Corrosion Characteristics And Improvement Of Electrolytic Zinc Aluminum Cathode Plate In Industrial ZnSO₄ Electrolyte

Hydrometallurgy is currently the main method for producing zinc metal in the world.In this process,zinc sulfate solution is used as the electrolyte and a pure aluminum plate is used as the cathode.However,the pure aluminum cathode plate is easy to corrode during its pretreatment and electrolyzation in the electrolyte due to the presence of the fluorine and chlorine ions,which may deteriorate electrolytic effect and shorten the lifespan of the pure aluminum cathode plate.The aluminum cathode plates produced by Hunan Hengjia Aluminum Co.Ltd.exhibit an unstable corrosion resistance with a short lifespan in the ZnSO4 electrolyte.In regard these issues,the corrosion process of the pure aluminum cathode plates produced by the company in the industrial ZnSO4 electrolyte was observed,and the corrosion mechanisms the cathodes were analyzed in order to find out the reasons for the fast corrosion rate of the aluminum plates.On the basis of it,the improved processes including heat treatment and cold rolling were proposed to promote the corrosion resistance of the aluminum cathode plates in the ZnSO4 electrolyte.Finally,the borate agents(Na2B4O7)were added to the industrial pure alumimum during it melting,and the the Fe impurities were removed effieciently,and the shape and the distribution of the impurite phases containing Fe were improved by controlling the content of B and the holding temperatures,which further promote the corrosion resistance of the aluminum cathode plates in the ZnSO4 electrolyte.The main research contents and results are as follows:1.The corrosion process of the pure aluminum cathode plates produced by the company in the industrial ZnSO4 electrolyte was observed,and the corrosion mechanisms and the reasons for the fast corrosion rate of the cathodes were analyzed.The results indicate that the pitting and crevice corrosion mainly occur in the cathode plates when they are immersed in the industrial ZnSO4 electrolyte.The analysis of the impurity concentrations and observations of the microstructures of the cathode plates imply that the relatively high concentration of the impurities of Fe and Si may produce the coarse impurity phases containing Fe and Si,leading to the micro cracks on the surface of the plates during cold rolling.It is responsible for the occurrence of the pitting and crevice corrosion of the cathode plates in the in the industrial ZnSO4 electrolyte.2.The aluminum ingots were annealed,and the effects of the annealing temperature and cold rolling processes on the corrosion behavior of the aluminum plates were investigated.It is found that,with the annealing temperature increasing,the contents of the coarse impurity phases gradually decrease though solution in the aluminum matrix.The samples annealed at 500℃ exhibit a superior corrosion resistance in the in the industrial ZnSO4 electrolyte.After annealing treatment,the ingots were cold in different directions,which can improve the corrosion resistance of the aluminum plates.3.The borate agents(Na2B4O7)were added to the Al melt at 780℃,and the cathode plates for the electrolytic zinc were prepared by casting method.The effects of the amount of the borate agents and the reaction time on the microstructure and corrosion behavior of the Al cathode plates were investigated.The results indicate that,with the additions of borate,agents increasing and the reaction time prolonging,the volume fraction of the Fe-rich phases decrease,and the shape of the Fe-rich phases changes from the needle-like shape to the fine granuar shape.Thus,the addition of the borate agents can reduce the Fe contains and change the shape of the Fe-rich phases.The corrosion behaviors of the A1 cathode plates obtained with the different borate agents additions and reaction durations in the simulated electrolyte were evaluated by immersion corrosion tests and elecrochemical property tests.The results indicate that,with the additions of borate agents increasing and the reaction time prolonging,the corrosion resistance of the A1 cathode plate increases.The A1 cathode plate obtained with 0.7%additions of Na2B4O7 and 60min reaction duration exhibits the highest corrosion resistance due to its low contain of the rich-Fe phase.4 By adding the Al-Sc intermediate alloy into the A1 melt at 850℃,the cathode plates for the electrolytic zinc were prepared.The effects of the Sc contents on microstructure and corrosion behavior of the Al cathode plates were investigated.The results indicate that,the additions of Sc can refine the microstructure and decrease the content of the impurities of the Al ingots.The as-cast Al sample is of very fine equiaxed grains,and the cathode plate fabricated by cold rolling it exhibits the superior corrosion resistance in the industrial ZnSO4 electrolyte.