Electrochemical corrosion is that metal and electrolyte form two electrodes to form a corrosion galvanic cell. For example, iron and oxygen, because the electrode potential of iron is always lower than that of oxygen, iron is the negative electrode and is corroded. It is characterized by the formation of many small bulges with different diameters on the surface where oxygen corrosion occurs, and the secondary layer is black powdery ulcer corrosion pits.
Basic Introduction
When the impure metal is in contact with the electrolyte solution, a galvanic reaction occurs, and the more active metal loses electrons and is oxidized. This corrosion is called electrochemical corrosion. The corrosion of steel in moist air is the most prominent example of galvanic corrosion. Steel does not corrode for a long time in dry air, but corrodes quickly in moist air. It turns out that in the humid air, a thin water film is adsorbed on the surface of the steel. This water film contains a small amount of hydrogen ions and hydroxide ions, and also dissolves gases such as oxygen. As a result, the surface of the steel is formed. A layer of electrolyte solution that happens to form countless tiny galvanic cells with the iron and a small amount of carbon in the steel. In these galvanic cells, 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 corrodes through 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 the negative electrode reaction process. The reaction product is the metal ion entering the medium or the metal oxide (or metal insoluble salt) covering the metal surface; The substance obtains electrons from the metal surface and is reduced, and the reaction process is called the positive reaction process. In the process of the positive electrode reaction, the substances that gain electrons and are reduced are customarily called depolarizers.
During uniform corrosion, there is no significant difference in the probabilities of the negative and positive reactions occurring anywhere on the metal surface, and the positions of the surfaces where the two reactions are performed are constantly changing randomly. If there are some areas on the metal surface that mainly carry out negative reactions, and the rest of the surface areas mainly carry out positive reactions, the former is called the negative area, the latter is the positive area, and the negative area and the positive area form a corrosion battery. It is the negative electrode reaction that directly destroys the metal material, so an external power supply or a wire is often used to connect the protected metal to another metal with a lower electrode potential, so that corrosion occurs on the metal with a lower potential.
Related Principles
There are many corrosion principles of metals, of which electrochemical corrosion is the most extensive one. When the metal is placed in an aqueous solution or in a humid atmosphere, a micro-battery will be formed on the metal surface, also known as a corrosion battery (the electrodes are customarily called negative and positive electrodes, not positive and negative electrodes). The oxidation reaction occurs on the anode, which dissolves the anode, and the reduction reaction occurs on the cathode, which generally only plays the role of transferring electrons. The main reason for the formation of corrosion batteries is that the metal surface adsorbs moisture in the air to form a layer of water film, so carbon dioxide, sulfur dioxide, nitrogen dioxide, etc. in the air are dissolved in this layer of water film to form an electrolyte solution. The metal in this layer of solution is always impure, such as industrial steel, which is actually an alloy, that is, in addition to iron, it also contains graphite, cementite and other metals and impurities, most of which are not active with iron. The anode of the corrosion cell thus formed is iron, and the cathode is impurity, and due to the close contact between iron and impurity, corrosion continues.
Common Form
Electrochemical corrosion of metals can be divided into general corrosion and localized corrosion according to the form of damage.
General Corrosion
General corrosion refers to corrosion that occurs over the entire metal surface. Generally speaking, the general corrosion is more evenly distributed, the corrosion rate is relatively stable, the life of the machinery and equipment can be predicted, the inspection of the equipment is also relatively easy, and generally no unexpected accidents will occur. The cathode and anode of the general corrosion battery are all micro-electrodes, and the area of the cathode and anode is basically equal, so the reaction speed is relatively stable.
Localized Corrosion
Localized corrosion refers to corrosion that only concentrates on a local area of the metal surface, and most of the remaining areas are hardly corroded. The amount of metal loss caused by localized corrosion is not large, but severe localized corrosion can lead to sudden damage to machinery and equipment, which is difficult to predict, often resulting in huge economic losses, and even catastrophic accidents. According to the survey results of Japan’s Mitsubishi Chemical Machinery Corporation on the destruction cases of chemical plants in the past 10 years, general corrosion and high temperature corrosion only account for 13.4%, while local corrosion accounts for more than 80%. This shows the severity of localized corrosion. Common local corrosion is a bit accidental corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking, etc.
