Concept According to the corrosion data of metal materials, selecting materials with low corrosion rate, low price and good performance for specific environments is a common and simple method for controlling corrosion, which can make the equipment obtain economical and reasonable service life. The addition of a small amount of corrosion inhibitor to a medium that may cause metal corrosion can greatly slow down the metal corrosion process.
Correct material selection and design According to the corrosion data of metal materials, it is a common and simple method to control corrosion to select materials with low corrosion rate, low price and good performance in specific environment, which can make the equipment obtain economical and reasonable service life. Since the structure of the equipment often affects corrosion, proper design is also important. In addition, the material selection personnel also need to have certain knowledge of corrosion and corrosion to better solve the problem of material selection.
Adjusting the media environment Corrosion can be aborted or slowed down if various factors that cause corrosion of metal materials and equipment in the environment are eliminated. However, most environmental conditions are uncontrollable, such as moisture in the atmosphere and soil, oxygen in seawater, etc. are impossible to remove. The production process can not be arbitrarily changed, and the local environment can be adjusted at this time. For example, the boiler in the water first deoxygenation (adding deoxidizers Na2SO3 and N2H4, etc.), can protect the boiler tube from corrosion; first remove the moisture from the closed warehouse into the air, can avoid the rust of the storage metal parts; often add alkali or acid in the water to Adjusting the pH to the optimum range (usually close to neutral) prevents scaling and perforation of the heat exchanger and other equipment by the cooling water; alkali or ammonia is often added to the process of refining the oil to keep the production fluid neutral to Weakly alkaline.
Adding Corrosion Inhibitors Adding a small amount of corrosion inhibitor to a medium that can cause metal corrosion can greatly slow down the metal corrosion process. Corrosion inhibitors can be classified into inorganic corrosion inhibitors, organic corrosion inhibitors and gas phase corrosion inhibitors.
Some inorganic corrosion inhibitors slow down the anode process and are called anode type corrosion inhibitors, such as oxidants (chromate, nitrite, Fe3+) or anode film formers (alkali, phosphate, silicic acid) that promote anode passivation. Salt, benzoate); another type of inorganic corrosion inhibitor promotes cathodic polarization, called cathodic corrosion inhibitors, such as Ca2+, Zn2+, Mg2+, Cu2+, Cd2+, Mn2+, Ni2+, etc., which can react with the cathode. The resulting OH- forms an insoluble hydroxide which covers the surface of the cathode in a thick film form, thereby retarding the diffusion of oxygen to the cathode and increasing the concentration polarization. There are also mixed corrosion inhibitors that simultaneously block the anode process and the cathode process. Some impurities in the solution, such as S, Se, As, Sb, Bi and other compounds, can suppress the cathode hydrogen evolution process, increase the cathode polarization and slow down the corrosion. The amount of corrosion inhibitor is generally determined by testing.
Organic corrosion inhibitors are adsorption-type corrosion inhibitors. They adsorb on the metal surface to form an invisible film with a thickness of several molecules. Generally, the anode and cathode reactions are blocked at the same time, but the blocking effect is not the same. Commonly used varieties include N, S-containing, O-containing, P-containing organic compounds, such as amines, heterocyclic compounds, long-chain fatty acid compounds, sulfur glands, aldehydes, organic phosphorus and the like. The types of slow release agents are electrostatic adsorption and chemical adsorption. Electrostatic adsorbents include aniline and its substitutes, pyridine, butylamine, benzoic acid and its substitutes such as benzenesulfonic acid; chemical adsorbents have nitrogen and sulfur heterocyclic compounds; some compounds have both electrostatic and chemical adsorption. In addition, some chelating agents can form a thin layer of organometallic compounds on the metal surface. In recent years, organic corrosion inhibitors have developed rapidly and are widely used. The use of these corrosion inhibitors also has disadvantages, such as possible contamination of products, which may adversely affect the production process.
