Moving Forward Every Day ——2010.01.29

The last edit to this post was made by JFTANG726 on 2010-1-28 at 20:44. The reference answers are as follows: There are two types of oxide films on metal surfaces. One type of oxide film is not only a poor ion conductor but also a poor electron conductor (an electrical insulator); this applies to the oxide films on metals such as aluminum and titanium； Another type, although an inferior ionic conductor, is an electronic conductor (semiconductor), such as the oxide films on metal materials like iron and chromium. The latter type of oxide film covers the surface of the metal material and does not have a significant effect on the metal’s electrical conductivity; however, as it is a poor ion conductor, it can prevent the anodic dissolution of the metal. This type of oxide film is known as a passivation film. The surface condition covered by a passivation film is called the passivated state, or simply the passive state. The properties of a metal surface in a passive state are known as passivity. Therefore, the metal surface state without a passivation film is referred to as an activated state. In less aggressive corrosive environments, stainless steel can maintain an intact passivation film on its surface, making it a typical metal material in a passive state.

A method of converting the metal surface into a state that is less susceptible to oxidation, thereby slowing down the rate of metal corrosion. 　　The phenomenon in which the chemical reactivity of an active metal or alloy **decreases**, resulting in it assuming a noble-metal state, is called passivation. 　　If the corrosion products formed on a metal as a result of the action of a medium have a dense structure, forming a thin film (often invisible) that covers the metal’s surface tightly, this changes the surface condition of the metal. As a result, the metal’s electrode potential shifts toward positive values, leading to a corrosion-resistant passive state. For example, when Fe turns into Fe2+, the standard electrode potential is –0.44 V; after passivation, this value jumps to +0.5–1 V, resulting in the corrosion-resistant properties typical of noble metals. This thin film is known as a passivation film. 　　The passivation of metals can also be a spontaneous process (such as the formation of an insoluble compound on the metal’s surface, namely an oxide film). In industry, passivators (mainly oxidizing agents) are used to passivate metals, forming a protective film.

Passivation creates an electron-conductive film on the metal surface that prevents the metal from dissolving; this film has very low solubility in the surrounding medium, which allows the rate of anodic dissolution of the metal to be kept at a very low level. Such a surface film is known as a passivation film. The passivation film on aluminum-titanium is relatively dense and silver-white, while iron-chromium passivation films can take on various colors

