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Discussion on Anti-corrosion Technology of Seawater Heat Exchanger

2014-09-24 10:25

Corrosion problems throughout various sectors and industries, the development of the national economy, human life and social environment have great harm. Due to corrosion, it often causes the shortening of the start-up cycle of the device, the premature scrapping of the device and equipment, the short service life, the pollution of the environment, etc.; at the same time, it will also affect the normal development of new technologies. According to statistics, the annual economic loss caused by corrosion damage in various countries accounts for about 1% to 5% of the GDP of the national economy in that year, which varies with the degree of economic development and the level of corrosion control in different countries. In our country, the loss caused by corrosion is particularly serious. According to the "China Corrosion Investigation Report", the annual corrosion loss in our country in recent years is about 500 billion yuan.

Seawater heat exchanger is one of the most widely used equipment in the operation of ships and offshore platforms. The heat transfer condition of the heat exchanger directly affects the overall smooth operation and comprehensive economic indicators of ships and operating platforms, and plays an important role in the safety, stability and long-term operation of production. Seawater has a high content of dissolved oxygen, chloride ions and microorganisms, which is very corrosive to metal heat exchangers. As we all know, the marine environment is a specific and extremely complex corrosive environment, which is very harsh. In this area, the atmosphere mainly contains water vapor, oxygen, nitrogen, carbon dioxide, sulfur dioxide, chloride salt and sulfate suspended in it. It has the characteristics of high humidity, high salt, high temperature and obvious dry and wet cycle effect, and is also relatively corrosive to metals.

1. Common corrosion mechanism of marine environment

1.1 electrochemical corrosion

Electrochemical corrosion is the most common, the most common corrosion, sometimes caused by corrosion alone, sometimes and mechanical action and other common corrosion. Carbon steel in the electrolyte solution (such as water) will form a micro battery, carbon steel basic metallographic organization for ferrite (Fe) and cementite (Fe3C), in the electrolyte solution will form a low potential of the ferrite as the anode, high potential cementite for the cathode corrosion battery, so that the steel is corroded.

Anode: 2Fe → 2Fe2 4e-

At the same time, the electrons are transferred from the anode to the cathode, where the oxygen in the water combines with the electrons to form hydroxide ions.

Cathode: 4e-2H2O O2 → 4 (OH)-

In solution, Fe2 + and (OH) -meet to generate Fe (OH) 2, which is deposited near the cathode-anode junction.

Total reaction equation: 2Fe O2 2H2O → 2Fe (OH) 2

Oxidation by oxygen in the air produces ferric hydroxide: 4Fe (OH) 2 O2 2H2O = 4Fe (OH) 3 ↓

Thus the steel produced rust.

1.2 chloride ion corrosion

Seawater, one of the medium of offshore metal heat exchanger, contains a large amount of chloride ions, which is very corrosive. There are two main types of corrosion caused by stainless steel heat exchanger: stress corrosion and pitting corrosion.

(1) Due to the chloride ions in seawater, the passivation film on the surface of the metal heat exchanger (stainless steel) for marine and offshore platforms is destroyed. Under the action of tensile stress, the area where the passivation film is destroyed will produce cracks and become the anode area of the corrosion cell. Continuous electrochemical corrosion may eventually lead to metal fracture. This corrosion has little to do with the concentration of chloride ions, and even a trace amount of chloride ions may cause stress corrosion.

(2) Chloride ions in seawater are easily adsorbed on the passivation film of the heat exchanger, squeezing out the oxygen atoms, and then combining with cations in the passivation film to form soluble chlorides. As a result, a small pit is corroded on the exposed metal surface of the heat exchanger. These small pits are called pitting nuclei. These chlorides are easy to hydrolyze, so that the pH value of the solution in the small pit decreases, the solution becomes acidic, and a part of the oxide film is dissolved, resulting in excess metal ions. In order to balance the neutrality in the corrosion pit, the external chloride ions continue to migrate into the hole, so that the metal in the hole is further hydrolyzed. This cycle, austenitic stainless steel corrosion, more and more fast, and to the depth of the hole, until the formation of perforation, resulting in pitting (pitting).

