process conditions
Low pH value, fast film formation, and easy detachment; High pH value, poor color, and prone to fogging; If the pH value is too low, the membrane will turn black. Under normal circumstances, the pH value is controlled between 1-2
It is appropriate.
High temperature, fast film formation, thick and loose film, easy to peel off; Low temperature, thin film with light color, long passivation time, slow film formation, but good adhesion. Usually conducted at room temperature. In winter, it is better to control the temperature between 15-30 ° C.
3. Improper operation
(1) In addition to strictly controlling the passivation time during operation, for parts of different sizes, poor swinging or shaking in the passivation solution can cause the edges of the parts to have dark colors and even film thickness to peel off; The color of the middle part is light or the color of the small parts is uneven, even light, and some films have peeled off. Therefore, it is necessary to swing or shake appropriately during operation. It is best to use air agitation for better results.
(2) During the passivation process, the parts cannot be stopped halfway. If the pause time reaches more than ten seconds, the passivation film will also fall off when passivated again.
(3) During the electroplating process, the current should be appropriate and not too high. Otherwise, there will be too much organic matter mixed in the coating, although the coating is bright, the adhesion of the film will weaken, and even the passivation film can be wiped off by hand, and the brittleness will increase. A slightly thicker coating is prone to peeling. So in operation, the current should be appropriate and the brightness of the coating should be moderate.
4. Out of light
Low chromium passivation, very low acidity, and almost no polishing effect on the coating. Therefore, before passivation, polishing must be carried out to avoid an increase in the pH value of the passivation solution, and more importantly, to chemically polish the galvanized layer and obtain a uniformly glossy coating, ensuring the adhesion and appearance quality of the passivation film.
5. Post passivation treatment
(1) Cleaning
After passivation, it should be carefully cleaned, otherwise residual acid on the surface of the parts can easily cause the passivation film to peel off. So it needs to be thoroughly cleaned with several layers of water, and finally cleaned with running water.
(2) Drying (aging)
The drying temperature should not exceed 65 ° C to improve the hardness and adhesion of the passivation film. Otherwise, the passivation film will lose too much water, turn red or even yellow, and the film is prone to detachment, cracking, and a decrease in corrosion resistance. The drying temperature should be kept basically the same.