What is electrochemical corrosion?
The metal material is in contact with the electrolyte solution and is corroded by the electrode reaction. The electrochemical corrosion reaction is a redox reaction. In the reaction, the metal loses electrons and is oxidized. The reaction process is called an anodic reaction process, and the reaction product is a metal ion entering the medium or a metal oxide (or a metal poorly soluble salt) covering the metal surface; The substance is reduced by obtaining electrons from the surface of the metal, and the reaction process is called a cathodic reaction process. In the cathodic reaction process, the substance obtained by obtaining electrons is conventionally referred to as a depolarizer. In the case of uniform corrosion, there is no significant difference in the probability of performing the anodic reaction and the cathodic reaction everywhere on the metal surface, and the surface positions at which the two reactions are carried out are constantly and randomly changed. If some areas of the metal surface are mainly subjected to anodic reaction, and the remaining surface areas are mainly subjected to a cathodic reaction, the former is referred to as an anode region, and the latter is a cathode region, and the anode region and the cathode region constitute an etched battery. Directly causing the destruction of the metal material is the anode reaction, so the externally connected power source or the wire is used to connect the protected metal to another metal having a lower electrode potential, so that the corrosion occurs on the metal having a lower potential.
Oxygen-suppressed metal in the acidity of a weak or neutral solution, the oxygen in the air dissolved in the water film on the surface of the metal caused by galvanic corrosion, called oxygen corrosion. For example, steel corrosion in near-neutral humid air is oxygen-absorbing corrosion, and its electrode reaction is as follows:
Negative electrode ( Fe ): Fe - 2e = Fe2+
Positive electrode ( C ): 2H2O + O2 + 4e = 4OH-
The galvanic corrosion of metals such as steel is mainly oxygen absorption corrosion.
Hydrogen is evolved in the case of galvanic corrosion in a more acidic solution, which is called hydrogen evolution corrosion . Carbon is generally found in steel products. In humid air, the surface of the steel absorbs water vapor to form a thin film of water. When carbon dioxide is dissolved in the water film, it becomes an electrolyte solution, which increases the H+ in the water . It is an infinite number of tiny primary batteries that use iron as a negative electrode, carbon as a positive electrode, and acidic water film as an electrolyte solution. The redox reaction that occurs in these primary batteries is
Negative electrode (iron): iron is oxidized Fe-2e=Fe2+ ; positive electrode (carbon): H+ in solution is reduced 2H++2e=H2↑
This forms a myriad of tiny primary batteries. Finally, hydrogen is released on the surface of the carbon, and the iron is corroded, so it is called hydrogen evolution corrosion.
Cathodic protection is an electrochemical protection technique used to prevent corrosion of metals in dielectrics (medium such as seawater, fresh water, and soil). The basic principle of this technology is to use a metal member as a cathode to apply a certain DC current to it. Cathodic polarization is generated. When the potential of the metal is negative to a certain potential value, the electrochemical non-uniformity of the metal surface is eliminated, and the cathodic dissolution process of the corrosion is effectively suppressed to achieve the purpose of protection. The polarization curve is used below to illustrate the principle of cathodic protection. To illustrate the problem, the cathode and anode polarization curves are simplified to a straight line, as shown in Figure 1 below.
Both the anodic reaction and the cathodic reaction on the metal surface have their own equilibrium points. In order to achieve complete cathodic protection, the potential of the entire metal must be lowered to the equilibrium potential of the most active point. The anode potential and the cathode potential of the metal surface are Ea and Ec , respectively . Due to the polarization of the metal corrosion process, the potentials of the anode and the cathode are close to the potential Ecorr (natural corrosion potential) corresponding to the intersection point S , and the corrosion current is Icorr. .
If cathodic polarization is performed, the potential will move from a more negative direction, and the anode reaction curve EcS is extended from S to C. When the potential is polarized to E1 , the required polarization current is I1 , which is equivalent to the AC line segment. wherein the line segment BC is applied to this portion, AB segment which is part of the current provided by the current of the anode reaction, at this time yet metal corrosion. If the metal cathode is polarized to a more negative potential, for example to Ea , then the potential of each region of the metal surface is equal to Ea , the corrosion current is zero, and the metal is fully protected. At this time, the applied current Iapp1 is the complete protection. Current is required.
Cathodic protection is divided into two types: sacrificial anode cathodic protection method and impressed current cathodic protection method.
In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to the experience of use, in addition to mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is local corrosion (ie stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion) . The failure cases caused by these localized corrosions account for almost half of the failure cases. In fact, many failures can be avoided through reasonable selection.
