Passivation is a method of transforming the metal surface into a state that is less susceptible to oxidation and slowing down the corrosion rate of the metal. In addition, the phenomenon of an active metal or alloy in which the chemical activity is greatly reduced and becomes a noble metal is also called passivation. Basic introduction Chinese name: Passivation Foreign name: Passivation; Inactivation; Deactivation Category: Chemical reaction, electrochemical reaction Common: aluminum and iron are passivated in concentrated sulfuric acid and concentrated nitric acid Commonly used agents: stainless steel passivation liquid, stainless steel passivation paste basic Meaning, corrosion definition, passivation principle, and anti-rust oil, passivation advantages, mechanism, phase film theory, adsorption theory, ship passivation, pickling passivation treatment, precautions, quality inspection, examples, long list of passivation Parts, passivation of long parts, passivation of flat parts, surface passivation, passivation of water-prone parts, passivation of small parts, basic meaning passivation, corrosion products generated by metal due to the action of the medium. If the metal has a dense structure, it will form A layer of thin film (often invisible) is formed and tightly covers the surface of the metal, which changes the surface state of the metal, causing the electrode potential of the metal to greatly jump to the positive direction and become a corrosion-resistant passive state. For example, when Fe → Fe 2+, the standard potential is -0.44V. After passivation, it jumps to +0.5~1V, showing corrosion-resistant noble metal properties. This thin film is called a passivation film. Passivation of metals may also be a spontaneous process (such as the formation of a layer of insoluble compounds, an oxide film, on the surface of the metal). In industry, passivating agents (mainly oxidants) are used to passivate metals to form a protective film. Common examples: cold concentrated sulfuric acid, cold concentrated nitric acid, iron and aluminum can all be passivated. Definition of Corrosion Corrosion is an electrochemical change process. There are a small number of electric ions on the surface of untreated metal. These electric ions will move from a high potential area (cathode) to a low potential area (anode), thus generating an electric current. Under the action of this current, the destructive attack on the metal will be accelerated and corrosion will occur in the weak parts of the metal potential. In layman's terms, it is rust. Passivation Principle The passivation mechanism can be explained by the thin film theory, that is, passivation is due to the interaction between metal and oxidizing substances, which generates a very thin, dense, good covering performance and firmly adsorbed on the metal surface. Passivation film on metal surfaces. This film exists as a separate phase, usually a compound of oxidized metals. It plays the role of completely separating the metal from the corrosive medium, preventing the metal from contacting the corrosive medium, so that the metal basically stops dissolving and forms a passive state to prevent corrosion. The main difference between passivation and anti-rust oil is that the products are different; anti-rust oil uses an oil film to seal the pores on the metal surface to isolate it from contact with oxygen and effectively prevent rust. In fact, no reaction occurs. . The oil film is relatively easy to be removed and destroyed as production progresses; passivation uses the oxidizing substances in the passivation liquid to produce a redox reaction with the metal to promote the formation of a layer of metal oxide on the metal surface to achieve effective The purpose of protecting metal. This process is a chemical reaction. The passivation film produced is dense, complete and difficult to be damaged. Advantages of passivation 1) Compared with the traditional physical sealing method, 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, making the operation more convenient; 2) Due to the passivation The process is carried out in a non-reactive state, and the passivating agent can be added and used repeatedly, so the life is longer and the cost is more economical. 3) Passivation promotes the formation of an oxygen molecular structure passivation film on the metal surface. The film layer is dense and stable in performance, and it also has a self-repairing effect in the air. Therefore, compared with the traditional method of applying anti-rust oil, the passivation formed Passivation film is more stable and 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 800-1250°C, the thermal oxidation process using dry oxygen, wet oxygen or water vapor has three continuous stages. First, oxygen in the ambient atmosphere enters the generated oxide layer, and then the oxygen passes through the oxygen dioxide layer. Silicon diffuses internally, and when it reaches the Si02-Si interface, it reacts with silicon to form new silicon dioxide. In this way, the entry-diffusion-reaction process of oxygen continues to occur, so that the silicon near the interface is continuously converted into silicon dioxide, and the oxide layer grows into the inside of the silicon wafer at a certain rate.
