The voltage curve of anodization can be roughly divided into three sections.
The first section: the voltage rises sharply to the maximum with time, which is called the critical voltage (or the voltage) in a few to ten seconds of the starting of the electrified layer. A continuous, non porous film layer (barrier) is formed on the anode. The film has a higher resistance, so as the film thickens, the resistance increases, and the tank voltage rises sharply. The presence of a non porous layer hinders the continuous thickening of the film, which is directly proportional to the formation voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte. In normal sulfuric acid anodizing, the thickness of the non porous layer is about 0.Ol to 0.Ol5 0.Ol5 m when the frequency of the high frequency oxidation power supply is 13 ~ 18V. The characteristic of this section is that the formation rate of oxide film is much larger than that of dissolution. The critical voltage is greatly affected by the temperature of the electrolyte, the temperature is high, the electrolyte has strong dissolution effect on the film, the thin layer has no pore, and the critical voltage is low.
The second paragraph: after the anodic potential reaches the highest value, the decrease of the membrane hole is 10% to 15% of the maximum value. This is due to the dissolution of the electrolyte on the film, resulting in holes in the thinnest part of the oxide film, and the resistance decreases, and the voltage drops. After the oxide film has pores, the electrochemical reaction can continue and the oxide film continues to grow.
The third stage: thickening of the porous layer is characterized by a gradual stabilization of the voltage at about 20s. At this time, the formation velocity of the barrier layer is balanced with the dissolution rate, and its thickness remains unchanged, but the oxidation reaction does not stop. The formation and dissolution of the oxide film continue to be carried out at the bottom of each hole, making the bottom of the hole moving to the inside of the metal, and with the prolongation of time, the hole and the hole are formed to form pore and hole wall. Because of the existence of the electrolyte in the pores, the conductive ions can be unimpeded here, so the change of the resistance value is not significant during the establishment of the porous layer, and the voltage has no obvious change, which is reflected on the characteristic curve as a stationary phase. The thickness of porous layer depends on the technological conditions, and the main factor is temperature. In the process of anodic oxidation, the corrosion of the film is increased because of the influence of various factors, and the corrosion of the film is increased. The corrosion rate of the diaphragm and the surface film layer is increased by the hole bottom, so the thickness of the porous layer increases slowly. The thickness of the porous layer will not continue to increase when the corrosion rate of the pore layer is equal to the velocity of the film forming at the bottom of the hole. The longer the arrival time of the balance, the thicker the oxide film is.