Corrosion of Magnesium and its Alloys

The various modes of degradation encountered are (i) general or uniform corrosion (ii) galvanic corrosion (iii) pitting corrosion (iv) crevice corrosion (v) filiform corrosion (vi) granular corrosion (viii) stress corrosion cracking (viii) corrosion fatigue. 

The overall corrosion reaction of magnesium in aqueous solution may be written as  

AZ91 AZ91 9.5 0.5 0.3 As sand-cast 95 135 2 General purpose 

QH21 QH21 0.7 1 1 2.5 As sand cast T6 185 240 2 Pressure-tight, weldable, high-proof stress up to 300°C 

WE54 WE43 0.5 3.25d 4 T6 190 250 7C room and elevated temperature, good corrosion resistance, weldable 

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In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

Aqueous solutions induce attack which varies not only with the solute but with the volume, movement, and temperature of the liquid (Tawil, 1987). Attack by cold pure water of low conductivity is very slow. A continuous film of such water can provide protection against the atmosphere and lead to higher fatigue endurance limits than are obtained in ordinary air. Reaction with water produces a film of sparingly soluble Mg(OH)2 (Chapter XX in Emley, 1966). Dissolved oxygen does not seem to be very important in the corrosion of magnesium and its alloys in chloride solutions (Froats et al., 1987), whereas salt solutions, or even distilled water saturated with CO2 are much more corrosive (Loose, 1946). 

Humidity plays a major part in the corrosion of magnesium and its alloys (Loose, 1946). Water vapor in air increases corrosion (Froats et al., 1987). The rate of attack is negligible at low humidity, but increases considerably above ca 90% relative humidity (RH) (Whitby, 1933 Froats et al., 1987). At RH up to 90% minor corrosion results from the formation of a nearly invisible film of amorphous Mg(OH)2. As the humidity increases beyond this level, heavier tarnish films develop, and the principal corrosion product is crystalline Mg(OH)2 (Froats et al., 1987). 

U Liu, M Schlesinger, Corrosion of magnesium and its alloys. Corrosion Science,... 

Hanawalt, Nelson, Peloubet Corrosion Studies of Magnesium and its Alloys , Trans Am. Inst. Mining Met. Eng. 147, 273-299 (1942)... 

The corrosion resistance of magnesium and its alloys depends upon the him formation on the surface and the stability of the him. 

Figure 22.1. Corrosion of magnesium in 3% NaCI, alternate immersion, 16 weeks, showing tolerance limit for iron and beneficial effect of alloyed zinc and manganese [Figure 4 from J. Hanawalt, C. Nelson, and J. Peloubet, Corrosion studies of magnesium and its alloys, Trans. AIME, Inst Metals Div. 147, 281 (1942)].

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. 

Hanawalt J D, Nelson C E and Peloubet J A (1942), Corrosion studies of magnesium and its alloys . Transactions of American Society of Mining and Metallurgical... 

Inoue H, Sugahara K, Yamamoto A and Tsubakino H (2002), Corrosion rate of magnesium and its alloys in buffered chloride solutions . Corrosion Science, 44, 603-610. 

The corrosion resistance of magnesium and its alloys is highly dependent on the purity of the material. A high-purity alloy can exhibit 10-100 times higher corrosion resistance than standard purity alloy in salt solutions (Song and Atrens, 1999). However, salt solutions do not represent the physiological... 

One of the main challenges in the use of magnesium (Mg) alloys, particularly for outdoor applications, is their poor corrosion resistance. Magnesium and its alloys are extremely susceptible to galvanic corrosion, which can cause severe localized damage in the metal resulting in decreased mechanical stability and an unattractive appearance. The mechanism of corrosion that occurs on the surface of magnesium and its alloys is shown in Fig. 15.1. 

H. Ardelean, P. Marcus and C. Fiaud, Enhanced corrosion resistance of magnesium and its alloys through the formation of cerium (and aluminium) oxide surface films . Materials and Corrosion, 52, 889 (2001). 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

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