As for the mechanism, we know that iron and aluminum can dissolve quickly in dilute HNO3 or dilute H2SO4, but the dissolution phenomenon almost completely stops in concentrated HNO3 or concentrated H2SO4. Carbon steel is usually easy to rust. If an appropriate amount of Ni is added to the steel , Cr, becomes stainless steel. The phenomenon that the chemical stability of metal or alloy is significantly enhanced due to the influence of some factors is called passivation. The metal passivation phenomenon caused by certain passivators (chemicals) is called chemical passivation. Oxidants such as concentrated HNO3, concentrated H2SO4, HClO3, K2Cr2O7, KMnO4, etc. can passivate metals. After the metal is passivated, its 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, metals can also be passivated using electrochemical methods. For example, Fe is placed in a H2SO4 solution as an anode, an external current is used to polarize the anode, and a certain instrument is used to increase the iron potential to a certain extent, and Fe is passivated. The metal passivation phenomenon caused by anodic polarization is called anodic passivation or electrochemical passivation. Metal in a passivated state can protect the metal from corrosion, but sometimes in order to ensure that the metal can participate in the reaction and dissolve normally, passivation must be prevented, such as electroplating and chemical power supplies. How is metal passivated? What is its passivation mechanism? First of all, it must be clear whether the passivation phenomenon is caused by the metal phase and the solution phase, or by the interface phenomenon. The effect of mechanical scraping on metals in a passivated state has been studied. Experiments show that if the metal surface is continuously scraped during measurement, the potential of the metal will violently move in the negative direction. That is, trimming the metal surface can cause the activation of the metal in the passive state. That is to say, it proves that the passivation phenomenon is an interface phenomenon. It changes at the interface between metal and medium under certain conditions. Electrochemical passivation is when the potential of the metal changes during anodic polarization and metal oxides or salts are formed on the electrode surface. These substances closely cover the metal surface and form a passivation film, causing metal passivation. Chemical passivation is the direct action of oxidants such as concentrated HNO3 on the metal to form an oxide film on the surface, or the addition of easily passivable metals such as Cr. , Ni, etc. caused by. During chemical passivation, the concentration of the added oxidant should not be less than a certain critical value, otherwise it will not 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 phase membrane or an adsorption membrane? There are two main theories, namely phase film theory and adsorption theory. The phase-forming film theory holds that when a metal dissolves and is under passivation conditions, a dense, well-covered solid material is generated on the surface. This material forms an independent phase, which is called a passivation film or a phase-forming film. This film mechanically isolates the metal surface from the solution, greatly reducing the dissolution rate of the metal and leaving it in a passive state. The experimental evidence is that on some passivated metal surfaces, the existence of a phase-forming film can be seen, and its thickness and composition can be measured. If you use a reagent that can dissolve metal but has no effect on the oxide film, and carefully dissolve and remove the metal under the film, you can separate the visible passivation film. How is the passivation film formed? As the metal anode dissolves, the composition of the nearby solution layer changes. On the one hand, the dissolved metal ions accumulate due to 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. As the electrolysis reaction continues, the electrolyte concentration in the solution layer close to the anode interface may develop to a saturated or supersaturated state. Therefore, a metal hydroxide or a certain salt with a small solubility product will be deposited on the metal surface and form an insoluble film. This insoluble film is often very loose and is not enough to directly cause passivation of the metal. It can only hinder the dissolution of metal, but the electrode surface is covered by it, and the contact area between the solution and the metal is greatly reduced. Therefore, the current density of the electrode must be increased, and the potential of the electrode will become more positive. This may cause OH- ions to discharge on the electrode, and its products (such as OH) react with metal atoms on the electrode surface to form a passivation film. Analysis shows that most passivation films are composed of metal oxides (such as iron Fe3O4), but a few are also composed of hydroxides, chromates, phosphates, silicates, and insoluble sulfates and chlorides. Adsorption theory believes that the metal surface does not need to form a solid product film to be passivated, 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 this adsorption layer is only a monomolecular layer thick, due