Passivation refers to the process in which the metal is oxidized by a strong oxidant or electrochemical method to make the surface into an inactive state, that is, passivation.
It is a method of slowing down the corrosion rate of the metal by converting the metal surface into a state that is not easily oxidized. In addition, the phenomenon of an active metal or alloy in which the chemical activity is greatly reduced and becomes a noble metal state is also called passivation. Cherry blossoms
Basic meaning
Passivation, if the corrosion product generated by the metal due to the action of the medium has a dense structure and forms a thin film (often invisible), which closely covers the surface of the metal, the surface state of the metal is changed, and the electrode of the metal is changed. The potential greatly jumps to the positive direction, and becomes a passive state of corrosion resistance.
For example, when Fe→Fe2+, the standard potential is -0.44V, and it jumps to +0.5~1V after passivation, and shows the noble metal performance of corrosion resistance. This film is called passivation film.
Passivation of metals may also be a spontaneous process (eg, the formation of a layer of insoluble compounds, ie oxide films, on the surface of the metal).
In industry, a passivation agent (mainly an oxidant) is used to passivate the metal to form a protective film.
Common examples: cold concentrated sulfuric acid, cold concentrated nitric acid, iron and aluminum can be passivated.
Corrosion Definition
Corrosion is an electrochemical process in which a small amount of ions are present on the untreated metal surface, and these ions move from a region of high potential (cathode) to a region of low potential (anode), thus generating an electric current.
Under the action of this current, it will accelerate the destructive attack on the metal and cause corrosion in the weak part of the metal potential, which is rust in popular terms.
Passivation principle
The mechanism of its passivation can be explained by the film theory, that is, it is believed that the passivation is due to the action of the metal and the oxidizing substance, and a very thin, dense, good covering performance is formed on the metal surface during the action, and it is firmly adsorbed on the metal surface. on the passivation film.
This film exists as a separate phase, usually a compound of metal oxides. It plays the role of completely separating the metal from the corrosive medium, preventing the metal from contacting with the corrosive medium, so that the metal basically stops dissolving and forms a passive state to prevent corrosion.
with anti-rust oil
The main difference between passivation and anti-rust oil is that the products produced are different; anti-rust oil uses the oil film to seal the pores on the metal surface to isolate and prevent rust from contacting with oxygen. In fact, there is no reaction.
The oil film is relatively easy to be removed and destroyed with the progress of production; passivation is the use of oxidizing substances in the passivation solution to produce redox reactions with metals, to promote the formation of a layer of metal oxides on the metal surface to achieve effective The purpose of protecting metal.
This process is a chemical reaction. The generated passivation film is dense and complete and is not easily damaged.
Passivation advantages
1) Compared with the traditional physical sealing method, the passivation treatment has the characteristics of absolutely not increasing the thickness of the workpiece and changing the color, improving the precision and added value of the product, and making the operation more convenient;
2) Since the passivation process is carried out in a non-reactive state, the passivation agent can be added and used repeatedly, so the service life is longer and the cost is more economical.
3) Passivation promotes the oxygen molecular structure passivation film formed on the metal surface, the film layer is dense, stable in performance, and has a self-healing effect in the air at the same time, so compared with the traditional method of applying anti-rust oil, passivation formed The passivation film is more stable and more resistant to corrosion.
Most of the charge effects in the oxide layer are directly or indirectly related to the thermal oxidation process. In the temperature range of 800-1250 °C, the thermal oxidation process with dry oxygen, wet oxygen or water vapor has three continuous stages. First, the oxygen in the ambient atmosphere enters the formed oxide layer, and then the oxygen passes through the dioxide Silicon diffuses inward, and when it reaches the SiO2-Si interface, it reacts with silicon to form new silicon dioxide.
In this way, the entry-diffusion-reaction process of oxygen continuously occurs, so that the silicon near the interface is continuously converted into silicon dioxide, and the oxide layer grows into the silicon wafer at a certain rate.
Previous: machining parts brushed Next: spray paint
Source: @cyanbat Author: cyanbat
