Corrosion Resistance And Pitting Initiationmechanism Of Fe-based Bulk Metallic Glasswith Sulfide Inclusions

In this paper,the 2 millimetre diameter Fe-based bulk cast rods with sulfide inclusions were prepared by using industrial raw materials and methods of arc melting and copper casting.The structure of the as-cast alloy was characterized by X-ray diffraction?XRD?.The thermal stability was tested by differential scanning calorimetry?DSC?.The structure of the amorphous cast rods and the composition of the sulfide inclusions were confirmed by scanning electron microscopy?SEM?and energy dispersive spectrometer?EDS?.In addition,potentiodynamic polarization curves,potentiostatic polarization curves and electrochemical impedance spectroscopy?EIS?were used to study the corrosion behavior of the amorphous cast rods and the pitting initiation mechanism induced by the sulfide inclusions.The results of potentiodynamic polarization curves showed that the potential polarization curves of the amorphous cast rods in H₂SO₄,HCl,NaOH and NaCl solutions at different concentrations and temperatures showed a wide passive zone,besides,there was a significant second passive zone in NaOH solution.With the increase of the solution concentration,the corrosion rate increased in H₂SO₄ and NaCl solution at first,and then decreased to a critical value.However,in HCl and NaOH solution,the corrosion rate of the amorphous casting rods increased gradually.In addition,with the increase of the solution temperature,the corrosion rate of the amorphous casting rods in the above four solutions gradually increased.Moreover,no sharply increasing current appeared in the passive zone of the anode polarization.The results of potentiostatic polarization curves showed that stable passive film formed on the corrosion surface of the amorphous casting rods in H₂SO₄ and HCl solutions with different concentrations and different temperatures,and no sharply increasing current peaks appeared.However,when tested in 0.1mol/L NaOH and0.1mol/L,4mol/L and 60? NaCl solutions,the sharply increasing current peaks emerged in the potentiostatic polarization curves,which indicated the initiating of pitting corrosion.The results of electrochemical impedance spectroscopy showed that all of the Nyquist curves were composed of just a single capacitive arc,and the electrode reaction was mainly controlled by the electrode potential.The results showed that with the extension of immersion time,the amorphous cast rods have excellent corrosion resistance in 0.5mol/L H₂SO₄,HCl,NaOH and NaCl solutions,the corrosion rate of the amorphous cast rods increased first and then tended to be stable.The relationship between the corrosion rate in solutions was as follows: the corrosion rate in H₂SO₄ solution was the largest,followed by HCl,and it was relatively low in NaOH and NaCl solution,in addition,that of the two was relatively close.Further observation of the SEM morphology by immersion corrosion tests showed that the sulfide inclusions did not dissolve obviously.A large number of cracks were produced on the surface of the amorphous casting rods after immersed in H₂SO₄ and HCl solutions for one week,while the corrosion surface had no obvious change except for a little corrosion products after immersed in NaCl and NaOH solutions.Pitting in NaCl solution is caused by the multiple effects of sulfide inclusions,erosive Cl-and dissolved oxygen.