to the adsorption of oxygen on the metal surface, the interface structure between the metal and the solution is changed, which increases the activation energy of the electrode reaction and decreases the reactivity of the metal surface, resulting in passivation. The main experimental basis for this theory is the measurement of interface capacitance and the amount of electricity required to passivate certain metals. Experimental results show that some metals can be passivated without forming a phase film. Both passivation theories can better explain some experimental facts, but they both have successes and shortcomings. The metal passivation film does have a phase film structure, but there is also a monolayer adsorption film. It is unclear under what conditions a phase-forming film is formed and under what conditions an adsorption film is formed. There is still a lack of direct experimental evidence for the combination of the two theories, so the passivation theory still needs to be studied in depth. Passivation of Stainless-steel Passivation of Stainless-steel In order to better transport highly corrosive goods, stainless steel tanks must be passivated. The passivation treatment of stainless steel should follow the recommended method of the stainless steel manufacturer. During the passivation process of stainless steel cargo tanks, operators should wear appropriate personal protective equipment, and operators should pay attention to coordination with each other; irrelevant personnel should stay away from the operating area. ***There are two treatment methods, namely nitric acid cleaning method and complete pickling method. Nitric acid treatment, often called passivation treatment, is a conventional treatment method. Complete pickling and passivation of the entire tank is usually only carried out during the construction phase and repair phase before delivery. 1. Use the cabin washing machine to circulate and passivate stainless steel. Passivation of SS C.O.T. using tank cleaning machine 1.1 Equipment required: Four stainless steel tank cleaning machines (316) equipped with 8 mm or 9.5 mm nozzles, plus four flushing machines. The washing machine does not need to be made of stainless steel. Four tank washing pipes can withstand 20% nitric acid solution and a safe working pressure of 10BAR (SS BW HOSE). Each cabin to be passivated requires 80 tons of fresh water for flushing. The more fresh water on board, the better. pH test kit or capable of testing pH values ??between 1 and 14 to 1/2 point. Blind plate adapter with four male connectors for tank cleaning pipes. The blind adapter should be valved for insulation. 1.2 Process of Passivation Nitric acid cleaning is often referred to as passivation, which means that a protective inert film is formed on the metal surface during this process. In fact, acid treatment mainly removes dirt that affects the formation of an inert oxidation film on the surface of stainless steel, and also helps to speed up the oxidation process. Clean the cabin to be passivated to the level of water white. Clean the entire cabin surface with about 15% nitric acid solution (10-20%). Remember to add the acid to the water, rather than the water to the acid, to minimize the heat generated by the mixing. To ensure a 15% solution concentration, the time it takes for the water flow to fill a 200-liter drum is measured to calculate the flow rate of the fresh water supply. Use this flow rate to inject the required amount of water into the cabin. It is recommended to use a sufficient volume of solution to allow uninterrupted pumping during the passivation process. Use a stainless steel pump to add the required amount of acid into the cabin, and directly introduce it into the water in the cabin through an appropriate HOSE. If a large number of cabins are to be passivated, it is recommended to prepare the solution in the first cabin and then transfer it from cabin to cabin. Note that there will be solution loss during transfer, so the solution may need to be refilled midway. When passivating a large number of cabins, the quality of the solution should be monitored, and the concentration and contamination of the solution should be controlled by measuring the pH value of the solution and observing the color of the solution. Use blind plate adapters to connect the specified number of tank washing pipes and machines. (Apply as many tank washing machines as possible) Hatch washing openings should be covered to prevent solution from flowing out onto the mild steel deck. Stainless steel pipe saddle covers work well to cover these openings. Water should be continuously flowed over the deck to dilute acid that occasionally reaches the deck. Start circulating the cleaning solution in the cabin as follows. The 10-inch drop from deck level takes a full hour of cycling. 15 inches above the cabin floor is enough to cycle for one hour. After the bottom plane circulation is completed, the valve on the circulation connection is closed and the solution is transferred to the next cabin. Before starting recirculation in each tank, measure the pH of the solution. If the pH is above 2, pour off the solution. After the circulation pipe is removed from the cabin, flush it with water.
