Electrochemical Characterization of Al 7075-T6 Surface Oxide after Alkaline Treatments


Electrochemical impedance spectroscopy was used to study the corrosion resistance of Al 7075-T6 after degreasing and one of three different alkaline activations. Electrochemical tests were conducted using an aqueous solution containing 0.35 w/v% NaCl and 0.70 w/v% (NH4)2SO4 at its natural pH of ∼5.5 or with its pH adjusted to ∼9.0. These tests showed increasing the electrolyte pH to 9.0 made the electrolyte less corrosive, which allowed impedance characterization at low frequency (1.00–0.01 Hz) after only 1500 s of stabilization prior to testing. The total impedance of the native oxide was ∼5.9 kΩ cm2 and degreasing did not alter this value. Degreasing followed by activation in 5 wt % Na2CO3 or 1 wt % NaOH decreased the total impedance by ∼20% to 4.8 kΩ cm2, whereas activation in 2 wt % NaOH decreased impedance by ∼30% to 4.2 kΩ cm2. The decreased impedance after activation allowed a cerium-containing solution to penetrate the surface oxide and react with the substrate to form a conversion coating. Thus, increasing the electrolyte pH made the electrolyte less corrosive, which enabled characterization of the decrease in surface oxide impedance after alkaline treatments using electrochemical impedance spectroscopy.


Materials Science and Engineering

Keywords and Phrases

Aluminum Alloys; Corrosion Resistance; Corrosion Testing; Electrochemical Impedance Spectroscopy; Electrolytes; Oxidation; PH; Surface Treatment

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Article - Journal

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© 2011 The Electrochemical Society (ECS), All rights reserved.

Publication Date

01 Jan 2011