Improving the Corrosion Resistance of Wrought ZM21 Magnesium Alloys by Plasma Electrolytic Oxidation and Powder Coating

Lavanya Rani Ballam,Silvia Franz,Carla Martini,Hamed Arab,Lorenzo Donati,Massimiliano Bestetti,Giulia Masi,Ramona Sola

doi:10.3390/ma14092268

Lavanya Rani Ballam, Silvia Franz + Show 6 more

Open Access

https://doi.org/10.3390/ma14092268

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Journal: Materials	Publication Date: Apr 27, 2021
Citations: 8	License type: CC BY 4.0

Affiliation: University of Bologna, Politecnico di Milano

Abstract

Plasma Electrolytic Oxidation (PEO) was applied to extruded ZM21 Mg alloys to improve their corrosion resistance in a chloride-containing environment. PEO was carried out in DC mode and voltage control in a fluoride-free electrolyte. Potentiodynamic polarization tests in 3.5 wt.% NaCl aqueous solution and neutral salt spray (NSS) tests were carried out. Microstructural and profilometric characterization, as well as NSS tests were performed in different conditions: (i) bare ZM21, (ii) PEO-treated ZM21, (iii) powder-coated ZM21 (without PEO interlayer), and (iv) PEO-treated ZM21 with powder coating top layer (carboxyl-functionalized polyester resin). The PEO + powder coating double layer was identified as the best-performing corrosion protection.

Highlights

Optical and scanning electron microscope (SEM) images showing the microstructure of the ZM21 alloy are shown in Figure 1a,b: a fully recrystallized microstructure was observed in the plane perpendicular to the extrusion direction
Similar observations were reported for the same alloy in the extruded condition,5 of
A polymeric topcoatresin) was deposited by powder coatingcoating onto Plasma Electrolytic Oxidation (PEO)

Summary

Introduction

Mg alloys attractive for lightweighting in automotive and aerospace [1,2]. Mg-Zn alloys are among the most promising Mg alloys, with the main advantage of high formability, combined with moderate strength and corrosion resistance [3]. Among Mg-Zn alloys, ZM21 is a promising candidate for automotive applications due to its extrusion speeds being higher than other medium strength alloys such as AZ31B [4], allowing for a cost reduction in the production of Mg extrusions and coping with one of the strategic objectives of the automotive industry [5]. Localized corrosion induced by non-homogeneous distribution of second-phase particles and impurities may significantly affect the ZM21 alloy in aggressive environments [6] and requires the use of surface modifications to achieve satisfactory service life and industrial application

Methods

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Conclusion