1.3 Cleaning by Fresh Water Select a cabin to store fresh water. The pump pipe for this hold should be connected to the tank washing pipe. Connect the required number of tank washing machines on the tank washing pipeline to the tanks that need to be flushed. The number of tank washing machines required is consistent with the number of machines used in the cycle. Use the same drop as when looping. Measure the pH of the used water every 15 minutes and record it. When the pH reaches an acceptable level (6-7), switch to a second drop rinse. Rinse on second level for 30 minutes. Completely drain the hold and remove the flushing machine. Ventilate the cabin. Conduct a visual inspection of the cabin and measure the pH of the surface in concealed areas. If you have a passivation meter on hand, record the readings from the passivation meter. Report: Submit a passivation treatment report to the appropriate fleet. It should be recognized that every vessel and every situation is different. 2. Passivation of SS C.O.T. by Steaming 2.1 Equipment required: 4 liters of nitric acid per 100 cubic meters of hold capacity. Stainless steel injector with suction connecting pipe. The connecting pipe should be equipped with acid-proof suction pipe and stainless steel ball valve. Acid-proof container for the containment and transfer of nitric acid on deck. A steam hose used to penetrate the ship's hold. 2.2 Process of Passivation: Install the steam injector into the cabin, either on a ladder or at the central wash hatch. Start adding steam to the cabin. After adding steam for a short time, open the suction ball valve installed on the acid container. Adjust the ball valve so that nitric acid is added to the steam slowly, evenly, and continuously for a period of at least 30 minutes. This is important, otherwise the nitric acid will not form a mist and blend into the steam. If the droplets of nitric acid are too large, they will fall directly to the bottom of the hold without producing any effect. When an appropriate amount of nitric acid is injected into the cabin (4 liters/100 cubic meters), stop injecting steam and close the cabin for 3-4 hours. After this time, clean the cabin with fresh water for about an hour. Before stopping the flushing process, check whether the pH of the rinsed water reaches an acceptable level (6-7). Report: Submit a passivation treatment report to the appropriate fleet. Regular steam passivation will produce satisfactory results. However, if the condition of the hold has deteriorated significantly, treatment using the recycling method is highly recommended. It should be recognized that every boat and every situation is different. Therefore, responsible persons are advised to evaluate these steps in light of their unique situation and use their best judgment. Pickling and passivation treatment Pickling and passivation treatment in the manufacturing process of stainless steel equipment 1. Cleaning and pickling and passivation after cutting. After cutting, contaminants such as iron filings, steel powder and cooling emulsion usually remain on the surface of stainless steel workpieces. , will cause stains and rust on the surface of stainless steel, so it should be degreased and degreased, and then cleaned with nitric acid, which not only removes iron filings and steel dust, but also passivates it. 2. Cleaning and pickling passivation before and after welding. Since grease is the source of hydrogen, pores will be formed in welds where grease is not removed, and low-melting-point metal contamination (such as zinc-rich paint) will cause cracking after welding, so stainless steel welding The groove and the surface within 20mm on both sides must be cleaned before installation. Oil stains can be scrubbed with acetone. Paint and rust stains should be removed with abrasive cloth or stainless steel wire brushes, and then wiped clean with acetone. No matter what welding technology is used in the manufacturing of stainless steel equipment, it must be cleaned after welding. All welding slag, spatter, stains and oxidation color must be removed. Cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing, sandblasting, etc. Carbon steel brushes should be avoided to prevent surface rust. In order to obtain the best corrosion resistance, it can be soaked in a mixture of HNO3 and HF, or pickled passivation paste can be used. In fact, mechanical cleaning and chemical cleaning are often combined. 3. Cleaning of forged castings. Stainless steel workpieces that have been thermally processed such as forging and casting often have a layer of oxide scale, lubricant or oxide contamination on the surface. The contaminants include graphite, molybdenum disulfide and carbon dioxide. It should be treated by shot blasting, salt bath and multiple pickling.
For example, the treatment process of stainless steel turbine blades in the United States is: salt bath (10min) → water quenching (2.5min) → sulfuric acid washing (2min) → cold water washing (2min) → alkaline permanganate bath (10min) → cold water washing (2min ) → sulfuric acid washing (1min) → cold water washing (1min) → nitric acid washing (1.5min) → cold water washing (1min) → hot water washing (1min) → air drying. Pickling and passivation treatment before new equipment is put into production. Many stainless steel equipment and pipelines in large chemical, chemical fiber, fertilizer and other equipment are required to be pickled and passivated before they are put into production. Although the equipment has been pickled in the manufacturing plant to remove welding slag and oxide scale, contamination by grease, mud, sand, rust, etc. will inevitably occur during storage, transportation, and installation. In order to ensure that devices and equipment test products (especially If the quality of chemical intermediates and refined products can meet the requirements, to ensure a successful test run, pickling and passivation must be carried out. For example, the stainless steel equipment