Corrosion Mechanism of Mg and Mg Alloys

Ming Liu (2010). Corrosion Mechanism of Mg and Mg Alloys PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

       
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Author Ming Liu
Thesis Title Corrosion Mechanism of Mg and Mg Alloys
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2010-07
Thesis type PhD Thesis
Supervisor Professor Andrej Atrens
Dr Guangling Song
Dr Patrik Schmutz
Total pages 113
Total colour pages 23
Total black and white pages 90
Subjects 09 Engineering
Abstract/Summary Magnesium alloys are very attractive due to their good strength/weight ratio, but magnesium usage has been limited largely due to the poor corrosion properties of magnesium alloys. So understanding their corrosion behaviour is an essential step towards improving their corrosion performance. In this doctoral dissertation, an effort was made to understand the following issues regarding the corrosion mechanisms and behaviour of pure magnesium and magnesium alloys: (1) The effect of crystallographic orientation on the active corrosion of pure magnesium; (2) The surface films formed on pure magnesium after immersion; (3) The surface films formed on Mg-Al intermetallics after immersion; (4) The impurity tolerance limit for corrosion of pure magnesium and magnesium alloy; (5) The corrosion of Mg-Y binary alloys. A range of advanced techniques were employed such as scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD), X-ray photoelectron spectroscopy (XPS), Time-of-Flight-Secondary Ion Mass Spectrometry (ToF-SIMS), optical microscopy, in-situ optical microscopy, electrochemical polarization, electrochemical impedance spectroscopy (EIS), gas collection of hydrogen evolution from corroding samples and calculated phase diagrams. For pure magnesium after active corrosion in 0.1 N HCl, it was found that grains near the (0001) orientation, which has the highest atomic density, are the most corrosion resistant compared with the grains near the (112􀴤0) or (011􀴤 0) orientations. Most grains exhibited a striated structure of long and narrow hillocks along the direction of {0001} and {202􀴤1} plane, which serve as a terrace plane in a hcp metal. For pure magnesium after short immersion times, the initial surface film has a duplex layer structure with an inner MgO layer next to the Mg metal and an external porous layer of hydroxide. The MgO layer is about 3 nm thick while the thickness of Mg(OH)2 outer layer is 3 to 5 nm with increasing immersion times. For Mg-Al intermetallics, Mg17Al12 and Al3Mg2 after a short period of immersion in water, their surface film is about 10 nm thick and is composed of Al and Mg hydroxide. For Al3Mg2, dealloying of Mg was found at the interfacial region with lateral rearrangement of Al atoms to form islands. Inside the surface film, a thin layer of MgH2 was also found covering the metallic Al islands. The calculated phase diagrams provide a metallurgical explanation for the tolerance limit of Fe in pure magnesium and the influence of Al and Mn content on the tolerance limit of Fe in magnesium alloys. A production technique for high-purity castings by means of control of pouring temperature was also proposed based on the current understanding. The corrosion behaviour of a series of Mg-Y binary alloys in chloride and sulphate solutions was studied. Although Mg-Y intermetallic can cause micro galvanic corrosion, yttrium incorporated in the surface film can improve the corrosion resistance. So yttrium can have a dual effect on the corrosion of Mg alloy. The overall effect is dependent on the electrolyte. In NaCl solutions, chloride ions can penetrate the surface film and cause active micro-galvanic corrosion. So the corrosion rate of two phase Mg-Y alloys increased with increasing Y content due to the increasing amount of Mg-Y intermetallic. In Na2SO4 solution, the protective nature of the surface film controls the corrosion behaviour, so the corrosion rate of two phase Mg-Y alloys decreased with increasing Y content due to the increasing amount of Y incorporated in the surface film. In present work, a mechanism is proposed for filiform corrosion on Mg alloys.
Keyword Corrosion mechanism
Magnesium
Magnesium Alloys
Intermetallics
EBSD
XPS
calculated phase diagram
crystallographic orientation
surface film
impurity tolerance limit
Additional Notes page numbers that should be printed in colour: 38-39, 43-45, 53, 55-56, 58, 71, 73, 80-81, 88-94, 96-97, 107

 
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Created: Thu, 18 Nov 2010, 17:34:32 EST by Mr Ming Liu on behalf of Library - Information Access Service