Corrosion is an omnipresent phenomenon in materials science and technology as well as in our everyday life. The need to fully understand corrosion processes and to search for novel coating protection to prolongate the lifetime of materials is high.
Scanning probe techniques, such as SECM can be employed to study local corrosion processes as well as the quality of coatings with sub-micrometer resolution (determined by the size of the used microelectrode). It can provide information about local reactivity in-situ and therefore identify precursor sites of pitting corrosion. The imaging of coated surfaces provides information about defects (cracks, pores, pinholes) within the coating or generally heterogeneous permeability. Also, active corrosion can be mapped by detection of released species.
Usually SECM works in amperometric mode tracking changes in current above the sample. If the concern arises that this might lead to local changes of conditions right above the investigated sample, potentiometric methods can be employed e.g. to study the local corrosion potential (open-circuit potential).
The local study of corrosion potentials can be performed by using a micropipette technique where the sample is only locally wetted by the drop forming at the microelectrode opening. The open circuit potential or corrosion potential can thus be measured between the sample (connected as working electrode) and an Ag/AgCl wire inside the micropipette (connected as quasi reference/counter electrode). Other measurements such as recording of local Tafel-plots or µ-EIS can be conducted.
Often, areas showing high electrochemical activity are precursor sites for pitting corrosion. By employing a redox mediator, such as I–/I3– the electrochemical activity of the material can be studied. With shear force SECM, simultaneous topography images are obtained directly linking high electrochemical activity to height differences and beginning of pitting corrosion.
The quality of a coating can be studied by SECM. Defects such as cracks or pores can be mapped due to regeneration of the redox mediator at spots where the metal surface is exposed.