The corrosion of Mg alloy Mg-10Gd-3Y-0.4Zr, in the as-cast (F), solution treated (T4) and aged (T6) conditions, was investigated
in 5% NaCl solution by immersion tests and potentiodynamic polarization measurements. The as-cast (F) condition had the highest
corrosion rate due to micro-galvanic corrosion of the α-Mg matrix by the eutectic. Solution treatment led to the lowest corrosion
rate, attributed to the absence of any second phase and a relatively compact protective surface film. Ageing at 250°C increased
the corrosion rate with increasing ageing time to 193h attributed to increasing micro-galvanic corrosion acceleration of
the Mg matrix by increasing amounts of the precipitates. Ageing for longer periods caused a decrease in the corrosion rate
attributed to some barrier effect by a nearly continuous second-phase network. Electrochemical measurements did not give accurate
evaluation of the corrosion rate in agreement with the immersion tests.
"If the volume fraction of Mg 17 Al 12 is sufficiently high, and there is a continuous Mg 17 Al 12 network, then the Mg 17 Al 12 network can act as a corrosion barrier and the corrosion rate can be lower than that of pure Mg in a typical solution like 3% NaCl. Otherwise, the Mg alloy has a corrosion rate significantly larger than that of pure Mg           . The corrosion rate of the Mg alloy is accelerated by Mg 17 Al 12 , because Mg 17 Al 12 acts as a efficient site for the cathodic reaction   , the evolution of hydrogen. "
[Show abstract][Hide abstract] ABSTRACT: A corrosion mechanism is proposed for Al3Mg2, based on electrochemical tests, XPS, and depth profiling using XPS and ToF-SIMS. After short (∼2 min) solution exposure, the surface consists of a surface film above dealloying. The dealloying is attributed to selective Mg dissolution and the surface rearrangement of Al into islands, although the metallic Al could alternatively be formed by two reduction reactions. The surface film thickness was ∼10 nm. After exposure to ultra-pure water, the composition was AlMg1.3O0.2(OH)5.1 corresponding to Al(OH)3·1.1 Mg(OH)2·0.2MgO. After exposure to 0.01 M Na2SO4, the composition was AlMg0.2O0.4(OH)2.5 corresponding to Al(OH)3·0.1Al2O3·0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH2. Three possibilities are identified for MgH2 formation. Al(OH)3 formation is attributed to a precipitation reaction. Bulk nanoporous Al3Mg2 formation is predicted to be possible by Mg dealloying of Mg17Al12.
"Magnesium alloys are gaining great interest in transport applications such as automobile construction due to their high strength–weight ratio. However, their relatively poor corrosion performance      is currently limiting their usage and there is currently significant research        to understand and improve the corrosion performance of magnesium alloys. The current model of the Mg corrosion mechanism [1–3,13–16] proposes that the important aspects are (i) a partially protective surface film, with the partial corrosion reactions occurring mainly at the metal surface at breaks in the partially protective film and (ii) the oxidation of metallic Mg to Mg 2+ occurs via the unipositive Mg ion, Mg + , and the unipositive Mg ion, Mg + can be oxidized to the Mg 2+ ion electrochemically or chemically. "
[Show abstract][Hide abstract] ABSTRACT: Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH(2) is a result of the Mg corrosion mechanism.
"All rights reserved. doi:10.1016/j.corsci.2009.03.014 it is essentially continuous and has a lower corrosion rate than the alpha-Mg matrix  ; otherwise there is the tendency for the corrosion rate to be accelerated, even for second phase particles as small as 40 nm . Intergranular stress corrosion cracking is expected for all creep resistant Mg alloys with a continuous second phase distribution along grain boundaries    ; the applied stress propagates stress corrosion cracking caused by micro-galvanic corrosion of the alpha-Mg by the adjacent continuous second phase. "
[Show abstract][Hide abstract] ABSTRACT: A first systematic investigation was carried out to understand the corrosion of common Mg alloys (Pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt.% NaCl by measuring hydrogen evolution and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3 wt.% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)2 surface film on Mg alloys is probably formed by a precipitation reaction when the Mg2+ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120–X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle. The electrochemical measurements of the corrosion rate, based on the “corrosion current” at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
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