Corrosion Prevention of Magnesium Alloys

Guang-Ling Song

• Publisher: Woodhead Publishing
• Year: 2013
• ISBN:
  9780857098962
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

Magnesium (Mg) alloys are receiving increasing attention due to their abundance, light weight, castability, formability, mechanical properties and corrosion performance. By selecting the appropriate combination of materials, coatings and surface modifications, their corrosion resistance can be greatly enhanced. Corrosion prevention of magnesium alloys is a comprehensive guide to the effective prevention of corrosion in these important light metals.Part one discusses alloying, inhibition and prevention strategies for magnesium alloys as well as corrosion and prevention principles. Part two reviews surface treatment and conversion. Beginning with an overview of surface cleaning and pre-conditioning, the book goes on to discuss the use of surface processing and alloying, laser treatments, chemical conversion and electrochemical anodization to improve the corrosion resistance of magnesium alloys. Coatings are then the focus of part three, including varied plating techniques, cold spray coatings, gel and electroless electrophoresis coatings. Finally, the book concludes in part four with a selection of case studies investigating the application of preventative techniques for both automotive and medical applications.With its distinguished editor and international team of expert contributors, Corrosion prevention of magnesium alloys is a key reference tool for all those working with magnesium and its alloys, including scientists, engineers, metallurgists, aerospace and automotive professionals, and academics interested in this field.

• Chapters provide an overview of surface cleaning and pre-conditioning
• Examines processes to improve the corrosion resistance of magnesium alloys, including laser treatments and chemical conversion and electrochemical anodization
• Discusses cold spray, sol-gel and electrophoretic coatings

Part I

Alloying and inhibition

1

Corrosion behavior and prevention strategies for magnesium (Mg) alloys

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

Abstract:

Magnesium (Mg) alloys have low corrosion resistance and exhibit unusual corrosion behavior in aqueous environments. Because of this unique corrosion performance, some special corrosion prevention techniques have to be employed for Mg alloys in their applications. This chapter briefly summarizes the corrosion characteristics of Mg alloys, and also presents a strategy and methodologies to mitigate the corrosion damage of Mg alloys in applications.

Key words

magnesium

corrosion

protection

1.1 Introduction

Note: This chapter is a revised and expanded version of Chapter 1 ‘Corrosion behaviour of magnesium alloys and protection techniques’ by G.-L. Song, originally published in Surface engineering of light alloys: aluminium, magnesium and titanium alloys, ed. Hanshan Dong, Woodhead Publishing Limited, 2010, ISBN: 978-1-84569-537-8.

Magnesium and its alloys have a high strength/density ratio and have found many successful applications, particularly in the automotive and aerospace industries (Makar and Kruger, 1993; Polmear, 1996; Aghion and Bronfin, 2000; Bettles et al., 2003; Song, 2005b, 2006). However, the poor corrosion resistance of existing magnesium alloys in some service environments has limited the further expansion of their application. Existing investigations have clearly suggested that the corrosion behavior of Mg alloys is very different from that of a convention metal, such as steel (Song, 2004a, 2005b, 2006, 2009a, 2009c; Winzer et al., 2005, 2007, 2008; Wan et al, 2006; Song and Atrens, 2007; Wang et al, 2007). For more successful applications of Mg alloys, it is important to understand their characteristic corrosion phenomena and have a practical corrosion prevention strategy.

1.2 Corrosion characteristics and implications in protection

The corrosion characteristics of Mg alloys critically concern their applications. Only if the corrosion behavior is comprehensively understood, can a Mg alloy be correctly used in suitable applications, and also the most effective protection be selected for this alloy.