It can prevent and slow down the dissolution of the anode; this film is generally referred to as a passivation film. When stainless steel parts are exposed to air, an oxide film forms on them, but this film does not provide sufficient protection. Usually, a thorough cleaning is required first, including alkali cleaning and acid cleaning, followed by passivation with an oxidizing agent, in order to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation, ensuring the formation of a high-quality passivation film. Because acid cleaning removes an average layer of 10μm thick from the surface of stainless steel, the chemical activity of the acid causes the dissolution rate in defective areas to be higher than in other parts of the surface; thus, acid cleaning helps to achieve a uniform balance across the entire surface, eliminating potential areas that are prone to corrosion. But more importantly, through pickling and passivation, iron and its oxides dissolve preferentially over chromium and its oxides, removing the chromium-poor layer and resulting in an accumulation of chromium on the surface of the stainless steel. The potential of this chromium-rich passivation film can reach +1.0V (SCE), which is close to the potential of precious metals, thereby enhancing the corrosion resistance. Different passivation treatments also affect the composition and structure of the film, thereby influencing its corrosion resistance. For example, through electrochemical modification, the passivation film can be given a multi-layered structure; CrO3 or Cr2O3 can be formed in the barrier layer, or a glassy oxide film can be created, enabling stainless steel to achieve maximum corrosion resistance. Beijing University of Science and Technology provides a brief overview using the photoelectron spectroscopy (XPS) study of the passivation film on 316L steel as an example. Stainless steel passivation involves the dissolution of the surface layer due to certain factors and the adsorption of water molecules; under the catalysis of oxidants, oxides and hydroxides are formed, which undergo reaction transformations with the Cr, Ni, and Mo elements that make up stainless steel. This process results in the formation of a stable protective film that prevents the destruction of the film and corrosion. When iron reacts with concentrated sulfuric acid, an oxide layer forms on the surface of the iron, preventing further reaction between the iron and the concentrated sulfuric acid; this is what is commonly referred to as passivation. When iron reacts with dilute sulfuric acid, ferrous sulfate and hydrogen are produced. The principle of passivation when aluminum reacts with concentrated nitric acid is the same as that for iron reacting with concentrated sulfuric acid; when aluminum reacts with dilute nitric acid, aluminum nitrate, nitrogen monoxide, and water are produced. Titanium is a precious metal with two allotropes: the stable alpha form at low temperatures (882.5°C) and the stable beta form at higher temperatures; the volume increases by 5.5% during the phase transition. Oxygen, nitrogen, and carbon are stabilizers of the α-type. The compatibility of titanium with various oxide phases plays a crucial role. Due to the extremely low passivation electrode potential of titanium, both chemical oxidation and atmospheric heating oxidation can be used to form a thick passivation film on the titanium surface, thereby enhancing its corrosion resistance; among these methods, atmospheric heating oxidation yields the best results. Furthermore, oxide films of different thicknesses on the titanium surface produce different colors due to light interference. Halogen elements, led by chloride ions, can cause pitting in the oxide film on the titanium surface. Titanium and oxygen can form oxides with various titanium/oxygen ratios and different structures, such as TiO2 (which has three structures: rutile, anatase, and brookite), TiO, Ti3O4, Ti2O3, etc. Under normal conditions, an oxide film 5–10 nm thick can be rapidly formed on the surface of fresh titanium. Under certain conditions, this oxide film can grow; it has been found that the thickness of the oxide film on the surface of dental implants used in the mouth for 6 years can reach 200 nm. Therefore, for titanium implants, what is most relevant to biocompatibility are the chemical properties of the oxide (i.e., ceramic) on the surface of titanium, rather than those of the metallic titanium itself. Thickening the oxide film on the titanium surface using certain methods can improve its corrosion resistance. Many studies by various authors have shown that chemical oxidation, anodic oxidation, and atmospheric heating oxidation can all result in the formation of a thicker passivation film on the titanium surface, thereby enhancing its corrosion resistance; among these methods, atmospheric heating oxidation yields the best results

