Electrogalvanizing is the most commonly used and widespread method of metal anti-corrosion treatment. After color passivation, the corrosion resistance of the galvanized layer is improved.
Author: Marisa
Electrogalvanizing is the most commonly used and widespread method for metal anti-corrosion treatment. After color passivation, the corrosion resistance of the galvanized layer is improved, and the colored passivation film contains a large amount of hexavalent chromium, which can automatically repair the damaged passivation film. Therefore, the color passivation process is widely used in production. Currently, low chromium or trivalent chromium passivation processes are used. Due to the long immersion time of low chromium passivation process, only with the appropriate ratio of activator and acidity can the same color as high chromium passivation be obtained. Otherwise, the color will become lighter, the appearance will be poor, and the passivation film will easily peel off.
Reasons for the detachment of passivation film
1. Passivation time
Low chromium passivation solution has low acidity, and the passivation film is formed in the solution. So under normal circumstances, the quality of the passivation film is related to the passivation time. As the passivation time prolongs, the color change of the film follows the following pattern:
Light yellow one → Golden yellow one → Rainbow one → Green one → Olive one → Earth yellow. The thicker the film, the poorer the color and adhesion, and the film is prone to detachment. So it is necessary to control the passivation time according to the size and quantity of the parts. This is crucial for avoiding delamination under normal circumstances.
2. Cleaning before passivation
Most galvanization methods use bright galvanization, and a large amount of additives are added to the solution. These organic substances are mixed in the coating or adhere to the surface of the coating. If not cleaned thoroughly, it can lead to the following adverse consequences:
(1) The organic matter adsorbed on the surface of the parts will be trapped in the passivation layer to isolate the passivation layer, greatly reducing the adhesion and corrosion resistance of the passivation film, resulting in the detachment of the passivation film or a lighter color.
(2) The organic matter adsorbed on the surface of the parts will reduce hexavalent chromium to trivalent chromium, and excessive trivalent chromium and organic matter will directly affect the adhesion of the film.
(3) Due to careless cleaning before passivation, a large amount of chloride ions may be introduced. Although chloride ions can accelerate film formation, they can make the film too thick and prone to detachment. Simultaneously causing difficulty in controlling passivation time.
(4) Due to the presence of acid on the operator's clothing, gloves, and items in contact with the plating, if not cleaned in a timely manner, unintentional contact with the plating during operation may cause the passivation film to peel off.
3. Pre plating treatment
Due to incomplete removal of oil stains and oxide scales during pre-treatment, the adhesion of the local coating is poor, resulting in the detachment of the passivation film.
4. Unstable plating solution
Due to improper process control, the plating solution is unstable; Poor local conductivity of the anode and cathode; Failure to maintain the plating solution in a timely manner can result in loose, porous, and partially dull coating; Or the coating has high brittleness,
There are pockmarks, small bubbles, and poor adhesion of the coating, resulting in poor adhesion and detachment of the passivation film.
5. Improper control of brightener
The brightener was not added according to the Hall cell test, nor was it operated according to the principle of adding less and more frequently, resulting in too much brightener added, the solution being too dirty, and the composition ratio being imbalanced. Only brightener was used to increase brightness. As a result, excessive organic matter is mixed in the coating, the coating becomes too smooth, and the adhesion of the film weakens and falls off. Excessive organic matter is mixed in the coating, and during the passivation process, the film is first dissolved, resulting in the exposure of organic matter. The inclusion acts as a barrier on the surface of the part, causing the film to peel off.