Study On Corrosion Inhibition And Protection Of Iron-contaminated Magnesium In Neutral NaCl Environment

Magnesium?Mg?and its alloys are the lightest structural metallic materials,which are characterized with high specific strength,high damping,electromagnetic shielding,and excellent casting,machinability and recyclability properties.They demonstrate significant potential in automotive,3C digital,aerospace,medical and health care as well as the field of national defense,which are known as“Green Engineering Materials of the 21st Century”.However,the standard potential of Mg is much lower than other engineering metallic materials resulting in corrosion of Mg even in ambient environment,which restricts the wide application in industry.Among the factors that affect the corrosion of Mg,transitional metallic impurities?e.g.iron,nickel and copper?can be easily coupled with Mg resulting in local galvanic corrosion of Mg and significantly increasing the corrosion rate of Mg alloys.However,the content of these elements in the magnesium matrix is usually very low?ppm,1 ppm=1?g/L?,and the evolutionary process of such impurity particles during the process of Mg corrosion can hardly be completely traced by conventional test methods.In this thesis,a series of testing and characterization techniques have been used for studying the corrosion behavior in neutral NaCl environment and corrosion prevention strategies of Fe-contaminated pure Mg.Two types of pure Mg containing different contents?54 and 342 ppm?of Fe impurity were used as substrate materials in present study.The shape and distribution density of Fe-rich particles on the surface of Mg are primarily identified using polarized light optical microscope?OM?,and the general corrosion rate of the two substrates were determined using hydrogen evolution.Simultaneously,four organic acids?i.e.fumaric acid,benzoic acid,3-methylsalicylic acid and2,5-pyridinedicarboxylic acid?were applied as corrosion inhibitors for these two substrates and their inhibition efficiency under varied concentrations were also tested through hydrogen evolution.Moreover,electrochemical impedance spectroscopy?EIS?and potentiodynamic polarization were used to study the effects of corrosion inhibitors on the corrosion kinetics and corrosion products layer on the surface of Mg.The results showed that the corrosion rate increased by around ten times higher when the content of Fe in Mg increased from 54 to 342 ppm.The corrosion behavior of both Mg substrates can be retarded by the four inhibitors,and the inhibition efficiency increases with increasing the concentration of inhibitors.A content of 0.05 M was adopted as an optimized concentration.Furthermore,the shift of open circuit potential?OCP?was highlighted in potentiodynamic polarization curves with addition of inhibitors,but the fundamentally corrosion kinetics of Fe-contaminated Mg remained unchanged.In order to explain the evolution of Fe-rich particles during the corrosion process of Mg and its impact upon the adsorption of inhibitor molecules on the surface of Mg,a combination of metallographic principles and transmission electron microscopy?TEM?were used for analyzing the identifying the composition and crystalline structure of Fe-rich particles.The presence and its indicative function of trace silicon?Si?was revealed.The variation of OCP in function of time for Mg-342Fe in 0.5 wt.%NaCl solution without and with addition of inhibitors was measured,and Volta potential difference between Fe-rich particles and Mg substrate before and after immersion in the same electrolyte systems was compared.The elemental distribution and microstructure of typical features on the corroded surfaces in-situ milled using a focused ion beam?FIB?were analyzed.Energy-dispersive X-ray spectroscopy?EDS?,X-ray diffraction?XRD?and UV-VIS-NIR spectroscopy were used for determining the composition of corrosion products and adsorption of inhibitors on Mg surface.The results found that some of Fe-rich impurities detached from Mg during the corrosion process and self-corroded forming Fe²⁺/Fe³⁺cations,which accelerated the corrosion of Mg.As characteristic corrosion inhibitors,fumaric acid and 2,5-pyridinedicarboxylic acid served as anodic or complex inhibitors preferentially impeding the oxidation of Mg within the corrosion cell,while 3-methylsalicylic was cathodic inhibitor that suppressed the re-deposition of Fe²⁺/Fe³⁺cations on Mg surface through forming Inh-Fe complex.In order to compensate the disadvantage of applying inhibitors in practice,plasma electrolytic oxidation?PEO?coatings,characterized by its porous surface microstructure,was applied as micro reservoir for storing inhibitors.The influence of electric parameters?including power frequency,duty ratio and current intensity?,treatment duration,and addition of micro/nano particles?hydroxyapatite,HAp?on the coating thickness and porosity was investigated.The results found that current intensity and treatment duration played the main role in determining the thickness and porosity of PEO coatings.The thickness and porosity increased with incre asing these parameters.On contrast,addition of HAp in electrolyte resulted in thin but compact coatings.Again,the storage capability of PEO coatings was evaluated employing UV-VIS-NIR,and it was found that increasing the thickness and porosity would benefit storing more inhibitor and achieving its slow-release.Epoxy polymer was used as sealing agent to be deposited on the surface of PEO coatings with fabricating a hybrid coating featured by active corrosion protection function.The influence of critical dip-coating parameters?including dipping repetition and delay duration with epoxy solution?on the thickness and surface microstructure was investigated.EIS,AC/DC/AC,salt spray and localized electrochemistry techniques were employed in studying the corrosion performance and active corrosion protection ability of the hybrid coating system.The results found that the liquid epoxy resin can penetrate into the PEO coatings through the pores and discharge channels in PEO coating during the dip-coating process.Increasing the repetition and/or the delay duration in epoxy solution favor the reduction of open pores on the surface of epoxy layer but reveal minor impact on the its thickness.As a result,a tough hybrid coating was fabricated,which survived from monthly immersion in 3.5 wt.%NaCl solution and 2000 h salt spray?5 wt.%NaCl?test.As for the mechanical properties,pull-off test were employed to study the bonding strength of epoxy layer on the pre-treated Mg surface,whereas the dry sliding wear performance and wear out mechanism of the hybrid coatings were evaluated by ball-on-disk test and subsequent characterization.It was revealed that the wear performance of PEO coatings was significantly strengthened after the introduction of the epoxy layer.The least wear consumption of hybrid coatings and the attack of the counter steel ball can be realized after the optimization of the dip-coating parameters.As a conclusion,the developed hybrid epoxy-PEO system provided a comprehensive protection strategy of Mg and its alloys against corrosion and wear attack,which meet the requirement of the general industrial fields.