XII Iberian Meeting of Electrochemistry XVI Meeting of the ...

XII Iberian Meeting of Electrochemistry & XVI Meeting of the Portuguese Electrochemical Society O C 07
Application of the Scanning Electrochemical Microscope to
study the corrosion mechanism in thin organic coatings with
defects
Andreia Marques, Alda Simões
ICEMS/ DEQB, Instituto Superior Técnico, Technical University of Lisbon - TULisbon, Portugal
alda.simoes@ist.utl.pt
Corrosion resistance of steel sheet is greatly increased when metallic and organic
coatings are applied together. In recent decades this concept was extended to coilcoating
or pre-painted metal sheet, which is of special interest for the automotive
industry. On the other hand, coil-coatings are difficult to weld, prone to cut-edge
corrosion and the shaping process may introduce micro-defects. Hence, the development
of coatings combining weldability and corrosion protection originated from self-healing
of damaged areas is of special interest.
Electrogalvanized steel sheet coil-coated with a 1.8µm weldable primer was choosen as
an object of study. Electrical conductivity is assured by graphite particles dispersed in
the polymeric matrix.
To understand the corrosion mechanisms in thin conductive coatings and the
mechanistic of self-repair of induced micro-defects of different sizes, samples were
tested in 0.05M NaCl solutions. The Scanning Electrochemical Microscope (SECM)
was used to study the localized corrosion kinetics of the thin protective coating at the
defect area. These studies were compared to data obtained from electrochemical
impedance spectroscopy.
The SECM ultra microelectrode (UME) was used for amperometric measurements, at a
constant cathodic potential of -0.70 V vs SCE. When the probe was located above the
defect, very low current densities were measured at the UME. At this potential, currents
correspond essentially to O 2 reduction. These results reveal local depletion of oxygen in
solution, thus corresponding to an active cathodic area. Thus, a map of surface reactivity
was obtained with good spatial resolution.
Impedance spectroscopy revealed the activity of the coating is low when compared to
metal-rich primers, probably because its conductive particles are graphite instead of
metal particles. This explains why the impedance remains practically constant with time.
Acknowledgments: The research was funded under contract RF CS-CT-2008-00028.
September, 811, 2010. ISEL - Lisbon 46