In-situ determination of thickness and electrochemical properties of barrier oxide film on impure aluminium in aqueous solution

Abstract

Lead is present as a trace element (ppm level) in nearly all commercial aluminium alloys. The objective of this work is to investigate the changes incurred by the presence of a small amount (20 ppm) of alloyed Pb and heat treatment on the properties of the barrier oxide in relation to those on pure aluminium, in chloride free acetate buffer. The potential range of interest was-1.1 to-0.1 V vs. saturated Hg/Hg2SO4. The methods used were electrochemical impedance spectroscopy (EIS) and chronoamperometry. The film growth data obtained by chronoamperometry was analysed by use of Cabrera-Mott inverse square logarithmic law. The activation energy for film growth, obtained from this analysis, decreased with increasing heat treatment time and temperature, along with deleterious changes in the electrochemical properties of the oxide, indicating reduced passivity. The steady state data obtained by EIS showed decreasing oxide resistivity with increasing heat-treatment temperature of the samples in the range 300-600°C. These changes were attributed to reduced passivity caused by increased segregation of Pb at the aluminium substrate-film interface.

Electrochemical behavior of active surface layers in AA8xxx aluminum alloys

“…The main factors controlling corrosion behavior of AA8xxx alloys are their chemical composition and the existence of an active surface layer, also referred as altered surface layer . Because of the relatively high Fe content in AA8xxx alloys, binary Al‐Fe and ternary Al‐Fe‐Si constituent particles are typically found in Al‐Fe‐Si alloys and quaternary Al‐Fe‐Mn‐Si particles in Al‐Fe‐Mn‐Si alloys .…”

“…The main factors controlling corrosion behavior of AA8xxx alloys are their chemical composition and the existence of an active surface layer, also referred as altered surface layer. 3,[5][6][7][8][9][10] Because of the relatively high Fe content in AA8xxx alloys, binary Al-Fe and ternary Al-Fe-Si constituent particles are typically found in Al-Fe-Si alloys and quaternary Al-Fe-Mn-Si particles in Al-Fe-Mn-Si alloys. 11 Fe-rich particles might increase the susceptibility to localized attack since they can behave as preferential sites for the initiation of localized types of corrosion like pitting attack.…”