Applicability of Electrical Equivalent Circuit Analysis Based on Voigt Model to Electrochemical Degradation Diagnosis of Heavy-duty Coating on Inner Surface of Oil-strage Tank and its Validation Using Depth Profile Analysis by IR and Raman Spectroscopy

Abstract

To investigating the long-term degradation of a heavy-duty coating, Fourier transform infrared spectroscopy （FT-IR） and Raman spectroscopy along the depth direction of a vinyl ester resin organic coating containing glass flakes was conducted. The organic coating sample was applied to the inner surface of the bottom plate of the oil storage tank, and used for approximately seventeen years. In addition, the degradation evaluation using the electrical equivalent circuit （EEC） model considering depth direction distribution of R//C time-constant （Voigt measurement model） was examined in detail.

In the FT-IR along the coating depth direction, no clear peak shift and/or disappearance of the spectrum was observed. On the other hand, the baseline of the Raman shift of the coating gradually increased from near the steel substrate interface toward coating surface. These results suggest that the long-term coating degradation appears as a difference in physical change along the depth direction, and supports the result that the impedance characteristics of the coating can be well-analyzed by Voigt measurement model. Furthermore, it is expected that the DC resistance value can be estimated accurately by the EEC analysis using Voigt measurement model.