1.3 atmospheric corrosion

The marine atmosphere refers to the atmospheric area above the sea surface splash zone and the coastal atmospheric area. In this area, it mainly contains water vapor, oxygen, nitrogen, carbon dioxide, sulfur dioxide, chloride salt and sulfate suspended in it. It has the characteristics of higher humidity, high salt, high temperature and obvious dry and wet circulation effect than ordinary atmosphere. Due to the high humidity of the marine atmosphere, water vapor is attached to the steel surface under the influence of capillary action, adsorption, and chemical condensation to form a layer of water film that is invisible to the naked eye. CO2, SO2 and some salts are dissolved in the water film. Make it a highly conductive electrolyte solution. Due to the different standard electrode potentials of the main element iron and trace element carbon of steel, when they are in the electrolyte solution at the same time, many primary batteries are formed. Iron is oxidized as an anode in the electrolyte solution (water film) and loses electrons. Become rust.

As the relative humidity of the marine atmospheric environment is relatively large, the water film is thicker, the salt content is higher, and the water film electrolysis ability is stronger. At the same time, the steel structure in the marine atmospheric environment is exposed to sunlight during the day, and water evaporation increases the surface salinity, and forms a wet surface at night. This dry-wet cycle greatly accelerates the corrosion rate. In addition, other substances dissolved in the water film, such as oxygen, carbon dioxide, sulfur dioxide and other chlorides and sulfates, are also deposited on the surface of the steel. On the one hand, salt is dissolved in the water film, and carbon dioxide and sulfur dioxide make the water film acidic, which improves the conductivity of the water film. On the other hand, chloride ions have a penetrating effect, which can accelerate local corrosion such as pitting, stress corrosion, intergranular corrosion and crevice corrosion. It can be seen that the marine atmospheric corrosion environment is far worse than the inland atmospheric environment.

2. Anti-corrosion measures

Due to the different heat exchanger working environment (temperature, pressure, medium and medium flow rate, etc.), the design of the material and heat exchanger structure is different, the main form of heat exchange surface corrosion failure is often different. Therefore, the technical methods of preventing corrosion are also different, which leads to the diversity and complexity of the surface treatment methods of heat exchange surfaces. However, most of the corrosion control methods are based on the following three ideas: ① to isolate the metal surface from the environmental medium; ② to improve the environment, so that the anode or cathode reaction in the controllable range; ③ the use of high corrosion resistance metal or non-metallic materials. So far, at home and abroad on the heat exchanger surface anti-corrosion treatment technology mainly includes: ① coating corrosion resistant coating; ② electrochemical protection; ③ adding corrosion inhibitor; ④ plating corrosion resistant layer; ⑤ comprehensive technical methods and other methods.

Selection of 2.1 Corrosion Resistant Materials

Material selection is the primary principle of heat exchanger anticorrosion design. Heat exchanger in order to achieve the purpose of corrosion resistance, in the equipment design and manufacturing process should consider the use of corrosion resistant materials instead of ordinary carbon steel. At present, heat exchangers with high corrosion resistance materials include stainless steel heat exchangers, titanium and titanium alloy heat exchangers, graphite heat exchangers, glass heat exchangers, plastic heat exchangers and ceramic heat exchangers. Marine refrigeration equipment also commonly uses aluminum brass and nickel-cupronickel as heat exchanger tubes. The end cover in contact with seawater is cast from aluminum brass or plastic. The tube plate in contact with seawater is made of stainless steel tube plate, and the tube plate made of aluminum brass or plastic and carbon steel. For the use of heat exchangers on offshore oil and gas drilling platforms, chromium-containing stainless steel has good corrosion resistance in CO2-containing oil and gas, but when the oil and gas also contains hydrogen sulfide and chloride, attention should be paid to sulfide stress cracking and chloride Stress cracking sensitivity is generally not applicable. The duplex stainless steel containing 22% to 25% chromium and the austenitic stainless steel containing high nickel have good corrosion resistance in high temperature and high chloride environment, and can resist hydrogen sulfide stress corrosion.

2.2 application of anticorrosive paint

Corrosion resistant coatings can not only make the heat exchange surface has anti-erosion, anti-penetration, moisture resistance and other properties, but also the isolation of the surface and the medium contact and scale inhibition, to a certain extent, can improve the performance and life of the heat exchanger, with this method there are two problems, one is the coating performance problem, the second is the coating process. Zinc-rich coatings and epoxy resin coatings are commonly used in the field of seawater or brine heat exchangers. Although some progress has been made in the heat exchanger coating corrosion resistant coatings, but still need to continue to improve and improve, low cost, no pollution, high performance is the goal of coating development; In addition, we must pay attention to the reasonable design of the structure and shape of the heat exchanger, because the reasonable structure of the heat exchanger can play a key role in simplifying the coating process and improving the quality of the coating.

