Impedance spectroscopy response of aluminum-copper-silicon alloys

The effect of increased Cu and Si additions and heat treatment on the polarization resistance, oxide-layer/double-layer capacitance, open circuit potential and breakdown potential of Al-Cu-Si alloys was evaluated. Electrochemical impedance spectroscopy and DC polarization techniques were combined with mechanical property evaluations to determine the relationship between the microstructure of Al-Cu-Si alloys and the ability of their oxide layer to provide protection against corrosion in an aqueous 5% NH$\sb4$Cl solution. Alloys were investigated in both the solution-treated and artificially aged conditions. The solution-treatment procedure involved heating the alloys to 550$\sp\circ$C for 30 minutes and subsequently water-quenching. Artificial aging was carried out at 250$\sp\circ$C. Increased aging times of 1, 2, 8 and 32 hours at 250$\sp\circ$C were employed on Al-Cu(2%), Al-Si(2%) and Al-Cu(2%)-Si(2%) alloys in order to correlate the observed electrochemical behavior with the precipitation strengthening reactions which occur during age-hardening. These precipitation reactions govern the resultant microstructure and determine the mechanical behavior, electrochemical behavior and type of corrosion attack observed in Al-Cu-Si alloys. In general, an increase in the susceptibility to pitting corrosion was correlated to a decrease in the protective oxide layer thickness brought about as a result of increased Cu and Si additions in solution-treated alloys and the precipitation of Guinier-Preston zones in age-hardened alloys. Cu and Si atoms at the metal/oxide interface inhibit the diffusion of Al$\sp{+3}$ ions to the oxide/electrolyte interface thereby limiting the oxide-layer thickness. Increased aging times, leading to the formation of Guinier-Preston zones and a concomitant age-hardening peak, also decrease the total number of Al$\sp{+3}$ ions available for passivation and thereby decrease the oxide-layer thickness. Consequently, the oxide-layer/double-layer capacitance and the polarization resistance of the alloys are correlated to changes in the oxide-layer thickness and the loss in continuity of the oxide-layer with overaging. In addition, the open-circuit and breakdown potentials and the type of corrosion attack observed during anodic DC polarization scans was related to the type of alloying element and their distribution.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Engineering

Grantor

Rice University

Year dc:date.issued

1991

Author dc:creator

• Griffin, Alfred Joseph, Jr.

Advisor dc:contributor.advisor

• Brotzen, Franz R.

• Engineering
• Materials science

Statement dc:rights

• Copyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/1911/16444

OAI identifier oai:identifier

oai:repository.rice.edu:1911/16444