The relationship between phosphating and passivation treatments

That’s not right; phosphorylation should be a subset of passivation

Generally, stainless steel is subjected to pickling and passivation, while carbon steel is treated with pickling and phosphating. Carbon steel can also be passivated

Phosphating is also known as phosphate passivation; it is indeed a type of passivation. However, the anti-corrosion effect of the phosphating layer is not very good, so it is often used as a base layer before applying paint.

What was said on the 4th floor is correct. Phosphating is primarily used as a primer to improve the adhesion and corrosion resistance of the paint film. Passivation is applied to all metal materials, regardless of whether they are stainless steel or not. This post was last edited by XXYY on 2009-4-15 08:32]

I believe it should be a parallel relationship; aside from the different processing techniques, the composition of the surface inert layer also differs

There are many passivation processes, which serve to prevent the surface of an object from changing color and to avoid further damage, allowing it to resist corrosion caused by air, acids, alkalis, salts, and other substances. Phosphating is just one of them; it is commonly used on carbon steel or ordinary iron sheets. It is generally not used alone, but rather in conjunction with painting processes afterward, or with sandblasting processes beforehand. Other passivation processes include anodizing and electropolishing.

Phosphating is now generally used as a pre-treatment before coating, in order to improve the adhesion between the coating and the substrate; it usually results in the formation of phosphates. Passivation generally aims to improve the corrosion resistance of the substrate. It’s my major from undergraduate studies; it’s a shame I haven’t used it in ages and have almost forgotten it.

What exactly does acid cleaning and purification mean?

