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Corrosion principle of metal products
Except for a few precious metals (such as gold and platinum), various metals have a tendency to interact with the surrounding medium, so metal corrosion is ubiquitous. According to the mechanism of metal corrosion, corrosion can be divided into two types: electrochemical corrosion and chemical corrosion. The corrosion of general metal products is mainly electrochemical corrosion.
First, the principle of metal electrochemical corrosion
Electrochemical rust refers to rust caused by the action of metal in contact with an electrolyte solution such as an acid, a base or a salt. It has a current generated during the rusting process, that is, a so-called microbattery. Electrochemical corrosion is the main form of destruction of metals.
1, the principle of electrochemical corrosion
The electrochemical corrosion of metals must be carried out under three conditions:
1 There is an electrode potential difference between each part of the metal (or between different metals);
2 each part having a potential difference is to be in a connected electrolyte solution;
3 Each part having an electrode potential difference must be associated.
The cause of electrochemical corrosion is mainly due to the role of the primary battery under the above three strips. When two metals are placed in an electrolyte and connected by wires, a current flows through the wires. This device is called a primary battery. The well-known volt battery device is the primary battery (as shown in Figure 6-1).
In a volt battery, the wires connect the plates together to form a free electron flow path. The electrolyte solution (sulfuric acid solution) maintains the two electrode plates at their respective electrode potentials while becoming a channel for cation flow.
It can be seen that the electrochemical corrosion of the metal is caused by the fact that when the metal is in contact with the electrolyte solution, the electrode potentials of the various portions of the metal surface are not the same, and as a result, a rusted battery is formed, wherein a portion having a lower potential becomes an anode and is susceptible to rust; The higher potential part becomes the cathode, which only acts as an electron transferr and is not rusted.
When the metal is in contact with air having a temperature higher than the surface temperature of the metal, the water vapor contained in the air can form liquid water, agglomerate on the surface of the metal, and wet the surface of the metal to form a water film. When a water film is present on the surface of the metal, the water film is actually an electrolyte solution due to the dissolution of certain gases in the atmosphere such as carbon dioxide, sulfur dioxide, nitrogen dioxide or salts. In this case, the metal surface naturally undergoes electrochemical corrosion, causing corrosion of the metal product.
2, polarization
The electrochemical corrosion of the metal material is the result of the action of the rust battery. The current generated during the rust process is called the rust current, and its magnitude is a function of the rust rate. According to Ohm's law, the rust current intensity I can be calculated:
Ec-Ea
I= -------
R
Where: I--corrosion current intensity
Ea--anode electrode potential
Ec--cathode electrode potential
R--Total resistance of rusted batteries (including internal resistance and external resistance)
After knowing the rust current intensity, the rust amount W and the rust rate can be calculated according to Faraday's law.
ItA
W= -------
Fn
Where: W--corrosion amount (g)
I--current intensity (A)
T--corrosion time (s)
F--Faraday constant
N--the order of metal in rusted batteries
A--the atomic weight of metal
The rust rate is the amount of rust per square meter per hour, ie
W 3600IA
---- = --------
ST SFn
Where: T--time (h)
S--anode area (m2)
The calculated rust rate is several tens to several thousand times larger than the actual rust rate. It has been experimentally proved that this is because the total resistance of the rusting process is not changed, but the cathode and anode potential difference is reduced to cause a result. Since the anode potential moves in the positive direction and the cathode potential moves in the negative direction when the rust starts, the potential difference is reduced. The change in electrode potential is mostly done in a short time from the start of rust. This change in electrode potential is called polarization. Polarization can reduce the rust rate of metal, so studying the polarization and its causes have certain practical significance for preventing corrosion of metal products.
The reason for the cathodic polarization is that during the anode reaction of the rusted cell, the metal ions dissolve into the electrolyte, leaving excess electrons in the anode region, and there is a potential difference between the interconnected anode and cathode regions. Then, excess electrons flow from the anode region into the cathode region. If the rate of cathodic reaction of electron absorption around the cathode is less than the rate at which electrons enter the cathode region, electrons accumulate on the surface of the cathode, with the result that the potential of the cathode gradually becomes negative. And the greater the current to the cathode, the more negative the cathode potential becomes. At the same time, the potential difference between the yin and yang poles is gradually reduced, so that it is close to zero, and the rust of the metal stops.
(2) Anodic polarization. The phenomenon that the anode potential is positive is called anodic polarization. The reason for the anodic polarization is that in some metals, the passivation film is formed mainly on the surface of the metal, which hinders the anode process from causing the anode potential to move in a positive direction; in addition, if the metal ions of the anode metal dissolved in the solution cannot diffuse, the accumulation Near the surface of the anode, the concentration of metal ions near the anode is gradually increased, and the anode potential is made positive, thereby retarding the progress of the anode process, which is called concentration polarization.
3. Passivation and activation of metal surfaces
If the metal is very chemically stable under certain conditions, it is very active in similar conditions. We call a state with very high chemical stability a passivation state. Corrosion products such as metal rust have a dense structure and tightly cover the surface of the metal to prevent further corrosion of the metal. The metal surface is passivated. This cover layer is called a passivation film. Almost all metals can be converted to passivation under appropriate conditions, and the passivation of iron-based metals (iron, aluminum, nickel) and metals such as chromium, niobium, tantalum, and molybdenum is the most representative.
