Introduction: Copper and its alloys have excellent conductivity, heat conductivity and corrosion resistance, as well as good mechanical properties and processing formability. They can also be recycled, so they are widely used in various fields of human production and life. They are one of the earliest metal materials used by mankind, and have a history of nearly 5,000 years. With the progress of the times and the development of science and technology, copper and its alloys are widely used in electronics, electric power, automobiles, ships, transportation, communications, home appliances, architecture, metallurgy, human life and so on.

1.Applications in naval vessels and ocean engineering

The key components used in naval ships and marine engineering need to have excellent corrosion resistance to seawater, while copper and copper alloys have excellent corrosion resistance to seawater and prevent the growth and adhesion of marine organisms, in addition, copper alloys also have other excellent comprehensive properties, which make them become indispensable, even irreplaceable materials in such projects. The main components of copper and copper alloys used in ships and ocean carriers are various wires, sea water pipes and valves, heat exchangers, condensers, heaters, propellers, etc. , the amount of copper is equivalent to 3% ~ 5% of the amount of steel.

Table 1 examples of important copper parts for naval vessels

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. Once they are corroded and destroyed, the normal operation of the equipment will be affected, which will lead to the reduction of navigation rate and the increase of operating cost, even threaten the safety of the whole ship. In recent years, the corrosion leakage accident of copper-nickel alloy pipe occurred in some power plants, which caused great economic losses.

2.Classification, designation and standardization of copper and its alloys

China's copper and copper alloys by color classification, generally divided into four categories. Brass is an alloy based on copper and zinc, which can be subdivided into simple brass and complex brass, of which the third component is also named, such as nickel brass, silicon brass, lead brass, iron brass; bronze, refers to in addition to copper-nickel, copper-zinc alloy copper base alloy, the main varieties of tin bronze, aluminum bronze, silicon bronze, special bronze (also known as High Copper Alloy) ; Cupronickel, refers to the copper-nickel alloy. The state has formulated four standards for these four categories of copper and copper alloys. One is the basic standard, Gb/t 5231-2012 specifies the chemical composition and shape of processed copper and copper alloy, the other is the standard of chemical analysis method, and the third is the test method of physical and chemical properties, including Resistance Coefficient, ultrasonic testing, Eddy current testing, residual stress, dezincification corrosion, oxygen content of oxygen-free copper, fracture, grain size, etc. , these include cathode copper, copper wire ingots for electrical purposes, cast brass ingots, cast bronze ingots, coarse copper, copper-beryllium master alloys, copper concentrates and copper alloy processing material standards.

3.Corrosion of copper alloys for naval vessels and marine engineering

(1) Corrosion characteristics of Marine Environment Sea water is a kind of electrolyte solution, which has certain oxygen, salt content, sea water conductivity, dissolved substance, Ph value, temperature, sea water current speed and sea creature, etc. , the electrochemical characteristics of seawater corrosion are determined as follows: 

01.Halogen Ions such as chloride ions 

02.in seawater can hinder and destroy metal passivation; 

03.The resistance retardation of seawater corrosion is small, and the contact of dissimilar metals can cause significant galvanic corrosion.

04.The pitting corrosion and crevice corrosion are easy to occur in seawater due to the local damage caused by passivation.

 The Marine Environment refers to any physical state from the marine atmosphere to the seabed mud, including the sea, the marine atmosphere area, the splash area, the tidal area, the total immersion area, the sea mud area, each zone has its own unique corrosion environment, the corrosivity of copper and copper alloys in different areas of the same sea area is different, as shown in Fig. 1. There are also variations in the corrosivity of the oceans in different parts of the world, due to the following corrosion factors: temperature, wind speed, current speed, sunshine, salinity, Ph, species of marine life, etc. , can Be summarized as chemical factors, physical factors, biological factors, as shown in table 4.

(2)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. 

01.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. 

02.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. 

03.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.

(3)Local corrosion types of copper alloys 

01.galvanic corrosion serious galvanic corrosion usually occurs when two metals with widely different galvanic orders are connected and exposed to the marine environment. In the connected galvanic couple, one metal is the anode, the other is the cathode, 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, consideration should be given to placing an insulating layer between the two metals or, if this is not possible, over the cathode of the couple.

02. 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.

03.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.

04.de-composition Corrosion de-composition corrosion is a special corrosion form of some copper alloys, such as brass with more than 15% zinc, especially + duplex brass, al-bronze with-2 phase and ni-cu 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.

