Corrosion of Magnesium Alloys

G L Song

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN: 9781845697082
• eBook ISBN: 9780857091413
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería minera

The use of magnesium alloys is increasing in a range of applications, and their popularity is growing wherever lightweight materials are needed. This book provides a comprehensive account of the corrosion of magnesium alloys. It covers not only the corrosion performances and mechanisms of Mg alloys in conventional environments, such as sodium chloride solutions, but also looks at their corrosion behaviours in special media, like engine coolants and simulated body fluids.Part one covers fundamentals such as the corrosion electrochemistry, activity and passivity of magnesium and its alloys. Part two then considers the metallurgical effect in relation to the corrosion of magnesium alloys, including the role of micro-structure and earth-rare elements, the corrosion behaviour of magnesium-based bulk metallic glasses, and the corrosion of innovative magnesium alloys. Part three goes on to describe environmental influences on the corrosion of magnesium alloys, such as atmospheric corrosion, stress corrosion cracking, creep and fatigue behaviour, and galvanic corrosion. Finally, part four is concerned with various means of protecting magnesium alloys against corrosion through the use of aluminium electrodeposition, conversion and electrophoretic coatings, and anodisation.With its distinguished editor and team of contributors, this book is an invaluable resource for metallurgists, engineers and designers working with magnesium and its alloys, as well as professionals in the aerospace and automotive industries.- Provides a comprehensive account of the corrosion of magnesium alloys covering fundamentals such as the corrosion electrochemistry, activity and passivity- Reviews the metallurgical effect in relation to the corrosion of magnesium alloys, including the role of micro-structure and earth-rare elements- Assesses environmental influences such as atmospheric corrosion, stress corrosion cracking, creep and fatigue behaviour, and galvanic corrosion

Part I

Fundamentals

1

Corrosion electrochemistry of magnesium (Mg) and its alloys

G.-L. Song, General Motors Corporation, USA

Abstract:

Magnesium (Mg) alloys are light, structural and functional engineering materials with a high strength to weight ratio which are increasingly being used in the automotive, aerospace, and electronic and energy industries. However, magnesium is chemically active with an electrochemistry differing from most conventional engineering metals. This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including: self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys.

Key words

Magnesium (Mg)

corrosion

electrochemistry

1.1 Introduction

Magnesium (Mg) and its alloys have many outstanding properties relative to other engineering materials such as: low density, high strength, great damping capability, excellent fluidity for casting, good electric shielding effect, non-magnetic, satisfactory heat conductivity, low heat capacity, negative electrochemical potential, acceptable recyclability and non-toxicity. These properties make Mg and its alloys attractive to many industries. Particularly in the automotive and aerospace industries, where the strength/weight ratio is a critical issue, Mg alloys have been regarded as a promising alternative to aluminum alloys (Aghion and Bronfin, 2000; Makar and Kruger, 1993; Song, 2005b, 2006; Song and Atrens, 2000; Song et al., 2005c) and have already found many applications (Aghion and Bronfin, 2000; Polmear, 1996). It is anticipated that a much wider application in the automotive, aerospace and electronic industries will be seen in the twenty-first century.

Currently, however, more ambitious Mg alloy applications in the automotive, aerospace and electronic industries are still unrealistic because of the poor corrosion resistance of the existing Mg alloys (Aghion and Bronfin, 2000; Bettles et al., 2003a,b). Before effective solutions to the corrosion problems of Mg alloys become available, a further expansion of Mg alloy applications appears to be unlikely. To date, a large number of studies have been carried out to address corrosion issues and to improve the corrosion performance of Mg alloys (Blawert et al., 2006; Hawkin, 1993; Hills, 1995; Gray and Luan, 2002; Jia et al., 2003a; Liu et al., 2008, 2009a; Nakatsugawa, 1996; Polmear, 1981; Shi et al., 2003b, 2006a,b; Skar and Albright, 2002; Shreir, 1965; Song, 2004a,b, 2005a,b, 2006, 2008a, 2009d; Song and Atrens, 1999, 2003, 2005, 2007; Song and Shi, 2006; Song and Song, 2006a,b,c; Song and StJohn, 2002, 2004, 2005a,b; Song et al., 1999, 2000, 2003, 2004a,c, 2005b,c, 2006b,c, 2007, 2010; Tawil, 1987; Wan et al., 2006; Wang et al., 2007; Zhao et al., 2008b; Zhu and Song, 2006; Zhu et al., 2005). The published results have clearly suggested that the corrosion of Mg is quite special in terms of its electrochemical behavior. In nature, the corrosion of Mg and its alloys is an electrochemical process and their corrosion performance or characteristics can be ultimately attributed to their electrochemical behavior. Therefore, revealing the electrochemical reactions involved in the corrosion process can provide a theoretical basis for understanding the characteristic corrosion phenomena for Mg and its alloys.

This chapter systematically summarizes the electrochemical characteristics and relevant corrosion behaviors of Mg and its alloys, in order to better understand their corrosion performance.

1.2 Thermodynamics

The thermodynamics of pure Mg is a foundation for understanding the electrochemical corrosion of Mg and its alloys. The stability of Mg in various environments can also provide clues for estimating the corrosion performance of Mg alloys in typical environments which is helpful towards understanding the thermodynamic behavior of Mg alloys. This section briefly touches upon the thermodynamics of pure Mg.

1.2.1 Thermodynamic tendency

Thermodynamically, Mg is very active. The standard Gibbs free energy changes (∆G⁰) for the following Mg oxidation reactions are quite negative (Ott and Boerio-Goates, 2000; Perrault, 1978; Wall, 1965; Weast, 1976–1977).

1.1

1.2

1.3

This means that Mg in natural environments has a great tendency to spontaneously transform into its oxidized states. Therefore, when Mg is exposed to environments containing oxygen or water, its surface always tends to be rapidly oxidized, thereby forming an oxide or hydroxide surface film (Alves et al., 2000; Nordlien et al., 1997).

The fact that Mg in an oxidized state is more stable than in its metallic state is also supported by the thermodynamic data of Mg compounds and species listed in Table 1.1. Corrosion is an oxidation process where various oxidized Mg species and compounds can be generated depending on the exposure media. The corrosion of Mg according to the tabulated data is a spontaneous process and thus Mg in most practical environments is thermodynamically unstable, as Mg in its oxidized states Mg^{2 +}, MgO or Mg(OH)₂ has a much more negative chemical potential. The more negative chemical potential of Mg(OH)₂ than that of Mg^{2 +} or MgO also suggests that in a solution Mg(OH)₂ is a more stable corrosion product than Mg^{2 +} or MgO. Ta...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Fundamentals
• Part II: Metallurgical effects
• Part III: Environmental influences
• Part IV: Corrosion protection
• Index