Metal passivation can protect the metal from corrosion, but sometimes it is necessary to prevent passivation, such as electroplating and chemical power supply, to ensure that the metal can participate in the reaction and dissolve normally.
How is the metal passivated? What is the passivation mechanism? First of all, it should be clear whether the passivation phenomenon is caused by metal phase and solution phase or interface phenomenon. Some people have studied the effect of mechanical scraping on passive metals. The experiment shows that when the metal surface is scraped continuously in the measurement process, the potential of the metal moves sharply in the negative direction, that is, trimming the metal surface can activate the passive metal. It is proved that passivation is an interface phenomenon. The interface between metal and medium changes under certain conditions. Electrochemical passivation is anodic polarization, and the potential of metal changes, forming metal oxide or salt on the electrode surface. These substances closely cover the metal surface and become a passivation film, which leads to metal passivation. Chemical passivation is caused by oxidants such as concentrated nitric acid directly acting on metals, forming oxide films on the surface, or adding easily passivated metals such as chromium and nickel. During chemical passivation, the concentration of added oxidant should not be less than a certain critical value, otherwise it will not lead to passivation, but will lead to faster metal dissolution.
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, the theory of phase-forming film and the theory of adsorption. It is considered that it is not necessary to form a solid product film to passivate the metal surface, but it is enough to form an adsorption layer of oxygen or oxygen-containing particles (such as O2- or OH-) on the surface or part of the surface. Although this adsorption layer is only as thin as a single layer, due to the adsorption of oxygen on the metal surface, the interface structure between metal and solution is changed, the activation energy of electrode reaction is increased, and the reaction ability of metal surface is reduced and passivated. The main experimental basis of this theory is to measure the interfacial capacitance and the amount of electricity needed to passivate some metals. The experimental results show that some metals can be passivated without forming phase films.
Both passivation theories can explain some experimental facts well, but they both have successes and shortcomings. The metal passivation film does have a phase-forming film structure, but it also has a single-layer adsorption film. It is not clear under what conditions the phase-forming film and the adsorption film are formed. The combination of the two theories lacks direct experimental evidence, so the passivation theory needs further study. Passivation of stainless steel
In order to better transport highly corrosive goods, the stainless steel cabin should be passivated, and the passivation treatment of stainless steel should follow the method recommended by the stainless steel manufacturer. During passivation of stainless steel cargo hold, operators should wear appropriate personal protective equipment and pay attention to mutual cooperation; Irrelevant personnel should stay away from the operation area.
* * * There are two treatment methods, namely, nitric acid cleaning method and complete acid soaking method. Nitric acid treatment, usually called passivation treatment, is a conventional treatment method. The complete pickling passivation of the whole cabin is usually only carried out in the construction stage and the repair stage before delivery.
1. Cyclic passivation of stainless steel with cabin washer. Passivating stainless steel C.O.T with oil tank cleaner
1. 1 requires the following equipment:
Four stainless steel cab washers (3 16) with 8 mm or 9.5 mm nozzles, plus four washers. The washing machine should not be made of stainless steel. Four tank washing tubes can withstand 20% nitric acid solution and a safe working pressure of 10 bar (stainless steel BW hose). Each cabin to be passivated needs 80 tons of flushing fresh water. The more fresh water on board, the better. The pH detection kit can detect the pH value from 1 to 14, with the accuracy of 1/2. Blind adapter with four male connectors for cab cleaning. Blind adapters shall be equipped with valves for thermal insulation.
1.2 passivation process
Nitric acid cleaning is usually called passivation, which means that a protective inert film is formed on the metal surface during this process. In fact, acid treatment is mainly to remove the dirt on the surface of stainless steel that affects the formation of inert oxide film, and it is also helpful to accelerate the oxidation process. Clean the cabin to be passivated to water white. Clean the entire engine room surface with about 15% nitric acid solution (10-20%). Remember to add acid to water instead of water to minimize the heat generated by mixing. In order to ensure the solution concentration of 15%, the time for filling 200-liter barrels with water flow was measured to calculate the flow of fresh water supply. This flow rate is used to inject the required amount of water into the engine room. It is recommended to use enough solution to ensure uninterrupted pumping during passivation. Use a stainless steel pump to add the required amount of acid to the cabin and introduce it directly into the water in the cabin through a suitable hose. If a large number of compartments are to be passivated, it is recommended to prepare the solution in the first compartment and then deliver it one by one. Note that the solution will be lost in transit, so the solution may need to be filled to half. When a large number of cabins are passivated, the quality of the solution should be monitored, and the concentration and fouling of the solution should be controlled by measuring pH value and observing the color of the solution. Connect the specified number of cabin cleaning pipes to the machine with blind adapters. (Use the cabin washing machine as much as possible) Cover the washing hatch to prevent the solution from flowing out on the low carbon steel deck. Stainless steel tube saddles cover these openings well. Water should be allowed to flow continuously across the deck to dilute the acid that occasionally flows onto the deck.
