Studies On Corrosion Electrochemical Behavior Of Nanocrystalline Ag-25Cu Bulk Alloy Prepared By Different Preparation Processes

Nanocystalline materials are new typed materials in the 20 th century. Because of they have some unique properties. There has attracted widespread attention in chemistry, physics, medicine and other research fields. And nancrysralline materials have been one of the widely used materials. Corrosion resistance and service life of materials are always some problems we often meet. It is very important to explore some factors that influence the corrosion behavior of nanocrystalline materials and find some ways to improve the corrosion properties of nanocrystalline materials.Two nanocrystalline bulk Ag-25Cu alloys are obtained by mechanical alloying（MA） and liquid phase reduction（LPR） methods. Compared with the corresponding coarse grained alloy obtained by powder metallurgy（PM） method, their corrosion behavior as well as the effect of preparation processes on them was investigatied by means of polarization curve, electrochemical impedance spectroscopies, the average activation energy curves, and Mott-Schottky curves in neutral Na₂SO₄ solutions, neutral NaCl solutions, acidic NaCl solutions, and H₂SO₄ solutions.When 0.02mol/L NaCl and 0.02mol/L H₂SO₄ solutions were respectively added to neutral Na₂SO₄ solutions, the corrosion rate increase significantly. Moreover, the corrosion current densities of three Ag-25Cu alloys increase and thus corrosion rates become fast with increment of NaCl and H₂SO₄ solution concentrations except LPRAg-25Cu alloy has a large corrosion current density in 0.02mol/L NaCl solution. The corrosion current densities of nanocrystalline LPRAg-25Cu alloy are evidently higher than those of coarse grained PMAg-25Cu, while the corrosion current densities of nanocrystalline MAAg-25Cu alloy are a little higher than those of coarse grained PMAg-25Cu. Electrochemical impedance spectroscopies contain a continuous capacitive arc at the high frequency stage. The trends of polar resistances are similar with those of corrosion current densities with the increment of the solution concentrations. There are no passivation phenomena for three Ag-25Cu alloys in neutral Na₂SO₄ solutions. When the concentrations of neutral NaCl and H₂SO₄ solutions are more than 0.10mol/L, there are passive phenomena and the passive current densities of PMAg-25Cu alloys is evidently higher than those of MAAg-25Cu or LPRAg-25Cu alloy. It shows that passive properties are improved with the decrease of grain size.In acidic NaCl solutions, the corrosion current densities of three Ag-25Cu alloys increasewith the increment of solution acidites. After the grain size is refined, corrosion current densities of LPRAg-25Cu alloy are evidently higher than those of PMAg-25Cu or MAAg-25Cu alloy except the corrosion current density of LPRAg-25Cu alloy is lower that of MAAg-25Cu in a NaCl solution containing 0.001mol/L H+, while MAAg-25Cu alloy is a little higher than the corrosion current densities of PMAg-25Cu alloy. Three alloys produce passive phenomena and passive current densities of MAAg-25Cu alloy are lowest among three Ag-25Cu alloys.In neutral Na₂SO₄ solutions, neutral NaCl solutions, acidic NaCl solutions, and H₂SO₄ solutions, the passive films formed by three Ag-25Cu alloys have good semiconductive properties. The refinement in the grain size is unable to change the semi-conductive types of the passive films. They are n-typed semiconductors. The densities of the carriers in passive films formed in MAAg-25Cu alloy is the lowest and therefore its passive property is best.