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Sensors 2009, 9, 10400-10410; doi:10.3390/s91210400
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sensors
ISSN 1424-8220 www.mdpi.com/journal/sensors Article

A New Experimental Method for in Situ Corrosion Monitoring Under Alternate Wet-Dry Conditions
Xinxin Fu, Junhua Dong *, Enhou Han and Wei Ke State Key Laboratory for Corrosion and Protection of Metals, Institute of Metal Research, Chinese Academy of Science,Shenyang 110015, China; E-Mails: xxfu@imr.ac.cn (X.F.); ehhan@imr.ac.cn (E.H.); kewei@imr.ac.cn (W.K.) * Author to whom correspondence should be addressed; E-Mail: jhdong@imr.ac.cn; Tel.: +86-24-2391-5912; Fax: +86-24-2389-1320. Received: 3 November 2009; in revised form: 16 November 2009 / Accepted: 17 November 2009 / Published: 21 December 2009

Abstract: A new experimental method was applied in insitu corrosion monitoring of mild steel Q235 under alternate wet-dry conditions. The thickness of the electrolyte film during the wet cycle was monitored by a high-precision balance with a sensibility of 0.1 mg. At the same time, an electrochemical impedance technique was employed to study the effect of film thickness on corrosion rates. Experimental results showed that there was a criticalelectrolyte film condition for which the corrosion rate reached a maximum during wet-dry cycles. For the substrate, the critical condition could be described by a film thickness of about 17 μm. For the rusted specimen, the critical condition could be described by an electrolyte amount of about 0.038 g, which is equivalent to a film thickness of 38 μm. This monitoring system was very useful for studyingatmospheric corrosion of metals covered by corrosion products. Keywords: alternate wet-dry condition; mild steel; electrochemical impedance spectroscopy (EIS); electrolyte film; electrical balance

Sensors 2009, 9 1. Introduction

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Atmospheric corrosion is an electrochemical process which occurs under thin dilute electrolyte. According to Tomashov, the atmospheric corrosion rate ofmetals depends on the thickness of electrolyte film on the surface [1]. This has attracted interests from many researchers. Mansfeld et al. [2] monitored the corrosion of metals covered with an electrolyte layer during drying using an atmospheric corrosion monitor (ACM), and found that the corrosion rate abruptly increased immediately before the surface dried out. Stratmann et al. [3] reported a setof equipments for monitoring the electrolyte layer thickness by measuring the potential difference between the electrode surface and a probe needle. Their study on oxygen reduction during dying of a platinum surface covered with a neutral solution layer indicated a maximum corrosion rate at a thickness of several tens of microns [4]. Nishikata et al. [5] reported that the corrosion rate of coppercovered with an acidic sodium sulfate solution film showed a maximum at the thickness of several tens of micrometers, which was measured with a similar method. The experimental results reported by Yamashita et al. [6] on the layer thickness dependencies of the corrosion rate of low alloy steel indicated that the corrosion rate reached a maximum when the layer thickness is about 10 μm. As the mostwidely used structural material and the largest source of losses caused by corrosion, the study of atmospheric corrosion of steels is of great significance. However, with low corrosion resistance, the surface of carbon steels can be severely rusted after numerous wet-dry cycles, which greatly influences the accuracy of traditional film thickness monitoring methods. Electrochemical techniques havebeen widely used in atmospheric corrosion studies. In traditional electrochemical measurements a tri-electrode system with a Luggin capillary is often employed, but the Luggin capillary causes the properties of the measured surface to change abruptly, thus producing great errors. Zhang and Lyon [7] measured the polarization curves of metal covered with a thin electrolyte layer with an improved...