2 - Magnesium Alloy Corrosion: Environmental Influences
2.1 Atmospheric Corrosion of Magnesium Alloys
While the influence on corrosion when Mg alloys are in bulk solutions is a major topic of
research, atmospheric corrosion of Mg alloys is of significant interest to many important areas,
such as automotive and aerospace technologies. When the interaction between a metal and the
atmospheric environment takes place, a complex process of corrosion may occur. The first step
of this corrosion is a formation of a water layer on the surface to the metal, which varies in
thickness depending on the climatic conditions, such as humidity. Due to the poor conductivity
of this thin electrolyte layer, there is a different influence on the corrosion of Mg alloys due to
the atmosphere compared to that in bulk solutions.
When exposed to ambient conditions, the formation of the aqueous layer occurs by adsorption on
the hydroxylated oxide present on the surface. The reason this layer is a prerequisite for
atmospheric corrosion is that this aqueous film formed on the surface acts as a medium for
common gaseous pollutants that affect the atmospheric corrosion of the Mg alloy. Some of these
pollutants include, sulphur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), carbon dioxide
(CO2) and nitric acid (HNO3) [1]. Other important particles in the atmosphere are sea salt and
ammonium sulphate ((NH4)2SO4) which, when dissolved into the water film, increase the
conductivity of the layer and facilitate electrochemical corrosion. Due to the many different
pollutants present in a variety of atmospheres, exposure conditions will be used to classify
different environments.
The presence of SO2 has been a historically important pollutant in urban and industrial
environments. SO2 being an atmospheric gas, is mainly emitted through the combustion of fossil
fuels, which is widely used as an energy source in many industries. Sulphur dioxide can convert
into sulphuric and sulphurous acids when in contact with moisture, which can affect atmospheric
corrosion of Mg alloys. However, because Mg alloy corrosion is still a new topic of research,
little information is known about the influence of gaseous pollutants on the atmospheric
corrosion. It has been observed that sometimes substantial amounts of magnesium sulphate are
found on Mg alloys after field exposure in polluted industrial environments [2]. Also, it was
discovered that the atmospheric corrosion of magnesium is mainly dependent on the time of
presence of the aqueous layer on the surface, rather than dependent on pollutant concentration [3,
4]. In a laboratory investigation [5] of the corrosion of magnesium effect due to SO2, it was
shown that the corrosion strongly increased with the SO2 concentration and that the deposition
rate of SO2 increased with the addition of NO2 and O3.
CO2 is another atmospheric gas that has an effect on the corrosion mechanism of magnesium. As
seen in these reactions:
Water reduction:
2H2O + 2e-  H2 + 2OHOxygen reduction:
O2 + 2H2O + 4e-  4OH-
OH- ions are formed on the surface of the metal during the corrosion process. This causes the
thin water films to often have a high pH. With the high pH in this electrolyte layer, the formation
of brucite (Mg(OH)2) is further facilitated and CO2 is attracted to a larger extent. The dissolution
of CO2 causes a decrease in pH according to [6]:
CO2(aq) + H2O  HCO3- + H+
HCO3- + OH-  CO32- + H2O
Mg(OH)2 transforms into magnesium hydroxyl carbonates due to the decreasing pH and the
presence of CO32-. Eventually the concentration of the ions in the layer will become
supersaturated, causing the precipitation of the ions. As the precipitated nuclei increases,
eventually covering the surface of the alloy, the forming of corrosion products happen.
The five major categories based on the potential corrosion rate are Rural, Urban, Industrial,
Marine and Indoor. A rural atmosphere refers to that of a rural countryside with little to no heavy
manufacturing industries present, meaning less pollutants and a generally low corrosion rate.
Urban atmospheres have more pollutants than rural, even in areas without heavy industries due
to the emissions from road traffic. These fossil fuels are responsible for oxides of nitrogen,
which can oxidise into nitric acid (HNO3), as well as sulphur dioxide (SO2). In an industrial
atmosphere, the pollutants due to fossil fuels are even greater due to the high emissions from
many manufacturing factories which in turn increase the corrosion rate. In marine atmospheres,
chloride-containing aerosols, such as NaCl, MgCl2 and other components in the sea salt, have a
large influence on corrosion rate. The chloride ions play a role in the break down of the passive
films on Mg alloys.
2.2 Stress corrosion cracking (SCC) of magnesium alloys
Another type of corrosion that can occur in magnesium alloys is stress corrosion cracking (SCC).
SCC causes a slow physical crack growth in the structure and can be extremely dangerous as
applications for Mg alloys are becoming more prevalent. When these cracks reach a critical size,
fast fracture occurs and can cause catastrophic failures of the alloy even when under safe loading
conditions. More detailed investigations in this sort of corrosion with existing and new Mg alloys
may increase because Mg parts are increasingly being used in load-bearing applications such as
engine blocks, transmission housings, engine oil pans, wheels, and structural body castings.