5.3 Passivation Theory Metal passivation is an interfacial phenomenon that does not alter the properties of the metal itself; it merely changes the stability of the metal surface in a medium. The reasons for passivation are complex, and there are currently different views on its mechanism; no complete theory exists to explain all cases of passivation. Below are brief introductions to two theories that are currently considered to be able to explain most of the experimental facts satisfactorily, namely the phase-formed film theory and the adsorption theory. 5.3.1 Phase formation film theory This theory suggests that when a metal anode dissolves, a dense and well-covered solid product film can be formed on the surface of the metal. This layer of product film forms a separate solid-phase film layer that separates the metal surface from the medium, hindering the progress of the anodic process and resulting in a **reduction in the rate of metal dissolution**, thereby putting the metal into a passive state. 5.3.2 Adsorption theory The adsorption theory suggests that metal passivation occurs due to the formation of an adsorption layer of oxygen or oxygen-containing particles on the surface, which alters the structure of the metal/solution interface and significantly increases the activation energy for the anodic reaction. That is, due to the adsorption of these particles, the reactivity of the metal surface is reduced, resulting in passivation. 5.3.3 Comparison of the two theories Both of these passivation theories can adequately explain most experimental facts; however, neither theory can comprehensively and fully account for all passivation mechanisms. The similarity between these two theories is that both hold that the formation of an extremely thin passivation film on the metal surface prevents the metal from dissolving; however, their explanations for the formation of this film differ. The adsorption theory suggests that a two-dimensional film with a single molecular layer is sufficient to cause metal passivation, whereas the phase-forming film theory holds that at least a three-dimensional film several molecular layers thick is required to protect the metal from dissolution; the initially formed single-molecular-layer adsorption film can only slightly reduce metal dissolution, while a thicker phase-forming film can achieve complete passivation. Furthermore, another difference between the two theories is the debate over adsorption bonds versus chemical bonds. In fact, during the passivation process of metals, under different conditions, the adsorption film and the phase-forming film can play a dominant role respectively. Some have attempted to combine these two theories to explain all metal passivation phenomena, suggesting that the adsorption of oxygen-containing particles is a prerequisite for the formation of a good passivation film; it is possible that an adsorption film is formed first, which then develops into a coherent film. It is believed that the ease of passivation mainly depends on the adsorption film, whereas the maintenance of the passivated state depends primarily on the phase-forming film. The growth of the membrane also follows a logarithmic law; the controlling factor for the adsorbed membrane is the electron tunneling effect, whereas the controlling factor for the phase-formed membrane is the movement of ions through the barrier. The general laws of electrochemical corrosion kinetics indicate that when a metal dissolves following the normal anodic reaction pathway, the higher the electrode potential, the greater the rate of metal dissolution. This is the case for nickel and iron when subjected to anodic polarization in hydrochloric acid. However, in many cases, opposite results can also be observed. If the electrode potential of a metal shifts in the positive direction due to an applied anodic current or a local anodic current, beyond a certain value the rate of dissolution of the metal decreases sharply. This phenomenon can be observed when iron and stainless steel are anodically polarized in sulfuric acid. This “anomalous” phenomenon during the dissolution of a metal anode is known as the passivation process of the metal. 　　If an iron sheet is placed in it, it will dissolve violently, and the dissolution rate of iron increases rapidly as the concentration rises. When the concentration reaches 30–92%, the solubility reaches its maximum value; however, if the concentration is increased further (above 50%), the solubility of iron drops by a factor of ten thousand, resulting in a special state of surface treatment. At this point, even if it is transferred to sulfuric acid, it will no longer be eroded by the acid, as the metal has become passivated. 　　If the medium contains strong oxidizing properties that cause the metal to become passivated. They are collectively referred to as passivators, although the occurrence of passivation does not depend solely on the strength of the oxidizing ability of the passivator. Obviously, this is related to the effect of anionic pairs on the passivation process. 　　Although the occurrence of passivation is usually associated with the action of oxidizing media, some metals can undergo passivation in non-oxidizing media as well. Molybdenum and niobium can be passivated in hydrochloric acid, while mercury and silver can be passivated in the presence of chloride ions. 　　In summary, if the passivation phenomenon occurs as a result of the natural interaction between the metal and the passivating agent, it is referred to as \"chemical passivation\" or \"auto-passivation\". Metals such as chromium, aluminum, and titanium are easily passivated by oxygen in air and in many oxygen-containing solutions, which is why they are known as \"self-passivating metals\". 　　Experimental results show that in electrolyte solutions free of active chloride ions, the passivation of metals can also be induced by anodic polarization; for example, 18-8 type stainless steel dissolves rapidly. However, when an external current is applied to cause anodic polarization, and the potential is polarized to -0.1V (SCE), the dissolution rate of the stainless steel drops sharply to a fraction of tens of thousands of its original value. Moreover, it maintains a high level of stability within the range of 0.1 to +1.2 V; this phenomenon is known as \"anodic passivation\" or \"electrochemical passivation\". Metals such as iron, nickel, chromium, and molybdenum can all undergo electrochemical passivation due to anodic polarization. 　　“\"Anodic passivation\" and \"chemical passivation\" are essentially the same, as both phenomena occur due to some kind of change in the surface of the metal that was originally activated and dissolved. This mutation causes the anodic dissolution process of the metal to no longer follow the Tafel law, and its dissolution rate drops sharply as a result. Therefore, passivation refers to such a sudden change in the surface condition of a metal. The state in which a metal is after passivation is called the passive state, and the properties possessed by a metal in its passive state are known as passivity. 　　There is also a term called \"mechanical passivation,\" which refers to the deposition of a thick, yet more or less porous layer of salts on the surface of a metal in certain environments. This layer of salt, which is usually non-conductive, actually acts as a mechanical barrier to separate the reactants, thereby reducing the rate of metal corrosion. Such passivation phenomena clearly do not require the metal’s electrode potential to shift in the positive direction. Even when the solubility product of salt is very low, the electrode potential can still shift in the negative direction. This is the case with lead in sulfuric acid, magnesium in aqueous solutions, and silver in chloride solutions. 　　