Why is acid washing and passivation required for stainless steel equipment?

1. Reason: Comprehensive acid washing and passivation of stainless steel to remove 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. This method is suitable for various types of stainless steel parts, sheets, and related equipment. 2. Materials used: Depending on the type of stainless steel and the severity of the oxide scale, it can be used in its pure form or diluted with water in 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 appears, 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. 3. Wash both the inside and outside. 4. Standard: SJ 20893-2003 Specifications for Acid Cleaning and Passivation of Stainless Steel

Well, the person upstairs explained it in great detail; I’ll learn from it*

2# hbwqlj: I have a question. Isn’t the process of removing grease done using alkaline solutions? And tasks like rust removal are also carried out with alkaline solutions, right? So how does that acid work here?

All of the speakers have covered this very thoroughly; therefore, acid washing of stainless steel is absolutely necessary.

Uh, thanks. I found it and took a look; I get it now. But why are there requirements for CL ions in the water when washing stainless steel equipment?

The mechanism of chloride ion-induced corrosion in stainless steel: In chemical production, corrosion occurs frequently during the use of pressure vessels, and it is one of the main factors that lead to various defects in these vessels. Ordinary steel has poor corrosion resistance, whereas stainless steel possesses excellent mechanical properties as well as good corrosion resistance. Cr and Ni are the main alloying elements that confer corrosion resistance to stainless steel. Cr and Ni enable stainless steel to form a highly dense oxide film in oxidizing media, thereby passivating the steel and reducing its corrosion rate in such media, which improves the steel’s corrosion resistance. The activation effect of chloride ions plays an important role in both the formation and destruction of the oxide film on stainless steel. Although there is still no definitive conclusion as to the mechanism by which chloride ions convert passivated metals into an activated state, it can generally be divided into two viewpoints. According to the theory of film formation, because chloride ions have a small radius and strong penetrating ability, they can most easily penetrate the extremely small pores in the oxide film to reach the metal surface. There, they interact with the metal to form soluble compounds, which alter the structure of the oxide film and cause corrosion of the metal. The adsorption theory suggests that the fundamental reason why chloride ions destroy the oxide film is their strong ability to be adsorbed by metals; they are preferentially adsorbed by the metals, thereby displacing oxygen from the metal surface. Since oxygen determines the passivation state of metals, chloride ions compete with oxygen for the adsorption sites on the metal surface; they can even replace the passivating ions that are adsorbed there and form chlorides with the metal. The adsorption of chlorides on the metal surface is not stable, resulting in the formation of soluble substances, which accelerates corrosion. Results of electrochemical studies on the passivation state of stainless steel show that the activating effect of chloride ions on the metal surface occurs only within a certain range; there is a specific potential value at which stainless steel begins to activate. This potential is the breakdown potential of the membrane; the higher the breakdown potential, the more stable the passive state of the metal. Therefore, the stability of the passivation state of stainless steel and its corrosion resistance in various media can be measured by the breakdown potential value. 2 Stress corrosion failure and protective measures 2.1 Mechanism of stress corrosion failure Among the causes of corrosion failure in pressure vessels, stress corrosion failure accounts for approximately 45%. Therefore, it is particularly important to study the stress corrosion failure of stainless steel pressure vessels. Stress corrosion refers to low-stress brittle fracture caused by the combined effect of tensile stress and a corrosive environment. Stress corrosion generally occurs under specific conditions: ①Only under the action of tensile stress. ② Environments that cause stress corrosion always contain specific corrosive agents; stainless steel is prone to stress corrosion in corrosive media containing oxygenated chloride ions as well as in solutions of H2SO4 and H2S. ③ Stress corrosion generally occurs easily in alloys and carbon steel. Studies show that the formation of stress corrosion cracks is primarily related to the concentration of chloride ions and temperature. Stress sources of pressure vessels: ① Tensile stress on the outer surface of the vessel caused by external loads. ②　Various residual stresses that arise during the manufacturing of pressure vessels, such as assembly residual stresses generated during the assembly process, and welding residual stresses produced during manufacturing. In chemical production, pressure vessels are exposed to a wide variety of media, many of which contain chloride ions; under such conditions, the pressure vessels suffer from stress corrosion failure. In an aqueous solution containing oxygen and chloride ions, chromium-nickel stainless steel first forms an oxide layer on its metal surface; this layer prevents further corrosion and causes the stainless steel to become passivated. Due to the tensile stress in the pressure vessel itself and the additional stress resulting from the thickening of the protective film, the protective film in certain areas ruptures. The base metal at those