Stress Corrosion Cracking ( SCC ): A general term for stress-bearing alloys that fail alternately in a corrosive environment due to the expansion of the striated lines. Stress corrosion cracking has a brittle fracture morphology, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking are tensile stress (whether residual stress or applied stress, or both) and the presence of a specific corrosive medium. The formation and expansion of the profile is approximately perpendicular to the direction of the tensile stress. This stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture without the presence of corrosive media. At the microscopic level, the crack passing through the grain is called a transgranular crack, and the crack along the grain boundary is called an intergranular crack. When the stress corrosion cracking spreads to a depth (here, the load is on the material section) The stress reaches its breaking stress in the air), and the material is broken by normal cracking (in the ductile material, usually by polymerization of microscopic defects). Thus, the section of the part that fails due to stress corrosion cracking will include a characteristic region of stress corrosion cracking and a " dimple " region associated with the polymerization of the microdefect .
Pitting corrosion : A form of localized corrosion that causes corrosion.
Intergranular corrosion : The grain boundary is a boundary between crystal grains with different crystallographic orientations. Therefore, they are favorable for segregation of various solute elements in steel or precipitation of metal compounds such as carbides and δ phases. District City. Therefore, in some corrosive media, it is not surprising that the grain boundaries may be corroded first. This type of corrosion is known as intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media.
Crevice Corrosion : A form of localized corrosion that may occur in the gap where the solution stagnates or in the surface of the shield. Such gaps may be formed at the junction of metal and metal or metal and non-metal, for example, where rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms are attached to the candle.
v General Corrosion: A term used to describe the corrosion phenomena that occur in a relatively uniform manner across the entire surface of an alloy. When total corrosion occurs, the village material becomes thinner due to corrosion, and even the material corrosion fails. Stainless steel may exhibit general corrosion in strong acids and bases. The failure problem caused by general corrosion is not very worrying because it can usually be predicted by simple immersion tests or by consulting the literature on corrosion.
2. Corrosion resistance of various stainless steel
304 is a versatile stainless steel that is widely used to make equipment and parts that require good overall performance (corrosion resistance and formability).
301 stainless steel exhibits significant work hardening during deformation and is used in a variety of applications requiring higher strength.
302 stainless steel is essentially a variant of 304 stainless steel with a higher carbon content , which allows it to achieve higher strength through cold rolling.
302B is a stainless steel with a high silicon content, which has high resistance to high temperature oxidation.
303 and 303Se are free-cutting stainless steels containing sulfur and selenium, respectively, for applications where the main requirements are easy cutting and high gloss. 303Se stainless steel is also used to make parts that require enthusiasm, because under these conditions, the stainless steel has good hot workability.
304L is a variant of 304 stainless steel with a lower carbon content for applications requiring soldering. The lower carbon content minimizes the precipitation of carbides in the heat affected zone near the weld, which may result in intergranular corrosion (weld erosion) in certain environments.
304N is a nitrogen-containing stainless steel added to increase the strength of the steel.
305 and 384 stainless steels contain high nickel and have a low work hardening rate, making them suitable for a variety of applications where high cold formability is required.
308 stainless steel is used to make the electrode.
The nickel and chromium contents of 309 , 310 , 314 and 330 stainless steels are relatively high in order to improve the oxidation resistance and creep strength of steel at high temperatures. The 30S5 and 310S are variants of the 309 and 310 stainless steels, except for the lower carbon content, in order to minimize the carbides precipitated near the weld. 330 stainless steel has a particularly high resistance to carburizing and thermal shock resistance.
Types 316 and 317 stainless steel contain aluminum and are therefore much more resistant to pitting corrosion in marine and chemical industrial environments than 304 stainless steel. Among them, variants of Type 316 stainless steel include low carbon stainless steel 316L , nitrogen-containing high-strength stainless steel 316N, and high -sulfur free-cutting stainless steel 316F .
321 , 347 and 348 are stainless steel stabilized by titanium, niobium and tantalum, respectively, and are suitable for use as welded members at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, which has certain restrictions on the combination of boring and drilling.
Electrochemical corrosion
1. When the impure metal is in contact with the electrolyte solution, the primary battery reaction occurs, and the more active metal loses electrons and is oxidized. This corrosion is called electrochemical corrosion. Corrosion of steel in moist air is the most prominent example of electrochemical corrosion.
