[China Aluminum Network] is a key step in the chemical cleaning process. Its purpose is to prevent corrosion of materials. If the boiler is pickled, rinsed and rinsed, the metal surface is very clean, very active, and susceptible to corrosion. Therefore, it must be immediately passivated to produce a protective film on the cleaned metal surface to slow the corrosion.

The passivation of metals caused by certain passivators is called chemical passivation. The metal passivation caused by anodic polarization is called anode passivation or electrochemical passivation. Passivation is an effective means of preventing metal from being corroded and protecting the metal. In chemical corrosion, the oxidant concentration should not be less than a certain critical value. What is the structure of the passivation film on the metal surface? At present, there are mainly two kinds of theory.

Passivation advantages

1) Compared with the traditional physical enclosure method, the passivation process has the characteristics of 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 performed in a non-reactive state, the passivation agent can be added and used repeatedly, so that the lifespan is longer and the cost is more economical.

3) Passivation promotes the formation of oxygen molecular structure passivation film on the metal surface, dense film layer, stable performance, and self-repair in the air, so compared with the traditional method of coating rust-proof oil, passivation formed The passivation film is more stable and more corrosion resistant.

Most of the charge effects in the oxide layer are directly or indirectly related to the thermal oxidation process. In the temperature range of 800-1250C, there are three continuous stages of the thermal oxidation process using dry oxygen, moist oxygen, or water vapor. First, the oxygen in the ambient atmosphere enters into the generated oxide layer, and then the oxygen passes through. The silica diffuses into the interior and reacts with silicon as it reaches the Si02-Si interface, forming new silica. In this way, the process of oxygen-diffusion-reaction takes place continuously, so that the silicon near the interface continuously transforms into silicon dioxide, and the oxide layer grows to the inside of the silicon wafer at a certain rate.

Through the study of high school chemistry, we all know that at room temperature, iron and aluminum can quickly dissolve in dilute HNO3 or dilute H2SO4, but it is insoluble in concentrated HNO3 or concentrated H2SO4. Ordinary carbon steel is usually easy to rust, if you add the right amount of Ni, Cr in steel, it becomes stainless steel. The phenomenon that the metal or alloy is influenced by some factors and the chemical stability is obviously enhanced is called passivation, and some people in the industry call it "blue". The passivation of metals caused by certain passivators (chemicals) is called chemical passivation. Concentrated HNO3, concentrated H2SO4, HClO3, K2Cr2O7, KMnO4 and other oxidants can passivate the metal. After metal passivation, the electrode potential moves in the positive direction, causing it to lose its original characteristics. For example, passivated iron cannot replace copper in the copper salt. In addition, electrochemical methods can also be used to passivate metals. For example, Fe is placed in an H2SO4 solution as an anode, and the anode is polarized by an applied current. The iron potential is increased to a certain degree by using a certain instrument, and Fe is passivated. The metal passivation caused by anodic polarization is called anode passivation or electrochemical passivation.

Aluminum alloys and other metals in the passive state can protect the metal to prevent corrosion, but in order to ensure that the metal can normally participate in the reaction and dissolved, but also must prevent the passivation, such as electroplating and chemical power.

How is the aluminum alloy and other metals passivated? What is the passivation mechanism? First of all, it should be clear that the passivation phenomenon is caused by the metal phase and the solution phase, or by the interface phenomenon. Someone has studied the effect of mechanical scraping on the metal in a passivated state. Experiments show that when the metal surface is continuously scraped during measurement, the potential of the metal moves sharply in the negative direction, that is, trimming the metal surface can cause activation of the passive metal. That is, the passivation phenomenon is proved to be an interface phenomenon. It is under certain conditions that the interface between metal and the medium is changed. When the electrochemical passivation is anodic polarization, the metal potential changes to form metal oxides or salts on the surface of the electrode. These substances are closely covered on the metal surface as a passivation film resulting in metal passivation, and chemical passivation is the formation of an oxide film on the surface by an oxidizing agent such as concentrated HNO3 directly to the metal, or adding an easily passivated metal such as Cr Caused by Ni, etc. In the chemical passivation, the concentration of oxidants added should not be less than a certain critical value, otherwise it will not only lead to passivation, but will cause the metal to dissolve faster.

What is the structure of the passivation film on the metal surface, is it an independent film or an adsorbent film? At present, there are mainly two kinds of theory, that is, phase film theory and adsorption theory. Phase film theory believes that when metals such as aluminum alloys are dissolved, under the condition of passivation, a dense, well-covered solid material is formed on the surface, and this material forms an independent phase, called a passivation film or A phase film, which mechanically separates the metal surface from the solution, so that the dissolution rate of the metal is greatly reduced and passive. The experimental evidence is that on some passivated metal surfaces, the presence of a phase-forming film can be seen and its thickness and composition can be measured. If a certain kind of reagent that can dissolve the metal and does not work with the oxide film is carefully removed to remove the metal under the film, the visible passivation film can be separated and how the passivation film is formed. When the metal anode dissolves, the composition of the solution layer around it changes. On the one hand, dissolved metal ions accumulate due to insufficient diffusion (fast dissolution). On the other hand, hydrogen ions in the interface layer also migrate toward the cathode, and 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 the electrolysis reaction, the concentration of the electrolyte in the solution layer immediately adjacent to the anode interface may develop into a saturated or supersaturated state. As a result, metal hydroxide or some kind of salt with a small solubility product will be deposited on the surface of metal such as aluminum alloy and form an insoluble film. This film is often loose, and it is not enough to directly lead to metal passivation. It can only hinder the dissolution of the metal, but the surface of the electrode is covered by it, and the contact area between the solution and the metal is greatly reduced. As a result, the current density of the electrode is increased, and the potential of the electrode becomes more positive. This may cause the OH-ion to discharge on the electrode, and its product (such as OH) reacts with the metal atoms on the surface of the electrode to form a passivation film. The analysis shows that most passivation films consist of metal oxides (such as iron oxide Fe2O3), but a few also consist of hydroxides, chromates, phosphates, silicates, and insoluble sulfates and chlorides. .

Adsorption theory holds that the surface of the metal does not need to be formed into a solid product film to passivate, but that an adsorbed layer of oxygen or oxygen-containing particles (such as O2- or OH-) forms on the surface or part of the surface is sufficient to cause passivation. Although this adsorbent layer is only monolayer thick, due to the adsorption of oxygen on the metal surface, the interface structure between the metal and the solution is changed, the activation energy of the electrode reaction is increased, and the reaction ability of the metal surface is decreased and passivated. The main experimental basis for this theory is to measure the interface capacitance and the amount of electricity required to passivate certain metals. The experimental results show that some metals can be passivated without forming a phase film.

Both passivation theories can explain some experimental facts well, but they all have successes and inadequacies. The metal passivation film does have a phase film structure, but at the same time, there is also a monolayer adsorption film. It is not yet clear under what conditions a phase film is formed and under what conditions an adsorption film is formed. The combination of the two theories still lacks direct experimental evidence, so the passivation theory remains to be studied in depth.

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