Characterization of coating systems by scanning electrochemical microscopy: Surface topology and blistering

doi:10.1016/j.porgcoat.2009.03.008

Progress in Organic Coatings

Volume 65, Issue 4, August 2009, Pages 435-439

https://doi.org/10.1016/j.porgcoat.2009.03.008 Get rights and content

Abstract

Operation of the scanning electrochemical microscope used in feedback mode over a coated metal allows changes in the state of the coating surface to be monitored during immersion in aqueous electrolytes. This paper reports changes in the coating induced by specific anions in the electrolyte in situ during immersion. Significant surface roughening is observed for immersion times shorter than 1 day when the electrolyte contains chloride ions. This effect is also observed when the oxygen dissolved in the electrolytic phase is employed as redox mediator for SECM imaging. The coated system exposed to chloride-free electrolytes containing sulphate or nitrate maintains a featureless topography within the same time scale. The observed features are due to the nucleation and growth of blisters at the metal/coating interface induced by chloride ions in the environment. The implication is that ionic migration occurs simultaneously with the absorption of water by the coating already from the beginning of exposure to the aqueous environment. The unique role of chloride ions compared with sulphate or nitrate ions towards coating performance has been established at a very early stage following immersion of the sample.

Introduction

The current availability of a variety of scanning microelectrochemical techniques for the in situ investigation of transformations occurring at coated metals during exposure to electrolytic environments is providing new spatially-resolved information concerning electrochemical and corrosion processes, typically at a microscopic and submicroscopic ranges [1]. In this way, our current knowledge on the origin and the mechanisms of those complex processes will be significantly extended in the near future.

Microelectrochemical techniques have been successfully employed in the last decade to gain new information contributing to the understanding of coating degradation from defects. The experimental data made available by these techniques correspond namely to corrosion potentials [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], ion concentrations [15], [16], [17], [18], [19], ionic fluxes [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] and impedances [30], [31], [32], [33], [34] as a function of the location from defects or cut edges in coatings. Conversely, the applicability of these techniques to study intact coatings has been very scarce, mainly due to the difficulties associated with the small responses produced by systems of such a low reactivity. In this context, scanning electrochemical microscopy (SECM) has appeared to be specially suited for such investigations [35], [36]. Operation of the SECM in the feedback mode has allowed the earlier stages of coating degradation through initial swelling of the coating and the subsequent nucleation of blisters at the metal-coating interface to be visualized [37], [38], making it possible to detect such processes at significantly shorter exposure times than those required during the application of conventional electrochemical techniques.

The feedback mode of the SECM requires the measurement of a faradaic current at the sensing microelectrode (i.e., the tip) [39], [40], and topographic changes in an insulating surface such as the intact coated metal originate from local hindrance in the diffusion-limited transport of a redox mediator as the tip is moved in close proximity to the surface. Usually, ferrocene–methanol is added to the solution to serve as the redox mediator [37]. But the addition of a freely diffusing redox mediator might be avoided by employing oxygen. Dissolved oxygen is present in the electrolyte phase in the naturally-aerated system, and indeed this chemical is necessary for the corrosion reaction to proceed once corroding microcells are established at the metal/coating interface.

In this paper we report data obtained by SECM on the characteristic behaviour of chloride ions towards coating damage, and a comparison between the effects of chloride, sulphate and nitrate ions on the topography of the coated samples is presented. The applicability of dissolved oxygen as redox mediator has also been tested. Surface mapping operating in the feedback mode of the SECM gives detailed information, such as the nucleation and growth of microscopic blisters under the polyester coating during its exposure to aqueous electrolytes at ambient temperature.

Section snippets

Scanning electrochemical microscopy

The SECM instrument used in these experiments was a CH900 scanning electrochemical microscope (CH Instruments, Austin, TX, USA), which is controlled by a control unit and software. The system was placed inside an active isolation workstation, which effectively isolated the system from electrical and acoustic noise as well as from mechanical vibrations. Control of the microelectrode was performed using inchworm type piezo motors (Burleigh) capable of reproducible motion. The motors were mounted

Results and discussion

Ferrocene was used as electrochemical mediator for the imaging of polymer coated metal samples with the SECM. On a 10 μm diameter microelectrode, the cyclic voltammogram recorded at a scan rate of 10 mV s⁻¹ in 0.1 M KCl naturally aerated solution depicts the electrochemical reactions associated with ferrocene dissolved in the electrolyte (see Fig. 1). A single voltammetric wave was found at positive potentials higher than ca. +0.20 V, which corresponds to the oxidation of ferrocene to ferrocinium.

Conclusions

Scanning electrochemical microscopy imaging in the feedback mode was used to characterize the influence of electrolyte composition on the performance of a metal-coating system following immersion in the aqueous environment. The polyester coated material acted as an almost ideally insulating surface, thus allowing changes in the current measured at the ultramicroelectrode to be directly related to topographic changes of the exposed surface as a function of elapsed time.

The method also allows

Acknowledgements

We gratefully acknowledge the financial support of this work by the Ministerio de Educación y Ciencia (Madrid, Spain) in the framework of Project CTQ2005-06446/BQU, and by the Gobierno de Canarias (Project No. PI2004/075). The awards by the University of La Laguna of an Invited Professorship to GTB and a research fellowship to Y.G.-G. are gratefully acknowledged.

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