Novel Electrochemical Techniques With Time/Spatial Resolution For Corrosion Investigations

Traditional electrochemical techniques have become more and more insufficientfor deeply studying complicated corrosion systems. Various novel electrochemicaltechniques with time/spatial resolutions have aroused a great deal of attention in thepast decades, such as scanning probes microscope (SPM), scanning electrochemicalprobes, electron probe micro-analyzer (EPMA), spectroscopic image, electrochemicalnoise (ECN) and some combined electrochemical techniques.In this paper, we made our focus on two main components: (1) applicationvarious the state of the art electrochemical techniques with time/spatial resolution,including in-situ optical microscope, in-situ AFM, EPMA, ECN and scanning Kelvinprobe force microscope (SKPFM), in investigations of corrosion of stainless steel indifferent environments; (2) home-establishment of a hybrid system of STM and SRETwhich is able to map both potential distribution in micro scale and atomic or nanotopographies of metals in aqueous solutions, and its applications in corrosion studies.The main results and progresses of this work are outlined as following:1.The morphological varieties of stainless steel during corrosion processes indifferent media have been in situ observed through an optical microscope with afield depth at least 20 times greater than the traditional optical microscopes. TheDifference Viewer Imaging Technique (DVIT) has been successfully achieved byusing the Photoshop software to deal with optical images which are collected inthe corrosion process. Accordingly, it is found that, there are three corrosion stageswhen the 18-8 stainless steel is immersed in HCl solution, including initiation,development and stable propagation respectively. These stages are wellcharacterized by the different dominant corrosion events. The local breakdown of passive film and metastable pitting nucleus occur during the initial stage. Somemetastable pitting continues and someone repassivate in the development. Finallyin the stage of stable pitting, some active pitting sites keep growth and form macropitting.2. In-situ AFM has been employed to investigate the initiation corrosion of 18-8stainless steel in HCl solution. It is found that an oxide film dissolves firstly andthen metastable pitting occurs at preferred sites. By using EPMA, the boundarysites of TiN intermetallic particle are recognized the corrosion susceptive sites.From results of SKPFM, it is concluded that a distinct difference of Volta potentialexist between the bulk matrix and the boundary sites of TiN particles in 18-8stainless steel, which is possible to result in micro galvanic corrosion.3. The ECN technique has been applied to the investigation of corrosion initiation inan early stage of 18-8 SS immersed in different solution, and the electrochemicalcurrent noise data has been analyzed using discrete wavelet transform (DWT).The ECN data have been decomposed according to the DWT to different waveletcoefficients (d₁, d₂,…,d₈) with corresponding time scales, which are associatedwith different electrochemical transient events. For the first time, the origin ofwavelet coefficients is discussed based on the correlation between the evolution ofthe energy distribution plot(EDP)of wavelet coefficients and topographic changesfrom OM, EPMA and AFM.4. A home-made hybrid instrument of SMET/STM has been successfully establishedin lab, and it has been applied to the studies of localized corrosion of 18-8 SS inFeCl₃ solution. A series scanning probes have also been developed, involving thePt metallic potential probe, Cl^- selective microprobe, pH selective microprobe andintegrated potential/topographic PtIr microprobe. Based on the accuratelycontrollable distance of sample-probe by STM function, the spatial resolution ofSMET on imaging for corrosion potential, Cl^- concentration and pH at interface ofmatel/solution is remarkably improved. The novel instrument has been employed,for the fist time, to detect the potential distribution in vertical range to electrodes, it is found that there is obviously different distribution on pitting site and onnonactive site. It is implied that the hybrid instrument is possible to image theinterfacial potential distribution for the corrosion system. Using the hybridinstrument, the integrated topography-activity images of 18-8 SS in FeCl₃ solutionhave been successfully recorded, which demonstrates that the hybrid instrument ofSMET/STM is able to become a powerful technique for in situ correlation studyon the surface structure and the electrochemical activity.