Follow the steps below to start circulating the cleaning solution in the cab. Descend from the deck plane 10 inch, and circulate for a whole hour. Hover for one hour above the cabin floor 15 inch. After the bottom plane circulation is completed, close the valve on the circulation joint and transport the solution to the next compartment. Before starting the recirculation of each compartment, measure the pH value of the solution. If the pH value is higher than 2, pour out the solution. After the circulating pipe is taken out of the cabin, it should be washed with water.
1.3 Wash with fresh water when washing.
Choose a cabin to store fresh water. The pump pipe of this cabin should be connected with the flushing pipe. Connect the required number of cabin washing machines on the cabin washing pipeline to the cabin to be washed. The number of cabin washers required is consistent with the number of machines used in the cycle. Use the same water drops as when riding a bike. Measure the pH value of the water used every 15 minutes and record it. When the pH value reaches an acceptable level (6-7), it will be changed to the second drip irrigation. Rinse the second drop for 30 minutes. Thoroughly drain the water in the cab and remove the washing machine. Ventilation the engine room. Visually inspect the cab and measure the pH value of the surface of the hidden area. If you have a passivator on hand, record the readings of the passivator. Report: submit the passivation treatment report to the corresponding fleet. It should be recognized that every ship and every situation is different.
2. steam passivation of stainless steel C.O.T by steam injection.
2. 1 The following equipment is required: 4 liters of nitric acid per 100 cubic meter of cabin capacity. Stainless steel syringe with suction connecting tube. The connecting pipe should be equipped with acid-resistant straw and stainless steel ball valve. Acid-proof container for storing and transferring nitric acid on deck. Used to detect the steam hose inside the cabin.
2.2 Passivation step: Install the steam ejector in the cabin, either on the ladder or on the washing hatch in the middle. Start to add 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 to slowly, evenly and continuously add nitric acid to the steam within at least 30 minutes. This is very important, otherwise nitric acid will not form fog and melt into steam. If the nitric acid droplets are too large, they will fall directly to the bottom of the engine room without any influence. When a proper amount of nitric acid (4 liters100 cubic meter) is injected into the engine room, stop injecting steam and close the engine room for 3-4 hours. After this time, clean the cabin with fresh water for about an hour. Before stopping the washing process, check whether the pH value of the washing water reaches an acceptable level (6-7). Report: submit the passivation treatment report to the corresponding fleet. If the steam method is used regularly for passivation treatment, satisfactory results will be produced. However, if the cabin condition has deteriorated seriously, it is strongly recommended to adopt the recycling method for treatment. It should be recognized that every ship and every situation is different. Therefore, it is suggested that the person in charge should evaluate these steps according to his own special situation and handle them according to his best judgment. Pickling passivation treatment in the manufacturing process of stainless steel equipment
1. Cleaning and pickling passivation after cutting
After the stainless steel workpiece is cut, dirt such as iron filings, steel powder and cooling emulsion will usually remain on the surface, which will cause stains and rust on the stainless steel surface. Therefore, it is necessary to degrease and degrease, and then clean with nitric acid, which not only removes iron filings and steel powder, but also passivates it.
2. Cleaning and pickling passivation before and after welding
Because grease is the source of hydrogen, pores will be formed in the weld if grease is not removed, and low melting point metal pollution (such as zinc-rich paint) will cause post-weld cracking. Therefore, before welding stainless steel, the groove and the surface within 20mm on both sides must be cleaned, the oil stain can be scrubbed with acetone, and the paint rust can be removed with emery cloth or stainless steel wire brush first, and then wiped with acetone.
