Principle of electrochemical corrosion?
1. Corrosive battery (primary battery or miniature battery)
Electrochemical corrosion of metals is a self-dissolution process when metals contact with media. In this process, the metal is oxidized, and the released electrons are completely consumed by the oxidant, forming a spontaneous short-circuit battery, which is called a corroded battery. Corroded batteries can be divided into three (or two) categories: (1) Different metals will contact with the same electrolyte to form corroded batteries.
For example, there is an iron rivet on the copper plate, which corrodes the battery.
Metal oxidation with iron as anode (cathode);
Fe→Fe2 ++ 2e-; (Fe→Fe2++2e)=-0.447V。
There may be the following two reduction reactions on cathode (anode) copper:
(a) When the partial pressure of oxygen in the air is 2 1 kPa, O2+4h+4e-→ 2h2o;
(O2+4H++4e-→2H2O)= 1.229V,
(b) During hypoxia, 2h++2e-→ H2 occurs; (2H++2e-→H2)=0V,
The electromotive force of the oxygen-containing battery is e1=1.229-(-0.447) =1.676 v; In the absence of oxygen, the electromotive force E2 of the battery is 0-(-0.447) = 0.447 V, which shows that oxygen corrosion is more likely to occur, and iron corrosion is particularly serious in the presence of oxygen. The copper plate and the nail are connected together, which is equivalent to the short circuit of the battery, so the above redox reaction continues to occur at the two poles.
Fe is oxidized into Fe2+ and enters the solution, and the surplus electrons turn to the copper electrode, where O2 and H+ undergo a reduction reaction, consuming electrons and H+, thus increasing the pH value of the solution.
Fe2+ ions generated in the water film react with OH- ions to generate Fe(OH)2, which is further oxidized by oxygen in the air, namely: Fe2++2OH-→ Fe (OH) 24Fe (OH) 2+2H2O+O2 → 4Fe (OH) 3.
Fe(OH)3 is the main component of rust. In this way, mechanical parts will be corroded.
(2) The metal contacted by the electrolyte will also be a battery with uneven surface or impurities.
For example, industrial steel contains impurities such as carbon. When its surface is covered with an electrolyte film, iron, carbon and electrolyte solution form a miniature corrosive battery.
The iron in the micro battery is the anode: Fe→Fe2++2e-
Carbon as cathode: if the electrolyte solution is acidic, hydrogen (2h++2e-→ H2) will be released on the cathode; If the electrolyte solution is alkaline, the reaction O2+2H2O+4e-→4OH- occurs on the cathode.
Summary: From the above analysis, it can be seen that the anodic reaction of corrosion battery is generally the dissolution process of metal:
M→Mz++ze-
Cathodic reactions vary under different conditions, and the following two reactions are the most common:
(1) Under anoxic conditions, H+ ions are reduced to hydrogen (hydrogen release corrosion).
2H++2e-→H2 .(=0.0V)
This reaction is usually easy to occur in acidic solutions and metal materials with small hydrogen overpotential.
② The reaction of oxygen reduction to OH- ion or H2O (oxygen consumption corrosion)
O2+2h2o+4e-→4oh- in neutral or alkaline solution. (=0.40 1V)
In acidic environment, O2+4h+4e-→ 2h2 (= 1.229V).
2. Once the corrosion current constitutes a corroded battery, when a current passes through it, the electrode will be polarized, so it is necessary to study the influence of polarization on corrosion. In the literature of metal corrosion, the polarization curve (the relationship between potential and current) is drawn as a straight line (the abscissa is logarithmic scale), which is called Evans polarization diagram (figure 10-8). The current density J corrosion in Evans polarization diagram represents the corrosion current of metal, and actually represents the corrosion rate of metal.
The main factors affecting the corrosion speed of metal surface (i.e. corrosion current j) are:
(1) EMF of corroded battery-The greater the potential difference between two balanced electrodes, the greater the maximum corrosion current. ② Polarization characteristics of metals —— Other things being equal, the greater the degree of polarization (i.e. the slope of polarization curve), the smaller the corrosion current.
③ Hydrogen overpotential —— When releasing hydrogen for corrosion, if the hydrogen overpotential precipitated on the metal surface is too large, the slope of polarization curve will be too large, but the corrosion current will be reduced.
Second, the stability of metals.
"What is the stability of metallic materials in the environment?" This is the primary problem that must be considered in the study of metal corrosion and anti-corrosion. Therefore, the potential -pH diagram of metal-water system is undoubtedly a very useful tool.