2.3 plating corrosion-resistant layer protection

Infiltration plating is a surface treatment method in which gaseous, solid or molten substances (metal or non-metal elements) to be infiltrated at high temperature penetrate into the interior from the surface of the metal to be infiltrated through diffusion to form a surface alloy coating. The formed coating is called infiltration coating, which is used to improve excellent properties such as oxidation resistance (corrosion resistance), heat resistance and wear resistance. Aluminizing is one of the commonly used plating varieties. The element to be infiltrated is aluminum. It can improve the resistance to high temperature oxidation and gas corrosion of steel, non-ferrous metals and alloys. It has good corrosion resistance in the atmosphere, hydrogen sulfide, carbon dioxide, atmosphere and seawater. Aluminizing process has been widely used in oil refining, metallurgy, chemical industry and so on. Compared with aluminizing method, zinc infiltration method has many advantages: the temperature of zinc infiltration method is relatively low, about 400 ℃ ~ 500 ℃, so the heat exchanger is not easy to deform; It can not only improve the corrosion resistance of metal materials in atmosphere, water, hydrogen sulfide and some organic media, but also make the surface of the workpiece obtain higher hardness and wear resistance than electrogalvanizing and hot galvanizing. The seepage layer is relatively uniform, when dealing with parts with complex shapes, the zinc seepage layer has outstanding advantages, regardless of the thread, inner wall or groove, the thickness of the seepage layer is almost the same; the zinc seepage layer is metallurgically combined with the substrate, and it is difficult to peel off. Moreover, the potential difference between the zinc permeation layer and iron is smaller than the potential difference between zinc and iron, and the zinc permeation layer has a better protective effect as an anodic protective layer. It is generally believed that the thicker the zinc seepage layer, the stronger the corrosion resistance. Zinc infiltration products have been gradually extended to petroleum, chemical, electric power, machinery, water conservancy, marine, post and telecommunications, construction, mining, transportation and other industrial fields. Electroless nickel-phosphorus plating technology is a chemical treatment method that uses a reducing agent to selectively reduce and precipitate nickel ions in a solution on a catalytically activated surface to form a metal coating. Electroless nickel-phosphorus plating is an amorphous alloy, I .e. metallic glass, which has high corrosion resistance (H2S and Cl-resistance), high temperature resistance (normal use at 380 ℃), erosion resistance and abrasion resistance (with certain hardness), good heat transfer, scaling resistance and other excellent characteristics. As nickel-phosphorus electroless plating heat exchangers are gradually favored by petrochemical enterprises, they can be used as protection for metal heat exchangers on offshore platforms.

2.4 Enamel Protection

Enamel is a kind of glassy magnetic layer formed by applying non-metallic organic substances on metal substrate materials by enameling method under high temperature conditions, and is closely combined with metal products. It has the strength and rigidity of metal materials, The appearance, smoothness and corrosion resistance of non-metallic inorganic substances. Process type enamel is the use of enamel technology and technology, will have industrial glass properties of acid-resistant glass material composite to the metal substrate material, the glass-metal composite, the coating has the properties of acid-resistant glass, such as strong corrosion resistance, high chemical stability, dense non-porous, but also has the characteristics of no metal ions, for the general other types of enamel. Used in industrial enamel called glass-lined, its products are glass-lined containers and glass-lined heat exchangers, etc., which is made of glass-lined glaze with high silicon content through multiple high-temperature calcination at about 900 ℃, so that the glass-lined glaze is tightly attached to the surface of the metal substrate. The thickness of enamel glass is generally 0.8~1.5mm. Due to the protection of the glass layer to the metal, the glass-lined container and the heat exchanger have excellent corrosion resistance. In the heat exchanger with seawater as the flowing medium, the glass lining on the inner side of the shell, the inner side of the end cover and the tube plate that can contact the seawater can effectively prevent seawater corrosion.

