Why Do Titanium Anodes Sometimes Undergo Passivation?

During the production process, a light cyan or gray-black film sometimes appears on the surface of the anode. In severe cases, the cell voltage will increase, the current will drop, and the current will not open too much. This phenomenon indicates that the anode is passivated, which causes passivation. What are the reasons for this?

1.Excessive current density: When the anode current density is greater than 4A/dm2, the alloy anode or single metal anode does not dissolve, a large amount of oxygen is evolved on the anode surface, and the anode plate becomes black and passivated. In actual operation, although the current density has not changed, sometimes because the cleaning work on the anode rod is not done well, some of the anodes are poorly contacted and non-conductive, which actually increases the current density of other parts of the anode, causing anode passivation The phenomenon appears.

2. Too little free sodium cyanide: On alloy anodes or copper anodes, when copper is dissolved, it reacts with free cyanide in the solution to generate copper cyanide complex ions. When the free sodium cyanide in the solution is insufficient, a black or light cyan passivation film is formed on the surface of the anode, which reduces the active surface of the anode, thereby increasing the anode current density, causing the anode to passivate and generating divalent copper ions on the anode surface. At this time, the surface of the anode was light blue, and the surface of the nearby solution was light blue.

3. Too little free sodium hydroxide: In alloy anodes or tin anodes, when tin is dissolved, it reacts with free sodium hydroxide in the solution to generate stannate. When the free sodium hydroxide in the solution is insufficient, a black metastannic acid film is formed on the anode surface to adhere to the anode, which reduces the active surface of the anode, thereby increasing the anode current density and causing anode passivation.