We know that steel is not easily corroded in dry air for a long time, but it is quickly corroded in moist air. It turns out that in the moist air, the surface of the steel absorbs a thin film of water containing a small amount of hydrogen ions and hydroxide ions, and also dissolves oxygen and other gases, resulting in the formation of steel surface. A layer of electrolyte solution, which combines with iron in steel and a small amount of carbon to form countless tiny primary batteries. In these primary batteries, iron is the negative electrode and carbon is the positive electrode. Iron loses electrons and is oxidized . Electrochemical corrosion is the main cause of steel corrosion.
The metal material is in contact with the electrolyte solution and is corroded by the electrode reaction. The electrochemical corrosion reaction is a redox reaction. In the reaction, the metal loses electrons and is oxidized. The reaction process is called an anodic reaction process, and the reaction product is a metal ion entering the medium or a metal oxide (or a metal poorly soluble salt) covering the metal surface; The substance is reduced by obtaining electrons from the surface of the metal, and the reaction process is called a cathodic reaction process. In the cathodic reaction process, the substance obtained by obtaining electrons is conventionally referred to as a depolarizer.
In the case of uniform corrosion, there is no significant difference in the probability of performing the anodic reaction and the cathodic reaction everywhere on the metal surface, and the surface positions at which the two reactions are carried out are constantly and randomly changed. If some areas of the metal surface are mainly subjected to anodic reaction, and the remaining surface areas are mainly subjected to a cathodic reaction, the former is referred to as an anode region, and the latter is a cathode region, and the anode region and the cathode region constitute an etched battery. Directly causing the destruction of the metal material is the anode reaction , so the externally connected power source or the wire is used to connect the protected metal to another metal having a lower electrode potential, so that the corrosion occurs on the metal having a lower potential .
2. There are many metal corrosion principles, of which electrochemical corrosion is the most extensive one. When the metal is placed in an aqueous solution or in a humid atmosphere, the surface of the metal forms a microbattery, also known as an etched battery ( the electrodes are conventionally referred to as the anode and the anode, not the positive and negative electrodes ) . An oxidation reaction occurs on the anode to dissolve the anode, and a reduction reaction occurs on the cathode, generally only functioning to transfer electrons. The reason for the formation of the corroded battery is mainly because the metal surface adsorbs the moisture in the air to form a water film, so that CO2 , SO2 , NO2, etc. in the air are dissolved in the water film to form an electrolyte solution, and the layer is immersed in the layer. The metal in the solution is always impure, such as industrial steel, which is actually an alloy, that is, in addition to iron, it also contains graphite, cementite (Fe3C) and other metals and impurities, most of which are not iron active. . The anode of the thus formed etching battery is iron, and the cathode is an impurity, and corrosion is continuously performed due to the close contact of iron with impurities.
(1) Hydrogen evolution corrosion ( when the surface of the steel surface is strongly acidic )
Anode (Fe) : Fe=Fe2+ + 2e-
Fe2+ + 2H2O=Fe(OH)2 + 2H+
Cathode ( impurity ) : 2H+ + 2e-=H2
Battery reaction: Fe + 2H2O=Fe(OH)2 + H2↑
Since hydrogen is evolved, it is called hydrogen evolution corrosion.
(2) Oxygen absorption corrosion ( when the surface of the steel surface is weakly acidic )
Anode (Fe) : Fe=Fe2+ + 2e-
Cathode: O2 + 2H2O + 4e-=4OH-
Total reaction: 2Fe + O2 + 2H2O = 2Fe(OH)2
Because of the absorption of oxygen, it is also called oxygen corrosion.
Fe(OH)2 formed by hydrogen evolution corrosion and oxygen absorption corrosion is oxidized by oxygen, and Fe(OH)3 is dehydrated to form Fe2O3 rust. The corrosion of steel products in the atmosphere is mainly oxygen absorption corrosion.
Fe + 2H2O=Fe(OH)2 + H2↑ O2 + 2H2O + 4e-→4OH-
2Fe + O2 + 2H2O=2Fe(OH)2 2H+ + 2e-→H2
The difference between chemical corrosion and electrochemical corrosion
According to the principle of corrosion, it can be divided into chemical corrosion and electrochemical corrosion. The difference between the two is that when electrochemical corrosion occurs, there are isolated cathodes and anodes on the metal surface, and a small current exists between the two poles, and pure chemical corrosion does not form a microbattery. In the past, high temperature gas corrosion (such as high temperature oxidation) was considered to be chemical corrosion, but the modern concept indicates that there are also isolated anode and cathode regions in high temperature corrosion, as well as electron and ion flow. Accordingly, another classification has emerged: dry corrosion and wet corrosion. Wet corrosion refers to the corrosion of metals in aqueous solutions, which is typical of electrochemical corrosion. Dry corrosion refers to corrosion in dry gases (usually at high temperatures) or non-aqueous solutions. Simple physical corrosion is rare for metals, and for non-metals, it usually produces pure chemical or physical corrosion, sometimes both at the same time.
