The law of weight loss, electrochemical process and corrosion-cavitation interaction of manganese brass, high manganese aluminum bronze and nickel aluminum bronze were tested and analyzed by using ultrasonic vibration cavitation equipment and electrochemical workstation, the morphology of cavitation damage was observed by SEM. The results show that the cavitation resistance is from high to low: Nickel aluminum bronze, high manganese aluminum bronze, manganese brass. At the beginning of cavitation erosion, the α phase of ni-al bronze and high mn-al bronze was plastic deformation, and the crack was initiated at the α/κ phase boundary. Cleavage cracking of β matrix phase in high manganese aluminum bronze. The matrix phase of Manganin brass has severe plastic deformation and cavitation damage. The corrosion potential of Manganin and permanganin bronze was shifted positively by cavitation erosion, and that of nickel-aluminum bronze was shifted negatively. The interaction of the three materials is mainly caused by corrosion promoting cavitation damage, and the mechanical damage is the dominant cavitation damage mechanism. Key words: propeller; Copper Alloy; cavitation erosion; interaction the 21st century is the ocean century. All countries in the world have put the development of the ocean field in an increasingly important position, the independence and modernization of marine equipment is the key to achieve this goal, which requires the high quality and long effective life of ships. As an important propulsion equipment of ships, propeller will suffer corrosion and cavitation in seawater. Cavitation damage is the cavitation process of bubble nucleation, growth and collapse due to the fluctuation of internal pressure of liquid. In addition, the interaction between cavitation erosion and corrosion accelerated the failure of materials. Therefore, the marine propeller material is required to have excellent mechanical properties and corrosion resistance. Manganese brass, high manganese aluminum bronze and nickel aluminum bronze are three kinds of copper alloys which are widely used in marine propeller. Manganese brass, which has good resistance to cold and hot working, has good corrosion resistance in seawater, chloride and superheated steam, and its cost is low, but it is prone to dezincification corrosion, and its mechanical properties and cavitation resistance are further reduced, used to make low-speed propellers. High Manganese aluminum bronze and nickel aluminum bronze are two kinds of aluminum bronze added with MN, FE and NI. Nickel-aluminum bronze is widely used in propeller materials because of its high strength, good fracture toughness, seawater cavitation resistance and seawater corrosion resistance. Compared with manganese brass, permanganate aluminum bronze has higher mechanical properties and seawater corrosion resistance, and better welding, hot working and casting properties compared with nickel-aluminum bronze. At present, a lot of researches have been carried out on cavitation and cavitation erosion, but cavitation and cavitation erosion involve many aspects such as fluid mechanics, materials, acoustics, etc. , 7 albums. For the cavitation erosion mechanism of copper alloys, Trethewey et Al. [8] considered that the work hardening ability of copper alloys was an important determinant, and Hucinska et Al. [9] considered that the stacking fault energy was the decisive factor, however, Suh et Al. [10] think that there is no relationship between the cavitation erosion resistance and the stacking fault energy, and Zhang et Al. [11] think that the microstructure of ni-al bronze has a great influence on the cavitation erosion resistance. In order to improve the cavitation resistance of propeller materials, it is of great significance to study the cavitation erosion mechanism of three kinds of copper alloys. In addition, devices in service in corrosive media are subject to corrosion, and there is usually an interaction between corrosion and Cavitation, which together cause material losses much greater than the sum of their individual actions [12-14] . The results of Song et Al. [15] show that the mass loss caused by the interaction between cavitation and corrosion is 31.45% of the cumulative mass loss of cast ni-al bronze in 3.5% (mass fraction) NACL solution. The results show that the mass loss caused by the interaction of mild steel is 66% of its cumulative mass loss in 3.5% NACL solution, while the mass loss caused by the interaction of stainless steel is negligible. The study on the Cavitation erosion-corrosion interaction of Copper Alloy for propeller is helpful to reveal the cavitation erosion damage mechanism. In this paper, the cavitation erosion behavior and electrochemical behavior of three typical copper alloys for marine propellers in 3.5% NACL solution were studied, and the interaction between corrosion and cavitation erosion was analyzed and discussed The mechanism of cavitation damage was revealed by observing the surface morphology of three copper alloys before and after cavitation erosion. The research results can provide a theoretical basis for the propeller manufacturing industry to select the material and further improve the performance and life of the propeller material.