General expression of 1. potential (E)-pH relation
If there is the following electrode reaction: xO (oxidation state) +MH+Ze-? -→ yr (reduced state) +nH2O
For example: Fe3O4+8h+2e-= 3fe2+4h2o.
Wherein O represents the oxidation state and R represents the reduction state; X, m, z, y and n are the measurement coefficients of each reactant and product. When t = 298. 15k.
E=-( 10— 14)
Since pH =-LG [A (H+)] and A (H2O) = 1, the above formula can be written as follows
E= - ( 10— 15)
When a (r) and a (o) are specified, the potential e has a linear relationship with the pH value. (1). Potential has nothing to do with pH reaction:
(2) These reactions only involve the gain and loss of electrons, without the participation of H+ or OH- ions.
For example, reaction Zn2+(AQ)+2e-= Zn (s);
e(Zn2 ++ 2e-→Zn)=-0.762+0.0295 LG[a(Zn2+)/a(Zn)].
When a (Zn2+) = 10-6 and a (Zn) = 1.0,
E(Zn2++2e-→Zn)=-0.939V
3. Hydrogen-oxygen electrode reaction in aqueous solution
Because the reaction is carried out in aqueous solution, it is related to H2, O2, H+ and OH-. Therefore, any reaction system with water as solvent must consider the hydrogen-oxygen electrode reaction.
Hydrogen electrode reaction (line ①): electrode reaction equation 2h+(a)+2e-→ H2 (p); When p(H2)=,
e(2 h++ 2e-→H2)=-0.0592 ph( 10— 13)。
On the E-pH diagram, it is a straight line with zero intercept and a slope of -0.0592.
Oxygen electrode reaction (line ②): electrode reaction formula O2(p)+2H+(a)+2e-→H2O(l).
At 298. 15k, when A (H2O) = 1, p(O2)=, e (O2+2h+2e-→ H2O) =1.229-0.0592 pH.
The formula shows that the E-pH linear slope of oxygen electrode reaction is the same as that of hydrogen electrode, but the intercept is different.
4. Application of potential -pH diagram
The point on each line in (1) diagram 10-9 represents the equilibrium state of zinc -H2O system. Any point that is not on the straight line is in an unbalanced state. The upper part of each line is the oxidation state stable region in the electrode reaction represented by this line, and the lower part is the reduction state stable region.
Therefore, the stable regions of Zn2+, Zn and Zn(OH)2 are obtained.
② Above the line is the stable region of O2 (oxidation state) and below it is the stable region of H2O (reduction state); Above line ① is the stable region of H+ (oxidized state), and below line ① is the stable region of H2 (reduced state).
(2) The electrode reaction represented by any two lines in the E-pH diagram can form a chemical reaction. For example, the chemical reaction formed by the electrode reaction indicated by lines ① and ② is O2(g)+2H2(g)=2H2O(l). This reaction can be regarded as a fuel cell consisting of an oxygen electrode and a hydrogen electrode.
Generally speaking, the oxidation state in the electrode reaction represented by the straight line in the high potential region can oxidize the reduction state in the reaction represented by the straight line in the low potential region, that is, [oxidation state ]up+[ reduction state ]down →[ reduction state ]up+[ oxidation state] down.
When two straight lines represent the reaction of electrodes to form a battery, the greater the electromotive force of the battery, the greater the trend of redox reaction.
For example, Zn2++2e-=Zn is the equilibrium system represented by line segment A, and this equilibrium is below line ①, indicating that Zn is unstable in aqueous solution. H+ in the solution is reduced to H2(g) and Zn is oxidized to Zn2+. The reaction of 2H ++ Zn = Zn ++H2 is spontaneous. Because the stable region of Zn is also below ② line of O2 reduction reaction, Zn is oxidized to Zn2: 0.5o2+2h+Zn = Zn2++H2O.
① Line reacts with line A to form a battery: ② Line and line A form a battery, which is farther away from line A than line ①, indicating that the thermodynamic stability of Zn in aqueous solution containing O2 is worse.
(3) E-pH diagram can be used to guide the research of corrosion protection and metal protection.
As can be seen from figure 10-9, when E.
