Department of Surface Modification and Nanostructures 1025 Budapest, Pusztaszeri út 59-67., HUNGARY, tel: +36-1-438-1166, fax: +36-1-438-1164 , http://www.chemres.hu
Scanning Electrochemical Microscopy (SECM) study of steel corrosion under scratched microencapsulated epoxy resin A. Pilbáth*, T. Szabó, J. Telegdi, L. Nyikos
Abstract electrochemical
Scanning electrochemical microscope (SECM (SECM)) has been applied to investigate the microscopic aspects of the reactions that
occur
on steel
under
scratched
epoxy epoxy
resin and microencapsulated
epoxy Fe2+
immersed in aqueous acidic 1 M sodium perchlorate (NaClO4) and 1 mM perchloric acid (HClO4) solution. The release of Fe
resin ionic
species into the solution phase from local anodic sites, as well as the consumption of dissolved oxygen at the corresponding cathodic locations was successfully monitored.
UreaUrea-Formaldehyde Microcapsules / Steel Samples Preparation Fig.1 SEM (A) and optical (B) micrograph of microcapsules Materials used: used: a) ShellShell-forming materials: A B urea resorcinol formaldehyde (37%) ammoniumammonium-chloride polyvinyl alcohol (PVA) - surfactant b) film forming material – linseed oil
Fig. Fig. 2 Diameter distribution of the microcapsules
Info on microcapsules: microcapsules: Tamá Tamás Szabó Szabó, szabo.tamas@ szabo.tamas@chemres.hu
The microcapsules were mixed into the epoxy resin in 4 w/w%. w/w%. The steel plates were coated with the above mentioned mixture and with the epoxy without microcapsules. The thickness of the dry coating was about 5050-60 µm. A scratch of about 1515-20 µm width was produced in the coating on the metal substrate. substrate. Fig. Fig. 3 SEM image: scratched steel surface of the microcapsulemicrocapsule-containing epoxy resin
SECM Instrumentation and Results B
Sensolytics Base SECM (Bochum(Bochum-Germany) operated with a 10 µm platinum tip as the probe, an Ag/AgCl/KCl Ag/AgCl/KCl (saturated) reference electrode and a platinum counter electrode were used. used. All potential values are referred to the Ag/AgCl/KCl Ag/AgCl/KCl (saturated) reference electrode. The corrosion of a scratched sample was studied by monitoring the oxidation of dissolved Fe2+ (Fig. Fig. 4A4A-B) and also the reduction of oxygen (Fig. Fig. 4C). The process of oxidation of Fe2+ showed that the current decreases decreases rapidly in time. time. This can be due to the precipitation of corrosion products, which block the active zones and decrease the concentration of soluble Fe2+ ions. Comparing the currents along the scratch (Fig. Fig. 4D) on a sample with microcapsules in the coating to the reference sample (no microcapsules) proves that the concentration of oxygen is higher in the the case of sample coated with microcapsules embodied epoxy which indicates a lower rate of corrosion. corrosion. Etip = 500 mV A
scratched epoxy coated steel samples samples B
C
with - (filled symbols) symbols) and without (opened symbols) symbols) microcapsules
D
Etip = -700 mV Fig.4. Fig.4. 3 D plots plots of line scans performed by SECM at different immersion time in the electrolyte solution on a scratched (20 µm) steel sample without (A,B,C),and with (D) microcapsules.
Conclusion
Immersion time: 10 min, min, 20 min, min, 30 min, 60 min, 90 min and 120 min The oxidation process (A, B) B) and the oxigen oxigen reduction process (C, D) D) is monitored.
The present study demonstrates, demonstrates, on the one hand, hand, that the SECM is a suitable technique to visualize and to investigate investigate the processes involved in the degradation of the damaged coating. coating. On the other hand, hand, the the selfself-healing effect of the microcapsules microcapsules was proven. proven. *e-mail: aranka.pilbath @chemres.hu aranka.pilbath@
This work was supported by Grant OTKA under Project Number of NK 68997.