The cathodic behaviour of Al–Mn precipitates during atmospheric and saline aqueous corrosion of a sand-cast AM50 alloy

The corrosion behavior of magnesium-aluminum (Mg-Al) alloy AM50 produced by a rheocasting (RC) technique was examined in the presence and absence of CO 2 at three temperatures −4, 4 and 22 • C. The slurry preparation in the RC material was performed with the newly developed RheoMetal process. For reference, 99.97% Mg was included in the corrosion exposures. The influence of the microstructure on the atmospheric corrosion of alloy AM50 produced by RC and high pressure die casting (HPDC) was investigated. The RC AM50 alloy showed better corrosion resistance than HPDC AM50 in all the exposure environments studied. For both materials, there was a strong positive correlation between temperature and the atmospheric corrosion rate. The superior atmospheric corrosion behavior of RC AM50 compared to HPDC AM50 is carefully discussed in relation to differences in the as-cast microstructure. This study demonstrates that producing the alloy AM50 by this type of RC technique opens the door to Mg-Al alloys as a promising candidate for various applications where corrosion resistance is of importance. Magnesium-aluminum (Mg-Al) alloys are being employed in the automotive and many other engineering sectors owing to their high specific strength, high specific stiffness, good castability and recycleability, excellent machinability and abundant resources.1-4 Thus, they offer several advantages from the weight reduction and energy savings points of view. However, the more widespread use of MgAl alloys is limited by their low corrosion resistance and poor high temperature mechanical properties. [5][6][7][8] The corrosion properties of MgAl alloys have therefore attracted scientific attention. Currently, the majority of Mg-Al cast components are produced by conventional high-pressure die casting (HPDC). One of the main problems of thickwalled Mg-Al components made by HPDC is the relatively high fraction of porosity caused by the turbulent die filling. The pores function as local stress concentrators and can severely degrade mechanical properties and porosity also interferes with the heat-treatment of cast components. 9,10 Semi-solid casting is an alternative manufacturing technique that can be used to produce castings with a high level of complexity. In this process, a semi solid slurry is used, which shows non-turbulent or thixotropic flow behavior. Semi-solid cast alloys offer some advantages over their HPDC counterparts. For instance, porosity is lower due to laminar mould-filling and lower solidification shrinkage. 11,12Thixo-forming and rheocasting (RC) are the main semi-solid manufacturing processes. In thixo-forming, a near-net shape forming process is achieved using a partially melted, non-dendritic alloy slug. 12In contrast, RC involves preparation of a semi-solid slurry from the liquid alloy by cooling. In the RheoMetal process (also called the RSF process or the Rapid S process) cooling is performed by using an enthalphy exchange material (EEM) attached to a stirrer. 13 Thus, the metal is cooled internally which eliminates t...

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confidence: 99%

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confidence: 99%

Effect of Rheocasting on Corrosion of AM50 Mg Alloy

Esmaily

Mortazavi

Shahabi-Navid

et al. 2014

“…The observed accumulation of domes of corrosion product on the microstructural features marked with arrows was expected from areas evolving H 2 due to the alkalinity produced (Equation 2) causing the precipitation of the Mg 2+ generated at adjacent anodic sites (Equation 1). 18,50 The presence of such a dome at the β-phase location, indicated by the white arrow, suggests this cathodic location was activated during the 4 h exposure period after the recording of the map in Figure 3C (recorded after only 2 h). Two possibilities exist for the increase in active site size detected in SECM: (1) the development of a smaller tip-to-substrate distance due to protrusion of the accumulating dome of corrosion product above the high H 2 flux regions; (2) increased radial diffusion of H 2 from the cathodic site.…”

Section: Resultsmentioning

confidence: 99%

Local Hydrogen Fluxes Correlated to Microstructural Features of a Corroding Sand Cast AM50 Magnesium Alloy

