To begin the anodizing process, three things are needed: an anodizing tank, a positive electrode (anode), and a negative electrode (cathode). First, the aluminum needs to be cleaned and its natural oxide layer removed. It’s then dipped into the anodizing tank, which has an electrically conductive solution in it. The solution is zapped with a current to release the aluminum’s electrons, leaving behind positively charged aluminum ions. During the process, the electrolyte completes the circuit between the anode and cathode, which can conduct electricity but won’t react with the electrolyte. Depending on the type of anodizing, that electrolyte will usually be something like sulfuric or chromic acid.
The electrons that leave the cathode are involved in producing negatively charged oxygen ions, which travel to the aluminum’s surface and join with the ions, creating a shiny new thin layer of aluminum oxide. You can adjust the thickness of that layer by controlling the density of the current, the time, temperature, and concentration of the electrolyte solution.
The first oxide layer that’s formed is often referred to as the barrier layer, and it’s continuous without any pores. But as the oxide layer continues to build up, it becomes increasingly porous because it starts restricting the flow of current, and this begins to create attachment points on the barrier layer which develop into small cylindrical pores that are orthogonal to that layer. As that happens, the current starts to spread out from the center of each of those pores, forcing the oxide layer to keep growing until it meets the oxide layers of nearby pores. The following image illustrates this process:
