Short CommunicationStudy of corrosion behavior of carbon steel under seawater film using the wire beam electrode method
Introduction
The relationship between corrosion rate of metals and the thickness of electrolyte film is probably the most important characteristic that researchers want to elucidate in atmospheric corrosion study. In the 1960s, Tomashov [1] presented a conceptual model for the variation of metal corrosion rate with the thickness of electrolyte layer. In that model, the corrosion rate increases with the decrease of the oxygen diffusion layer thickness (cathodic control was believed), and a maximum of corrosion rate appears in the transition from cathodic control to anodic control. After that, several other mathematical models have been proposed [2], [3], [4]. Most parameters used for establishing these models were selected from electrochemical data published in various literature and some of them were assumed. More experimental data are still needed to enrich our knowledge. Experimentally investigating atmospheric corrosion with some new electrochemical techniques is not only important to verify these models [2], [3], [4], but also helps us to further understand this corrosion process.
To date, only a few electrochemical techniques have been employed to study this specific corrosion process. Zhang and Lyon [5] investigated the cathodic polarization curves for copper, zinc and iron electrodes under thin electrolyte layers. In Ref. [5], a specially designed cell was used, and the Luggin capillary was used to connect the normal reference electrode to the electrolyte layer covered on the electrode surface. Nishikata et al. [6] used an electrochemical cell in which the working electrode, reference electrode (Ag/AgCl) and counter electrode (Ag/AgCl) were embedded in parallel in the same epoxy resin, and the cathodic polarization curves were performed under thin electrolyte layers. Still in Ref. [6], a two-electrode cell (two identical iron electrodes, 0.1 mm × 10 mm, mounted 170 μm apart in parallel in the epoxy resin) was used for EIS test. Stratmann et al. [7], [8], [9], [10] firstly introduced the Kelvin probe technique to the atmospheric corrosion research and several important papers have been published by the Stratmann’s group. Tsuru et al. [11] measured the diffusion-limiting current density of oxygen under electrolyte film of various thickness with Kelvin probe and the result was consistent with Tomashov’s model [1]. After decades of study, the electrolyte layer used in atmospheric corrosion research has been changed from static and even to dynamic and uneven, such as wet–dry cyclic test. The EIS method may be the most popular technique employed in atmospheric corrosion study. Tsuru’s group has performed a lot of work using the EIS method [11], [12], [13]. However, fitting EIS data measured under extremely thin electrolyte film and estimation of k value in Stern–Geary equation on rust covered surface still need further investigation [13]. Although atmospheric corrosion has been investigated by these electrochemical methods for many years, as noted in Ref. [2], the development of new electrochemical methods is still an important field of research.
The WBE (also named multi-electrode array) method has been employed in corrosion research for several years [14], [15], [16], [17], [18], [19], [20]. The remarkable characteristic of this method is that potential/current distributions can be measured under complex surface conditions. Tan firstly investigated localized corrosion and heterogeneous electrochemical processes with WBE method [19]. Muster et al. [17] investigated the influence of droplet characteristics on the atmospheric corrosion of zinc by multi-microelectrode approach. Wang developed a WBE testing equipment based on modular instruments [14]. Wang et al. [15], [16] studied local corrosion under different size of droplets with this device. Corrosion of metals under droplets is a typical form of atmospheric corrosion, the results of these work show that the WBE method is a potential local electrochemical technique for atmospheric corrosion research. To our knowledge, the application of this method in atmospheric corrosion investigation under thin electrolyte film has not been reported yet.
The present work aims to characterize the relationship between the corrosion behavior of carbon steel and seawater film with various thickness using the WBE method. The results of this study were compared with electrochemical data in literature and reasons for the difference were discussed.
Section snippets
Experimental
The 121 microelectrodes in the WBE were fabricated from A3 carbon steel wire (1.2 mm in diameter). These microelectrodes were regularly arranged as a 11 × 11 matrix and embedded in epoxy resin with an interval of 0.2 mm from each other. Each microelectrode acted as a galvanic current sensor and substrate. The area of the total 121 microelectrodes was 1.37 cm2. The surface of WBE was gradually ground down to 1000 grid with SiC paper, degreased with acetone and rinsed with deionized water.
As shown in
Results and discussion
In order to eliminate the current fluctuation during the initial period, the WBE was immersed in seawater for about 60 min before current distribution measurement under seawater film. Fig. 2 presents the current distributions of WBE under seawater film with various thickness. The current distribution is relatively homogeneous and the value is low (reflected from the color) when seawater film is relatively thick, as shown in Fig. 2 (a) and (b). We can also see that most electrodes reveal anodic
Conclusion
The WBE method was employed to investigate the effect of seawater film thickness on corrosion behavior of carbon steel in this study. The corrosion rate increased significantly under thin seawater film. The diffusion layer thickness of oxygen was about 500 μm and the critical thickness was about 40 μm at which corrosion process transited from cathodic control to anodic control. The variations of the sum of anodic current and the sum of the cathodic current with seawater film thickness were
Acknowledgments
This work is supported by NSFC (Nos. 51131005, 21203172). This is MCTL Contribution No. 22.
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2022, Corrosion ScienceCitation Excerpt :Fig. 3 shows the current maps of the WBE covered by uncracked mortar with time. Similar to the previous literature [10,13,14], the positive current values represent the anodic area, and negative current values represent the cathodic zone. The current distribution of the WBE is uniform, and no obvious current peak appears before seawater wetting and drying cycles.