No matter what welding process is used in the manufacture of stainless steel equipment, it should be cleaned after welding to remove all welding slag, spatter, stains and oxidation color. Cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing and sandblasting. Carbon steel brushes should be avoided to prevent the surface from rusting. In order to obtain the best corrosion resistance, it can be soaked in the mixed solution of HNO3 and HF, or it can be used with pickling passivation paste. In fact, mechanical cleaning is usually combined with chemical cleaning.
3. Cleaning of forged castings
After forging, casting and other hot working, the surface of stainless steel workpiece is often polluted by a layer of oxide scale, lubricant or oxide, and the pollutants are graphite, molybdenum disulfide and carbon dioxide. Shot peening, salt bath treatment and multi-pass pickling treatment should be adopted. For example, the treatment of stainless steel turbine blades in the United States is as follows:
Salt bath (1 0min) → water quenching (2.5min) → sulfuric acid washing (2min) → cold water washing (2min) → alkaline permanganate bath (10min) → cold water washing (2min) → sulfuric acid washing (1min).
Pickling and passivation treatment before the new unit is put into production.
Many large-scale chemical, chemical fiber, chemical fertilizer and other stainless steel equipment and pipelines need pickling passivation before they are put into production. Although the equipment has been pickled before leaving the factory to remove welding slag and scale, it will inevitably cause pollution such as grease, mud and rust during storage, transportation and installation. In order to ensure the quality of commissioning products (especially chemical intermediates and refined products) of devices and equipment meet the requirements and ensure the success of one commissioning, pickling passivation must be carried out. For example, the stainless steel equipment and pipelines of H2O2 production plant must be cleaned before production, otherwise the heavy metal ions in the dirt will poison the catalyst. In addition, if there are grease and free iron ions on the metal surface, it will cause H2O2 to decompose and release a lot of heat violently, causing fire and even explosion. Similarly, for the oxygen pipeline, the existence of trace oil and metal particles may also produce sparks, causing serious consequences.
Treatment of pickling passivation in field maintenance
Austenitic stainless steels 3 16L, 3 17 and 304L are widely used for refining equipment materials such as terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fiber and acetic acid. Because all materials contain harmful ions such as Cl-, Br-, SCN- and formic acid, or because dirt and materials are caked, spots will be caused on the equipment. When stopping for maintenance, all or part of equipment or parts can be passivated by pickling, and the passivation film can be repaired to prevent local corrosion from spreading. For example, the stainless steel tube of the dryer in PTA plant of Shanghai Petrochemical Company and the stainless steel heat exchanger in acrylic fiber plant have been passivated by pickling.
Descaling and cleaning of in-use equipment
Stainless steel equipment in petrochemical plants, especially heat exchangers, will deposit various scales on the inner wall after a certain period of operation, such as carbonate scale, sulfate scale, silicate scale, iron oxide scale, organic scale and catalyst scale. , will affect the heat transfer effect, resulting in corrosion under the scale. Appropriate cleaning agents should be selected for descaling, such as nitric acid, nitric acid+hydrofluoric acid, sulfuric acid, citric acid, EDTA and water-based cleaning agents. , and add an appropriate amount of corrosion inhibitor. After descaling and cleaning, it can be passivated if necessary. Chemical treatment, for example, stainless steel heat exchangers in PTA, acetic acid and acrylic fiber plants of Shanghai Petrochemical Company have been descaled and cleaned. Matters needing attention in stainless steel pickling passivation
1. pickling passivation pretreatment
If there is dirt on the surface of stainless steel workpiece before pickling passivation, it should be cleaned mechanically first, and then degreased. If pickling solution and passivation solution can't remove grease, the presence of surface grease will affect the quality of pickling passivation. Therefore, degreasing cannot be omitted, and alkali solution, emulsifier, organic solvent and steam can be used.
2. Control of Cl- in pickling solution and cleaning water
For some stainless steel pickling solutions or pickling pastes, it is not suitable to prevent stress corrosion cracking by using chlorine-containing corrosive media such as hydrochloric acid, perchloric acid, ferric chloride and sodium chloride as main agents or additives to remove the surface oxide layer and chlorine-containing organic solvents such as trichloroethylene to remove grease. In addition, industrial water can be used as initial washing water, but the halide content of final washing water is strictly controlled. Deionized water is usually used. For example, the content of C 1- in the water for hydrostatic test of austenitic stainless steel pressure vessels in petrochemical industry should be controlled within 25mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to make it meet the requirements. If the content of C 1- exceeds the standard, the passivation film of stainless steel will be destroyed, which is the root of pitting corrosion, crevice corrosion and stress corrosion cracking.