Gas phase corrosion inhibitors are mostly volatile substances and are also adsorption type corrosion inhibitors. Its vapor is decomposed by the water in the atmosphere to form an effective corrosion-inhibiting group, which is adsorbed on the metal surface to slow down the corrosion, and is generally used for the protection, storage and transportation of metal parts. It must be used in a sealed package and the inner tank of the marine tanker can also be used to protect it. Common effective vapor phase inhibitors are alicyclic amines and aromatic amines; polymethyleneamine; mixtures of nitrites and thioureas; urotropine and ethanolamine; nitrobenzene and nitronaphthalene.
Cathodic protection During the electrochemical corrosion of metals, the cathode of a miniature battery is an electrode that accepts electrons to produce a reduction reaction. The anode is an electrode that loses the oxidation reaction of electrons, and only the anode is corroded. The cathodic protection method is to treat the metal to be protected as the cathode of the etching battery (the positive electrode of the primary battery) or as the cathode of the electrolytic cell without being corroded. The former is called the sacrificial anode method, and the latter is called the impressed current method. Sacrificial anode method is the connection of a negative (more active) metal or its alloy to a protected device, such as a steel device connected to a piece of Zn, Mg or Al alloy, so that they are used as anodes in the formed galvanic cells. Corrosion, while the metal equipment is protected as a cathode, the sacrificial anode must be replaced at regular intervals. The impressed current method is to connect a flow guiding electrode (graphite, platinum or rhodium, titanium, high silicon iron, scrap steel, etc.) as an anode in the system. When the cathode current is externally introduced, the local cathode current is equal to the local anode current. When the directions are opposite and cancel each other, the metal corrosion stops and the purpose of protecting the equipment is achieved. Cathodic protection is widely used in metal structures and devices in soils and seawater, such as pipelines, cables, sea vessels, harbor terminal facilities, drilling platforms, reservoir gates, oil and gas wells, etc. In order to reduce the current input and prolong the service life, the cathodic protection method is generally combined with the metal surface coating method, and is an economical, simple and effective metal anticorrosion method.
Anode protection Some metals that can be passivated, when a current is supplied from the outside, the potential rises with the current, and after reaching a blunt potential, the corrosion current drops rapidly, and then the potential rises, and the corrosion current does not change until the blunt region is reached. Using this principle, the current is introduced into the anode with the device to be protected, so that the potential is maintained in the middle of the passivation region, and the corrosion rate can be kept low. In the potential interval in which the bluntness is maintained, the dissolution rate of the anode is determined by the dissolution rate of the passivation film. Therefore, the passive state of the metal is not a thermodynamically stable state, but a state of dissipative structure far from equilibrium. The anodic protection method requires a potentiostat to control the potential of the device (to avoid entering the active or overpassing regions when fluctuating). The application of this method is limited because it is only applicable to passivable metals. The anode protection method is industrially used for the production and processing of stainless steel "target=_blank> stainless steel or carbon steel containers and equipment such as H2SO4, H3PO4, NH4HCO3 solution, and NH4NO3 compound fertilizer.
Alloying A certain proportion of the alloy component that promotes passivation is added to the base metal to obtain a material with excellent corrosion resistance. For example, when Cr is added to Fe, when the amount of Cr is more than 12%, it becomes stainless steel "target=_blank> stainless steel, and its surface can form a passivation film in an oxidizing environment, which has high corrosion resistance. Ni can expand the passivation range and improve the mechanical properties. Cr-Ni stainless steel with Cr 18% and Ni 9% "target=_blank> stainless steel is the most widely used corrosion-resistant alloy in industrial and civil applications. When the nickel in the nickel-copper alloy is more than 30% to 40%, they are superior to the pure Cu and pure Ni in some environments. A series of Ni alloys are commonly used corrosion resistant materials, such as nickel cast iron with excellent alkali resistance; Ni-Mo-Cr alloy is one of the few alloys that can withstand high temperature non-oxidizing acids (such as HCl); Ni-Al-Cr-Fe alloy High temperature resistant oxidizing acid, hypochlorite, seawater, etc., better than ordinary stainless steel "target=_blank> stainless steel.