How is metal passivated? What is its passivation mechanism? Is the passivation phenomenon caused by the metal phase and the solution phase, or by interfacial phenomena? Some studies have been conducted on the effect of mechanical abrasion on metals in a passivated state. Experiments show that continuously scraping the metal surface during measurement causes the metal’s potential to shift sharply in the negative direction; in other words, polishing the metal surface can lead to the activation of metal in a passive state. This proves that the passivation phenomenon is an interfacial phenomenon. It occurs at the interface where a metal and a medium come into contact under certain conditions. Electrochemical passivation occurs when, during anodic polarization, the potential of the metal changes, leading to the formation of metal oxides or salts on the electrode surface. These substances cover the metal surface tightly to form a passivation film, thereby causing the metal to become passivated. Chemical passivation occurs when oxidizing agents such as concentrated HNO3 act directly on the metal to form an oxide film on its surface, or by adding metals that are prone to passivation such as Cr and Ni. During chemical passivation, the concentration of the oxidizing agent added must also be above a certain critical value; otherwise, not only will a passive state not be achieved, but the metal will instead dissolve more rapidly. What is the structure of the passivation film on metal surfaces? Is it a **phase film or an adsorptive film?** Currently, there are mainly two theories, namely the phase-formed film theory and the adsorption theory. The phase formation film theory suggests that when a metal dissolves, under passivation conditions, a dense and highly covering solid substance is formed on the surface. This substance constitutes a **phase, which is known as a passivation film or phase formation film. This film mechanically separates the metal surface from the solution, thereby **reducing the rate of metal dissolution and putting the metal in a passivated state. Experimental evidence shows that on certain passivated metal surfaces, a phase-formed film can be observed, and its thickness and composition can be measured. By using a reagent that can dissolve metal without affecting the oxide film, and carefully dissolving the metal beneath the film, the visible passivation film can be separated. How is the passivation film formed? When the metal anode dissolves, the composition of the solution layer surrounding it changes. On the one hand, the dissolved metal ions accumulate because their diffusion rate is not fast enough (the dissolution rate is fast). On the other hand, hydrogen ions in the interfacial layer also migrate toward the cathode, while the anions in the solution (including OH-) migrate toward the anode. As a result, OH- ions and other negative ions accumulate near the anode. As the electrolytic reaction proceeds, the electrolyte concentration in the solution layer adjacent to the anode surface may reach a saturated or supersaturated state. As a result, metal hydroxides or certain salts with a low solubility product deposit on the metal surface, forming an insoluble film. This film is often quite porous, and it is not sufficient to cause passivation of the metal on its own; it can only hinder the dissolution of the metal. However, by covering the electrode surface, it significantly reduces the contact area between the solution and the metal. Therefore, the current density at the electrode must be increased, and the potential of the electrode will become more positive. This can lead to the discharge of OH- ions at the electrode, and the products resulting from this (such as OH) react with the metal atoms on the electrode surface to form a passivation film. Analysis shows that most passivation films are composed of metal oxides (such as Fe2O3 for iron), but a few are also made up of hydroxides, chromates, phosphates, silicates, as well as insoluble sulfates and chlorides. According to the adsorption theory, it is not necessary for a solid product film to form on the metal surface in order for passivation to occur; rather, the formation of an adsorption layer of oxygen or oxygen-containing species (such as O2- or OH-) on the surface or part of the surface is sufficient to induce passivation. Although this adsorption layer is only as thick as a single molecular layer, the adsorption of oxygen on the metal surface alters the interface structure between the metal and the solution, increasing the activation energy for electrode reactions and reducing the reactivity of the metal surface, thereby causing passivation. The main experimental basis for this theory is the measurement of interfacial capacitance and the amount of charge required to passivate certain metals. Experimental results show that some metals can be passivated without the need to form a phase-forming film. Both passivation theories can explain some experimental facts well, but each has its strengths and weaknesses. Metal passivation films do have a phase-formed film structure, but there are also adsorptive films in the form of monolayers. It is not yet clear under what conditions a phase-forming film is formed, and under what conditions an adsorbed film is formed. There is still a lack of direct experimental evidence supporting the combination of these two theories; therefore, the passivation theory requires further investigation. 1# JFTANG726

