Enhancement of stress corrosion cracking of AZ31 magnesium alloy in simulated body fluid thanks to cryogenic machining

Abstract

Magnesium and its alloys have recently attracted great attention as potential materials for the manufacture of biodegradable implants. Unfortunately, their inadequate resistance to the simultaneous action of corrosion and mechanical stresses in the human body have hampered their use as implant materials. This work aims at evaluating the Stress Corrosion Cracking (SCC) susceptibility of the AZ31 Mg alloy after being machined under cryogenic cooling. The SCC behaviour was evaluated by means of Slow Strain Rate Tests (SSRTs) in Simulated Body Fluid (SBF) at 37 °C. Prior to testing, a full characterization of the machined surface integrity, including microstructural observations, residual stress, nano-hardness measurements and surface texture analysis was carried out together with the assessment of the corrosion properties through potentiodynamic polarization curves. In addition, the morphology of the fracture surfaces after SSRTs was analysed by means of 3D optical profiler and Scanning Electron Microscopy (SEM). The improved corrosion resistance due to the increased extension of the nano-surface layer and to the compressive residual stresses represents the reason of the reduced SCC susceptibility of cryogenically machined AZ31 samples as compared to dry machined ones.

Stress corrosion cracking behavior of zirconia ALD–coated AZ31 alloy in simulated body fluid

“…The strain rate value was chosen in order to render the Mg alloy susceptible to SCC according to 41 , and the experimental set‐up is described in details in Ref. 31. In order to quantify the AZ31 SCC sensitivity, the susceptibility indices I UTS and I ɛ were calculated according to Equations () and () 42 : where UTS is the ultimate tensile strength and ε the elongation at failure, both evaluated during tests conducted in SBF and air.…”

“…The tests were carried out using cylindrical dog-bone-shaped samples, whose dimensions are reported in Ref. 31, at a strain rate of 3.5 × 10 -6 s -1 in SBF solution at body temperature (37 ± 1 C). The tests were carried out according to the standard 40 .…”

“…The strain rate value was chosen in order to render the Mg alloy susceptible to SCC according to 41 , and the experimental set-up is described in details in Ref. 31. In order to quantify the AZ31 SCC sensitivity, the susceptibility indices I UTS and I ɛ were calculated according to Equations (2) and (3) 42 :…”

“…However, literature in this regard is quite lacking. Peron et al investigated the effect of cryogenic machining on the SCC of AZ31 alloys and reported that the nanosurface layer obtained by the cryogenic machining reduced the SCC susceptibility of more than 10% 31 . More effective in the reduction of the SCC susceptibility has revealed to be the applications of coatings, with a reduction of about 80% in the SCC susceptibility provided by hydroxyapatite coating containing multiwalled carbon nanotubes 32 .…”

Clinoenstatite/Tantalum Coating for Enhancement of Biocompatibility and Corrosion Protection of Mg Alloy

“…Recently, magnesium based-alloys have received a great deal of consideration as potential biodegradable materials for implants because of their excellent mechanical characteristics and biodegradability performance [1][2][3]. It is noted that the Mg containing Ca and Zn has the necessary criteria to be employed as an orthopedic biodegradable material and Zn and Ca are capable of improving the healing rate of broken or impaired bones by creating hydroxyapatite throughout the degradation of the alloy in the system [4][5][6][7]. Having said that, the high degradation rate of Mg alloys in physiological medium lessens the mechanical characteristics of implants ahead of time and generates Mg (OH) 2 and H 2 gas [8,9].…”