Effects Of Medium Conditions And Heat Treatment On The Electrochemical Behavior Of Aluminum Alloy Sacrificial Anodes

Aluminum(Al)alloy sacrificial anodes have been applied widely in various industrial areas for their unique features,including low density,high theoretical capacitance,high electrical conductivity,cheap and easy to obtain,long life,etc.However,they encountered problems,such as accelerated self-corrosion,shortened service life,and reduced current efficiency,in some special medium conditions,e.g.,high-temperature,high-pressure deep oil wells,low-temperature,high-pressure deep sea,and high-temperature,atmospheric pressure media.At present,the changing laws and mechanisms of the electrochemical behavior of aluminum alloy sacrificial anodes in these special media are not clear,which has hindered their application in these special media and the development of new anode materials.Therefore,it has important theoretical and practical significance to study the influence of particular relevant media on aluminum alloy sacrificial anode's electrochemical behavior and further explore methods to improve their electrochemical performance.In this paper,the Al-Zn-In-Mg-Si-RE anode(as-cast AR anode)was taken as the research object,and its microstructure was characterized.Using electrochemical,weight loss,and current efficiency measurements,combined with 3D morphology observation and microanalysis,the effects of temperature and pressure on the electrochemical behavior of the as-cast AR anode in 3.5%Na Cl solution were studied,and the related mechanism was discussed.The effects of CO₂ and corrosion inhibitors in the formation water saturated with CO₂ on the as-cast AR anode's electrochemical behavior were preliminarily explored.The as-cast AR anode's microstructure was modified by solution aging heat treatments(T6treatment),and then the change of their electrochemical behavior was studied.The main findings are as follows:1.Characterization of as-cast AR anode's microstructureThe as-cast AR anode has clear grain boundaries and coarse grains.Its microstructure was composed of?-Al phase,Mg Zn₂ phase,and Al₂Ce Zn₂phase.Mg Zn₂ and Al₂Ce Zn₂ phases distributed in the grain boundaries and grain centers,had a relatively negative potential,and produced a strong corrosion micro-galvanic effect with the Al matrix.The uneven distribution of alloying elements leads to the uneven anode microstructure.2.The influence of temperature and pressure on the as-cast AR anode's electrochemical behaviorIn a the 3.5%Na Cl solution,increasing the temperature(T)or pressure(P)impeded the as-cast AR anode's cathodic process significantly,causing its self-corrosion potential(E_corr)to shift positively and corrosion current density(i_corr)to increase.When T<40?or P<2.0MPa,the change of pressure or temperature will significantly affect the as-cast AR anode's polarization and impedance behavior.Increasing T accelerated the as-cast AR anode's dissolution.The uneven dissolution of the as-cast AR anode at high temperatures(>40?)decreased its current efficiency.Although increasing pressure made the as-cast AR anode's dissolution more uniform,its current efficiency still decreased.3.The influence of CO₂ and corrosion inhibitors in oil formation water on the as-cast AR anode's the electrochemical behaviorThe presence of CO₂ in the formation water slowed down the as-cast AR anode's self-corrosion and reduced its current efficiency.However,as the temperature rises,the influence of CO₂ was weakened.Adding 8-hydroxyquinoline(8-HQ)and 2-Mercaptobenzothiazole(2-MBTH)in the formation water saturated with CO₂ at different temperatures have had different effects on the as-cast AR anode's anodic process.They promoted the anodic process at 25?and hardly affected the anodic process at 40?,but both inhibited the cathodic hydrogen evolution process,thereby slowing down the as-cast AR anode's self-corrosion.In the formation water saturated with CO₂,adding 8-HQ further decreased the anode current efficiency at 25?but increased it at 40?,while adding 2-MBTH has little effect on the current efficiency.Different inhibitors have different effects on the as-cast AR anode's electrochemical performance.4.The effect of T6 heat treatment on the as-cast AR anode's electrochemical behavior The T6 treatment at 470?and 510?improved the AR anode's microstructure,making the grains refined with uniform size,the second phase amount increased,alloying elements'distribution uniformed,and the surface potential difference reduced.The T6 treatment at 510?had a more apparent effect.The T6 treatment changed the AR anode's polarization and impedance behavior in the formation water,making it more prone to passivation and enhanced its corrosion resistance.But it had little effect on the AR anode's dissolution morphology and slightly improved the current efficiency.It can be considered that the T6treatment has no apparent effect on the AR anode's electrochemical performance.