PSI - Issue 18

Mirco Peron et al. / Procedia Structural Integrity 18 (2019) 538–548 Author name / Structural Integrity Procedia 00 (2019) 000–000

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surgery are permanent metallic materials, such as stainless steel, titanium, and cobalt-chromium alloys (Q. Chen and Thouas 2015). Because of their high strength and good corrosion resistance, they have been widely used as the load bearing implants for bone healing and repair of damaged tissues (Hanawa 2010; Albrektsson et al. 1981). The key problems with these permanent implants are two-fold. Firstly, the great difference in elastic modulus of these materials compared to that of human bone results in the occurrence of the stress-shielding phenomenon. This is a consequence of stress distribution changes between the bone and the implant (Bauer and Schils 1999; Dujovne et al. 1993; Engh and Bobyn 1988): bones adapt to the reduced stress field according to the Wolff’s law (Wolff 1986), resulting in the bone either becoming more porous (internal remodelling) or thinner (external remodelling), increasing the possibility of implant failure. Secondly, due to the arise of long-term complications (Pound 2014; Jacobs, Gilbert, and Urban 1998), the permanent implant must be removed when the healing process is completed. However, the additional surgeries necessary to remove the implant cause an increase in costs to the health care system, as well as emotional stress to the patient. In order to solve these drawbacks, biodegradable materials have been studied, in particular Mg and its alloys (Peron, Torgersen, and Berto 2017; Li and Zheng 2013). Among metallic engineering materials, Mg is one of the best candidates for biomedical applications (Singh Raman, Jafari, and Harandi 2015) due to its best bio mechanical compatibility with human bones (Peron, Torgersen, and Berto 2017; Staiger et al. 2006; Hänzi, Sologubenko, and Uggowitzer 2009). Despite their highly attractive properties, Mg and its alloys have not yet been used as implants materials because of their high corrosion rates in the physiological environment, which may result in a loss of mechanical integrity and in hydrogen evolution at a rate that is too fast for the bone tissue to accommodate it. In addition, in orthopaedic applications, the implant must possess adequate resistance to failure when the corrosive human body fluid acts concurrently with the mechanical loading characteristics of the human body. Corrosion-assisted cracking phenomena, such as stress corrosion cracking (SCC) and corrosion fatigue (CF), were in fact reported to cause the failure of several traditional implants (Teoh 2000; Akahori et al. 2000; Jafari, Harandi, and Singh Raman 2015). In particular, SCC is particularly dangerous because it leads to a sudden and catastrophic fast failure under mechanical loading conditions otherwise considered to be safe, and magnesium and its alloys have been reported to be susceptible to it in simulated physiological conditions (Jafari et al. 2017; Kannan and Raman 2008; Jafari, Raman, and Davies 2018). Therefore, it is important to develop Mg-based implants granting both strength and corrosion resistance in the human body without causing corrosion-assisted cracking phenomena. However, most studies have focused on improving the electrochemical properties of Mg and its alloys, whereas the literature on improving their resistance to corrosion-assisted cracking phenomena is very limited. In fact, while different procedures have been applied in the recent years to improve their corrosion resistance, from alloying to surface modification techniques, only few of them have been assessed regarding their effects on the susceptibility to corrosion-assisted cracking phenomena. Mohajernia et al. (Mohajernia et al. 2018) reported that hydroxyapatite coating containing multi-walled carbon nanotubes reduced the corrosion current density of AZ31 alloy of three order of magnitude. In addition, they reported the elongation to failure of AZ31 samples subjected to slow strain rate tests (SSRT) in simulated body fluid (SBF) at 37 °C to be increased about 70% with the application of the coating. These results agree with those obtained by Chen et al. (L. Chen et al. 2018). They coated Mg-4Zn-0.6Zr-0.4Sr with a composite coating consisting of a poly (lactic-co-glycolic acid) (PLGA) superimposed to a micro-arc oxidation (MAO) layer and they reported this composite coating to increase the elongation to failure of the bare alloy subjected to SSRT in modified simulated body fluid. Recently, a new coating technique has been introduced. Atomic Layer Deposition (ALD) is a coating technique based on chemical vapor deposition (CVD) which prepares thin films with high conformality and offers thickness control on atomic level (Graniel et al. 2018). The resulting film is dense and pin-hole free. In fact, since the reactions are surface-reactions and not gas-reactions, ALD provides superior film uniformity (Chalker 2016). This technique has been shown to provide an efficient improvement in the corrosion resistance of Mg alloys (Marin et al. 2012). For example, Liu et al. reported that a 10 nm thick ZrO 2 coating reduced the corrosion current density of a commercial AZ31 Mg alloy from 5.1 ꞏ 10 -7 A/cm 2 to 2.7 ꞏ 10 -7 A/cm 2 (X. Liu et al. 2018). The results agreed with those obtained by Yang et al. (Yang et al. 2017b) that reported a decrease of three orders of magnitude in the corrosion current density after the application of a 40 nm thick ZrO 2 coating on a Mg-1Sr alloy. In addition they reported that a thicker coating layer provides a better corrosion resistance. However, at the best of the authors’ knowledge, the assessment of the effect of ALD coatings on the SCC susceptibility is still missing, and thus this work aim to fill this gap. In order to do this, a 100 nm thick layer has been coated on AZ31 Mg alloy cylindrical dog-bone samples and SSRTs at strain rate equal to 2.6 10 -6 s -1 have been carried out, and the samples were immersed in simulated body fluid at 37 °C for the whole duration of the tests. The same tests have been carried out also in air as reference. In addition, potentiodynamic polarization curves and hydrogen evolution experiments have been carried out to assess the corrosion