1.2.1 Electrochemical corrosion mechanism

Mg alloys follow a corrosion mechanism different from other engineering metallic materials. On pure Mg or a Mg alloy, the overall corrosion reaction can be written as (Makar and Kruger, 1993; Song and Atrens, 1999; Song, 2005b, 2006):

[1.1]

or

[1.2]

This overall corrosion can be decomposed into anodic and cathodic reactions. The cathodic process is (Song, 2005b, 2006):

[1.3]

or

[1.4]

and the anodic process (Song, 2005b, 2006):

[1.5]

[1.6]

y is the ratio of further anodic reaction over hydrogen production reactions. The detailed anodic and cathodic reactions under a steady corrosion condition have been illustrated previously (Song et al, 1997a, 1997b; Song and Atrens, 1999, 2003; Song, 2005b). In general the anodic dissolution occurs mainly in a film-free area, while in a film-covered area the anodic dissolution is negligible. The cathodic reaction, which is mainly a hydrogen evolution process, can take place in both film-free and film-covered areas, and the reaction rate in a film-free area is much faster than the rate on a film-covered surface, particularly if impurity particles are present there.

Both the anodic and cathodic reactions occur simultaneously on Mg. In fact, more detailed reaction steps can be involved in the anodic and cathodic processes, which have been demonstrated by Song (2011) and are schematically illustrated in Fig. 1.1.

1.1 Schematic illustration of anodic and cathodic reactions involved in the self-corrosion of Mg.

Based on this corrosion model, the subsequent corrosion phenomena and behavior can be predicted. Correspondingly, what these corrosion characteristics imply can also be understood.

1.2.2 Hydrogen evolution

The overall corrosion reactions [1.1] and [1.2] suggest that the dissolution of Mg is always accompanied by hydrogen evolution. This corrosion-related hydrogen evolution is also applicable to Mg alloys in aqueous solutions (Song et al, 1998, 2001, 2005a; Song and Atrens, 1998, 2003; Song and St John, 2002), including engine coolants (Song and St John, 2004, 2005) and simulated body fluids (SBF) (Song and Song, 2006, 2007; Song, 2007b). In a severe corrosion process, Mg particle undermining may take place (Mg particle falling into solution due to the surrounding material becoming completely corroded). However, this process has been shown to have no influence upon either reaction [1.1] or [1.2] (Song et al., 1997b). Therefore, for Mg and Mg alloys in an aqueous solution, the hydrogen evolution phenomenon is one of the most important features and includes cathodic hydrogen evolution (CHE) and anodic hydrogen evolution (AHE).

Having realized that hydrogen evolution is closely associated with the corrosion of Mg and its alloys, a simple hydrogen evolution measurement technique was first employed by Song et al. (1997b) to estimate the corrosion rate of Mg. Following further illustration of the theory and analysis of possible errors inherent to this method (Song et al., 2001; Song, 2005b, 2006), it has also been widely used on many Mg alloys (Krishnamurthy et al, 1988; Hallopeau et al., 1999; Bonora et al.; 2000; Eliezer et al., 2000; Mathieu et al., 2000; Song and St John, 2002; Song, 2005a; Pu et al, 2012; Song and Xu, 2012).

According to reaction [1.1] or [1.2], dissolution of one Mg atom always corresponds to the generation of one hydrogen gas molecule. In other words, if there is one mole of hydrogen evolved, then there must be one mole of magnesium dissolved (Song et al, 2001; Song, 2005b, 2006). Measuring the volume of hydrogen evolved is equivalent to measuring the weight-loss of a corroding Mg alloy, and the measured hydrogen evolution rate is equal to the weight-loss rate if they are both converted into the same unit (e.g. mole per minute).

The experimental set-up for measuring hydrogen evolution is straightforward (Song et al, 2001) and can simply consist of a burette, a funnel and a beaker (see Fig. 1.2a). This set-up can also be combined into an electrolytic cell for electrochemical measurements (see Fig. 1.2b) (Song and St John, 2002). The overall theoretical error of this technique is less than 10% (Song et al.; 2001). In practice, the corrosion rates of various Mg alloy specimens measured by the hydrogen evolution method have been found to be in very good agreement with those measured by weight-loss (Song et al., 2001). Furthermore, the hydrogen evolution measurement has several advantages over the trad...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Alloying and inhibition
• Part II: Surface treatment and conversion
• Part III: Coatings
• Part IV: Case studies
• Index