Influence of Surface Pretreatment on Coating Morphology and Corrosion Performance of Cerium-based Conversion Coatings on AZ91D Alloy

Abstract

The corrosion protection of cerium-based conversion coatings (CeCC) formed on AZ91D magnesium alloy has been studied using potentiodynamic polarization measurements, electrochemical impedance spectroscopy (EIS), and salt fog testing. The effect of acid (sulfuric acid [H2SO4]) and alkaline (sodium carbonate [Na2CO3]) pretreatments on the coating morphology and corrosion resistance of the cerium conversion layer was investigated. Significant change in corrosion resistance occurred with different pretreatments. Samples with combined acid and alkaline pretreatments showed better salt fog performance than samples pretreated with an acid or alkaline solution only. The corrosion rate (mm/y) calculated from potentiodynamic polarization measurements were 0.77 for bare AZ91D alloy, while cerium coated rates were 0.16 for acid-cleaned, 0.09 for alkaline-cleaned, 0.04 for acid then alkaline, and 0.18 for alkaline then acid pretreatments. Potentiodynamic and impedance results were correlated with that of salt fog testing. Low calculated corrosion rates corresponded to few pits and tails during salt fog testing.

Department(s)

Materials Science and Engineering

Keywords and Phrases

AZ91D Alloy; Cerium Conversion Coatings; Magnesium Corrosion; Surface Pretreatments

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2010 NACE International, All rights reserved.

Publication Date

01 Oct 2010

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