In offshore engineering, copper alloys are mainly used in desalination, salt production, offshore oil extraction, and power stations in Binhai. The copper alloy condenser tube is the key material in the multi-stage flash desalination unit. Demand for copper pipes has increased dramatically in the past decade as desalination plants have proliferated, with an estimated total demand of about 80,000 tons in the decade from 2010 to 2020. However, the manufacture of copper alloy parts such as heat exchangers, condensers and propellers for ships in our country still needs to be improved at present. The quality level of domestic copper alloy parts for ships is relatively low compared with the products of well-known foreign manufacturers, and their corrosion resistance is slightly poor, short service life.

1.Corrosion resistance of copper and copper alloys in marine environment

There are many reasons for the excellent corrosion resistance of copper and copper alloys in seawater. 1 The thermodynamic stability of copper, that is, the difficulty of copper ionization. The Standard Potentials for CU2 + and CU + formation are 0.337V (She) and 0.521V (SHE) , respectively. When copper corrodes in seawater, the CATHODIC reaction is not the CATHODIC depolarization of hydrogen evolution, but the CATHODIC depolarization of oxygen. The corrosion of copper is influenced by the ionization process of oxygen. 2 The copper alloy is deposited in seawater directly on the surface of the Cuprous oxide, with other corrosion products such as copper chloride, Copper hydroxide, copper carbonate or malachite, and calcium containing substances. Copper Ions are toxic and can prevent the aggregation of marine animals and plants and the corrosion of marine organisms. The corrosion of copper in seawater mostly belongs to uniform corrosion. However, due to the complexity of the marine environment and the great change of the working conditions of the parts used in ships and marine engineering, all kinds of local corrosion of copper alloys are likely to occur, and their harm is far greater than that of uniform corrosion.

2.Local corrosion types of copper alloys

(1) galvanic corrosion serious galvanic corrosion usually occurs when two metals with large differences in galvanic order are connected and exposed to the marine environment. In the connected galvanic couple, one metal is the anode and the other is the cathode, and the anode will be corroded. The corrosion degree depends on the potential difference between the two metals. The larger the potential difference is, the more serious the corrosion is. A typical example of galvanic corrosion is corrosion between the bronze propeller of an ocean-going vessel and the exposed surface of a steel hull. The potential of bronze is about-0.31 V (SCE) and the potential of steel plate is about-0.61 V. if there is an exposed piece of steel plate, it is the anode, and the propeller is the cathode. To control galvanic corrosion, several principles must be observed. First, an insulating layer should be considered between the two metals. If it is not possible, the cathode of the couple should be covered with a non-conducting protective layer. Furthermore, the area of the cathode can be reduced.

(2) crevice corrosion of metal parts is inevitable when they are installed together. In seawater, such crevice corrosion is likely to occur for the metal which can produce oxide film. In crevice corrosion, oxygen is insufficient and passivation film gradually degenerates, crevice oxygen is sufficient, passivation film integrity, so the crevice outside is the corrosion of the Cathode, crevice is the corrosion of the anode. As a result of the design and installation features (such as gasket, gasket, rivets, etc.) , metal parts of the gap will be inevitable, when the marine organisms attached or coating local fall off the gap will be generated. The crevice corrosion of some copper alloys has different characteristics, that is, the copper ion outside the crevice is removed by flowing seawater, and the concentration of copper ion in the crevice is higher, forming a copper concentration difference battery, the crevice is a cathode, the crevice is an anode. The measures to control crevice corrosion are improving design, minimizing crevice, implementing Cathodic protection to reduce corrosion, reducing the area of metal outside crevice and reducing the area of cathode to control the corrosion inside crevice.

(3) there are many kinds of defects on the surface of pitting copper alloy products, such as non-uniformity of chemical composition, non-uniformity of metallographic structure, inclusion, surface attachment or deposit, which will destroy the oxide film on the surface of copper alloy and form the pitting source, these pitting sources and surface film integrity of the formation of a galvanic corrosion, pitting source is the anode, continuous corrosion, the final parts can be perforated leakage. The way to prevent pitting is to reduce the surface defects and clean the components frequently. For some pipe fittings, the method of early pre-filming is very effective.

(4) decanting corrosion decanting corrosion is a special corrosion form of some copper alloys, such as brass with more than 15% zinc, especially + duplex brass, there is also dealumination corrosion of aluminum bronze with-2 phase and de-nickel corrosion of copper-nickel Alloy. Brass is the solid solution of copper and zinc, and zinc is the anode component of the solid solution. Zinc is selectively dissolved, and copper alloy becomes zinc-free sponge copper, which causes material damage. When-2 phase is contained in aluminum bronze, the most serious dealuminizing corrosion occurs when-2 phase forms a network along the grain boundary. The method of inhibiting dezincification corrosion of brass is to select brass with lower Zn content and to add alloy elements such as as as, b, sn, P or SB which can inhibit dezincification. The method to restrain the dealuminization of aluminum bronze is to eliminate the precipitation of-2 phase along grain boundary by heat treatment, or to add l% ~ 2% iron or more than 4.5% nickel. Friction stir treatment can also greatly improve the structure of aluminum bronze and inhibit the dealumination corrosion.

