Development And Application Of High Throughput Micro-electrochemical Characterization Technique For Corrosion Study

The composition-structure-property of metallic materials is the core issue of corrosion research,but conventional electrochemical test methods and devices are difficult to meet the requirements for characterizing the electrochemical property of metal on a local microscale.To acquire the electrochemical property of matrix phase,inclusion and other special microstructure is helpful for in-depth understanding the micro-galvanic corrosion behavior of metal,especially for the local corrosion,such as pitting,and intergranular corrosion.It not only estimates the corrosion resistance of different phases by their polarization behaviors,but also provide the input parameters for numerical simulation of complex micro-galvanic corrosion.Then,connection between the micro-electrochemical property and the macroscopic corrosion behavior of metal will be established.In order to perform the electrochemical tests at micro scale,a test platform has been successfully developed in combination with lithography and capillary technologies.It composed of a high-throughput sample library(a lithographic mask preparation system),an automatic microinfusion pump,a high-recision motorized control platform,a microscope,an electrochemical test system,and a control system.Compared with conventional microcapillary test systems,this platform exhibited the clear advantages,including accurate control of the working electrode,lower ohmic resistance,lower risk of leakage or blocking,lower probability of crevice corrosion,less influence of compositional variation near the tip,suitable for low conductivity solution system.Importantly,this test platform is more suitable for high throughput microelectrochemical characterizations.In addition,the test platform was employed to investigate the corrosion behavior of a SA508-309L/308L welded joint in 3.5wt%NaCl solution,and the effect of microstructure of duplex stainless steel 2205 on its corrosion property was deliberated based on the high throughput EBSD data and microelectrochemical data.During the research,experiments and analysis methods such as optical microscope(OM),scanning electron microscope(SEM),energy spectrometer(EDS),electron backscatter diffraction(EBSD),and scanning Kelvin probe force microscope(SKPFM)were applied.Characterization on composition and microstructure of the weldment shows that:the ratio of Creq/Nieq decreases gradually when close to the fusion line,making the solidification mode changes from FA to AF in stainless steel.It leads to the morphology changes from lathy,skeletal to vermicular and island with decreasing ferrite content,and forms complex microstructure composed of ferrite,austenite,inclusion,carbide and a phase.In low alloy steel,the microstructure is determined by the thermal cycle history,which is subdivided into five regions:coarse-grain heat affected zone(C1,composed of coarse-grained ferrite),fine-grain heat affected zone(C2 and C3,composed of mixed bainite),intercritical heat affected zone(C4,composed of coarse ferrite and mixed bainite)and subcritical heat affected zone(C5,composed of tempering bainite).The inclusions on the stainless steel side are mainly oxides containing silicon,titanium and aluminum.There are three main types of inclusions in the low alloy steel:sulfide,oxide and composite inclusions,which are diverse in form,variety and composition.The 309L-SA508 fusion boundary was characterized by multi-analytical approaches.The fusion line region is composed of austenite(A),martensite(M)and coarse ferrite(CF),and the M zone is subdivided into three regions:A/M transition zone,M zone and M/CF transition zone.The corrosion resistance of martensite is better than that of coarse ferrite due to its high content of low-activity?3 coincidence site lattice(CSL)grain boundary and the higher content of Cr and Ni,while its surface potential measured by SKPFM is lower than that of coarse ferrite.The inconsistency between the surface potential and the corrosion tendency of the electrochemical property of martensite is that the metal/medium interface conditions of the two measurement processes are different.The residual strain of the former plays a major role,and the latter is related to the composition and structure of the metal/solution interface which will change in the solution.The microelectrochemical properties of SA508-309L/308L welds were studied.It was found that the welds exhibited 5 kinds of electrochemical impedance responses,8 kinds of potentiodynamic polarization curves and 6 orders of magnitude of polarization resistance values in 3.5wt%NaCl solution.At the same time,the corrosion process with high throughput information of corrosion morphology variation of the weldment was recorded by OM.It shows that inclusion was the main cause of pitting corrosion,and the lateral propagation did not occur in the pits.At the initial stage of immersion test,pitting was mainly triggered in the fine-grained heat-affected zone(C2).At the later stage of immersion,uniform corrosion occurs gradually from the heat affected zone near the tempering zone(C5)to the fusion line,accompanied by pitting corrosion.The substrate around the pitting is protected by corrosion products,which changes the development of corrosion during the immersion process.Combined with the microelectrochemical properties and immersion test results,it reveals a corrosion tendency of low alloy steel is as follows:coarse-grained heat affected zone(C1)>fine-grained heat affected zonel(C2)>fine-grained heat affected zone 2(C3)>incomplete recrystallization zone(C4)>tempering zone(C5).The open-circuit potential and corrosion potential of C2,C3 and C4 regions(including inclusions)are lower than those of the C1,showing a higher corrosion tendency.Statistical analysis shows that the corrosion current density on the side of low alloy steel shows the following sequences:C2>C3>C5?C4>C1.In the heat affected zone,there are some test areas showing a high corrosion rate,which is related to the active dissolution caused by inclusions.At the initial stage of immersion,the corrosion behavior of welded joints is mainly pitting corrosion in the C2.At the later stage of immersion,uniform corrosion gradually occurs from the C4 to the fusion line area,accompanied by pitting,showing a corrosion resistance as following:C5>C4>C3>C2.Due to the protection of corrosion products,C1 and the fusion line area do not begain to dissolve until the late immersion period.The corrosion rate observed in the immersion test is highly consistent with the statistical results of the local electrochemical property at micro scale.It can be inferred that the inclusions in the low alloy steel play an important role in the corrosion behavior of the whole welded joint.The microstructure of DSS 2205 was characterized by high-throughput EBSD technique.Combining with the results of micro-electrochemical high-throughput characterization,the influence of microstructure on the corrosion resistance of DSS 2205 was discussed.Statistically,the average corrosion resistance of austenite is better than that of ferrite.Due to the galvanic effect of the two phases,the corrosion resistance of the dual phase is better than that of single one.The corrosion resistance of austenite and ferrite single grains generally follows the sequences of decreasing gradually from low grain plane index to high grain plane index.The analysis shows that different electrochemical activities of grains with different orientations are similar to the galvanic effect between the two phases,and the corrosion resistance of coupling effect on sveral grains with same phase structure is better than that of single grains.Residual strain has limited effect on the properties of oxide film on the surface of austenite and ferrite,which makes the open circuit potential increase slightly.The effect of residual strain on the corrosion behavior of austenite is greater than that of ferrite.