SORPTION OF SULFATE AND CHLORIDE ANIONS ON A WELL-CHARACTERIZED AL-2024 ELECTRODE

We studied sorption processes, adsorption and incorporation, of sulfat e and chloride anions on the surface of well-characterized aluminum 20 24 alloy using electrochemical, radiochemical, and ultrahigh-vacuum sp ectroscopic techniques. The measurements were carried out at an open-c ircuit potential and in the electrode potential range on the negative side of the open-circuit potential (cathodic polarization conditions), at various pH values. The focus was on sorption reversibility as well as on the relationship between anion's surface concentration and the electrode potential. We have found that sorption of sulfate anion is c ontrolled by pH, surface charge, and the stability of aluminum oxide f ilms. We have also found that adsorption of chloride is weaker than su lfate and is more irreversible since chloride incorporation occurs mor e readily than sulfate. The change in the alloy surface composition an d morphology induced by the electrochemical treatment and anion adsorp tion was monitored by scanning Auger microscopy and energy-dispersive X-ray spectroscopy. The characterization exhibits copper-rich intrusio ns and extrusions that may act as either local cathodes or anodes in t he overall alloy dissolution process. The distribution and evolution o f such Cu-rich inclusions under studied experimental conditions were m onitored and are reported. The dissolution of aluminum from the alloy affects both sulfate and chloride adsorption/incorporation processes. While sulfate and chloride adsorption have no effect on cathodic curre nt measured in the studied electrode potential range, the high anion s urface concentration may have a detrimental effect on the alloy stabil ity, particularly when the beneficial influence of the cathodic polari zation (protection) ends.