(5)Stress Corrosion cracking (SCC) is the phenomenon of fracture (or cracking) of a metal material subjected to tensile stress in certain media due to the synergistic effect of corrosion and stress. Cracking and fracture correspond, respectively, to cracking, where the former highlights the beginning of cracking, and cracking, where the latter includes fracture to fracture. It is generally considered that there are three basic conditions for the occurrence of stress corrosion fracture, namely, sensitive material, specific medium and tensile stress. This indicates that stress corrosion is a complex phenomenon: when the stress does not exist, the sensitive material corrodes very little in this particular medium environment; after applying the stress, after a period of time, the sensitive material will fracture under the condition of not serious corrosion and not enough stress. It is generally believed that stress corrosion cracking does not occur in pure metals, while stress corrosion cracking in each alloy is only sensitive to some specific media. With the development of the application environment of alloys, it has been found that there are a wide range of environments that can cause stress corrosion of various alloys.

Under the action of external tensile stress or self-residual stress, copper alloys may be subjected to stress corrosion when exposed to the matching corrosion medium (solution containing NH4 + or vapor or mercury salt solution) , it's extremely dangerous. Among the four kinds of copper alloys, red copper and white copper have the best resistance to stress corrosion in marine environment, brass is the most sensitive to stress corrosion, and bronze is the second. The "quarter crack" of brass is a typical stress corrosion phenomenon. Moisture, oxygen-containing ammonia, ammonium salt and mercury salt can cause stress corrosion of brass, and SO2 can accelerate it. The mechanism of stress corrosion of brass is that firstly, a protective film is formed on the surface of copper, then the protective film cracks under stress, which promotes the anodic dissolution along the crystal, and then forms a protective film at the dissolving place, then cracks, then dissolves, the anodic dissolution along the grain boundary is caused by the selective dissolution of zinc in brass. When brass does not form film in the medium, stress corrosion cracking (SCC) may occur. In general, the SCC of brass is intergranular, brass is transgranular, and + brass can be intergranular and intergranular. Phase boundary extended. In addition to reducing residual stress and improving the environment medium, measures to reduce the stress corrosion sensitivity of brass, such as reducing the content of zinc, or adding an appropriate amount of stress corrosion resistant micronutrient, are also effective. Some bronze, such as manganese bronze, aluminum bronze, beryllium bronze and composite bronze for propeller, also have stress corrosion in polluted seawater. But the stress corrosion resistance of bronze is higher than that of brass.

(6) corrosion fatigue corrosion fatigue is the brittle fracture of materials or components under the action of alternating stress and corrosion environment. The damage caused by alternating stress combined with corrosive environment is much more serious than that caused by alternating stress alone (i. e. fatigue) or corrosion alone. This kind of damage often occurs in marine propeller, turbine and turbine blade, pump shaft and pump Rod, offshore platform and so on. Corrosion fatigue and stress corrosion both involve the interaction of stress and corrosion media, but they are quite different: corrosion fatigue occurs under alternating stress, while stress corrosion usually occurs under tensile stress; Corrosion fatigue can also occur in pure metals, and it does not need a special combination of material and medium environment for corrosion fatigue to occur in metal components. Under normal circumstances, the designer will always give the dynamic copper alloy key components high Factor of safety, corrosion fatigue fracture is less likely, but once occurred, the consequences are very serious. In order to avoid corrosion fatigue, attention should be paid to the following aspects: 1 reasonable selection of materials, in general, materials with high resistance to pitting corrosion, materials with high resistance to corrosion fatigue, materials sensitive to stress corrosion, and materials with poor resistance to corrosion fatigue. Also note that the material strength, corrosion fatigue strength is not necessarily high. Carefully designed to minimize the stress level of the components, avoid sharp gaps in the components, reduce stress concentration. If possible, heat treatment can be used to eliminate the internal stress, while shot peening can be used to make the surface of the workpiece have residual compressive stress is mostly beneficial. The application of coating, corrosion inhibitor or electrochemical protection can also produce good results. (7) cavitation erosion and the uneven distribution of fluid pressure around copper alloy parts with relatively high velocity of fluid movement, such as the inlet and outlet of marine propellers, pump valves or heat exchanger tubes, in the local area of the metal surface in the low pressure zone, bubbles of fluid are formed, which then collapse downstream, producing a high-pressure shock wave or microjet, pressure up to 400 ATM (LATM = 101325PA) , or even higher, damages the protective film on the metal surface, speeding up the process of corrosion. The cumulative damage caused by repeated cycles of cavitation formation and collapse is called cavitation corrosion.

3.Experiment

The corrosion behavior of copper alloy in marine atmosphere was studied by a series of experiments. Copper and H62 brass were selected as test objects. The corrosion behavior of copper and brass H62 exposed to marine atmosphere for 56 days was studied by exposure experiments on the deck of a sailing ship. The corrosion rate of copper and brass H62 was studied by weight loss method, and the surface morphology of the corrosion product was observed by Scanning electron microscope, x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were used to analyze the phase composition and element valence of the corrosion products. The results show that the average corrosion rates of red copper and brass H62 are 0.0090 MM/A and 0.0024 MM/A respectively. The surface corrosion products of red copper are irregular lamellar and round granular in shape, and the H62 brass surface corrosion products form a dense network corrosion product layer. The main corrosion products of red copper are Cu2O and brass H62 are Zno, Cu2O, ZN5(OH)8cl2h2o and Cu (OH)2h2o. 

Source: China Hownet, the network collates

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