Corrosion and stress corrosion cracking of magnesium alloys

Abstract

Abstract Magnesium (Mg) alloys are attractive for light-weight applications such as in the aerospace and automobile industries, due to their high strength-to-weight ratio. The widespread application of Mg alloys in automobiles can decrease fuel consumption through light-weighting, which benefits our environment. Mg alloys are also regarded as promising biodegradable implants for use in the human body. However, the poor resistance of corrosion and stress corrosion cracking (SCC) limits their more wide-spread application in both industry and medical application. It is therefore necessary to better understand the mechanisms and the important factors, which control Mg corrosion and SCC, and to find better ways to improve their corrosion and SCC performance. In this doctoral dissertation, an effort was made to understanding the following issues regarding the corrosion and SCC mechanisms, and behaviour of pure magnesium and magnesium alloys: (1) The corrosion behaviour of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2 (2) The corrosion behaviour of as-cast and solution-heat-treated binary Mg-X alloys in salt spray and 3.5% NaCl solution saturated with Mg(OH)2 (3) Influence of hot rolling on the corrosion behaviour of several Mg-X alloys (4) The influence of casting porosity on the corrosion behaviour of Mg0.1Si (5) Stress corrosion cracking of several solution heat-treated Mg-X alloys (6) Stress corrosion cracking of several hot-rolled Mg-X alloys A range of advanced techniques were employed such as optical microscopy (OM), scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electrochemical polarization, electrochemical impedance spectroscopy (EIS), gas collection of hydrogen evolution from corroding samples, and linearly increasing stress testing (LIST). For the ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2, the intrinsic corrosion rate measured with weight loss, PW = 0.25 ± 0.07 mm y-1. The average corrosion rate measured from hydrogen evolution, PAH, was lower than that measured with weight loss, PW, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism was suggested. For solution-heat-treated Mg–X alloys (X = Mn, Sn, Ca, Zn, Al, Zr, Si, Sr), corrosion rates did not meet the expectation that they should be equal to or lower than those of high-purity Mg. There was circumstantial evidence that the higher corrosion rates were caused by the particles in the microstructure; the second phases had been dissolved. For the hot-rolled Mg–X alloys (X = Gd, Ca, Al, Mn, Sn, Sr, Nd, La, Ce, Zr or Si) in 3.5% NaCl solution saturated with Mg(OH)2, the corrosion rate for all Mg–X alloys (except Mg0.1Zr and Mg0.3Si) decreased after hot rolling, attributed to fine-grained alloys having a more homogeneous microstructure, and fewer, smaller second-phase particles. For Mg0.1Zr and Mg0.3Si, the corrosion rate increased after hot rolling. There were a number of possible reasons, one of which was a greater sensitivity to the precipitation of deleterious Fe-rich particles. The influence of casting porosity on the corrosion behaviour of Mg0.1Si was studied. Specimens with porosity had higher corrosion rates attributed to the corrosion associated with the pores activating significant corrosion over the whole specimen surface, wherein important aspects were (i) the breakdown of a partly protective surface film, and (ii) micro-galvanic acceleration of the corrosion by Fe-rich particles. For SCC behaviour of solution-heat-treated Mg0.1Zr, Mg1Mn, Mg0.1Sr, Mg0.3Si, Mg5Sn, Mg5Zn and Mg0.3Ca in distilled water (DW), SCC susceptibility was related to the stress rate for all the Mg-X alloys except for Mg0.1Sr. In DW, Mg5Zn and Mg0.3Ca suffered the most serious trans-granular SCC. Some specimens tested in DW preferred to crack at solute atoms, second phase particles, grain boundaries and defects, attributed to H trapping. It was confirmed that hydrogen atom played a significant role in the SCC behaviour of Mg alloys. For SCC behaviour of hot rolled Mg0.1Zr, Mg0.1Sr, Mg1Mn, Mg0.3Si, Mg5Sn, Mg0.7La, Mg0.9Ce, Mg0.6Nd, Mg6Al, Mg5Gd and Mg0.3Ca, all the alloys (except for Mg1Mn and Mg0.7La) had good SCC resistance in DW, and the hot-rolled Mg1Mn and Mg0.7La had acceptable SCC resistance. The increase of SCC resistance by hot rolling was related to improvement of the microstructure. There was no obvious difference of the fractography between the specimens tested in air and in DW for each hot rolled Mg-X. They all cracked through trans-granular cracking, presenting either smooth trans-granular feature or rough trans-granular feature, as a combination result of tensile stress and shear stress.