and pipelines of H2O2 production equipment must be cleaned before being put into production. Otherwise, if there are contaminants and heavy metal ions, the catalyst will be poisoned. In addition, if there are grease and free iron ions on the metal surface, it will cause the decomposition of H2O2, violently releasing a large amount of heat, causing fire or even explosion. Similarly, for oxygen pipelines, trace amounts of oil dirt and metal particles may cause sparks and serious consequences. Pickling and passivation treatment in on-site maintenance In the equipment materials of production equipment such as refined terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fiber, acetic acid, etc., austenitic stainless steel 316L, 317, and 304L are widely used. Materials contain harmful ions such as Cl-, Br-, SCN-, formic acid, etc., or due to dirt and material agglomeration, pitting corrosion, crevice corrosion and weld corrosion will occur on the equipment. When shutting down for maintenance, the equipment or components can be fully or partially pickled and passivated to repair the passivation film to prevent the expansion of local corrosion. For example, the stainless steel pipes of the Shanghai Petrochemical PTA plant dryer were updated and overhauled, and the stainless steel heat exchangers of the acrylic fiber plant were overhauled and pickled and passivated. Descaling and Cleaning of In-Service Equipment Stainless steel equipment in petrochemical installations, especially heat exchangers, after a certain period of operation, various dirt will be deposited on the inner wall, such as carbonate scale, sulfate scale, silicate scale, iron oxide Scale, organic scale, catalyst scale, etc. affect the heat exchange effect and cause corrosion under the scale. It is necessary to choose a suitable cleaning agent for descaling. You can use nitric acid, nitric acid + hydrofluoric acid, sulfuric acid, citric acid, EDTA (ethylenediaminetetraacetic acid), water-based cleaning agents, etc., and add an appropriate amount of corrosion inhibitor. After descaling and cleaning, passivation treatment can be performed if necessary. For example, the stainless steel heat exchangers of Shanghai Petrochemical PTA, acetic acid, acrylic and other equipment have been descaled and cleaned. Precautions for stainless steel pickling and passivation 1. Pre-treatment for pickling and passivation If there is surface dirt on the stainless steel workpiece before pickling and passivation, it should be cleaned mechanically, and then deoiled and degreased. If the pickling solution and passivating solution cannot remove grease, the presence of grease on the surface will affect the quality of pickling and passivation. Therefore, oil removal and degreasing cannot be omitted. Alkali solution, emulsifier, organic solvent, steam, etc. can be used. 2. Control of Cl- in pickling liquid and rinse water. Some stainless steel pickling liquids or pickling pastes use corrosion media containing chloride ions such as hydrochloric acid, perchloric acid, ferric chloride and sodium chloride as the main agent or auxiliary agent. Agents are used to remove the surface oxide layer, and chlorine-containing organic solvents such as trichlorethylene are used to remove grease, which is not suitable from the perspective of preventing stress corrosion cracking. In addition, industrial water can be used for preliminary flushing water, but the final cleaning water requires strict control of halide content. Deionized water is usually used. For example, when using water for hydrostatic testing of petrochemical austenitic stainless steel pressure vessels, the C1- content should not exceed 25 mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to make it meet the requirements. If the C1- content exceeds the standard, it will Destroying the passivation film of stainless steel is the root cause of pitting corrosion, crevice corrosion, stress corrosion cracking, etc. 3. Process control in pickling and passivation operations. Nitric acid solution alone is effective in removing free iron and other metal contaminants, but it is ineffective in removing iron oxide scale, thick corrosion products, tempering films, etc. HNO3+ should generally be used For HF solution, for convenience and safe operation, fluoride can be used instead of HF[2]. HNO3 solution alone does not need to add corrosion inhibitor, but when HNO3+HF pickling, Lan-826 needs to be added. Use HNO3+HF pickling. To prevent corrosion, the concentration should be maintained at a ratio of 5:1. The temperature should be lower than 49°C. If it is too high, HF will volatilize.
For the passivation solution, HNO3 should be controlled between 20% and 50%. According to electrochemical tests, the quality of the passivation film treated with an HNO3 concentration less than 20% is unstable and prone to pitting [8], but the HNO3 concentration should not be greater than 50%, to prevent over passivation. Although the one-step method for oil removal, pickling and passivation is easy to operate and saves man-hours, the pickling and passivation solution (paste) will contain corrosive HF, so the quality of the final protective film is not as good as the multi-step method. During the pickling process, the acid concentration, temperature and contact time are allowed to be adjusted within a certain range. As the use time of the pickling solution increases, attention must be paid to changes in acid concentration and metal ion concentration. Care should be taken to avoid over-pickling. The titanium ion concentration should be less than 2%, otherwise severe pitting corrosion will occur. Generally speaking, increasing the pickling temperature will speed up and improve the cleaning effect, but may also increase the risk of surface contamination or damage. 