The passivated metal, in the dry state, can be kept passivated for a long time in vacuum-dried air or dry oxygen. However, in humid air, especially in the absence of impurities such as carbon dioxide, nitrogen dioxide, and sulfur dioxide, the passivated surface can be activated quickly.
The stability of the passivation state determines the conditions of the passivation, the structure of the metal surface, the state, and the time of action of the passivating agent. After the metal is passivated, the potential of the electrode is increased, that is, the potential is moved in the positive direction, so the electrochemical corrosion rate is greatly reduced to almost zero. When the external conditions change, the passivation of the passivated metal to an activated state is called passivation or activation.
Second, the principle of chemical corrosion
Chemical rust refers to rust caused by direct chemical action of metals in external media. It is characterized in that no current is generated during the rusting process, and the rust product precipitates on the metal surface. The rust product film, that is, the oxide film covers the surface of the metal, can reduce the reaction speed of the metal to the medium to a certain extent. If the formed film is very tight and complete, the metal and the medium can be isolated, hindering them. Contacting each other, the metal is protected from further rust. For example, the Al2O3 film formed by the simultaneous oxidation of aluminum is much thinner than the Fe3O4 film formed by the simultaneous oxidation of iron, but because of its tightness and completeness, the protection performance is good. A film composed of a layer of rusted product that covers the metal surface to reduce the rate of metal rust, which we call a surface protective film. Oxidation of metals at high temperatures or by the action of gases such as oxygen, carbon dioxide, hydrogen sulfide, chlorine, hydrogen chloride, carbon dioxide, hydrogen, etc., and the action of metals with acids, bases, and salts in a normal temperature dry environment are chemical rust. At normal temperature, chemical rust is not dominant, but at high temperatures, chemical rust is more pronounced.
Films produced by chemical rust can be divided into five cases:
(1) A protective film is not formed after chemical rust, but a volatile product is formed. In this case, the rust rate only determines the speed of the chemical reaction, that is, the sublimation rate of the oxide. The amount of corrosion is linear with time. Metals belonging to such cases are molybdenum, ruthenium, osmium, iridium and the like.
(2) Forming an incomplete film on the metal surface, such as many cracks or large pores in the film, the film does not have a protective effect. The amount of corrosion is linear with time. Metals belonging to such a situation are potassium, sodium, calcium, magnesium, and the like.
(3) Form a complete film on the metal surface. The rust rate determines the diffusion and chemical reaction rate, which can be represented by Figure 6-2. The curve in the figure is basically a parabola. It starts to indicate that the rust rate is determined by the chemical reaction speed. It is linear. Because the start time is short, the linear shape is not obvious. Then the rust rate is determined by the diffusion speed. The curve is parabolic and finally indicates rust. The speed is determined by both the chemical reaction rate and the diffusion rate. Metals belonging to such a kind are tungsten, iron, cobalt, copper, nickel, manganese, lanthanum, cobalt, titanium, and the like.
(5) Many metals interact with oxygen in the air to form a protective film of oxide film on the surface of the metal. The thickness of this film depends on the nature of the metal, the surface state, the oxidation temperature and the composition of the medium. The film formed in dry air is very thin under normal temperature conditions. Some metals such as silver, copper and copper alloys darken the surface of oxide films formed in dry air containing sulfides, which is often referred to as tarnishing.
It has been proved by experiments that the metal surface is oxidized into a film in the air, or is passivated into a film in a solution. If the growth process of the protection is a straight line law, the surface protective film does not have a protective property, and it will be continuously damaged and peeled off. If the growth of the surface protective film follows a logarithmic law, the protective film has good protection properties. Therefore, studying the formation process of the surface film is also very beneficial for further study of the corrosion resistance of the metal.
Anti-rust packaging technology
Metal corrosion due to chemical or electrochemical action of the surrounding medium is called metal corrosion. According to different corrosive media, it can be divided into atmospheric corrosion, seawater corrosion, underground corrosion, and bacterial corrosion. The most encountered in packaging engineering is atmospheric corrosion. Corrosion has a serious destructive effect on metallic materials and products. According to the test, if the steel is rusted by 1%, its strength will be reduced by 5% to 10%, and the steel sheet is more likely to lose its use value due to rust and perforation. The loss of metal products due to rust far exceeds the value of the materials used. Therefore, in order to reduce the loss caused by metal corrosion, it is very important to study the corrosion law of metal products and its protection technology.
Even if a piece of metal is not in contact with other metals, placing it in an electrolyte solution will produce a rust-like battery similar to the above. Because the metals used in general industry are not composed of the same metal element, they often contain a small amount of impurities. In addition, a large amount of impurities are also distributed on the metal surface. When it comes into contact with the electrolyte solution, each particle impurity becomes a cathode for the metal itself, so there are inevitably many tiny cathodes and anodes on the entire surface. Many tiny primary cells are formed on the metal surface. These tiny primary batteries are called microbatteries.
(1) Cathodic polarization: The phenomenon that the cathode potential shifts in the negative direction is called cathodic polarization.
(4) Some membranes formed by metal corrosion have greater resistance to diffusion, and the corrosion rate is logarithmically related to time. The metals belonging to this type are zinc, silicon, aluminum, chromium, and the like. The oxidation of iron at lower temperatures also obeys the logarithmic curve relationship.