| Due to the excellent performance in corrosion resistance,cupronickel alloys have been widely used as construction materials for seawater piping,ship hulls,heat exchangers and condensers in ships,power plants,desalination plants,etc.for numerous years.However,a serious corrosion always appears in either quiet or hydrodynamic conditions after a long exposure time.Adding corrosion inhibitors into the medium is one of the most economic and effective methods for protecting a metal against corrosion.In recent years,amino acids has been functioned as environment-friendly corrosion inhibitors for stainless steel,aluminum alloy ect.In this study,methionine(MET),an environment-friendly amino acid containing a heteroatom(sulfur atom),was selected to explore its inhibition performance for Cu-10 wt.% Ni alloy and 3.5 wt.% NaCl solution were used.The corrosion inhibition behavior of an environment-friendly amino acid-methionine for copper-nickel alloy(Cu-10 wt.% Ni)in 3.5 wt.% NaCl solution was investigated by using the electrochemical impedance spectroscopy(EIS),potentiodynamic polarization and cyclic voltammetry(CV),Also the morphologies and compositions of the surface films were investigated by scanning electron microscope(SEM)and Fourier transform infrared spectrum(FTIR),thus to study its mechanisms of action.To enhance the inhibitive efficiency,two inorganic corrosion inhibitors sodium silicate(SS)and sodium phosphate(SP)are studied to combine with the MET,The inhibition mechanism and synergy effect will be discussed in more detail at the same time.The main results are as follows:(1)MET has some inhibition effects for Cu-10 wt.% Ni alloy in 3.5 wt.% NaCl solution.The highest inhibitive efficiency achieved 62% at an additive concentration of 40 ppm.It proves that MET would function as a cathodic corrosion inhibitor.As the process of the dissolved oxygen diffusion from the bulk solution to the surface of the copper electrode was controlled,hence to control the dissolution of copper of the whole corrosion process.The SEM results of the film reveals that because of the adsorption reaction between the MET molecule with the alloy surface,A flat but also show loose quality film was formed on the surface of the alloy.The film can prevent the aggressive ions to the metal surface to some extent thus to surpress the alloy parts effectively from corrosion.(2)There is a synergetic action between the two corrosion inhibitors(MET and SS)for Cu-10 wt.% Ni alloy in 3.5 wt.% NaCl solution.Only with a total of 40 ppm of the two corrosion inhibitors,the inhibitive efficiency will achieve to 90%.It is far larger than the single corrosion inhibitor.The results demonstrate there exists synergetic effect between the compound corrosion inhibitor.The complex inhibitor functions probably as the cathodic inhibitor.It is attributed to a more compact and uniform film formed on the alloy surface as can be seen in the SEM results thus to suppress the aggressive ions to the alloy surface.The results demonstrate there is a good synergistic effect of the MET with the SS for Cu-10 wt.% Ni alloy's anti-corrosion in 3.5 wt.% NaCl solution.(3)There is a synergetic anti-corrosion action between the two corrosion inhibitors(MET and SP)for Cu-10 wt.% Ni alloy in 3.5 wt.% NaCl solution.The highest inhibitive efficiency of 98% will be achieved at the concentration of 40 ppm MET and 200 ppm SP.It is larger than the single corrosion inhibitor.The combining inhibitors function as a mixing type inhibitor,so the cathodic reaction of the oxygen diffusion is controlled and making anodic alloy in passivation state.The SEM results demonstrate that combined with the two inhibitors can form different physical shielding of the products.The advantage of the different film are fully used by adding the two different inhibitors.They complement each other nicely form a flat,densely and thick physical film on the alloy surface to prevent the metal parts effectively from corrosion,thus to achieve the synergy effect. |
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