Electrochemical corrosion
Metal corrosion is an urgent problem to be solved, and the annual damage due to corrosion of metals is enormous. Workers worked hard to refine iron ore coal into iron and steel, but these steels were corroded every day and turned into an iron ore brother, iron oxide. Therefore, understanding the causes of corrosion and finding ways to prevent corrosion, both theoretically and practically, have great significance. Introduced here is the corrosion caused by electrochemical reactions.
Immerse the two nails in concentrated hydrochloric acid. The soaking time is preferably longer (about one day) to remove the rust on the zinc or old nails on the surface of the nail. The hydrochloric acid on the nail was then washed off with water and allowed to air dry. Put the pure zinc sheet (can wash the outer skin of the waste dry battery, polish it with sandpaper, cut into long and narrow zinc sheets) around the middle of the first nail, the zinc sheet should be tightly wound, and minimize the zinc sheet. The gap between them.
Wrap the copper wire (or the enamelled wire with the peeled patent leather) around the middle of the second nail and also tightly.
The fine iron wire was immersed in 6 M hydrochloric acid to remove zinc plated on the surface. It is then washed with water, dried, and wrapped around the middle of a long piece of zinc. Then, another piece of treated fine iron wire is wound on a thick copper wire (or a thick enameled wire to scrape off the patent leather).
After the above materials are ready, put four test tubes on the test tube rack, fasten the cotton threads on the nails and zinc wire, and hang them in four test tubes so that they are in the middle of the test tube. However, care should be taken not to allow nails, zinc sheets and copper wires to come into contact with the inner wall of the test tube, but to hang in a test tube.
Add 100 ml of distilled water and 0.7 g of agar to the beaker and heat to dissolve all the agar. Slightly cold, add 2 ml of phenolphthalein indicator to the agar solution and mix well. Wait until the agar solution becomes warm, but before it freezes, turn the agar solution into warm, but before freezing, pour the agar solution into four tubes until it is full. After the agar solution is solidified, the nails, zinc sheets and copper wires are fixed in the jelly. At this time, it must be noted that air bubbles cannot be left in the jelly. Â
Soon, the metal and agar jelly in the four tubes changed. In the first tube, the middle of the nail was wrapped around the zinc sheet, and a precipitate was formed on the zinc sheet because the zinc was corroded:
Zn=Zn 2+ + 2e
The resulting Zn2+ reacts with potassium ferricyanide in the jelly to form a white zinc ferricyanide precipitate:
　　　　　　　　　　 Zn 2+ +2K 3 [Fe(CN) 6 ]=Zn 3 [Fe(CN) 6 ] 2 +6K +
The nail remains the same, but the jelly around it turns pink.
Why is this change happening? After the zinc sheet is wound around the nail, zinc and iron form a primary battery, zinc is the negative electrode, and iron is the positive electrode. The following reaction occurs on the negative electrode:
　　　　　　　　　　　　　　　　 Zn=Zn 2+ +2e
Hydrogen is evolved on the positive electrode:
　　　　　　　　　　　　　　　　 2H+2e=H 2
Due to the precipitation of hydrogen gas, the concentration of hydrogen ions around the positive electrode is reduced, and the concentration of hydroxide ions is relatively increased, so that the gel around the positive electrode (iron) is alkaline, so the phenolphthalein is pink.
In the second test tube, the middle of the nail is wound around the copper wire. Pink appears around the copper wire. A blue precipitate appears on the nails (especially at both ends of the nail). This is because the iron and copper contact also constitute a primary battery. But iron is the negative electrode and copper is the positive electrode, so the iron is corroded. The reaction of the negative electrode is:
　　　　　　　　　　　　　　　　 Fe=Fe 2+ +2e
　 Fe 2 + and the potassium ferricyanide in the jelly form a deep blue Teng's blue precipitate ( Fe 3 [FeCN 6 ] 2 ):
　 3Fe 2 ++2K 3 Fe(CN) 6 =Fe 3 [Fe(CN) 6 ]2+6K +
Hydrogen is generated on the positive electrode (copper):
　 2H++2e=H 2 Â
As in the above experiment, the concentration of hydroxide ions around the copper wire is greater than the hydrogen ion concentration, and the phenolphthalein in the jelly appears pink.