Studying the passivation phenomenon has great practical significance. Since a metal in a passive state has a very low dissolution rate, it can potentially be used to slow down metal corrosion. By adding certain metals that are prone to passivation to iron, various stainless steels are produced; these steels can easily become passivated in strongly oxidizing environments. Therefore, such alloy steels can be used as substitutes for precious metals in the manufacture of chemical equipment that comes into contact with strongly oxidizing media. Phosphating is one of the important methods for preventing corrosion in metal materials; its purposes include providing corrosion protection for the base metal, serving as a primer before painting, enhancing the adhesion and corrosion resistance of coating layers, and acting as an anti-friction lubricant during metal processing. Based on their application, they can be divided into three categories: 1. Coating-type phosphating, 2. Cold extrusion lubrication phosphating, 3. Decorative phosphating. Classified by the phosphate used, they include: zinc phosphate series, zinc calcium phosphate series, iron phosphate series, zinc manganese phosphate series, and manganese phosphate series. Phosphating can be classified by temperature as: high-temperature phosphating (above 80 °C), medium-temperature phosphating (50–70 °C), low-temperature phosphating (around 40 °C), and room-temperature phosphating (10–30 °C). I. Mechanism of phosphating film formation Phosphating involves the following processes: (1) The dissolution of the metal, that is, the reaction between the metal and the free acids in the phosphating solution: M + H3PO4 = M(H2PO4)2 + H2↑. (2) The acceleration process carried out by accelerants is as follows: M(H2PO4)2 + Fe²⁺ → M3(PO4)2 + FePO4. Due to the oxidizing effect of oxidants, the gradual deposition of insoluble salts is accelerated, which isolates the metal substrate from the bath solution and can limit or even stop the acid etching process. (3) Hydrolysis of phosphoric acid and its salts The basic components of phosphating solutions are acid phosphates of one or more heavy metals, with the molecular formula Me(H2PO4)2. These acid phosphates dissolve in water, and under certain concentrations and pH values, they undergo hydrolysis to produce free phosphoric acid: Me(H2PO4)2 → MeHPO4 + H3PO4; 3MeHPO4 → Me3(PO4)2 + H3PO4; H3PO4 → H2PO4- + H+; H2PO4- → HPO4^2- + 2H+; HPO4^2- → PO4^3- + 3H+. Due to the sharp decrease in the H+ concentration on the surface of the metal workpiece, the dissociation equilibria of phosphate ions shift to the right, resulting in the formation of phosphate ions. (4) Formation of the phosphating film: When the PO3-4 ions released from the metal surface reach saturation in combination with the metal ions Zn2+, Mn2+, and Fe2+ present in the phosphating solution, they crystallize and deposit on the surface of the metal workpiece. The crystal grains continue to grow until a continuous, water-insoluble, and strong phosphating film is formed on the surface of the metal workpiece: 3M2+ + 2PO3-4 + 4H2O → M3(PO4)2·4H2O ↓; 2M2+ + Fe2+ + 2PO3-4 + 4H2O → M2Fe(PO4)2·4H2O. Some of the Fe2+ ions released from the metal workpiece are used as components of the phosphating film, while the remaining Fe2+ ions in the phosphating solution are oxidized to Fe3+, resulting in the formation of FePO4 precipitates, which are one of the main components of the phosphating sludge. The aforementioned phosphating principle can explain the film-forming process of zinc-based phosphatings, zinc-calcium-based phosphatings, and manganese-based phosphatings; it can also explain the film-forming process in the case of zinc parts and aluminum parts. However, the phosphating film on zinc parts consists only of zinc phosphate, while for aluminum parts, more fluorides need to be added in order to form AlF3 and AlF3·6H2O. II. Various Applications of Phosphating 1. Priming for Painting Since metals are polar substances, while paint is an organic polymer compound that is non-polar, applying paint directly to the surface of steel parts results in poor adhesion, and the paint tends to peel off easily. Phosphating before painting solves this problem; during phosphating, a reaction occurs with the iron on the metal surface, causing phosphates to be firmly deposited there. Moreover, the phosphating layer contains tiny pores, and when paint is applied, its polymers can penetrate these pores, thereby enhancing the adhesion of the paint and preventing it from peeling off. This extends the corrosion resistance period. For painting purposes, zinc-based or zinc-calcium-based phosphatings are generally used, and high-quality phosphatings that can be applied at room temperature also yield good results. 2. Decorative Phosphating After phosphating, a phosphating layer is formed on the surface of steel parts, which isolates them from air, oxygen, etc. By sealing the pores in this phosphating layer, excellent corrosion resistance is achieved. Common decorative phosphatings include manganese-based, zinc-based, zinc-calcium-based, and zinc-manganese-based types. Among them, the manganese-based phosphating coating has the darkest color, being black-gray, while the zinc-based coating has the lightest color, being gray. For decorative phosphating, manganese-based phosphating is the preferred choice. Due to the stability of manganese phosphates, their corrosion resistance is **better than that of zinc-based phosphatings. Moreover, manganese-based phosphatings have a darker color; their particle crystals are hemispherical in shape, giving them a smooth texture. Additionally, the wear resistance of manganese-based phosphating films is higher than that of zinc-based ones, making them suitable for applications where higher wear resistance is required. 3. Lubricating phosphating for cold extrusion: The phosphating layer contains tiny pores that can store saponification fluid, thereby providing lubrication and preventing the workpiece from sticking to the surface of the die during cold extrusion, which would otherwise cause damage to the expensive die. III. Black phosphating: In addition to producing gray to dark gray phosphating films, it is also possible to use black phosphating. Currently, black phosphating technologies can be divided into two types. One method is to modify the formula of the original phosphating solution in order to change the color of the phosphating film to black; another method is to treat the steel’s surface in two steps: by normal-temperature blackening first and then phosphating. First, the steel is blackened using a normal-temperature blackening process to achieve a good black appearance, followed by phosphating treatment to enhance the adhesion and corrosion resistance of the surface coating. The black phosphating technology for steel at room temperature is still in the research stage. A approach that combines phosphating with room-temperature blackening, in order to make full use of the advantages of each method and obtain a surface film with good adhesion and corrosion resistance, is a route worth exploring. Even when done in two steps, it is much simpler than the high-temperature alkaline phosphating process.

This post was last edited by hanermin on 2009-11-7 at 12:31. Purpose: To carry out a thorough acid washing and passivation of stainless steel, removing various types of oils, rust, scale, weld spatter, and other contaminants; after treatment, the surface becomes a uniform silver-white color. This process **improves the corrosion resistance of stainless steel and 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 anti-fogging 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 specific 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 appears, with a uniform and dense passivation film formed. After treatment, remove it and rinse it thoroughly with clean water; it is advisable to rinse it further with alkaline water or lime water to neutralize it. 2. The phenomenon in which the chemical reactivity of an active metal or alloy **decreases, resulting in it assuming a precious-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 transforms to 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.