Advantages and principles of anodic passivation an

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Advantages and principles of anode passivation and chemical passivation of metals

the last process step in chemical cleaning is a key step, and its purpose is to prevent corrosion of materials. For example, after acid pickling, water washing and rinsing, the metal surface of the boiler is very clean, very activated and easy to be corroded, so passivation treatment must be carried out immediately to form a protective film on the cleaned metal surface to slow down corrosion

metal passivation caused by some passivators is called chemical passivation. The phenomenon of metal passivation caused by anodic polarization is called anodic passivation or electrochemical passivation. Passivation is an effective means to prevent metal from being corroded and protect metal. During chemical corrosion, the concentration of oxidant shall not be less than a certain critical value. There are two theories about the structure of passive film on metal surface

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) because the passivation process is carried out in a non reactive state, the passivator can be added and used repeatedly, so the service life is longer and the cost is more economical

3) passivation promotes the formation of oxygen molecular structure passivation film on the metal surface. The film is dense and stable, and has self repairing effect in the air at the same time. Therefore, compared with the traditional method of coating antirust oil, the passivation film formed by passivation is more stable and corrosion resistant

most of the charge effects in the oxide layer are directly or indirectly related to the process of thermal oxidation. 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 environmental atmosphere enters the generated oxide layer, and then the oxygen diffuses internally through silicon dioxide. When it reaches the Si02 Si interface, it reacts with silicon to form new silicon dioxide. In this way, oxygen enters, diffuses and reacts continuously, so that silicon close to the interface is continuously converted into silicon dioxide, and the oxide layer grows into the silicon wafer at a certain rate

through the study of high school chemistry, we all know that iron and aluminum can quickly dissolve in dilute HNO3 or dilute H2SO4 at room temperature, but not in concentrated HNO3 or concentrated H2SO4. Ordinary carbon steel is usually easy to rust. If an appropriate amount of Ni and Cr is added to the steel, it will become stainless steel. The phenomenon that the chemical stability of metals or alloys is significantly enhanced by some factors is called passivation, and it is also called "bluing" in industry. The phenomenon of metal passivation caused by some passivators (chemicals) is called chemical passivation. Oxidants such as concentrated HNO3, concentrated H2SO4, hclo3, K2Cr2O7 and KMnO4 can passivate metals. After metal passivation, its electrode potential moves in the positive direction, making it lose its original characteristics. For example, the passivated iron cannot replace copper in the copper salt. In addition, the metal can also be passivated by electrochemical methods, such as placing Fe in H2SO4 solution as an anode, polarizing the anode with an applied current, and using a certain instrument to raise the iron potential to a certain extent, Fe will be passivated. The phenomenon of metal passivation caused by anodic polarization is called anodic passivation or electrochemical passivation

passivation of metals such as aluminum alloy can protect metals from corrosion, but sometimes passivation must be prevented in order to ensure that metals can participate in the reaction and dissolve normally, such as electroplating and chemical power supply

aluminum alloy and other metals are recycled plastic granulator operations that touch a wide range of areas of the national economy. How can they be passivated? What is the passivation mechanism? First of all, it should be clear whether the passivation phenomenon is caused by the metal phase and solution phase, or by the interface phenomenon. Someone has studied the effect of mechanical scraping and grinding on the passivated metal. Experiments show that when the metal surface is scraped continuously during measurement, the potential of the metal moves violently in the negative direction, that is, trimming the metal surface can cause the activation of the passive metal. That is to say, passivation is an interface phenomenon. It changes on the interface between metal and medium under certain conditions. Electrochemical passivation refers to the formation of metal oxides or salts on the electrode surface when the potential of the metal changes during anodic polarization. These substances closely cover the metal surface and become a passive film, resulting in metal passivation. Chemical passivation is caused by the direct action of oxidants such as concentrated HNO3 on the metal and the formation of an oxide film on the surface, or the addition of metals that are easy to passivate, such as Cr, Ni, etc. During chemical passivation, the concentration of oxidant added should not be less than a certain critical value, otherwise it will not lead to passivation, but will cause faster dissolution of metal

what is the structure of the passive film on the metal surface, is it an independent phase film or an adsorption film? At present, there are mainly two theories, namely, phase forming film theory and adsorption theory. According to the theory of phase forming film, when aluminum alloy and other metals are dissolved, under passivation conditions, close and well covered solid substances are formed on the surface. This substance forms an independent phase, which is called passive film or phase forming film. This film mechanically separates the metal surface from the solution, greatly reducing the dissolution rate of the metal and presenting a passive state. The experimental evidence is that the existence of phase forming film can be seen on some passivated metal surfaces, and its thickness and composition can be measured. For example, a visible passive film can be separated by carefully dissolving and removing the metal under the film with a reagent that can dissolve the metal but does not work with the oxide film. How is the passive film formed? When the gold property is anodic dissolution, the composition of the solution layer around it changes. On the one hand, the dissolved metal ions accumulate due to the insufficient diffusion speed (fast dissolution speed). On the other hand, the hydrogen ions in the interface layer also migrate to the cathode, and the negative ions (including oh-) in the solution migrate to the anode

as a result, OH ions and other negative ions are enriched near the anode. With the continuation of electrolytic reaction, the electrolyte concentration in the solution layer adjacent to the anode interface may develop to saturated or supersaturated state. Therefore, metal hydroxide or some salt with small solubility product will be deposited on the surface of aluminum alloy and other metals and form an insoluble film. This film is often very loose, which is not enough to directly lead to the passivation of the metal, but can only hinder the dissolution of the metal, but the electrode surface is covered by it, and the contact area between the solution and the metal is greatly reduced. Therefore, it is necessary to increase the current density of the electrode, and the potential of the electrode will become corrected. This may cause OH ions to discharge on the electrode, and its products (such as oh) are related to the classification of electric spring fatigue testing machine and how to choose suitable equipment? The metal atoms on the electrode surface react to form a passive film. Analysis shows that most passive films are composed of metal oxides (such as iron oxide Fe2O3), but a few are also composed of hydroxide, chromate, phosphate, silicate, insoluble sulfate and chloride

according to the adsorption theory, it is not necessary to form a solid product film on the metal surface before passivation, but as long as an adsorption layer of oxygen or oxygen-containing particles (such as o2- or oh-) is formed on the surface or part of the surface, it is enough to cause passivation. Although the adsorption layer is only a single experiment with a lower speed and a thinner molecular layer, the adsorption of oxygen on the metal surface changes the interface structure between the metal and the solution, which increases the activation energy of the electrode reaction, reduces the reaction ability of the metal surface and passivates. The main experimental basis of this theory is to measure the interface capacitance and the amount of electricity required to passivate some metals. The experimental results show that some metals can be passivated without forming a phase forming film

both passivation theories can better explain some experimental facts, but both have successes and shortcomings. Metal passive films do have a phase forming film structure, but there are also monolayer adsorption films. At present, it is not clear under what conditions the phase forming film is formed and under what conditions the adsorption film is formed. The combination of the two theories still lacks direct experimental evidence, so the passivation theory needs to be further studied

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