05.Stress Corrosion cracking (SCC) is the phenomenon that a metal material subjected to tensile stress is fractured (or cracked) due to the synergistic effect of corrosion and stress in certain media. 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 (NH4 + containing solution or vapor or mercury salt solution) . 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, the copper surface produces protective film, then the protective film cracks under stress, which promotes the anodic dissolution along the crystal, then forms the protective film at the dissolving place, then cracks, then dissolves, and the anodic dissolution along the crystal 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.

06. 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. To avoid corrosion fatigue, the following points should be noted:

001.reasonable material selection, in general, the material with high pitting corrosion resistance, corrosion fatigue resistance is also higher, stress, corrosion sensitive material, corrosion fatigue performance is also poor. 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.

07.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. Cavitation corrosion is caused by the synergistic effect of mechanical factors of shock wave or micro-jet and corrosion medium. The mechanical action can destroy the protective film of Copper Alloy and promote the corrosion, and the further corrosion produces the pit or makes the pit deeper and Coarser, which in turn promotes the nucleation of cavitation bubbles. The relative cavitation erosion rates of some common copper alloys are given in Table 5, and the cavitation erosion test results of some propeller copper are given in Table 6.

The main measures to restrain cavitation corrosion are: 1 improving the design, reducing the pressure difference of fluid and reducing the formation of cavitation from the hydraulic angle. 2. Increase the Surface finish of the component and reduce the probability of cavitation. The shock wave is absorbed by a rubber or plastic coating with high elasticity. Using CATHODIC protection, a hydrogen bubble is generated in the workpiece to buffer the shock wave of the cavitation bubble. 5. Choose the right materials. (8) the erosion corrosion of metal surface and corrosive fluid due to high-speed relative motion between the metal damage phenomenon, known as erosion corrosion. The corrosive fluid may be a single-phase flow or a multiphase flow with both gas and solid phases. Generally speaking, the higher the relative velocity is, the more solid particles are suspended in the fluid, and the harder and heavier the mass is, the more serious the erosion corrosion is. Table 7 shows the corrosion rates of several copper alloys in single-phase seawater at several flow rates.

The data in the table show that the corrosion rate is high when the flow rate is high, and the erosion corrosion resistance of different copper alloys is very different. In general, every copper alloy has a limit flow rate of erosion corrosion resistance, called critical flow rate, over the critical flow rate, the failure rate of materials will suddenly increase significantly. Of course, the value of the critical velocity is related to the material and the parameters of the corrosive fluid. The literature shows the critical flow rates of several copper alloys in a simple fluid, I. E. IN CLEAN SEAWATER. The critical flow velocity of red copper is 0.9 m/s, that of arsenic-bearing Navy brass is 1.8 m/s, that of arsenic-bearing aluminum brass is 3 m/s, that of 90 Cu-10ni (1.5 fe) is 3.6 m/s, that of 70 Cu-30ni (0.7 fe) is 4.5 m/s, and that of typical seawater in condenser is 2.4 m/s, then only the last three copper alloys can be used to make seawater condensers. If the above-mentioned clean seawater fluid temperature increases, Ph changes, seawater is polluted, contains sediment, and so on, then the critical flow rate will also reduce. In the final analysis, the mechanism of erosion corrosion is that the metal surface has a protective film in the corrosive medium, and the erosion makes the film thin or destroyed, thus making the bare metal further corrode, when the flow rate is much higher than the critical flow rate, the metal will be mechanically stripped away. In view of this mechanism, the measures to reduce erosion corrosion are as follows: 1 selection of erosion-resistant materials, as shown in Table 7, in the copper alloy 8m/s high-speed erosion, erosion-resistant, corrosion performance ranking for 70Cu-30Ni (0.5Fe) , 90Cu-10Ni (0.8Fe) , aluminum brass, navy brass, 70Cu-30Ni (0.05 fe) , aluminum bronze, silicon bronze. The erosion corrosion can be reduced by changing the corrosion medium, adding corrosion inhibitor, filtering the suspended solid particles and lowering the operating temperature. Improve design, reduce flow rate, reduce turbulence, thicken vulnerable parts or make these parts easy to replace and repair. Appropriate sacrificial anode or electrochemical Cathodic protection is also an effective measure. 

Source: Caiyitong

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