2.5 adding corrosion inhibitor

The corrosion of metals is the result of anodic and cathodic processes in the electrolyte solution. The addition of corrosion inhibitor can block the progress of any process or block the progress of two processes at the same time, thus slowing down the corrosion rate. The selection of corrosion inhibitor should be determined according to the concentration of salt in water, pH value, the concentration of dissolved oxygen and the concentration of interfering substances. Soft water corrodes steel very slightly, and in this case, very small amounts of corrosion inhibitors such as sodium chromate, sodium nitrite, polyphosphate, sodium benzoate or borax are effective. For circulating water with low salt content, the corrosion of steel can usually be controlled by adjusting the pH to the alkaline range. Sodium chromate or sodium nitrite is an effective corrosion inhibitor for steel, but sodium nitrite is not suitable for copper or brass, copper or brass can use mercaptobenzothiazole and other corrosion inhibitors to inhibit corrosion. Water containing a large amount of organic matter, such as sea water, will consume a large amount of corrosion inhibitors through the oxidation of organic matter, so it is not suitable to use oxidizing corrosion inhibitors such as chromate and nitrite. At this time, the protection of organic corrosion inhibitor is better.

2.6 electrochemical protection

There are two electrochemical protection methods: cathodic protection method and anodic protection method. Cathodic protection is to adjust the potential of the metal to the negative, so that the metal enters the non-corrosive area of the E-pH diagram, thereby reducing or inhibiting the corrosion of the anode, which can be achieved by impressed current and sacrificial anode. Anodic protection refers to the use of external power to protect the passive metal potential to move in the positive direction (I. e. anode polarization), so that its potential into the passivation area of the E-pH diagram, thereby inhibiting metal corrosion. The uniform distribution of cathodic protection current is the key to the quality of protection. It must be that the protected parts reach the potential of complete protection everywhere, so the layout of the anode and the magnitude of the applied current are the key to cathodic protection. In addition, the use of cathodic protection method should pay attention to the following two points: ① in the acidic medium in the hydrogen corrosion environment, the use of cathodic protection power consumption, and easy to cause hydrogen embrittlement; ② sacrificial anode cathodic protection is limited to the limited length of the entrance of the heat exchanger tube, the depth of the tube is still difficult to achieve cathodic protection. At present, the general small seawater heat exchanger mostly adopts the cathodic protection of sacrificial anode, and the large heat exchanger mostly adopts the impressed current cathodic protection. When using the sacrificial anode protection method, it should also be noted that: ① in the environment of high chloride ion concentration, because the chloride ion can locally destroy the passivation film and cause pitting corrosion, the anode protection method can not be used in general; ② the equipment is more and the cost is higher. At present, zinc is generally selected as the sacrificial anode, and Mg-based metals and Al-based alloys are also used as sacrificial anodes.

3. Prospect of Anti-corrosion Technology of Seawater Heat Exchanger

There are many forms of corrosion failure of seawater heat exchanger. It is necessary to analyze the specific problems, grasp the main forms of corrosion failure, and take corresponding solutions. The current seawater heat exchanger adopts almost all the above anti-corrosion measures, but none of them fundamentally solves the corrosion problem of the heat exchanger.

From the mechanism analysis of heat exchanger corrosion, it can be seen that electrochemical corrosion and stress corrosion are the main corrosion types of seawater heat exchanger. Therefore, the anti-corrosion of the heat exchanger should be mainly from the electrochemical protection and reduce the stress level of the equipment. With regard to electrochemical corrosion, cathodic protection law has been well applied in offshore platform heat exchangers abroad. Generally speaking, large heat exchangers often use impressed current cathodic protection, while small seawater heat exchangers use sacrificial anode cathodic protection, which can slow down the corrosion of heat exchangers to a certain extent. In the aspect of cathodic protection, the potential distribution of heat exchange equipment should be deeply studied, and the corresponding measures should be studied to make the potential distribution more uniform, meet the needs of passivation, and develop a highly reliable controller. With regard to the stress in the equipment, in addition to the pressure load in the heat exchange equipment, the problem gradient in the equipment, the vibration of the equipment, the impact of the fluid, the local stress concentration on the equipment and the residual stress generated in the manufacturing process of the equipment will produce high stress areas on the equipment, and the existence of these areas will accelerate the corrosion of the equipment. Therefore, it is necessary to optimize the operating environment of the equipment from three aspects: design, manufacture and operation. In terms of design, the maximum stress amplitude is reduced by optimizing the structure and analyzing the thermal-solid coupling stress in detail. The residual stress is strictly controlled in manufacturing, and the vibration of the equipment and the excessive impact of the fluid are reduced by controlling the operating parameters in operation. Through the above work, will effectively extend the operation cycle of equipment, greatly improve economic efficiency.