Influence factors of corrosion resistance of Copper-Nickel alloy in seawater
The corrosion rate of cu-Ni alloy in seawater mainly depends on the properties of the corrosion product film on its surface. The corrosion resistance of cu-Ni alloy in seawater can be influenced by the factors that can affect the formation process, formation rate, film composition, structure and protection performance of the film. At present, domestic and foreign scholars have conducted a large number of effective and detailed studies on factors affecting the corrosion resistance of copper-nickel alloy in seawater [48, 50, 53, 55, 58, 98-104]. From the perspective of environment, these factors include high salinity of seawater medium, high sand content in some sea areas, seawater acidification caused by Marine organisms and long-term high-velocity seawater scouring. From the point of view of materials, the content of alloy elements, deformation, microstructure changes after heat treatment and grain boundary structure will affect the formation and properties of the corrosion product film of the copper-Nickel alloy in seawater, thus affecting the corrosion resistance of the copper-nickel alloy in seawater environment.
Material influencing factors
Salinity in seawater environment directly affects the oxygen content and electrical conductivity of seawater medium. As the salinity of sea water increases, its electrical conductivity increases and its oxygen content decreases. Reverse changes in seawater conductivity and dissolved oxygen result in a peak corrosion rate at a certain salinity. Therefore, the corrosion rate fluctuates greatly in different sea areas due to the difference of salt content. The tidal changes and photosynthesis of Marine plants will increase the dissolved oxygen content in seawater. For passivation of metal materials in seawater are less, the higher the dissolved oxygen content in seawater, metal corrosion rate is higher in the water medium, for in seawater are prone to passivation or produce protective corrosion product film of metal material, increase the oxygen content is conducive to the formation of the passivation film or corrosion product film and repair. For copper-Nickel alloys, a protective oxide film can be rapidly formed in adequately oxygenated seawater, while the film formation rate is almost zero in deoxygenated seawater.
The temperature fluctuation leads to the change of corrosion rate of metal materials, which may be caused by many reasons. With the change of temperature, the bioactivity capacity of the middle sea in the Marine environment will change, and the type and concentration of seawater ions will also change dramatically. Wang studied the effect of temperature on the corrosion behavior of B30 cu-Ni alloy in seawater at 20℃ ~ 80℃, and found that the corrosion rate of B30 cu-ni alloy increased with the increase of temperature, and the phenomenon of selective dissolution of copper element appeared. The nickel content in the corrosion product film increased significantly and was higher than that in the matrix. Ezuber temperature of B10 copper nickel alloy was studied, the influence of sulfur pollution in seawater corrosion rate found within the scope of the 25℃ to 80℃, as temperatures rise, B10 copper nickel alloy corrosion rate increased significantly, at 25℃ sulfide pollution when promoting the improvement of the corrosion rate, and within 50℃to 80℃ and the role of sulphide from improve corrosion rate to reduce the corrosion rate.
The pH level of seawater is controlled by the dissociation balance of seawater carbonate system. The pH value of ocean seawater is generally maintained between 7.8 and 8.4, which is relatively stable compared with other natural water sources. However, the pH value of seawater will be affected by physical chemistry and biogeochemistry, especially in the coastal estuarine area, due to the input of terrestrial nutrients and polluted organic matter, a large number of Marine plankton will multiply and die, resulting in the periodic change of the pH value of seawater with the growth and death of organisms. In seawater environment, cu - Ni alloy must face the corrosion of acidic, neutral and alkaline seawater. Chi Chang-wan studied the corrosion behavior of The samples of B30 cu-Ni alloy in sodium chloride solution with different pH values after different potential polarization, and found that the corrosion resistance of B30 cu-Ni alloy in basic solution was better than that in acidic and weak base solution. In the strong acid solution of 3, the surface of B30 material was mainly corroded by denickel, and protective oxide film could not be formed. The pH value & gt; In the strong alkali solution of 12, the surface of B30 material has two corrosion forms of copper and nickel removal. Because the high concentration of OH- inhibits the reduction reaction of oxygen, the surface of the sample cannot form a corrosion micro battery, and the corrosion rate drops. Dense protective oxide film is easily formed on the surface of B30 under pH value of 5 ~ 9.