ruptured areas is then exposed directly to the corrosive medium. The electrode potential at these locations is lower than that of the areas where the protective film remains intact, which results in the formation of the anode of a microcell and thus anodic dissolution. Because the anode is small while the cathode is large, the dissolution rate of the anode is very high. Once corrosion reaches a certain level, a new protective film forms; however, under tensile stress, this film can be damaged again, leading to further anodic dissolution. During this repeated formation and breaking of the protective film, corrosion in certain local areas intensifies, eventually leading to the formation of holes. The presence of these holes causes stress concentration, which further accelerates the plastic deformation of the hole surfaces and the breakdown of the protective film. The combined effect of this tensile stress and the corrosive medium leads to stress corrosion cracks. 2.2 Measures to prevent stress corrosion failure: Methods to control stress corrosion failure include addressing the internal factors by selecting appropriate materials, and addressing external factors by controlling stress, the medium, or potential, among others. The actual situation is ever-changing, and it can be applied according to the specific circumstances. (1) Use of stress-corrosion-resistant materials: In recent years, various stainless steels resistant to stress corrosion have been developed, including high-purity austenitic chromium-nickel steels, high-silicon austenitic chromium-nickel steels, high-chromium ferritic steels, and ferritic-austenitic duplex steels. Among them, ferrite-austenite duplex steel has the best resistance to stress corrosion. (2) Control of stress: During the assembly of pressure vessels, stress concentration should be minimized, and the areas in contact with the medium should have as little residual stress as possible to prevent damage from impacts or scratches; welding procedures must be followed strictly. (3) Strictly adhere to operating procedures. Process operations and process conditions have a significant impact on the corrosion of pressure vessels. Therefore, it is necessary to strictly control process parameters such as raw material composition, flow rate, medium temperature, pressure, and pH value. Add a corrosion inhibitor within the limits permitted by the process conditions. When chromium-nickel stainless steel is used in chlorides containing dissolved oxygen, the mass fraction of oxygen should be reduced to below 1.0 × 10^-6. Practice has shown that in water with a chloride ion mass fraction of 500.0 ×10^-6, adding a mixture of nitrate with a mass fraction of 150.0 ×10^-6 and sodium sulfite with a mass fraction of 0.5 ×10^-6 yields good results. (4) Maintenance and management: To ensure the long-term safe operation of pressure vessels, it is necessary to strictly comply with relevant regulations and rules regarding pressure vessels. Defects that are permitted to exist in operational pressure vessels must be rechecked regularly, so as to keep track of how these defects develop over time and take appropriate measures to reduce corrosion of the equipment. 3 Corrosion failure due to pitting and preventive measures 3.1 Mechanism of corrosion failure due to pitting: In localized areas on the surface of pressure vessels, small holes form that cause corrosion to progress deeper; other areas experience no corrosion or only slight corrosion. This type of corrosion is known as pitting corrosion (also referred to as spot corrosion). Pitting generally occurs easily in stationary media. Metals with self-passivation properties often suffer from pitting in media containing chloride ions. Erosion holes usually develop along the direction of gravity or horizontally; once formed, pitting has a tendency to dig deeper, that is, it accelerates automatically as it goes deeper. In aqueous solutions containing chloride ions, the oxide film on the surface of stainless steel begins to dissolve. This is because chloride ions preferentially adsorb onto the oxide film, displacing oxygen atoms; they then combine with the cations in the oxide film to form soluble chlorides. As a result, small pits with a diameter of 20μm to 30μm are formed on the base metal, and these small pits serve as the nuclei for pitting corrosion. Under applied anodic polarization, as long as the medium contains a certain amount of chloride ions, it is possible for an erosion nucleus to develop into an erosion hole. In corrosion under natural conditions, the presence of oxygen or cationic oxidants in media containing chloride ions can promote the growth of corrosion nuclei into corrosion pores. Oxidizing agents can accelerate the anodic polarization process, raising the corrosion potential of the metal above the pitting critical potential. The metal surface inside the corrosion pit is in an activated state with a relatively negative potential, while the metal surface outside the pit is in a passivated state with a relatively positive potential. As a result, a microelectrode couple corrosion cell consisting of an active state and a passive state is formed inside and outside the pit. This cell features a large cathode-to-anode area ratio; the anode current density is high, which causes the corrosion pit to deepen rapidly. Meanwhile, the metal surface outside the pit is protected by cathodic protection, allowing it to remain in a passivated state. Anodic dissolution primarily occurs within the pores: Fe → Fe2+ + 2e, Cr → Cr3+ + 3e, Ni → Ni2+ + 2e. When the medium is neutral or slightly alkaline, the main reaction outside the pores is: O2 + H2O + 2e → 2OH-. 　　