Phenomenal Hazard
Due to the potential difference between the metal surface and the iron scale, local corrosion of the metal is caused, and this corrosion is generally pit corrosion, which mainly occurs below the water wall tube with deposits and where the heat load is high. For example, near the burner, on the fire side of the furnace tube, etc., it is very easy to cause metal perforation or over-temperature explosion. Although the high oxides of copper and iron can corrode steel, the corrosion effect is limited, but when oxygen is supplemented, the corrosion will continue and aggravate. The hazard is very large. On the one hand, it will cause a large area of corrosion on the metal surface of the deactivated equipment in a short period of time. On the other hand, the iron content of the feed water increases when the unit starts and runs due to the formation of deposits on the metal surface and the rough state of the metal surface due to the deactivation of corrosion. It not only aggravates the formation of iron scale in the furnace tube, but also aggravates the corrosion of thermal equipment during operation.
Common Anticorrosion Methods
There are many reasons for the formation of electrochemical corrosion of metals, many influencing factors, and different environmental factors, so that one anti-corrosion measure cannot be used to solve all corrosion problems. Commonly used methods in metal anticorrosion are: cover layer protection, electrochemical protection, corrosion inhibitor protection.
Overlay Protection
Cover layer protection is to cover the surface of the material with poor corrosion resistance with metal or non-metallic materials with good corrosion resistance, and isolate the base material from the corrosive medium to achieve the purpose of controlling corrosion. The surface coating protection method can not only improve the corrosion resistance of the base metal, but also save a lot of precious metals and alloys.
Regardless of whether a metal cover layer or a non-metal cover layer is used, it must have a good bonding force with the base metal, and the surface of the equipment must be cleaned before construction, and then the construction of the cover layer is carried out. The main aspects of surface cleaning are degreasing and rust removal. The methods of degreasing include chemical degreasing and electrochemical degreasing. Chemical degreasing is mainly cleaning with organic solvents and lye. Now there are some new types of synthetic detergents. A small amount of synthetic detergent is added to the high-temperature and high-pressure water flow to clean the oil stains on the metal surface, which has the advantages of fast speed and cleanliness, but requires special cleaning equipment. The methods of removing rust on the metal surface include mechanical descaling method and pickling descaling method. With the advancement of science and technology, a new type of rust removal method has emerged, which is to use pickled acid plus corrosion inhibitors and fillers to make pickling paste, smear it on the metal surface, and rinse it off with water after rust removal. , and then apply passivation paste to passivate the metal without rusting.
There are two types of surface covering layers: metallic covering layers and non-metallic covering layers. Metal coating generally includes bimetal, metal lining, electroplating, chemical plating, thermal spraying (spraying), hot dip plating, etc. Non-metallic covering layers include paint covering layers, glass fiber reinforced plastic linings, rubber linings, brick linings, etc. Both bimetal and metal lining have a certain thickness. According to the operating rules, the corrosion resistance of the material can be achieved. The metal coating generally referred to refers to electroplating, electroless plating, thermal spraying (spraying), hot dip plating, etc. Most of these coatings are porous and thin. Metal overlays can be divided into anode overlays and cathode overlays according to their electrochemical behavior in the medium. The electrode potential of the anode cover layer is negative than that of the base metal. In use, the complete covering layer has a good protective effect on the base metal; even if the integrity is destroyed, it can continue to protect the base metal from corrosion as a sacrificial anode. In general, zinc, cadmium, and aluminum are the anodic coatings for carbon steel. The electrode potential of the cathode coating is more positive than that of the base metal. Only mechanically protect the base metal from corrosion during application. Once the integrity of the cover layer is destroyed, it will form a corrosion cell with the base metal, which will accelerate the corrosion rate of the base metal. Nickel, copper, lead, tin, stainless steel, etc. generally used are anodic coatings for carbon steel. The non-metallic coating mainly protects the base metal from the following three aspects: isolation, corrosion inhibition, and electrochemical effects. The selection of non-metallic coating should be based on five aspects: the adaptability of the coating to the environment, the adaptability of the protected base material and the coating, the possibility of construction conditions, the compatibility of the coating, and the economic rationality. Considering. Phosphate coating is a new anti-corrosion method. It is formed by brushing, spraying or immersing in an acidic orthophosphate solution containing iron, zinc and magnesium for a long time to form an oil film, which is composed of thick and porous phosphate fine grains, which are tightly bonded to the steel. superior. These films do not provide good corrosion resistance and are therefore not used alone, but they provide a good base for oils, waxes, and lacquers to help prevent the spread of rust under the paint film. When the coating protection is generally combined with electrochemical protection and corrosion inhibitor protection, the protection effect of the coating directly affects the combined protection effect.