The sulfur compounds in seawater environment mainly come from decaying plants and animals in seawater, industrial wastewater and waste residue, sulfate reducing bacteria. In both deoxidized and undeoxidized sulfide polluted seawater, the failure of cu-Ni alloy showed accelerated corrosion. Macdonald studied the sulfide concentration of B10 and B30 corrosion behavior of copper nickel alloy in sea water flow, the results found that the sulfide pollution generated on the surface of copper nickel alloy in seawater porous, the protective sulfide copper film, under the condition of oxygen copper nickel alloy from corrosion potential negative shift, the reduction of hydrogen ions become the main form of cathodic reaction. Eiselstein compared the corrosion patterns of copper-nickel alloys in deoxygenated, sulfated, unoxygenated, uncontaminated, and first exposed to contaminated seawater and then exposed to uncontaminated seawater. The results show that sulfide polluted seawater is more corrosive to cu-Ni alloy in the seawater without oxygen removal, and the dissolved sulfide not only accelerates the corrosion rate of cu-Ni alloy rapidly, but also prevents the formation of normal protective oxide film by forming porous cuprous sulfide film. Alhajji studied the influence of sulfide concentration on the corrosion rate of cu-Ni alloy, and found that the corrosion rate of cu-Ni alloy gradually increased with the increase of sulfide concentration. In this process, the dissolved sulfide played a role in accelerating the anode and cathode reaction on the surface of cu-Ni alloy, and the corrosion product film gradually changed from CuS to Cu2S.
Marine biological corrosion in seawater environment is mainly caused by three types of surface of Marine biological adhesion and fouling materials. The three types of sea creatures are bacteria and algae, soft growths such as caverns, and hard Marine creatures such as barnacles. Sea creatures corrosion occurred mainly comes from microbial adhesion of material surface seawater environment, microbial reproduction form microbial film, after macro sea biological larvae are attached to the microbial membrane surface gradually grow, cause the material's surface is covered by sea creatures, macro sea biological decay after death, the microbial multiply, hinder the material surface water flow in the seawater environment, increase the chances of getting turbulence, at the same time by the sea uneven distribution of biofilm and its structure, lead to the generation of oxygen concentration cell. The metabolism of Marine organisms produces sulfides and acidifies seawater, changing the interface between metals and seawater and causing severe local corrosion. Efird B10 and B30 two kinds of copper and nickel alloy was studied in the practical environment exposure after 3 years and 5 years of corrosion status of sea creatures, it is found that exposure to 3 years only slightly macro sea creatures, exposed 5 years sea creatures adhesive film is covered on the surface of the copper and nickel alloy 70% Yuan a Marine microorganisms was studied, such as pseudomonas of B30 copper nickel alloy corrosion status of sea creatures, the results show that this kind of pseudomonas exists B30 copper nickel alloy corrosion speed obviously improved, the Marine microorganisms attached to the alloy surface membrane, inducing tiny pits,The structure of protective corrosion product film on the surface of cu-Ni alloy was changed and the protective product film was weakened.
Flow rate is an important factor affecting erosion corrosion. Increase the flow velocity on the one hand can accelerate the mass transfer process, will more depolarization agent (O2, H +) or corrosion inhibitor molecules to the surface, so as to accelerate the corrosion of materials or promote passivation and accelerate corrosion inhibitor in adsorption film on the surface of the material, on the other hand the fluid on the material surface shearing action, leading to the breakage of the material surface membrane, more corrosion products from the materials and corrosive medium interface, have the effect of accelerated corrosion. The seawater velocity has an important effect on the corrosion and protection of copper-Nickel alloy. When the flow rate of seawater does not reach the critical flow rate, oxygen or corrosion inhibitor molecules transported by seawater are conducive to the formation of cu-Ni alloy surface film. When the seawater velocity exceeds the critical velocity, the surface shear stress caused by the flow will lead to the rapid failure of the cu-Ni alloy surface film.
In some seawater environments near estuaries, due to the large amount of sand in terrestrial rivers, there are a large number of solid particles in seawater. The hardness, shape, quantity and Angle of attack of solid particles in corrosive media are important factors affecting erosion corrosion. The higher the hardness of the solid particles, the larger the particle size and the greater the erosion corrosion damage. Because of the "shielding effect" between the solid particles, the increase of the concentration of the solid particles does not necessarily cause the continuous increase of the erosion corrosion rate. The included Angle between the particle and the sample surface along with the incident direction of the fluid medium is called the Angle of attack. The erosion corrosion rate of ductile and brittle materials varies with the Angle of attack. The maximum erosion rate of the ductile material was reached at an Angle of attack of 15 ~ 40°, while that of the brittle material was reached at an Angle of attack of 90°.
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