In the third tube, the middle of the zinc sheet is wound around the wire, and the zinc and iron form the primary battery. Zinc is a negative electrode and iron is a positive electrode. Therefore, a white precipitate is formed on the zinc sheet, and the periphery of the wire becomes pink.
In the fourth test tube, the middle of the copper wire is wound around the wire, the iron is the negative electrode, the copper is the positive electrode, the Tengshi blue is formed on the wire, and the pink wire appears around the copper wire.
Finally, it can be seen that bubbles appear in the jelly, indicating that hydrogen is actually generated during the metal corrosion process.
Through the above four experiments, you can understand that one of the reasons for metal corrosion is that when there are impurities in the metal, the two metals form the primary battery, causing galvanic corrosion.
If the impurity metal is more active than iron, the impurities are first corroded. If iron is more active than the impurity metal, the iron is corroded. As you carefully observe the phenomena that occur in your daily life, you will find examples of these galvanic corrosion.
White iron is made by plating zinc on iron sheets. It can be used as a chimney, iron basin, kettle, etc. Zinc plating acts as a protective iron on the iron sheet because zinc not only isolates iron from air (oxygen and moisture), but even if the white iron is destroyed, zinc is first corroded.
Now that you understand the truth of galvanic corrosion, you know that you should never let iron come into contact with inactive metals such as copper or silver, or iron will soon be corroded.
In addition, the tinplate used to make the cans is plated with tin on the outside of the iron. Since the tin is relatively stable, it can be prevented from being corroded after being plated on the iron. However, if the tin coating has a crack, the can is worse. The reason is very simple, because tin is not as active as iron. In the primary battery they are composed of, iron is the negative electrode and tin is the positive electrode. Of course, the iron is first corroded. In this way, tin not only does not protect, but makes the iron rotten faster.
Reference electrode
Reference electrode
An electrode used as a reference for comparison when measuring various electrode potentials. The electrode to be measured and the reference electrode of the precisely known electrode potential value constitute a battery, and the electrode potential of the electrode to be measured can be calculated by measuring the value of the battery electromotive force. The reference electrode must be a single reversible reaction of the electrode reaction, and the electrode potential is stable and reproducible. A slightly soluble salt electrode is often used as a reference electrode, and the hydrogen electrode is only an ideal but not easily achievable reference electrode. A list of commonly used reference electrodes is as follows.
1. Research electrode: The research electrode is also called the working electrode or the test electrode. The electrode process occurring on this electrode is our research object. It is therefore required to study the surface properties of the electrode, such as the composition of the electrode and the surface state of the electrode. As a research electrode widely used in electrochemical research, there are a solid metal electrode and a liquid metal electrode.
2. Auxiliary electrode: The auxiliary electrode is also called the counter electrode. It is only used to pass the current to achieve the polarization of the research electrode. When the cathode process was studied, the auxiliary electrode was used as the anode, and when the anode process was studied, the auxiliary electrode was used as the cathode. The area of ​​the auxiliary electrode is generally larger than that of the research electrode, thus reducing the current density on the auxiliary electrode so that it is not substantially polarized during the measurement, and thus a platinum black electrode is often used as the auxiliary electrode. Sometimes, for the convenience of measurement, the auxiliary electrode can also be used to study the same metal fabrication of the electrode. If the product of the auxiliary electrode has an effect on the research electrode, the anode and cathode regions can be separated by a plain porcelain or microporous sintered glass plate ( D ) in the middle of the H -type electrolytic cell (see Fig. 2-3 ).
3. Reference electrode: The reference electrode is a comparison standard for measuring the electrode potential. It has a known and stable electrode potential during the measurement process. Therefore, the reference electrode and the electrode to be tested are used to measure the battery, and the measurement battery is measured. The electromotive force can calculate the electrode potential of the electrode to be tested. Generally, the performance of the reference electrode is strict. The reference electrode is a reversible electrode. Its potential is an equilibrium potential. It conforms to the Nernst electrode potential formula. In principle, the reference electrode should be an unpolarized electrode. In other words, when a current flows. The electrode potential change is very small. In use, the stability and reproducibility of the reference electrode are required to be good, that is, the electrode potential value of the reference electrode should not change after a certain period of time, and the potential of the reference electrode produced each time should also be the same. The potential of the reference electrode changes little with temperature, and can quickly return to the original electrode potential after power-off, without hysteresis. Finally, the reference electrode is easy to prepare, use, and maintain.
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