3. Process control in pickling passivation operation
Nitric acid solution alone is effective for removing free iron and other metal pollutants, but it is ineffective for removing mill scale, thick corrosion products, tempered films, etc. HNO3+HF solution is generally used, and fluoride can be used instead of HF[2] for convenience and safety. No corrosion inhibitor can be added to nitric acid solution alone, but Lan-826 needs to be added to nitric acid+hydrofluoric acid pickling. Use nitric acid+hydrofluoric acid for pickling. To prevent corrosion, the concentration should be kept at 5: 1. The temperature should be lower than 49℃. If it is too high, HF will volatilize.
For passivation solution, nitric acid should be controlled between 20% and 50%. According to electrochemical test, the quality of passivation film treated with nitric acid concentration less than 20% is unstable and pitting corrosion is easy to occur [8], but the nitric acid concentration should not be greater than 50%, so it is necessary to prevent over-passivation.
Using one-step method to treat degreasing pickling passivation is simple and saves working hours, but there will be corrosive HF in pickling passivation solution (paste), so the quality of the final protective film is not as good as that of multi-step method.
During pickling, it is allowed to adjust the acid concentration, temperature and contact time within a certain range. With the increase of pickling time, we must pay attention to the changes of acid concentration and metal ion concentration to avoid excessive pickling. The concentration of titanium ions should be less than 2%, otherwise it will lead to serious pitting corrosion. Generally speaking, increasing the pickling temperature will accelerate and improve the cleaning effect, but it may also increase the risk of surface pollution or damage.
4. Pickling control of stainless steel under sensitized condition.
Some stainless steels are sensitized due to poor heat treatment or welding, HNO &；; HF pickling may cause intergranular corrosion, and cracks caused by intergranular corrosion will concentrate halides during operation, cleaning or subsequent processing, leading to stress corrosion. These sensitized stainless steels are generally not suitable for rust removal or pickling with HNO3+HF solution. If pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used.
5. Pickling of stainless steel and carbon steel components
For stainless steel and carbon steel components (such as stainless steel tubes, tubesheets and carbon steel shells in heat exchangers), if HNO3 or HNO3+HF is used for pickling passivation, carbon steel will be seriously corroded. At this time, appropriate corrosion inhibitor such as Lan-826 should be added. When the assembly of stainless steel and carbon steel can't be washed with HNO3+HF in sensitized state, glycolic acid (2%)+ formic acid (2%)+ corrosion inhibitor can be used at 93℃ for 6h or ammonium EDTA-based neutral solution+corrosion inhibitor at 12 1℃ for 6h, then it can be washed with hot water and immersed in 65438.
6. Post-treatment of pickling and passivation
After pickling and water washing, stainless steel workpiece can be soaked in 1 crude permanganate solution containing 10% (mass fraction) NaOH+4% and 4% (mass fraction) KMnO4 at 7 1 ~ 82℃ for 5 ~ 60 min to remove pickling residue, and then thoroughly cleaned with water. Spots or stains appear on the back surface of stainless steel pickling passivation, which can be removed by scrubbing with fresh passivation solution or high concentration nitric acid. Stainless steel equipment or components that are finally passivated by pickling shall be protected and covered or wrapped with polyethylene film to avoid contact between dissimilar metals and nonmetals.
The treatment of acidic and passivation waste liquid shall conform to the national environmental protection discharge regulations. For example, fluorine-containing wastewater can be treated with lime milk or calcium chloride. Passivation solution does not need 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, and oxygen can be removed by heat treatment (heating to 200℃ for a period of time) if necessary. Stainless steel pickling passivation quality inspection
Because chemical inspection will destroy the passive film of the product, it is usually carried out on the sample plate. Examples of methods are as follows:
(1) copper sulfate titration test
Drop 8g CuSO4+500ml H2O+2 ~ 3ml H2SO4 solution on the surface of the sample plate and keep it moist. If copper does not precipitate within 6 minutes, it is qualified.
(2) Potassium ferricyanide titration test
2 ml HCl+1ml H2SO4+1gk3fe (cn) 6+97 ml H2O solution was dropped on the surface of the template, and the quality of the passive film was judged by the number of blue spots produced and the length of appearance time.