Corrosion behaviour of AZ91D and AM50 magnesium alloys with Nd and Gd additions in humid environments
Highlights
► Mg alloys with additions of Nd and Gd were exposed to high humidity atmosphere. ► The increase of Nd or Gd diminished the effect of micro-galvanic couples. ► Corrosion resistance of the AM50 alloy improved with the addition of Nd or Gd by 43%. ► Nd and Gd had no significant effect on the corrosion resistance of the AZ91D alloy.
Introduction
AM and AZ series alloys are amongst the most attractive structural magnesium alloys in automotive industry and other industries concerned with weight reduction of the components due to a combination of corrosion resistance, strength, and ductility. However, magnesium alloys are susceptible to severe corrosion in chloride containing solutions [1], therefore their use is mainly limited to the atmospheric environment, where magnesium alloys has been reported to perform comparably to mild steel [2].
Corrosion of magnesium in solutions is influenced by concentration of aluminium in the alloy, its distribution in the α-Mg phase and the amount/morphology of the β-Mg17Al12 phase [1], [3]. Surface film of Mg–Al alloys enriched in Al2O3 increases the stability and the corrosion resistance of the passive film [4]. Atmospheric corrosion of magnesium alloys involves a different mechanism than the one that takes place in solutions. Water reduction is the main cathodic reaction in solutions; whereas during atmospheric corrosion thin electrolyte layers form on the surface [5], and the main cathodic reaction is oxygen reduction [6]. Further, atmospheric corrosion of magnesium alloys depends strongly on the impurities of alloys such as Fe, Cu and Ni [7], [8]. Magnesium in atmosphere forms a protective film composed of a mixture of magnesium hydroxide, carbonate and sulfate, which is much influenced by relative humidity (RH) and the temperature, due to the dissociation of water vapor which induces surface oxidation process [9]. The presence of CO2 gas (up to 350 ppm) is known to inhibit the pitting corrosion of magnesium in atmosphere (95% relative humidity) due to formation of Mg2(OH)2CO3·3H2O, whereas in the absence of CO2 localized attack, strongly related to distribution of noble inclusions, proceeds with formation of magnesium hydroxide [10], [11], [12].
The authors of the present work previously reported that corrosion behaviour of AZ31, AZ80 and AZ91D magnesium alloys is influenced by the aluminium content and alloy microstructure for relative humidity (RH) values above 90% [13], [14]. The initiation of corrosion was localized around the Al–Mn inclusions in the AZ31 alloy and at the centre of the α-Mg phase in the AZ80 and AZ91D alloys. The β-Mg17Al12 phase acted as a barrier against corrosion [13]. Fraction of magnesium carbonate on the surface was shown to increase with time whereas fraction of magnesium oxide decreased [14], however no substantial enrichment of aluminium in the corrosion products film on the AZ31 alloy was found after 30 days of testing.
Regarding the β-Mg17Al12 phase, low volume fraction and agglomerated microstructure of this phase serves as galvanic cathode and accelerates the corrosion process, whereas nearly continuous phase of a relatively high volume fraction acts as a barrier against corrosion [15].
Substantial improvement of corrosion resistance of magnesium alloys can be achieved through the addition of rare earth (RE) elements. The refinement of the microstructure and formation of RE-containing phases that are more stable than the β-Mg17Al12 phase also result in improved mechanical properties at high temperatures [16]. The beneficial effects of RE elements on the corrosion behaviour of Mg–Al alloys is determined by three main factors: (i) reduction of the effect of the impurities due to their entrapment in intermetallic compounds [17]; (ii) formation of Al-RE phases with low cathodic activity, which reduces the volume fraction of the Al–Mn intermetallics and, therefore, suppress micro-galvanic couples [18], [19], [20], [21]; (iii) refinement of the microstructure facilitating the formation of a more uniform β-Mg17Al12 phase network [22], [23], [24], [25], [26].
The present work investigates the corrosion behaviour of AM50 and AZ91D alloys with Nd or Gd additions in laboratory controlled humid environments with the aim of determination of the role of RE in atmospheric corrosion of these alloys.
Section snippets
Test alloys
The raw original AM50 and AZ91D alloys were supplied by Magnesium Elektron Ltd. The alloys were fabricated by gravity casting followed by an extrusion process. Nd and Gd (99.9%) were added to molten alloys at 740 °C protected by a MAGREX 60 slag. Cylindrical casting ingots of 400 mm length and 45 mm diameter were homogenised at 350 °C for 24 h and air cooled. The resulting ingots, rectified to 40 mm in diameter, were extruded at 350 °C with an extrusion ratio of 4:1 and extrusion rate of 0.4 mm/s.
Microstructural characterization
Optical micrographs of AM50 and AZ91D alloys reveal equiaxial grains of α-Mg phase with an average size of 6.6 and 6.0 μm, respectively, and precipitation of β-phase (Mg17Al12) (Fig. 1a and b). The presence of aluminium and manganese in both alloys results in the formation of intermetallic Al–Mn phases, such as Al10Mn3 as shown in TEM micrograph and respective electron diffraction for AM50 alloy (Fig. 2a and b). The area fraction of Al–Mn intermetallics, fAl–Mn, in these alloys is ∼0.2%. A
Discussion
All materials have demonstrated an excellent corrosion performance with no or negligible visible signs of surface degradation in atmospheres with RH of 80% and 90% at 50 °C. Notable changes and differentiation in corrosion behaviour of various alloys manifested themselves at 98% RH in the form of localized corrosion. Firstly, as expected, AZ91D alloy revealed smaller surface degradation than AM50 alloy due to higher aluminium content in its α-phase and a β-phase acting as physical barrier
Conclusions
- 1.
The addition of Nd or Gd to AM50 and AZ91D alloys has modified the microstructure of the mother alloys resulting in the formation of Al2RE and Al–Mn-RE intermetallic compounds and consequent reduction of the fraction of β-Mg17Al12 phase.
- 2.
Gd- and Nd-containing intermetallic particles with increased content of RE elements and reduced content of Mn are less noble than Al–Mn inclusions but slightly more noble than the β-phase.
- 3.
At 80, 90 and 98% RH, the AZ91D alloy revealed lower corrosion rate than
Acknowledgements
The authors are grateful to the MCYT (Spain, Project MAT 2009-09845-C02-01) and the MANOEQ of Departamento de Metalurgia Física of CENIM (CSIC) for supply of the test materials. R. Arrabal and E. Matykina are grateful to the MICINN (Spain) for financial support via the Ramon y Cajal Programme (RYC-2008-02038, RYC-2010-06749). K. Paucar is grateful to the Fundación Carolina for funding a grant.
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