Semiconducting Behavior of Epoxy Resin/Carbon Steel in Sulfuric Acid Solution

Abstract

Semiconductor behavior of epoxy resin coated on carbon steel immersed in 0.5 mol·L-1 sulfuric acid aqueous solution during its degradation was investigated. Potential (U)-capacitance (C) measurement and Mott-Schottky analysis technology were utilized to understand the coat′s conduction mechanism during its degradation in the electrolyte. The epoxy resin film was still an insulator in the initial immersion stage (10 min), but its outer layer (the side that contacts with solution) gradually changed into n type semiconductor due to the corrosion with growing immersion time. Charge carrier density of this semiconductor film increased by degrees with extension of immersion time from 1010 cm-3 at 7 h to 1012 cm-3 at 48 h approximately. Between 7 h and 48 h, the organic film had not entirely transformed into semiconductor but only the outer side, and the inner was still insulator. Therefore, an MIS structure (metal-insulator-semiconductor) was generated. The result indicated that this MIS structure was in inversion state between -0.5 V and 0.5 V, and carrier charges were electron holes in inversion layer. The measured space-charge capacitance was series capacitance of inversion layer capacitance and depletion layer capacitance at relatively low frequency. The capacitance value decreased with growing polarization potential. The measured capacitance at relatively high frequency was only the depletion layer capacitance and remained constant in the total scanning potential region. An anodic drift occurred about the C-U characteristic curve of the MIS structure.