Screen-printed electrode as a cost-effective and miniaturized analytical tool
for corrosion monitoring of reinforced concrete
Herein, we report the first electrochemical sensor based on a screen-printed electrode designed to evaluate the corrosion level in iron-reinforced concrete specimens. The combination of an Ag pseudoreference electrode with a gel polymeric electrolyte allows for fast, stable and cost-effective potentiometric measurements, suitable for evaluating the corrosion of iron bars embedded in concrete samples. The sensor was found to be capable of discriminating between a standard non-corroded sample and samples subject to corrosion due to the presence of chloride or carbonate in the concrete matrix. The potential in concrete-based specimens containing carbonate (pH 9, −0.35 ± 0.03 V) or chloride (4% w/w, −0.52 ± 0.01 V) was found to be more negative than in a standard concrete-based sample (−0.251 ± 0.003 V), in agreement with the ASTM standard C876 method which uses a classical Cu/CuSO4 solid reference electrode. Our results demonstrate that a printed Ag pseudoreference electrode combined with KCl agar provides an efficient and reliable electrochemical system for evaluating the corrosion of iron bars embedded in concrete-based structures.
Results and discussion
Steel embedded in concrete encounters a favourable environment (alkaline conditions) for iron passivation as a consequence of the production of an iron oxide film (i.e. Fe2O3) on the surface; this acts as a protective layer. However, the resistance to corrosion due to this protective layer tends to drastically decrease when the reinforced concrete is exposed to chlorides or CO2 (carbonation reaction), resulting in the steel becoming depassivated and the observation of active corrosion. To evaluate the suitability of our new sensor system, several studies were carried out using the different concrete specimens prepared in the
presence and absence of CaCl2 and NaHCO3, as described below.
First, the sensor was tested with a pristine concrete specimen and a potential value equal to 0.218 V was obtained. This figure is in agreement with the value reported by the ASTM standard C876 method corresponding to only 10% probability of corrosion. It should be taken into account that we have used a Ag pseudo-reference electrode which has a slightly lower potential (0.285 vs NHE) with respect to the Cu/CuSO4 reference electrode (0.314 V vs NHE).
To deliver a successful sensor, the repeatability and the robustness of the analytical device are of prime importance and these aspects were evaluated for the reinforced concrete sample in the absence of CaCl2 and NaHCO3.
We thus tested the same device in 10 successive measurements, obtaining an intra-electrode repeatability (RSD%) equal to 0.5%.
We also evaluated the repeatability in an interelectrode procedure by testing the same concrete-based sample with different gel-based electrolytes and different printed reference electrodes, obtaining a RSD% equal to 1% Thus, outstanding repeatability was demonstrated, especially when bearing in mind that both the gel-based electrolyte and the printed reference electrodes were produced manually.
Conclusion
In this work, a cost-effective and miniaturized electrochemical sensing tool has been designed and produced with the aim of evaluating the corrosion of steel bars embedded in concrete. The combination of a silver-based printed pseudoreference electrode with a gel-based electrolyte (KCl in agar gel) makes it possible to perform the measurement
directly on the concrete sample without any sample treatment. Since the printed electrochemical reference sensor has been conceived as a miniaturized electrode, the sensing tool can easily be embedded in a concrete structure for on-line monitoring at low cost. Since concrete is the most widely used manufactured material globally, and climate
change can accelerate the deterioration of concrete infrastructure, much work is being done to reduce the economic impact of concrete testing. In this overall scenario, the sensing tool that we have presented here could pave the way for a cost-effective sensing system, expanding the application of printed electrodes to the construction-engineering sector.
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