Enhanced corrosion resistance of AZ91 magnesium alloy through refinement and homogenization of surface microstructure by friction stir processing

Electrochemical behavior of a magnesium ZK60 alloy processed by high-pressure torsion

“…It may be hypothesized that the significant increase in corrosion activity in the early stages of HPT processing originates from the variation of the imposed strain across each circular disk from zero at the center to a maximum at the periphery since this variation leads to an inhomogeneity in the refined grains [18,86]. Moreover, the inductive loop in the Nyquist spectra appears when the corrosion products are fragmented [21,[87][88][89], and the dissolved Mg generates intermediate species which remain adsorbed but unstable on the surface. The shrinking inductive loop and appearance of a diffusion impedance through corrosion product in the Nyquist spectra also confirms this proposal [55,58,60].…”

“…Plastic deformation creates many lattice defects, interstitials and vacancies [21], and in addition more grain boundaries with lower atomic densities are produced compared to within the grains [75,76]. Recent studies show nano-sized second phase precipitation from the highly supersaturated…”

“…However, high-pressure torsion (HPT) is an SPD process that has an established capability in producing significant grain refinement in a range of Mg alloys and, more important, the processing can be effectively conducted at room temperature without the development of segmentation or cracking in the base metal [17,18]. Although the effect of various SPD processes on corrosion of magnesium was investigated thoroughly [5,6,8,12,[19][20][21], nevertheless little attention was devoted to the development of electrochemical behavior in the HPT-processed Mg alloys [9,10,[22][23][24] where reports have centered mainly on noting that more refined grains lead to a more homogenous corrosion. In practice, processing by HPT permits a study of corrosion behavior over a wide range of grain sizes and grain size distributions [25,26].…”

Microstructural Characteristics, Mechanical and Corrosion Properties of an Extruded Low-Alloyed Mg-Bi-Al-Zn Alloy

“…Recently, many reports indicate that second phases that function as cathodic phases could accelerate Mg substrate to be corroded and dissolved (Feng et al, 2017), and Zeng et al pointed out that decreasing volume fraction of particles of the second phase led to improving resistance of Mg alloys to corrosion (Zeng et al, 2015). In addition, Liu et al reported that the impedance of AZ91 alloys with the presence of finer nanometer particles of Mg 17 Al 12 phase in 0.6 M NaCl solution increased from 300 to 500 Ω due to the decreased susceptibility of microgalvanic corrosion (Liu et al, 2018). In addition, a previous report (Hagihara et al, 2016) indicates the dependence of corrosion properties of Mg and its alloys upon crystal orientation, with resistance to corrosion increasing according to the order (102) < (113) < (100) < (110) < (0001).…”

Corrosion Behavior of the As-Cast and As-Solid Solution Mg-Al-Ge Alloy

“…Corrosion of magnesium alloys can be due to external factors such as environmental media, as well as internal factors such as composition and microstructure, especially the precipitated phase [ 12 , 13 , 14 , 15 ]. In regard to precipitation phases of magnesium alloy, it is generally believed that the precipitated phases can contribute to galvanic corrosion with the matrix, accelerating the corrosion of magnesium alloy.…”