(4) E-pH diagram of water and iron:
Fe2++2e-→ Fe (line 1)
Fe2O3+6h+2e = 2fe2++3H2O (2 wires)
Fe3++E-= Fe2+ (three wires)
Fe2O3+6h+= 2fe3++3H2O (4 wires)
Fe3O4+8h+2e-= 3fe2++3H2O (5 wires)
3Fe2O3+2h+2e-= 2Fe3O4+H2O (6 wires)
Fe3O4+8h+8e-= 3fe+4h2o (7 wires)
In a word, E-pH diagram is widely used to solve a series of reaction and equilibrium problems in aqueous solution, such as element separation, hydrometallurgy, metal anticorrosion, metal electrodeposition, geological problems and so on.
Thirdly, electrochemical protection.
1. Anode protection (for metals with passivation curves)
In principle, the anodic protection method can be used to prevent the corrosion of metals passivated by a certain anodic current in some chemical media.
For example, anode protection has been widely used in the carbonization tower of ammonium bicarbonate production in China's chemical fertilizer plant, and achieved good results, effectively protecting the carbonization tower and the cooling water tank in the tower.
Attention should be paid to this method: the potential range in the passivation zone should not be too narrow, otherwise, due to improper control, the anode potential will be in the activation zone, which will not only promote the dissolution of the metal, but also accelerate the corrosion of the metal.
2. Cathodic protection is to add an anode to the protected metal component, so that the component itself becomes a cathode and is protected, and a reduction reaction occurs. Cathodic protection can be achieved in two ways.
(1) is called sacrificial anode protection method: it is called protector, that is, the metal with relatively large negative potential, that is, the metal that can be easily dissolved in the anode (for example, adding a zinc block to an iron container) is connected as a more effective anode. At this time, the dissolution of the protective agent basically replaced the dissolution of the anode in the original corrosion system, thus protecting the original metal. The disadvantage of this method is that the anode used as protector consumes more.
(2) impressed current cathodic protection method: At present, this method is widely used to protect gates and underground metal structures (such as underground storage tanks, oil pipelines and cables). ), seawater and fresh water corrosion equipment, chemical equipment crystallization tank and evaporation tank, is currently recognized as one of the most economical and effective anti-corrosion methods. In this method, the protected metal is connected to the negative pole of the external power supply, and another auxiliary anode is introduced into the system and connected to the positive pole of the external power supply.
The current from the auxiliary anode (composed of metal or nonmetal conductor) enters the cathode and anode areas of the corroded battery, and then returns to the DC power supply B. When the cathode area in the corroded battery is polarized to the open-circuit potential of the anode in the corroded battery by external current, all metal surfaces are at the same potential, and the corrosion current disappears. Therefore, as long as a certain applied current is maintained, the metal can no longer be corroded.
(3) Gas phase cathodic protection. Whether electrochemical methods can be used in gas phase environment is a problem that people have been hoping to solve. 1988, China developed cathodic protection technology in gas phase environment, which was used to protect the splash zone on overhead metal pipelines, bridges, rails and offshore engineering components. In the actual test of overhead metal pipelines, it achieved very good protection effect, prolonged the service life of materials by more than 20 times, and provided a new way for component protection in gas phase environment. The principle of gas-phase cathodic protection is the same as that of cathodic protection in solution, except that the solution is replaced by solid dielectric and becomes the main ion migration channel for cathodic protection current to flow from anode layer to cathode layer. The applied cathode current flows in from the auxiliary anode and reaches the cathode (i.e. the protected structural material) through the solid dielectric, thus protecting the structure in the gas phase environment.
3. Anti-corrosion function of corrosion inhibitor
Adding a small amount of corrosion inhibitor (such as a few ten thousandths) to the corrosive medium can significantly slow down the corrosion rate of metals. This method of using corrosion inhibitor to prevent metal corrosion is one of the most widely used methods in corrosion prevention.
Next, we will explain the basic principle of corrosion inhibitors to inhibit metal corrosion according to the polarization diagram. The rate of electrochemical corrosion is determined by the polarization characteristics of anode process and cathode process. As long as the added corrosion inhibitor can inhibit one or both of the above processes, the corrosion rate will decrease. According to corrosion inhibitors and the process that corrosion inhibitors can inhibit, we can divide corrosion inhibitors into anode corrosion inhibitors, cathode corrosion inhibitors and mixed corrosion inhibitors.
Corrosion inhibitor is added to accelerate polarization and reduce corrosion current. The main mechanism is to form a passivation film or an adsorption film on the electrode surface.
There are many kinds of corrosion inhibitors, including nitrite, chromate, dichromate and phosphate. Organic corrosion inhibitors include amines, aldehydes, heterocyclic compounds and imidazolines. When it is used, it should be determined by screening test according to the type of protected metal and corrosive medium.