The phenomenon in which the chemical reactivity of an active metal or alloy **decreases**, resulting in it assuming a noble-metal state, is called passivation. If the corrosion products formed on a metal as a result of the action of a medium have a dense structure, forming a thin film (often invisible) that covers the metal’s surface tightly, this changes the surface condition of the metal. As a result, the metal’s electrode potential shifts toward positive values, leading to a corrosion-resistant passive state. For example, when Fe turns into Fe2+, the standard electrode potential is –0.44 V; after passivation, this value jumps to +0.5–1 V, resulting in the corrosion-resistant properties typical of noble metals. This thin film is known as a passivation film. Mechanism: Iron and aluminum dissolve rapidly in dilute HNO3 or dilute H2SO4, but dissolution almost completely stops in concentrated HNO3 or concentrated H2SO4. Carbon steel tends to rust easily; however, by adding an appropriate amount of Ni and Cr to the steel, stainless steel is obtained. The phenomenon in which the chemical stability of a metal or alloy increases significantly as a result of certain factors is known as passivation. The metal passivation phenomenon caused by certain passivators (chemical substances) is called chemical passivation. Oxidizing agents such as concentrated HNO3, concentrated H2SO4, HClO3, K2Cr2O7, and KMnO4 can all cause metal passivation. After metal passivation, its electrode potential shifts in the positive direction, causing it to lose its original properties; for example, passivated iron cannot displace copper from copper salts. Furthermore, metal passivation can also be achieved through electrochemical methods; for example, by placing Fe in an H2SO4 solution as the anode and polarizing the anode with an external current, certain instruments are used to raise the iron potential to a certain level, thereby causing the iron to become passivated. The metal passivation phenomenon caused by anodic polarization is called anodic passivation or electrochemical passivation. When metal is in a passivated state, this protects it from corrosion; however, in situations where the metal needs to dissolve in order to participate properly in reactions, it is necessary to prevent passivation, as in cases such as electroplating and chemical batteries. How is metal passivated? What is its passivation mechanism? First of all, it is necessary to determine whether the passivation phenomenon is caused by the metal phase and the solution phase, or by interfacial phenomena. Some studies have been conducted on the effect of mechanical abrasion on metals in a passivated state. Experiments show that continuously scraping the metal surface during measurement causes the metal’s potential to shift sharply in the negative direction; in other words, polishing the metal surface can lead to the activation of metal in a passive state. This proves that the passivation phenomenon is an interfacial phenomenon. It occurs at the interface where a metal and a medium come into contact under certain conditions. Electrochemical passivation occurs when, during anodic polarization, the potential of the metal changes, leading to the formation of metal oxides or salts on the electrode surface. These substances cover the metal surface tightly to form a passivation film, thereby causing the metal to become passivated. Chemical passivation occurs when oxidizing agents such as concentrated HNO3 act directly on the metal to form an oxide film on its surface, or by adding metals that are prone to passivation such as Cr and Ni. During chemical passivation, the concentration of the oxidizing agent added must also be above a certain critical value; otherwise, not only will a passive state not be achieved, but the metal will instead dissolve more rapidly. Classification: Chemical passivation – The passivation phenomenon that results from the natural interaction between metals and passivators. Metals such as chromium, aluminum, and titanium are prone to being passivated by oxygen in air and many oxygen-containing solutions; therefore, these metals are known as self-passivating metals. Using it can help certain metals slow down corrosion. As is common for steel, solutions such as nitric acid, potassium dichromate, and sodium nitrite are used for passivation treatment ； By adding easily passivatable metal components to iron, corrosion-resistant stainless steels and similar materials can be produced. Pickling and passivation: Used for the thorough pickling and passivation of stainless steel, removing various types of oils, rust, oxide scales, weld spatter, and other contaminants. After treatment, the surface becomes a uniform silver-white color, **which enhances the corrosion resistance of stainless steel. It is suitable for various types of stainless steel parts, sheets, and related equipment. Features: Simple to operate, easy to use, cost-effective. It also contains high-efficiency corrosion inhibitors and antifog agents to prevent excessive corrosion and hydrogen embrittlement in metals, as well as to suppress the formation of acid fog. It is particularly suitable for small, complex workpieces in situations where coating is not feasible, and it outperforms similar products on the market. Treatment: Passivation treatment is the final process step in chemical cleaning and is a crucial one. After being pickled, rinsed with water, and washed out, the metal surface of the boiler becomes very clean and highly activated, making it susceptible to corrosion. Therefore, passivation must be carried out immediately to form a protective layer on the cleaned metal surface, thereby reducing corrosion. Purpose: To carry out a thorough acid washing and passivation treatment of stainless steel, removing various types of oils, rust, scale, weld spatter, and other contaminants; after this treatment, the surface becomes a uniform silver-white color. This improves the corrosion resistance of stainless steel, and it is suitable for various types of stainless steel parts, sheets, and related equipment. Features: Simple to operate, easy to use, cost-effective. It also contains high-efficiency corrosion inhibitors and antifog agents to prevent excessive corrosion and hydrogen embrittlement in metals, as well as to suppress the formation of acid fog. It is particularly suitable for small, complex workpieces in situations where coating is not feasible, and it outperforms similar products on the market. Usage: Depending on the type of stainless steel and the severity of oxidation, it can be used in its pure form or diluted with water at a ratio of 1:1 to 1:4 ； Ferritic, martensitic, and austenitic stainless steels with low nickel content (such as 420, 430, 200, 201, 202, 300, 301, etc.) are used after dilution, while austenitic stainless steels with higher nickel content (such as 304, 321, 316, 316L, etc.) are soaked in their original form ； It is generally used at room temperature or after being heated to 50–60 degrees. Soak it for 3–20 minutes or longer (the exact time and temperature should be determined by the user based on their own testing), until the surface dirt is completely removed and a uniform silver-white color is achieved, with a uniform and dense passivation film formed. After treatment, take it out and rinse it thoroughly with clean water; it is advisable to rinse it further with alkaline water or lime water to neutralize it. Passivation: The process of using a chromate solution to react with a metal’s surface, thereby forming a trivalent or hexavalent chromium layer, is known as passivation, or chromating. It is commonly used in the treatment of aluminum, magnesium, and their alloys; it can also form a chromium layer on steel. However, it is rarely used alone and is usually employed in combination with phosphating to seal the pores in the phosphating layer, thereby passivating the exposed steel within that layer and suppressing the corrosive effect of residual phosphating accelerants, thus enhancing the protective properties. For passivation, a potassium dichromate solution (2–4 grams per liter) is generally used; sometimes 1–2 grams of phosphoric acid is added as well. The part is immersed in this solution at 80–90 degrees Celsius for 2–3 minutes before being removed and washed with water.

