| Zinc is a versatile metal material that ranks fourth in terms of the world metallic production and consumption.Atmospheric corrosion is a common corrosion type for zinc and numerous works have been done to investigate its atmospheric corrosion behaviors and mechanisms.However,atmospheric corrosion is a very complicated corrosion process and there are still many corrosion mechanisms remaining unclear.The tropical marine atmosphere is recognized as an aggressive environment to metals because of its high relative humidity,high temperature and high salt deposition rate conditions.Zn5?OH?8Cl2·H2O is the main corrosion product of zinc under marine atmosphere and is protective to the zinc substrate.However,its evolution process and effect on the corrosion process in the marine atmosphere are still needed to be clarified.NaCl and SO2 are two well-known stimulators of the zinc atmospheric corrosion and their synergistic effect on zinc has been demonstrated.However,further work needs to be done to study the effect of their deposition rate ratio on the zinc atmospheric corrosion.MgCl2 is a main factor affecting the metallic corrosion in Salt Lake Environment in Western China.There have been some researches concerning the atmospheric corrosion behavior of zinc in this special environment,and the effect of MgCl2 on zinc corrosion behavior was observed.But the corrosion mechanism of MgCl2 on the zinc corrosion process was not discussed in details.Electric field is a main factor affecting the metallic atmospheric corrosion under electric environments.It is vital to gain a good knowledge of zinc atmospheric corrosion behaviors under electric field to ensure the security of the regular power grid working.Unfortunately,few investigations have been carried out to study the effect of electrical field on the zinc atmospheric corrosion.In this work,the atmospheric corrosion mechanism of zinc regarding the four main factors proposed above were further studied in details.The initial corrosion behavior of pure zinc in a simulated tropical marine atmosphere was investigated by gravimetric method,X-ray diffraction?XRD?,Fourier transform infrared spectrometer?FTIR?,scanning electron microscope combined with energy dispersive spectroscopy?SEM-EDS?and electrochemical impedance spectroscopy?EIS?.The results showed that the kinetics of corrosion process was a decelerating process following the typical empirical equation D=Atn.The corrosion rate of zinc was significantly influenced by the deposition rate of NaCl.Their relationship could be described by a power function D=BxC+D0.The main corrosion products detected in this study were Zn5?OH?8Cl2 H2O,Zn?OH?2,Zn5?CO3?2?OH?6 and ZnO.The structure and protectiveness of the corrosion product layer were examined.It suggested that the protectiveness of the corrosion product layer formed on zinc could mainly be attributed to the compact structure composed of Zn5?OH?8Cl2 H2O,which could inhibit the charge transfer process during the corrosion period.Corrosion of zinc in the simulated tropical marine atmosphere was pitting along some grain boundaries and crystal plains.The synergistic effect of NaCl and SO2 toward the atmospheric corrosion of zinc was investigated with different molar deposition rate ratios by using gravimetric method,XRD,FTIR and SEM-EDS.The results showed that the corrosion rate of zinc was dramatically influenced by the molar deposition rate ratio of NaCl and SO2 under wet-dry conditions.The thickness loss of zinc increased in the order of SO2<1:3<3:1<NaCl<1:1.The synergistic effect of NaCl and SO2 towards zinc was related to their molar deposition rate as well:when the molar deposition rate ratios were not equivalent?3:1 and 1:3?,the corrosion rate of zinc was synergistic decelerated by the two media;when the molar deposition rate ratio was equivalent?1:1?,the corrosion rate of zinc was synergistic accelerated by the two media.The composition of corrosion products on zinc samples varied with the molar deposition rate ratios of NaCl and SO2.NaZn4SO4?OH?6Cl·6H2O formed when both NaCl and SO2 were in presence.Zn5?OH?8Cl2·H2O was generated when the molar deposition rate of NaCl was higher than SO2 while soluble corrosion products,ZnSO4·H2O and Na2Zn?SO4?2 4H2O formed when the deposition of NaCl was lower than SO2.The corrosion form was also affected by the molar deposition rate ratios of NaCl and SO2:the corrosion is pocking corrosion when the molar deposition rate of NaCl and SO2 were not equivalent?NaCl,3:1,1:1 and SO2?while the corrosion is patchy corrosion when the molar deposition rate of NaCl and SO2 were equivalent?1:1?.The effect of MgCl2 on the atmospheric corrosion of zinc under wet-dry conditions was studied by using gravimetric method,XRD and SEM-EDS.The gravimetric results showed that the corrosion rate was significantly decelerated by MgCl2.The XRD results indicated that the corrosion products formed on zinc samples were Zn5?OH?8Cl2·H2O?Zn4CO3?OH?6 H2O and Zn?OH?2 in the presence of NaCl while was only Zn5?OH?sCl2·H2O in the presence of MgCl2.The inhibition effect of MgCl2 on the zinc atmospheric corrosion could be explained by the decrease of pH induced by MgCl2,which could promote the formation of protective corrosion product Zn5?OH?8Cl2·H2O.The effect of direct current electric field?DCEF?on the initial atmospheric corrosion of zinc in the presence of NaCl salt particles were investigated by using gravimetric method,XRD,FTIR,ion chromatography?IC?and SEM-EDS.The results showed that the corrosion behaviors were obviously influenced by the direction of DCEF.When the direction of the DCEF was vertical down to the zinc sample surface,the corrosion rate of zinc was accelerated slightly while the corrosion was more uniform;the corrosion products detected were Zn5?OH?8Cl2·H2O,Zn4?OH?6CO3·H2O,Zn?OH?2 and NaZn?OH?3;protuberances with chloride distributed inside and connected by Cl-poor compounds formed on the zinc sample surface.In contrast,when the direction of the DCEF was vertical up to the zinc sample surface,the corrosion rate of zinc was decelerated significantly,however,the corrosion was more local and pitting corrosion was more severe;the corrosion products detected were only Zn5?OH?sCl2 H2O and Zn4?OH?6CO3 H2O;notably,isolated protuberances with chloride distributed in uniformly formed on the zinc sample surface.More attentions need to be focused on the corrosion rate and corrosion form under DCEF with different directions. |
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