The stress corrosion cracking mechanism is often postulated to a process called hydrogen
embrittlement (HE). HE is caused by lone hydrogen atoms that can diffuse into the alloy. Due to
the extreme small size of the lone hydrogen atoms when compared to the metal, these atoms can
re-combine into H2 molecules in the voids of the crystal lattice and create pressure from inside
the void. This pressure amplifies the stress from the loading and can cause catastrophic fractures
at much lower loads than the yield strength of the material.
With many different compositions of Mg alloys, differences on the susceptibility of SCC can be
seen. Al in Mg alloys is vulnerable to SCC [7,8,16] in air, distilled water and chloride-
containing solutions and at stresses as low as 50% of the yield strength, SCC-induced fractures
may occur. Makar et al. [9], however, observed that (i) increasing Al from 1% to 9% increased
the repassivation rate of a protective film of Mg alloys, and (ii) Al increased the overall pH range
over which Mg alloys form a protective film. These observations explain the perceived increase
of corrosion resistance of Mg-Al alloys. Unlike Al, Zn was reported to increase the susceptibility
of SCC [10]. It was also considered that Mg-Mn alloys are immune in chloride solutions and
chloride-chromate soluons [10], but are vulnerable in distilled water and solutions containing
both chloride and sulphate ions [11]. The influence of Fe was shown to significantly increase the
corrosion rate [13,14] above the tolerance limit, however, Fairman and Bray [12] stated that Fe
contributes to general corrosion and has a small effect on SCC. Other elements such as Li, Ag,
Nd, Pb, Cu, Ni, Sn and Th have little to no influence on SCC [8,10].
Stress corrosion cracking has been witnessed in many different environments. One of these is in
the presence of gaseous H [15]. This follows that HE occurs and is responsible for increased
SCC susceptibility. SCC has been reported in damp and outdoor air atmospheres as well,
although dry air is generally considered to be inert for pure Mg [12,16]. Other than atmospheric
environments, SCC was also seen to occur in aqueous solutions containing chloride and
chromate ions. The chromate ions promote a protective film growth which inhibits corrosion, but
the chloride ions break down the film and may cause SCC. However, SCC does not occur in
chloride-chromate solutions with pH values greater than 12.
References
1. C. Leygraf, in Corrosion Mechanisms in Theory and Practice, P. Marcus Editor, p. 529,
Marcel Dekker, New York (2002)
2. I. L. Rozenfeld, Atmospheric Corrosion of Metals, p. 221, National Association of
Corrosion Engineers, Huston (1972)
3. G. K. Berukshtis and G. B. Klark, Corrosion Resistance of Metals and Metallic Coatings
in Atmospheric Conditions, Nauka, Moscow (1971)
4. Y. Mikhailovskii, A. Skurikhin, M. Czerny, R. Wellesz and M. Zaydel, Protection of
Metals, 15, 419 (1979)
5. D. Bengtsson Blucher, Carbon dioxide: the unknown factor in the atmospheric corrosion
of light metals, a laboratory study, Doctoral thesis, Department of chemical and
biological engineering, Chalmers, Goteborg (2005)
6. R. Lindstrom, J.E. Svensson and L. G. Johansson, Journal of the Electrochemical
Society, 149, B103-B107, (2002)
7. ASM Handbook, Vol 13, ‘Corrosion’ ASM International, fourth printing J.R. Davised,
Metals Park, Ohio, 1992
8. ASM Specialty Handbook: Magnesium and Magnesium Alloys, ASM International,
Metals Park Ohio, 1999, 211
9. GL Makar, J Kruger, K Sieradzki, Stress corrosion cracking of rapidly solidified
magnesium alloys, Corrosion Science, 1993, 34, 1311-1342
10. WK Miller, in Ed RH Jones, Stress Sorrosion Cracking: Materials Performance and
Evaluation, ASM International, Metals Park, Ohio, 1992, 251
11. ND Tomashov, VN Modestova, in Ed. IA Levin, Intercrystalline Corrosion and
Corrosion of Metals Under Stress, London, 1962, 251-262
12. L. Fairman, HJ Bray, Transgranular SCC in magnesium alloys, Corrosion Science, 1971,
11, 533-541
13. G. Song, A. Atrens, Corrosion mechanisms of magnesium alloys, Advanced Engineering
Materials, 1999, 1, 11
14. GL Song, A. Atrens, Understanding magnesium corrosion mechanism: a framework for
improved alloy performance, Advanced Engineering Materials, 2003, 5, 837
15. N Winzer, A Atrens, W Dietzel, VS Raja, G Song, KU Kainer, Characterisation of stress
corrosion cracking (SCC) of Mg-Al Alloys, Materials Science and Engineering A, 2008,
427, 97-106
16. RS Stampella, RPM Procter, V Ashworth, Envinronmentally induced cracking of
magnesium, Corrosion Science, 1984, 24, 325-341