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Volume 2 Extended Abstract 20

Submitted 26th August 1999

Comparison of the data from two electrochemical techniques for the evaluation of protective coating performance; Current interrupter and AC impedance methods

Hiroyuki Tanabe and Masanori Nagai, 
Fundamental Research Laboratory,Dai Nippon Toryo Co., Ltd.
1382-12, Shimoishigami, Ohtawara, Tochigi 324, Japan Isao Sekine and Makoto Yuasa Department of Industrial Chemistry, Science University of Tokyo,
2641 Yamazaki, Chiba 278, Japan

Keywords: underfilm corrosion, current interrupter, alkyd, epoxy,

Introduction

Electrochemical methods have been studied for the quantitative analysis of degradation of coated metal so far. We developed an under-film corrosion tester using the current interrupter technique. The data obtained from the under-film corrosion tester were discussed comparing that from the electrochemical impedance spectroscopy in this paper. Both electrochemical impedance spectroscopy and current interrupter method give parameters of natural potential(E), film resistance(Rf), film capacitance(Cf), resistance of charge transfer(Rct) and capacitance of double layer(Cdl).

Experimental

The parameters mentioned above were measured for evaluating protective properties of the following coated films Alkyd clear (La), lead cyanamid alkyd paint(La-P) TiO2 pigmented epoxy paint Mild steel(JIS G 3141) was used as a substrate. The sample paints were coated by an applicator and a spray. Dry film thickness of 20µm, 60µm and 100µm was obtained for each paint. Sodium chloride solution of 3 wt% was used as a test solution.

Conclusions

It was found that there was a good co-relation between the data from the two methods. However, the maximum value measurable in case of film resistance of both the current interrupter and electrochemical impedance spectroscopy were more than 1010 ?cm2 and 5x108 ?cm2 respectably. The current interrupter method had advantages and that is, gave some parameters which means useful not only for anti-corrosive performances but also for the analysis of corrosion mechanism.

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