Erosion-Corrosion Behavior Of Copper And 90/10 Cu-Ni Alloy In Flowing Seawater

Copper and 90/10 Cu-Ni alloy are among the most important commercial metals in the marine enviroment due to their excellent electrical and thermal conductivities, good corrosion resistance and ease of manufacture. They have good corrosion resistance to flowing seawater and are easily worked and welded. Copper, however, has a lower resistance to flow induced corrosion than 90-10 copper-nickel . Both of them are widely used for pipe works, tube plates, water boxes, flanges and pump casings, in cooling systems and so on.The corrosion resistance of Copper and 90/10 Cu-Ni alloys to seawater is associated with formation of protective layers on the metal surface. However, they are known to be susceptible to enhanced corrosion in seawater when there is sufficient relative motion between the metal and the fluid. It is possible to overestimate the corrosion resistance of a metal if the testing is done under static conditions while service is under non-static congditions. In this work , the erosion-corrosion behavior of copper and 90/10 Cu-Ni alloys in flowing seawater and sand presented seawater.In quiet seawater, it's well known for copper and 90/10 Cu-Ni alloy, the protective film is mainly made up of Cu₂O and Cu₂(OH)₃Cl. It has been reported that the atacamite(Cu₂(OH)₃Cl) film protects the underlying Cu₂O layer which contributes the resistance to erosion-corrosion mostly. The protective surface films will be more thicker with the exposure time in quiet seawater. These films prevent the charge and mass transfers in the electrochemical reaction. In flowing seawater, the atacamite will be removed by continuously filtering the solution to prevent its deposition onto the metal surface and the inner oxide layer Cu2O becomes thiner and thiner with the increasing velocity of flow. For copper and 90-10 Cu-Ni alloys, the rate of electrochemical reaction in quiet or flowing seawater is controlled by the rate of charge and mass transfer through the protective surface film, in quiet seawater , it is also controlled by the rate of diffusion of oxygen(O₂) in the solution. The corrosion rate of copper and 90/10 Cu-Ni alloys will decrease continually as the protective films continue to improve with time. Of course, the corrosion rates tend to be much more higher under condition of flow than those measured in quiet seawater , after eroding the same time, they also be more and more higher with the increasing velocity of flow. If the velocity of seawater increases beyond a critical point, however, the protective film and the metal itself will be damaged seriously by erosion-corrosion , then the metal will be invalidation quickly. The critical breakaway velocity of copper is about 0.9～1.2m/s, and for 90/10 Cu-Ni alloy, a velocity beyond 3m/s comes to be its critical breakaway velicity . Copper is more sensitive to the velocity of flowing seawater than 90-10 Cu-Ni alloy.The protective surface films of copper and 90-10 Cu-Ni alloy will be damaged greatly when there is sand presented in the flowing seawater and the metal itself can also be impacted by the sand. The damaged protective films make the surface of copper and 90-10 Cu-Ni alloy be rough. It seems likely that the increased turbulence induced by a rougher surface will increase the surface shear stress and the mass transfer rate and so increase the susceptibility to erosion-corrosion, the weight-loss of the copper and copper alloy will become much higher. Comparatively, 90-10 Cu-Ni alloy is more susceptibility to erosion-corrosion induced by sand-presented flowing-seawater than copper .The erosion-corrosion behavior of copper is a little similar to the 90-10 Cu-Ni alloy's. But the latter's resistance to erosion-corrosion is absolutely much more better than copper. It should be to choose the right metal to use on the basis of the metal's performance and the demand of the production in practice.