1. Methods the experimental materials were three typical copper alloys for marine propeller materials, namely ZHMn55-3-1 manganese brass (Mn-brass) , ZQMn12-8-3-2 high manganese aluminum bronze (MAB) and ZQAl9-4-4-2 nickel aluminum bronze (NAB) .

Three kinds of copper alloy samples were etched with 5gFeCl3 + 2mLHCl + 95mLC2H5OH solution, and the microstructure was observed by optical microscope. Cavitation experiments were carried out with Qsonica700 ultrasonic vibration cavitation equipment according to ASTMG32-10 standard. The vibration frequency is 20kHz and the amplitude is 60μm. The ultrasonic vibration probe is placed 0.5 mm above the sample, the sample is immersed in the test medium, and the distance between the sample surface and the liquid surface is 15 mm. In order to analyze the interaction between corrosion and Cavitation, cavitation experiments were carried out in distilled water. Three parallel specimens were selected for each material. The surface morphology of the sample after cavitation erosion was observed by scanning electron microscope (SEM) . Electrochemical test using Gamry1000E electrochemical workstation. The corrosion medium is 3.5% NACL solution prepared by analyzing pure NaCl reagent and distilled water, the platinum plate is used as auxiliary electrode, the Saturated Calomel electrode (SCE) is used as reference electrode, and the sample is used as working electrode. In order to study the effect of cavitation erosion on the corrosion potential, the corrosion potential was monitored under the condition of static-cavitation alternation, which lasted for 30 min. First, the samples were kept in static state and cavitation state for 30 minutes to obtain stable potential, then the polarization curve was measured. The potential scanning rate was set to 0.5 MV s-1, and the scanning potential range was set to-0.25 ~ 0 v relative to the open-circuit potential. Three parallel specimens were selected in each group to reduce the experimental error. 2. Results and analysis of 2.13 kinds of copper alloy microstructure manganese brass, high manganese aluminum bronze and nickel aluminum bronze optical microstructure as shown in figure 1. Fig. 1A shows the optical microstructure of manganin brass. The bright white irregular block or strip is α phase, the solid solution β phase is based on CuZn except α phase, and the Black Small particle phase is κ phase with high hardness, mainly in the Beta phase, but also in the Alpha phase. The high manganese aluminum bronze is composed of α rich Cu matrix phase, β phase with irregular shape and κ phase distributed in α phase. The α phase is a face-centered cubic cu-based solid solution, the β phase is a body-centered cubic structure based on Cu3Al or Cu2MnAl, and the κ phase is a Fe-and Mn-rich phase, as shown in Fig. 1B. Figures 1C and D show the optical microstructure of the nickel-aluminum bronze. The Bright White Strip in figure 1C is the Alpha phase, β ‘is a high-density NiAl-based precipitated phase, which is divided into four different types of Fe-rich intermetallic compounds. Kappa I is a rose-shaped precipitate with a diameter of 5 ~ 10 μm. κ II is a rose-like phase with Dendrites, mainly distributed on the α/β phase boundary, and its diameter ranges from 1 μm to 2 μm. κ III is a fine lamellar eutectoid structure. κ IV phase is a fine precipitation phase, with the highest iron content in the intermetallic compound and different sizes (diameter < 0.5 μm) dispersed in α phase [18] , as shown in Fig. 1D.

2.2 cavitation erosion result and analysis 2.2.1 cavitation weight loss figure 2A is the time-dependent curve of cavitation weight loss of three materials in distilled water and 3.5% NACL solution. It can be seen that ni-al bronze has the best cavitation resistance, followed by high mn-al bronze and low mn-brass. The weight loss rate of manganese brass is 3.4417 MG cm-2 h-1, which is 2.82 and 3.82 times of that of high manganese aluminum bronze and nickel aluminum bronze respectively. Fig. 2B shows the time-dependent curves of cavitation erosion weight loss rates for the three materials in distilled water and 3.5% NACL solution. The incubation periods of manganese brass, high manganese aluminum bronze and nickel aluminum bronze are about 0.5,1 and 2 H, respectively. The weightlessness in distilled water is caused by the mechanical shock caused by the collapse of the bubble. The weight loss of the three copper alloys in 3.5% NACL solution was higher than that in distilled water, which was due to the interaction of corrosion and cavitation erosion. The weight loss rate of manganese brass is 4.4167 MG cm-2 h-1 after cavitation in 3.5% NACL solution for 5 H, which is 3.07 and 4.06 times of that of high manganese aluminum bronze and nickel aluminum bronze, respectively. The incubation period of cavitation erosion of the three copper alloys was less than 0.5 h.