Electrochemical Protection
Electrochemical protection is divided into cathodic protection and anodic protection.
Cathodic protection is a protection method that connects the protected metal to the negative electrode of the applied current power supply, and passes enough cathodic current to the metal surface to make the potential of the metal negative, thereby reducing the rate of metal dissolution. The application of cathodic protection technology has been relatively mature. The cathodic protection devices that have been used in my country include post and telecommunications system cable devices, buried and underground pipelines, buried and underground storage tanks, oil pipelines, natural gas pipelines, bridge piles, gates, platforms, etc., all use cathodic protection. . Cathodic protection is used in conjunction with other anti-corrosion methods when applied. The cathodic protection is combined with the covering layer, so that most of the area is covered by the covering layer, the current consumption is greatly reduced, and at the same time, it overcomes the shortcomings such as pinholes and local damage that are prone to occur when the covering layer is used alone; cathodic protection and corrosion inhibition The combined use of anti-corrosion inhibitor can solve the shortcomings of ineffective corrosion inhibitor alone or large drug consumption, and can also solve the disadvantage of poor effect of cathodic protection alone due to complex structure.
Anodic protection is to connect the protected metal component to the positive pole of the external DC power supply. In the electrolyte solution, the metal component is anodic polarized to a certain potential to establish and maintain a stable passive state, so that the anodic dissolution is inhibited and the corrosion rate is reduced. lower so that the device is protected. Active-passive metals such as titanium, stainless steel, carbon steel, nickel-based alloys and other metals can be protected by anodic protection, which can not only control the overall corrosion of these metals, but also prevent localized pitting corrosion, stress corrosion cracking, and intergranular corrosion. corrosion. However, anodic protection can only be applied to metals with specific electrolyte compositions and in the liquid phase. The concentration of halogen ions in the medium cannot exceed a certain critical value, otherwise these active ions will destroy the passive state of the metal, thereby destroying the anode protection function. The equipment with remarkable application effect of anode protection in my country are: carbon steel storage tanks, various heat exchangers, sulfur trioxide generators, etc. in the production of sulfuric acid; carbonization towers, ammonia storage tanks, etc. in the production of ammonia water and ammonium salts. The combination of anode protection and cover layer protection only needs to passivate the places where the coverage is not strict, the critical passivation current is greatly reduced, and the investment cost is greatly reduced; because the anode area is greatly reduced, re-passivation after activation is also easy. Anodic protection and corrosion inhibitor combined protection can reduce the critical current density and reduce investment costs. For example, sodium chromate is added to the mixed solution of ammonium nitrate and urea, inorganic corrosion inhibitors such as sodium thiocyanate are added to urea and ammonia water, and they are used in combination with anode protection, and the effect is very good.
Corrosion Inhibitor Protection
Corrosion inhibitor protection is a method of protecting metals by adding small amounts of substances that prevent or slow metal corrosion. Corrosion inhibitor protection is characterized by low investment, quick return and easy use. However, the application of corrosion inhibitors also has certain limitations: corrosion inhibitors should not be used at high temperatures, can only be used in closed and circulating systems, have strong pertinence, pollution and waste liquid recycling and treatment issues should also be carefully considered . Therefore, the corrosion inhibitor should be strictly selected according to the specific situation when it is used. Corrosion inhibitor is one of the most important anti-corrosion methods in my country and is widely used in petroleum, chemical, steel, machinery, power, transportation and other sectors. Corrosion inhibitors are used in combination with other anti-corrosion methods to achieve better results.