Dauphin‐Ducharme

Asmussen

Tefashe

et al. 2014

Self Cite

Successful in situ spatiotemporal tracking of corrosion processes occurring at heterogeneous Mg alloy microstructures was achieved through tandem analyses involving electron and electrochemical microscopies. Through cross-correlation of scanning electron microscopy and scanning electrochemical microscopy images and subsequent analytical transmission electron microscopy, the morphology and chemical composition of microstructural components on the surface of a sand-cast AM50 Mg alloy were related to their respective local evolution of H 2 with micron scale resolution prior to, during and post corrosion. The results confirm that the preferential water reduction sites in the initial stages of corrosion are the Al 8 Mn 5 intermetallics while a β-Mg 17 Al 12 precipitate contaminated with Ni becomes cathodically active at a later stage of corrosion. This approach demonstrates the power of correlative approaches to probe and understand local electrochemical phenomena. Owing to their light weight, high strength-to-weight ratio, good castability and high damping capacity, Mg alloys are ideal candidates for automotive structural components.1-5 The compromise of formability, room temperature ductility and price represent major challenges in commercializing these materials. Importantly, Mg alloys also suffer from poor corrosion resistance in aqueous environments 6-8 and due to their positions in the electrochemical activity series, are highly susceptible to accelerated corrosion rates when in galvanic contact with a more noble material.9 At open circuit, the overall galvanic current is null due to a balance between the anodic and cathodic current densities, while on a microscopic scale, a difference in electrochemical potential is obtained. In Mg alloys, galvanic coupling between microstructural components of the alloy (commonly referred to as microgalvanic coupling) stems from an inherent electrochemical potential difference between the Mg matrix and its secondary phases, [10][11][12] and can be tracked using local scanning probe methods.Local scanning probe techniques such as several modes of scanning electrochemical microscopy (SECM), [13][14][15][16][17][18][19] scanning Kelvin probe force microscopy (SKPFM), 20-22 the scanning vibrating electrode technique (SVET) [23][24][25] and local electrochemical impedance spectroscopy (LEIS) 26 have been used to probe the heterogeneous surfaces of Mg alloys and to assess the electrochemical behavior of the different microstructural constituents responsible for microgalvanic corrosion. Among these techniques, SECM has the ability to quantify electrochemical fluxes in situ with high lateral resolution using a microelectrode (ME). 27 This technique has been successfully applied in corrosion research to provide in situ analyses of pitting corrosion at oxide films, [28][29][30] to assess lateral variations in corrosion kinetics, 31 and to detect defects within coated metal samples. 32The feedback mode of SECM has been employed to probe corroding AM60 15 and AZ31 17 Mg alloys. The incre...

“…The influence of microstructure on the atmospheric corrosion of Mg-Al alloys has also been widely studied. [22][23][24][25][26][27][28][29][30][31] Table I lists the typical ambient concentrations of sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ) and ozone (O 3 ). 32,33 O 3 and NO 2 are among the most important trace oxidants in the outdoor atmo-sphere, which often occur at higher concentration than SO 2 , see for e.g.…”

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confidence: 99%

The Influence of SO₂on the Corrosion of Mg and Mg-Al Alloys

Esmaily

Blücher

Lindström

et al. 2015

The SO 2 -induced atmospheric corrosion of some magnesium-aluminum (Mg-Al) alloys, including Mg alloy AZ91D, and commercially pure Mg (CP Mg) was investigated using well-controlled laboratory exposures and included real-time measurements of SO 2 deposition. The influence of SO 2 concentration, alloy composition, humidity, and ppb level additions of O 3 or NO 2 on the rate of SO 2 deposition was investigated. SO 2 accelerates the corrosion of Mg and Mg alloys causing localized corrosion, MgSO 3 6H 2 O being the dominant corrosion product. At 60% RH, traces of O 3 or NO 2 strongly increased both the SO 2 deposition and the corrosion rate. The rate of SO 2 deposition was strongly dependent on humidity; at 70% RH and higher the SO 2 deposition rate was very rapid and constant in time while it was transient below 50% RH. At 60% RH, a change from transient to rapid, steady-state, SO 2 deposition occurred. The sudden activation is explained by the onset of electrochemical corrosion. The activation behavior was shown to depend on SO 2 concentration, the thickness of the surface film and by the presence of ambient O 2 . © The Author Magnesium-aluminum (Mg-Al) alloys are widely used as structural materials in many engineering sectors as a result of their good castability and good mechanical properties at room temperature. Due to their low weight/strength ratio, these types of Mg alloys have found widespread applications; from portable microelectronics to automobiles and aircraft. However, Mg alloys are sensitive towards corrosion and exhibit the lowest corrosion potentials among the engineering metals.1-4 Also, Mg and Mg-Al alloys exhibit high corrosion rates when immersed in NaCl (aq) solution, corrosion being more severe compared to other engineering alloys, including carbon steel. Nevertheless, Mg alloys exhibit somewhat better corrosion properties than steels under atmospheric conditions. [5][6][7][8] The causes behind the relatively slow corrosion of Mg-Al alloys in the atmosphere are still not fully clear.Because Mg-Al alloys are often used in atmospheric conditions, a better understanding of the atmospheric corrosion properties is of importance. A large number of parameters affect the atmospheric corrosion process of Mg-Al alloys. Therefore, systematic studies are required to clarify their role on the corrosion. There have been extensive researches on the atmospheric corrosion behavior of Mg alloys using field studies that correlate corrosion rate to selected environmental parameters. [9][10][11][12][13][14][15][16][17][18][19] The variability and complexity of the outdoor atmosphere constitute a major problem when trying to correlate the corrosion rate of a metal to the concentration of, e.g., a particular trace gas in the environment. Laboratory studies under controlled conditions provide the opportunity to study the effect of a specific parameter and/or the synergic effects of atmospheric variable on the corrosion of metallic materials. For example, laboratory investigations have shown that carbon dioxide (CO 2 ...