Effect Of Grain Boundary Segregation On Electrochemical Performance And Dissolution Behaviour Of Aluminium Alloy Sacrificial Anode

The surface microstructure and electrochemical behaviour in 3% sodium chloride solution of Al-5%Zn-0.03%In alloy, submitted to different heat treatments (namely as-cast, water quenched, air-cooled and furnace-cooled), were investigated by metallographic microscope, electron probe microanalysis (EPMA), electrochemical impedance spectroscopy (EIS) and galvanostatic polarization techniques. The experiments focused on the influence of grain boundary segregation on electrochemical performance of alloys. Empirical relationships between morphological structure (solid solution, segregation phases, intermetallic compounds, inclusions and shrinkage cavity) and electrochemical performance (open circuit potential, operating potential, polarization behaviour and current efficiency) were established in the paper.The as-cast Al-Zn-In sacrificial anode exhibit a typical dentritic structure, the alloying element such as zinc and indium mainly are dissolved inα-Al solid solution, and a few are distributed as segregated second-phase particles, intermetallic compounds or inclusion. The second-phase particles, enriching in zinc and other metallic impurities such as iron and silicon, commonly segregate in the interdentritic region or concentrate along the grain boundaries. It was found that the heat treatment at 510 degree C for 10 hours retards the grain boundary segregation and produces a homogenous distribution of elements and impurities in the matrix. All the three different homogenization treatments decrease the hydrogen evolved corrosion and promote the anode dissolve uniformly, and therefore improve the current efficiency, but not to exert any influence on the open circuit potential, operating potential and dissolve behaviour. The main efficiency loss can be ascribed to the hydrogen evolution. The simulated Al2Zn segregation alloys were developed according to the average elements distribution of zinc enriched zones in Al-Zn-In alloy. The zinc rich alloy exhibited a corrosion potential of -1040mV, which is considerably more cathodic than the value of Al-Zn-In alloy (-965mV). It is also believed that the zinc segregation in Al-Zn-In alloy can be attributed to the formation of local galvanic cells as coupled with alloy base, which lead to the preferential dissolution of active site and disruption of the oxide film. Among the tested alloys, the water-quenched after heat treatment alloy exhibited the best electrochemical performance which improved the current efficiency from 69.43% of as-cast sample to 85.02%.