Due to the separation of the positive and negative poles, secondary corrosion products will form at the pore openings, providing little protection. The medium inside the pore is in a stagnant state relative to the medium outside the pore; dissolved metal cations cannot diffuse outward easily, and dissolved oxygen cannot diffuse inward either. As the concentration of metal cations in the pore increases, chloride ions migrate in to maintain electrical neutrality, thereby forming a concentrated solution of metal chlorides within the pore. This concentrated solution helps keep the metal surface in the pore in an activated state. Furthermore, as a result of chloride hydrolysis, the acidity of the medium inside the pores increases, which accelerates anode dissolution and further promotes the development of these pores. The pH value of the medium at the pore openings gradually rises, and the soluble salts in water are converted into precipitates; as a result, rust and scale deposits together at the pore openings, forming a closed battery. After the occluded battery is formed, material exchange inside and outside the pores becomes more difficult, leading to a greater concentration of metal chlorides within the pores. The hydrolysis of these chlorides further increases the acidity of the medium, and this increased acidity accelerates the rate of anode dissolution. As a result, the erosion of the pores progresses at a faster pace, and it is possible for the metal surface to be completely eroded through. This effect of acidification within the pores caused by a closed circuit, which accelerates corrosion, is known as autocatalytic acidification. There are many factors that affect pitting corrosion; the properties of the metal or alloy, its surface condition, the properties of the medium, pH value, temperature, and so on are all major factors influencing pitting corrosion. Most pitting corrosion occurs in media containing chloride ions or chlorides. Metals with self-passivation properties are more sensitive to pitting; the stronger their passivation ability, the higher their sensitivity. Experiments show that under anodic polarization conditions, the presence of chloride ions in the medium alone is sufficient to cause pitting in metals. Moreover, as the concentration of chloride ions increases, the pitting potential decreases, making pitting more likely to occur, and it further accelerates the process of pitting. A stationary medium can accelerate pitting more than a flowing medium. The flow rate of the medium has a dual effect on reducing pitting: an increased flow rate (while still in a laminar state) facilitates the delivery of dissolved oxygen to the metal surface, allowing oxide films to form more easily; on the other hand, it reduces the chances of deposits accumulating on the metal surface, thereby decreasing the likelihood of pitting occurring. 3.2 Measures to prevent pitting corrosion: (1) By adding elements such as molybdenum, nitrogen, and silicon to stainless steel, or by increasing the chromium content while adding these elements, steel grades with excellent properties can be obtained. Corrosion-resistant stainless steels can be basically divided into 3 categories: ferritic stainless steels; ferritic-austenitic duplex steels; and austenitic stainless steels. Materials resistant to pitting corrosion should be given priority in design. (2) Reduce the concentration of chloride ions in the medium, and take strict precautions to prevent phenomena such as leakage, spillage, dripping, and seepage during operation. (3) Corrosion inhibitors can be added if the process conditions permit. The requirement for corrosion inhibitors is to increase the stability of the passivation film or to facilitate the repassivation of a damaged passivation film. For example, adding 3% NaNO2 to a 10% FeCl3 solution can prevent pitting in 1Cr18Ni9Ti steel over the long term. (4) External cathodic current protection is employed to suppress pitting. The corrosion of stainless steel pressure vessels by chloride ions has a significant impact on the safety of these vessels. Even with reasonable design and precise manufacturing to avoid or reduce defects in the container itself, over time, due to the combined effect of various complex factors, the container will still suffer some degree of corrosion. Although the methods for preventing chloride ions from corroding stainless steel are not yet fully developed, it is still essential to know some of the most basic protective measures in order to ensure the smooth progress of production. In addition, it is necessary to follow the operating procedures strictly, strengthen equipment management, and conduct regular inspections of containers to ensure their safe operation within a reasonable service life. Hehe! Found it on my own, sharing here

1. Alkaline solutions are generally used for the degreasing of carbon steel. High-concentration NaOH solutions can corrode stainless steel; therefore, alkaline solutions are not typically used to degrease stainless steel. 2. Use water with a chloride content of less than 25PPM to wash stainless steel, as high chloride levels can cause hydrogen embrittlement and pitting in stainless steel. 3. Stainless steel can be cleaned using either strong acids or weak acids, and the choice of acid depends on the requirements of the client. After all, the stainless steel is meant to be used in practice, not just for display; it needs to meet certain standards, and there are certain ions that must not be present. 4. As for whether to clean the inside or the outside, it is generally necessary to clean the inside; of course, if financial conditions permit, cleaning the outside as well is better.

There are pickling pastes available on the market. Go and ask.

Have those stainless steel railings on the streets been pickled? Which model is this usually? It’s rare to see any rust on them. But the equipment is quite rusty.