If the corrosion products formed on a metal as a result of the action of a medium have a dense structure, forming a thin film (often invisible) that covers the metal’s surface tightly, this changes the surface condition of the metal. As a result, the metal’s electrode potential shifts toward positive values, leading to a corrosion-resistant passive state. For example, when Fe turns into Fe2+, the standard electrode potential is –0.44 V; after passivation, this value jumps to +0.5–1 V, resulting in the corrosion-resistant properties typical of noble metals. This thin film is known as a passivation film. Generally, passivation films can be divided into three categories: 1 Films with colors ranging from light yellow to brown: with a film weight of 3.2~11 g/dm2; 2 Yellow films used as a base layer for painting: with a film weight of 1.1~3.8 g/dm2; 3 Colorless decorative films or light yellow films with low contact resistance

There are two types of oxide films on metal surfaces: one type of oxide film is not only a poor ion conductor but also a poor electron conductor (an electrical insulator), such as the oxide films on metal materials like aluminum and titanium； Another type, although an inferior ionic conductor, is an electronic conductor (semiconductor), such as the oxide films on metal materials like iron and chromium. The latter type of oxide film covers the surface of the metal material and does not have a significant effect on the metal’s electrical conductivity; however, as it is a poor ion conductor, it can prevent the anodic dissolution of the metal. This type of oxide film is known as a passivation film. The surface condition covered by a passivation film is called the passivated state, or simply the passive state. The properties of a metal surface in a passive state are known as passivity. Therefore, the metal surface state without a passivation film is referred to as an activated state. In less aggressive corrosive environments, stainless steel can maintain an intact passivation film on its surface, making it a typical metal material in a passive state.

Passivation involves the formation on a metal surface of an electron-conducting film that prevents the metal from dissolving; this film has very low solubility in the surrounding medium, which allows it to keep the rate of anodic dissolution of the metal at a very low level. Such a surface film is known as a passivation film. Oxide films on metal materials such as aluminum and titanium are not only poor ion conductors but also poor electron conductors (electrical insulators) ； Although the oxide films on metal materials such as iron and chromium are poor ion conductors, they are electron conductors (semiconductors).

Passivation involves the formation on a metal surface of an electron-conducting film that prevents the metal from dissolving; this film has very low solubility in the surrounding medium, which allows it to keep the rate of anodic dissolution of the metal at a very low level. Such a surface film is known as a passivation film. The passivation films of aluminum and titanium are formed due to the oxide layer on their surfaces. The passivation film of iron-nickel is formed due to a chromium-rich oxide film on its surface.