Ⅰ.Study On The Corrosion And Diffusion Behaviour Of Cu Thin Film And The Surface Of Fe Induced By Mechanical Attrition Treatment Ⅱ.Localized Corrosion Of X-65 Steel Under The Defected High Performance Composite Coating

Part 1:Corrosion and diffusion are two important properties of materials.They are involved in many fields,including preparation,application,failure analysis and reliability evaluation of nanomaterials.Up to date,majority studies on nanomaterials have been mainly focused on the preparation of nanomaterials,characterization of nanostructure and the properties of nanomaterials including mechanical,electric, optical,magnetic and catalytic properties.However,few studies have been concerned with corrosion and diffusion properties of nanomaterials.It is of significant importance to clarify the corrosion and diffusion behaviour of nanomaterials both in a scientifical and practical way.In this paper,two different typical nanomaterials were selected to investigate the corrosion and diffusion behaviour in nanomaterials.One is nanoscale Cu thin film prepared by vacuum deposition.The structure,composition and surface morphology change of Cu thin film after oxidation were studied.Two new methods for characterizing oxidation kinetics of thin films were established by using sheet resistance measurement or optical transmittance measurement.The oxidation behaviour of Cu thin films with different thicknesses was investigated,and an abnormal accelerated transport phenomenon controlled by tunneling effect was observed.Another is surface nanocrystallized Fe prepared by surface mechanical attrition treatment(SMAT).The effect of SMAT on the electrochemical corrosion behaviour of Fe was investigated,and the results showed that the corrosion rate increased with the increasing degree of nanocrystallization.The diffusion behaviour of A1 in SMAT Fe in low temperatures was studied by secondary ion mass spectroscopy(SIMS).The relevant diffusion parameters were obtained and accelerated diffusion behaviour of A1 in nanocrystalline surface layer of Fe was observed.The main results are as follows:1.The oxidation behaviour of Cu thin films with 7～25 nm thicknesses at 140℃was investigated.Cu thin films with smooth surface morphology were prepared by vacuum deposition with controlled evaporation rate.The oxidation kinetics of Cu thin films was characterized by using the sheet resistance and optical transmittance measurements.The results showed that the oxidation behaviour of Cu films was accelerated obviously when the thickness of Cu film was ultra thin(dcu＜14 nm) and the oxidation kinetics followed inverse logarithm law.2.The oxidation kinetics of Cu thin films was also characterized by measuring the sheet resistance of Cu films during oxidation.It was found that when the film was relatively thick,the oxidation kinetics followed traditional parabolic law.Furthermore, the diffusion mechanism in Cu film during oxidation was discussed based on the value of the diffusion energy.The results showed that the method was effective for characterizing oxidation kinetics of nanoscale thin films.3.The corrosion resistances of both Fe and 316 stainless steels were decreased after surface mechanical attrition treatment(SMAT).Grain refinement was the main reason for decreasing corrosion resistance of Fe.For 316 stainless steels,besides the reason of grain refinement,the martensitic transformation during SMAT was another reason for deterioration of corrosion resistance.4.Within a temperature range of 300～380℃,the diffusivity of A1 in the nanocrystalline Fe prepared by SMAT was 6-7 orders of magnitude higher than that in coarse-grained Fe.The diffusion activation energy of A1 diffusion in nanocrystalline Fe was 1.40 eV,which was much smaller than that for the A1 diffusion inα-Fe (2.28eV).The enhanced diffusivity of A1 was originated from high density dislocations and a large volume fraction of nonequilibrium grain boundaries in nanocrystalline surface layer of Fe.5.Powder aluminizing could be achieved at a much lower temperature,i.e.,400℃,for SMAT sample.Furthermore,a much thicker(about 20μm) and more continuous aluminized layer was obtained by an optimized successive aluminizing process.The current density of the aluminized SMAT sample decreased significantly, and obvious passivation behaviour was found for the SMAT sample processed by successive aluminizing.The results suggested that lower aluminizing treatment of SMAT sample could significantly improve the corrosion resistance of SMAT sample.Part 2:Electrochemical impedance spectroscopy(EIS) technique has been used extensively to characterize the coating performance and the corrosion of steel under the coating.However,EIS measurement is associated with a major shortcoming,i.e., the measured impedance result is attributed to the electrochemical response of the whole electrode,reflecting an"averaged"behaviour of the macroscopic electrode.As a consequence,local electrochemical process occurring at micro-defect such as pinhole in the coating is"averaged"out.Therefore,analysis of EIS data is incapable of revealing the process and mechanism of localized electrochemical corrosion reaction of steel under coating with micro-scaled defects.Without such information,it will be impossible to develop a complete understanding of localized coating degradation and the resultant corrosion of the steel.Localized EIS(LEIS) measurements provide a promising alternative to investigate microscopically the coating degradation and localized corrosion of steel under coating.To date,there has been very limited work to characterize electrochemically the coating property for prevention of corrosion of steel,especially the localized corrosion process normally occurring at microdefects,such as pinholes in the coating.Therefore,in this work,LEIS measurements were performed on HPCC-coated pipe steel to investigate the localized corrosion process and mechanism of steel under the defected coating in a near-neutral pH solution.LEIS plots were measured either directly above the defect over a certain frequency range or by mapping the defected area at a single measurement frequency.For comparison,conventional EIS measurements were also conducted on the macroscopic electrode.It is anticipated that this research provides an essential insight into the mechanism of localized corrosion of steel under defected coating in near-neutral pH solution.The main results are as follows:1.The LEIS responses measured at the defected coating is dependent on the size of the defect.For small defects,e.g.,less than 200μm diameter,localized corrosion process and mechanism of steel,as indicated by the measured LEIS plots,change with time.The diffusion process dominates the interfacial corrosion reaction,which is due to the block effect of the deposited corrosion product combined with the geometrical factor of a large coating thickness/defect width ratio.In the presence of a big defect, e.g.,up to 1000μm,the LEIS responses measured at the defect are always featured by a coating impedance in the high-frequency range and an interfacial corrosion reaction in the low-frequency range.The block effect of corrosion product does not apply due to the relatively open geometry associated with the big defect.2.Conventional EIS measurements on a macroscopic-coated electrode reflect the"averaged"impedance results from both coating and defect.The information of the localized electrochemical corrosion processes and mechanisms at the small defect is lost,and the coating impedance information is"averaged"out when a big defect is contained.