The Corrosion Behaviors Of In-situ Zr-based Metallic Glass Matrix Composites In Chloride-containing Solutions

Bulk metallic glasses?BMGs?have attracted significant attentions as a promising engineering material due to their excellent mechanical properties,such as high strength,high hardness,great wear resistance and low elasticity modulus.However its application in the engineering filed was hindered by its poor room-temperature plasticity.Among the variety of proposed methods to improve the plasticity of bulk metallic glasses,the most common and effective way is to synthesize in-situ second-phase dendrites,i.e.in-situ metallic glass matrix composites?MGMCs?by dispersion strengthening.In-situ MGMCs greatly improves its plasticity whereas reducing the strength of bulk metallic glasses rather moderately.To ensure the successful implementation of this new type engineering material,the research regarding its corrosion behavior is much desired.Currently,very few research has been conducted on the corrosion behaviors and corrosion resistance of MGMCs.In this thesis,MGMCs with nominal composition of Zr_58.5Ti_14.3Nb_5.2Cu_6.1Ni_4.9Be₁₁1 is employed to study the corrosion behaviors and corrosion resistance in various aqueous solutions such as acid,alkali and salt.The electrochemical impedance spectroscopy?EIS?,potentiodynamic polarization and chemical immersion tests of Zr_58.5Ti_14.3Nb_5.2Cu_6.1Ni_4.9Be₁₁MGMCs were carried out in KCl,CaCl₂ and NaCl solutions with equal molar Cl^-concentration.The result of potentiodynamic polarization curves showed that Zr_58.5Ti_14.3Nb_5.2Cu_6.1Ni_4.9Be₁₁1 MGMCs had the lowest corrosion current density and the highest pitting potential in 0.5 M CaCl₂ solutions,indicating that the in-situ MGMCs have better corrosion resistance and pitting resistance in 0.5 M CaCl₂ solutions.Analysis suggested that the Ca²⁺of relatively small radius might penetrate the passive films preferentially hence inhibited the penetration effect of Cl^-on the passive films.The chemical immersion tests of the Zr-based MGMCs in three chloride-containing solutions showed that the corrosion rate of these samples was the lowest in NaCl solutions,whereas the highest in CaCl₂solutions suggesting that the corrosion resistance of the MGMCs was the best in NaCl solutions and the worst in CaCl₂ solutions.With the further XPS evidence,it is inferred that this observation is related to the highest ZrO₂,TiO₂ and Nb₂O₅of the Zr-based MGMCs surface in NaCl solutions.The electrochemical impedance spectroscopy?EIS?and potentiodynamic polarization curves of in-situ Zr_58.5Ti_14.3Nb_5.2Cu_6.1Ni_4.9Be₁₁1 MGMCs were obtained in acid,alkali and salt solutions.The result of potentiodynamic polarization curves showed that the corrosion resistance of Zr-based MGMCs in H₂SO₄ solutions was better than that in KOH solutions.The dynamic polarization curves of specimen were similar that in K₂SO₄ and KCl solutions,the difference only lied in the current density.It is noteworthy that the electrochemical and chemical corrosion rates of Zr-based MGMCs are almost same in 1 M KOH solutions,which indicates that a dense and stable passive films are formed on the Zr-based MGMCs surface.Then,Potentiodynamic polarization curves were tested in the mixed solutions of H₂SO₄ and KCl concentration.The results showed that the corrosion resistance of Zr-based MGMCs decreased with the increase of chloride concentration.Further XPS data suggested that on the surface of sample the decrease of Nb content,the increase of Cu and Ni content,and the little change of Zr and Ti content occurs.A detailed discussion was conducted involving a series of chemical reactions initiated and the possible mechanism of passive films formation by Cl^-.