4. Control of pickling under sensitized conditions of stainless steel. Some stainless steels are sensitized due to poor heat treatment or welding. Pickling with HNO&HF may cause intergranular corrosion. Cracks caused by intergranular corrosion may occur during operation, cleaning, or Subsequent processing can concentrate halides and cause stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling with HNO3+HF solution. If such pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used. 5. Pickling of stainless steel and carbon steel assemblies. For stainless steel and carbon steel assemblies (such as stainless steel tubes, tube sheets and carbon steel shells in heat exchangers), if HNO3 or HNO3+HF is used for pickling and passivation, it will cause serious corrosion. For carbon steel, appropriate corrosion inhibitor such as Lan-826 should be added at this time. When the stainless steel and carbon steel assembly is in a sensitized state and cannot be pickled with HNO3+HF, glycolic acid (2%) + formic acid (2%) + corrosion inhibitor can be used at a temperature of 93°C for 6 hours or ammonium EDTA. Base neutral solution + corrosion inhibitor, temperature: 121°C, time: 6h, then rinse with hot water and immerse in 10mg/L ammonium hydroxide + 100mg/L hydrazine [3]. 6. After pickling and rinsing, the pickled and passivated stainless steel workpieces can be treated with an alkaline permanganate solution containing 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 at 71 to 82°C. Soak in water for 5 to 60 minutes to remove pickling residue, then rinse thoroughly with water and dry. If spots or stains appear on the surface of stainless steel after pickling and passivation, they can be eliminated by scrubbing with fresh passivation solution or higher concentration of nitric acid. Stainless steel equipment or components that are finally pickled and passivated should be protected and covered or wrapped with polyethylene film to avoid contact between different metals and non-metals. The treatment of acidic and passivation waste liquids should comply with national environmental protection discharge regulations. For example, fluoride-containing wastewater can be treated with lime milk or calcium chloride. The passivation solution should not use dichromate as much as possible. If there is chromium-containing wastewater, ferrous sulfate can be added for reduction treatment. Pickling may cause hydrogen embrittlement of martensitic stainless steel. If necessary, heat treatment can be used to remove oxygen (heat to 200°C and keep warm for a period of time). Quality inspection Stainless steel pickling passivation quality inspection Since chemical inspection will destroy the passivation film of the product, inspection is usually performed on a sample plate. Examples of methods are as follows: (1) Copper sulfate titration test: Drop 8g CuS04 + 500mL H20 + 2~3mL H2SO4 solution onto the surface of the sample and keep it wet. If no precipitation of copper occurs within 6 minutes, it is qualified. (2) For potassium ferricyanide titration test, use 2mL HCl + 1mL H2SO4 + 1g K3Fe(CN)6 + 97mL H20 solution to drop on the surface of the sample, and identify the quality of the passivation film by the number of blue spots generated and the length of time they appear. Good or bad. Example: Passivating a long string of parts: When passivating a long string of parts, the upper and lower ends of the plated parts enter and exit the passivation solution in sequence. At the same time, when the workpiece swings in the solution, the lower end of the plated part swings much larger than the upper end; another When the surface stays in the air after passivation, the solution flows downward from the upper end. The surface of the lower end plated piece has more solution than the upper end plated piece, and the lower end has more chemical reaction time with the solution. All these make the color of the lower end passivation film darker than the upper end. In order to reduce the color difference, it is recommended that these parts enter and exit the passivation solution laterally (hook the lower end with a hook), and prevent the anode from being too long. Passivation of long parts If the passivation tank cannot accommodate the passivation of long parts, temporary measures can be taken. Bricks or wooden strips are used to process a frame that can accommodate the plated parts. The frame is lined with plastic cloth and the passivation solution is injected. It can be used immediately. This method is both convenient and can avoid quality problems such as uneven film layers or joint marks.
Passivation of flat parts Generally, when passivating flat parts, the contact between the edge part and the passivation solution will be more severe than the middle part when swinging in the passivation tank, causing the color of the passivation film in this part to be uneven. This problem can be solved by compression. Solved by air stirring, the effect is very good. In order to improve the uniformity of the passivation film, attention should be paid to the uniformity of current distribution during the galvanizing process. If necessary, the edges of the plated parts should be prohibited to prevent the coating from being roughened due to excessive current and affecting the color of the passivation film. Surface passivation: After passivation of surface-clean parts, due to the smooth surface, the passivation solution is difficult to adsorb on the surface and drains quickly. Therefore, the residence time in the solution and in the air during passivation must be appropriately extended, otherwise the workpiece will be damaged. The passivation film appears lighter. For passivation of water-prone parts, it is necessary to avoid pocketing the solution, so as not to cause excessive loss, consumption and environmental pollution of the passivation solution, and to ensure the quality of the passivation film in this part. For passivation of small pieces, the entire string of tied workpieces can be put in a plastic basket for passivation to prevent them from being separated from the group due to shaking during passivation and falling into the tank causing repairs.