Metal Coatings and Coatings Steel coatings are often used to protect steel products by plating a thin layer of more corrosion-resistant metals such as Cr, Ni, Pb, etc. If a metal Zn, Cd or the like is used as the plating layer, the polarity of the etched battery is opposite to that described above, the bare steel in the plated micropores is the cathode, and the Zn or Cd plating layer is the anode, and the steel is cathodically protected by the sacrificial anode. Sn-plated Fe (tinplate) is widely used in food cans. Although the standard electrode potential of Sn is higher than Fe, it is lower than Fe in food organic acids, and can also act as a sacrificial anode. When the plating layer is a noble metal (Au, Ag, etc.), an easily passivable metal (Cr, Ti), and Ni, Pb, etc., since their electrode potential is higher than Fe, if there is damage, these metals become cathodes in the electrode reaction. It will accelerate the corrosion of the underlying iron, so this type of coating is not suitable for corrosive environments (such as acid), but it can be used in the atmosphere, water and other environments. In addition to electroplating, hot dip plating (melt immersion plating), flame spraying, vapor plating, and integral metal sheet plating are also commonly used.
Coatings The use of organic coatings to protect metal structures in the atmosphere is the most widely used traditional method of corrosion protection. All kinds of paints and varnishes are available in this category. They are mainly made up of synthetic resins, vegetable oils, rubber slurries, solvents, dry aids, pigments, fillers, etc., and there are many varieties. In the past, vegetable oil-based paints were used. It is now mostly replaced by synthetic resin paint. These coatings cover the metal surface and form a porous film after drying. Although the metal cannot be completely isolated from the medium, the diffusion resistance of the medium through the micropores and the solution resistance are increased, so that the corrosion current is lowered. In a moderate environment (such as the atmosphere, seawater, etc.), the microporous bottom metal is slowly corroded, and the corrosion products can block the micropores and have a long service life. This method is not suitable for strong corrosive solutions (such as acids) because the metal corrodes quickly and produces hydrogen, which can rupture the paint film.
Coating coating process: First of all, surface treatment, this is the most important part, surface rust, oil, etc. should be completely removed, otherwise it will affect the adhesion of the coating and metal; secondly, the primer is selected, generally added red Dan, lead calcium silicate, zinc chromate and zinc powder and other corrosion inhibitors can inhibit corrosion when the micropores penetrate into the medium; finally, the topcoat, in addition to corrosion resistance, aesthetics is also an important purpose. It is common to apply several layers of paint to minimize micropores.
Commonly used synthetic resin varieties are: epoxy resin, polyurethane, chlorosulfonated polyethylene, chlorinated polyether, phenolic, furan (sterol). Asphalt is a cheap but high performance coating that is often used in combination with epoxy resins in underground pipelines. Natural resin lacquer is a special product in China. It has excellent acid resistance and alkali resistance and is an advanced coating.
Zinc-rich zinc-based paints are widely used in inorganic coatings. Synthetic resins are used as binders. The zinc film on the surface of the dry surface is electrically conductive. It has the same function as cathodic protection and can be used in the atmosphere for a long time. It can also be used in the atmosphere. Higher temperature environment.
The lining is suitable for the interior of equipment that is in contact with strong corrosive media, such as rubber or plastic lining for storage tanks of hydrochloric acid and dilute sulfuric acid, stainless steel for steel tanks for storing nitric acid, "target=_blank> stainless steel sheet lining, etc. Acid-resistant bricks (silica brick) It is also widely used in lining. The refractory brick lining can be used for heat insulation. The enamel is highly resistant to acid and is widely used in food, medicine and other industries to ensure product quality.
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