2.2.2 the electrochemical test chart 3 shows the potential variation of three copper alloys under static and alternate cavitation erosion conditions. It can be seen that cavitation erosion has different effects on the corrosion potential of the three copper alloys. The corrosion potential of the manganese brass and the high manganese aluminum bronze has a positive shift of 118.3 MV and 103.7 MV, respectively, and the corrosion potential of the nickel aluminum bronze has a negative shift of 25.7 MV. Corrosion potential and corrosion current density are determined by both anodic and CATHODIC processes. On the one hand, cavitation damages the oxide film on the surface of the material and accelerates the anodic process; on the other hand, it accelerates the diffusion rate of products and reactants in the solution and accelerates the CATHODIC process. In the former, the corrosion potential is decreased, while in the latter, the corrosion potential is increased. In 3.5% NACL solution, the CATHODIC process of copper and its alloys is the oxygen reduction reaction, and the anodic process is the dissolution of copper and the formation of copper oxide. The results of Song et Al. [15] show that in static 3.5% NACL solution, the β-phase of Zn-rich Matrix of Mn-brass will be corroded preferentially, and the κ phase of Fe-rich Mn-rich dendrites and Fe-rich Mn-rich dendrites of high Mn-al bronze will be corroded locally, the difference between the oxides of Fe and Cu in the surface corrosion product film can also reduce the compactness and protection of the film. Therefore, for these two materials, the corrosion product film formed on the surface is not very protective or can not be formed quickly in a short time. cavitation mainly accelerates the diffusion of oxygen in the solution, thus accelerating the CATHODIC process, so the potential is positive, the oxygen diffuses slowly, the potential shifts negatively, and reverts back to its static value. The protective film [21,22] with Cu2o in the outer layer and Al2O3 in the inner layer was rapidly formed on the surface of ni-al bronze in 3.5% NACL solution, the surface rapidly forms a protective film which causes the potential to move forward to a static value. Fig. 4 shows the polarization curves of the three copper alloys in static and cavitation conditions in 3.5% NACL solution. Table 2 shows the corrosion current density and corrosion potential. It can be seen that the corrosion potential sequence under static and cavitation corrosion conditions is consistent with the results in Fig. 4, and cavitation corrosion increases the corrosion current density of the three materials by an order of magnitude. In both static and cavitation conditions, the corrosion rate depends on the oxygen reduction process, which accelerates the electrochemical reaction process and reduces the charge transfer resistance at the interface between the material and the solution, the passivation film on the surface of copper alloy is destroyed by the shock wave or micro-jet produced by the collapse of the cavitation bubble, and the local surface is in the state of active dissolution. In addition, a large amount of energy is released at the moment of cavitation collapse, and a local high temperature is generated on the surface of the material. As cavitation progresses, voids and micro cracks are formed on the surface of the material, and the size of the Surface roughness increases, which will accelerate the electrochemical reaction process, cavitation therefore increases the corrosion current density. The results show that the corrosion current density of the three copper alloys increases and the increase amplitude is close to each other.

2.2.3 interaction analysis of cavitation erosion process of materials in corrosive medium, the surface not only has mechanical damage but also electrochemical corrosion. The electrochemical corrosion and the mechanical impact of cavitation erosion are not independent, they will interact, and the material damage is more serious than the sum of them. The interaction between corrosion and cavitation can be expressed in the following equation [23] :

Wt is the total weight loss of cavitation erosion, that is, the total weight loss of cavitation erosion in corrosive medium, WE is the pure weight loss of cavitation erosion caused by mechanical factors, that is, the weight loss of specimen in distilled water, and WC is the pure weight loss of corrosion under static conditions WS is the loss caused by corrosion-cavitation interaction, Weic is the increment of corrosion weight caused by cavitation factor, WC and WEIC are calculated by Faraday’s law and the corrosion current density under static and cavitation conditions respectively. WCIE is the increment of cavitation weight loss caused by corrosion factors, which is obtained from total weight loss minus other components. Using Faraday’s law to convert the corrosion current density to the corrosion rate, the equation is as follows:

