| Zn–Al alloy coating has been highly applied for the long lasting corrosion protection of steel by combining the satisfactory erosion resistance of aluminum with a great galvanic protection of zinc.However,the corrosion resistance of Zn-15 Al alloy coating will be weakened exposed to the environments with high temperature,high humidity,high salt fog and other bad conditions,and the service life of the coating will be obviously reduced.Therefore,it is of great significance to develop some new Zn-Al alloy coatings and to analyze the anticorrosion mechanism of these coating,which is of great importance to improve its corrosion resistance and service life.In this paper,Zn-15 Al and Zn-50 Al alloy coatings were prepared by arc spraying,and Zn-50Al-1Si,Zn-50Al-1Si-0.05 RE and Zn-50Al-1Si-0.05RE-0.7Mg alloy coatings were prepared by hot dip galvanizing.The corrosion resistance of different alloy layers was tested by salt spray corrosion and electrochemical measurements,and the corrosion resistance mechanism was explored in depth.The equivalent conversion coefficient was calculated according to the polarization curve and corrosion current density obtained by the electrochemical test.The results show that the surface of hot sprayed Zn-Al alloy coating was smooth and flat without any defects such as fine lines and bulging.But the Zn-50 Al alloy coating has brighter color and less surface defects compared with Zn-15 Al alloy coating.The microstructure of the two kinds of coatings was uniform and compact,and the coating was covered on the surface with lamellar structure,which can hinder the invasion of corrosive medium effectively.In salt spray atmosphere,the corrosion mechanism of Zn-15 Al is mainly the self-sealing effect of zinc corrosion products,while the corrosion mechanism of Zn-50 Al is mainly the shielding effect of aluminum oxide.With the increase of aluminum content in the coating,the polarization potential is moving forward,and the corrosion tendency decreases.Due to the addition of RE and magnesium,hot-dip Al–Zn–Si–RE–Mg coating with finer microstructure buffered the pH at the cathodic sites,hindered the formation of zinc oxide and thereby inhibited oxygen reduction,which led to an increase in the level of zinc and aluminum oxide and an enhanced improvement in its ability to resist corrosion.All the three coatings could provide a sufficient driving force to substrate for cathodic protection which is at least 300 mV of corrosion potential more negative than that of steel substrate with-0.643 V vs SCE.The polarization potential of Al–Zn–Si–RE–Mg coating was the highest one,which indicating that this coating consumed more slowly as the sacrificial anode material and developed a longer effective protection to the substrate in the corrosion process.The corrosion potential of the five kinds of alloy layer decreases with the increase of NaCl solution concentration,and the corrosion potential of hot-dip Zn-Al coating is relatively stable concentrated in the range-1.00 V to-1.02V(vs.SCE),which is less affected by the concentration of NaCl solution.According to the equivalent conversion coefficient method,the conversion relationship between five kinds of alloy layers at different concentrations of NaCl solution and water medium were obtained,which provide a basis for the compilation of accelerated environmental spectrum in laboratory. |
PDF Full Text Request