V is the corrosion rate, MG cm-2 h-1, a is the atomic weight of the metal (Cu is 64g mol-1) , N is the valence number, that is the number of electrons in the metal anodic reaction equation (Cu + 1) , ICR is the corrosion current density, a cm-2, F is the Faraday constant, the value is 96500C/mol. The results of interaction analysis of the three materials are shown in Table 3. It can be seen that the ratio of pure cavitation corrosion to total weight loss of the three materials, i. e. , the value of fs is very large, which indicates that all three materials exhibit the mechanism of cavitation damage dominated by mechanical damage, and the proportion of pure corrosion is very small. This is due to the excellent corrosion resistance of copper and its alloys due to the formation of oxide films in seawater. The proportion of total weightlessness caused by interaction (FS) from large to small is: Manganese brass (21.67%) , nickel aluminum bronze (16.35%) , high manganese aluminum bronze (13.70%) . The fCIE of the three materials is larger than that of FEIC, that is, the interaction is mainly caused by the increment of cavitation erosion caused by corrosion, which is due to the decrease of the adhesion between the different phases, the increase of the Surface roughness, and the decrease of the mechanical properties of the metal surface, therefore, the damage caused by cavitation stress is aggravated.

2.2.4 figure 5 shows the surface morphology of Manganin, permanganin and ni-al bronze after cavitation for 1 and 5 H in 3.5% NACL solution, respectively. After 1 H of Cavitation, the soft α phase of mn-cu was damaged seriously, the κ phase on β phase was lost, and the β phase was corroded obviously, as shown in Fig. 5a1. The α phase of high manganese aluminum bronze is plastic deformed, and the stress concentration at the α/κ phase boundary is the first to occur cavitation erosion, which results in the detachment of κ phase, see figure 5b1. The corrosion damage of ni-al bronze is the least, the properties of α phase and κ phase are different greatly, and cracks are easy to appear at the boundary of Α/κ III phase, as shown in Fig. 5c1. After 5 hours of Cavitation, the damage of manganin brass was aggravated, and the original structure could not be identified. The surface of Manganin brass was covered with large and deep cavitation pits, which were honeycomb like, as shown in Fig. 5a2. There is still some β phase on the surface of high manganese aluminum bronze, and cavitation damage is intensified, and cavitation holes are distributed all over the surface, as shown in Fig. 5b2. The ni-al bronze has the least damage and the EUTECTOID lamellar structure with high hardness is partially present on the surface, as shown in Fig. 5c2. Because the matrix phase of manganese brass is β phase, cleavage cracking occurs under cavitation stress, and corrosion occurs preferentially. The interaction between corrosion and cavitation will aggravate the damage under cavitation stress, so the cavitation corrosion resistance of manganese brass is the worst. The major reasons for the low cavitation corrosion resistance of high manganese aluminum bronze are the large scale kappa phase exfoliation and cleavage cracking of β phase. In addition, it has been shown that α phase in ni-al bronze has better work hardening ability than that in high mn-al bronze, which is one of the reasons for its better cavitation resistance.

3. Conclusion (1) under the experimental conditions, the cavitation resistance of the three materials is from high to low: Nickel aluminum bronze, high manganese aluminum bronze, manganese brass. The weight loss rate of manganese brass is 4.4167 MG cm-2 h-1 after cavitation in 3.5% NACL solution for 5 H, which is 3.07 and 4.06 times of that of high manganese aluminum bronze and nickel aluminum bronze, respectively. The incubation period of cavitation erosion was less than 0.5 h. (2) cavitation corrosion makes the corrosion potential of manganese brass and high manganese aluminum bronze move positively, and the corrosion potential of nickel aluminum bronze move negatively. Cavitation corrosion increased the corrosion current density of the three materials by an order of magnitude. (3) the proportion of weight loss caused by pure cavitation corrosion of manganese brass, high manganese aluminum bronze and nickel aluminum bronze is 77.92% , 84.94% and 82.70% , respectively, the results show that the mechanical impact damage caused by cavitation stress is the dominant factor of cavitation damage. The weight loss caused by corrosion-cavitation interaction of manganese brass, high manganese aluminum bronze and nickel aluminum bronze accounted for 21.67% , 13.70% and 16.53% of the total weight loss. The dezincification corrosion of manganese brass will deteriorate the mechanical properties of the material surface and increase the Surface roughness, so the corrosion significantly promotes cavitation corrosion. (4) for ni-al bronze and high mn-al bronze, the plastic deformation of α phase occurs under cavitation stress, and cavitation cracks preferentially initiate at the interface of κ phase and α phase. In addition, cleavage cracking occurred in β phase of high manganese aluminum bronze. However, cleavage cracking occurs in β phase of Manganin Matrix, and severe plastic deformation occurs in the softer α phase, with the largest Surface roughness and the worst cavitation erosion resistance. 

Source: Chinese Journal of Corrosion and protection

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