A chemo-damage-transport model for chloride ions diffusion in cement-based materials: Combined effects of sulfate attack and temperature

The corrosion process of concrete usually accompanies pH changes of the pore solution. Thus, monitoring the pH value of concrete has great significance for the prediction of concrete corrosion. However, the most commonly used methods for measuring the pH value of concrete pore solution are destructive methods, which are very inefficient and cannot provide the pH value for real-time. In this study, an all-solid-state pH sensor constituted with iridium oxide (IrOx) working electrode and manganese/manganese dioxide (Mn/MnO2) reference electrode was developed. The IrOx electrode fabricated with carbonate melt oxidation method and Mn/MnO2 fabricated with powder compaction method exhibited a dense and homogeneous micro structure. The as-prepared pH sensors showed remarkable long-term stability, ideal potential-pH response, low hysteresis, fast pH response time and strong resistance to ionic interference in solutions. Furthermore, the pH sensor was embedded in the mortar samples and used to monitor pH changes during the accelerated carbonation test. The results indicate that the pH sensor can provide accurate pH value variations of the mortar samples during the carbonation process, and it has great potential for non-destructively monitoring the corrosion process of concrete. 

All-solid-state, long term stable, and embedded pH sensor for corrosion monitoring of concrete

In this work, the morphology, chemical constitution, and pH sensing performance of IrO x electrode prepared by cyclic voltammetry (hydrous) and thermal oxidation (anhydrous) are investigated. The results suggest that, IrO x film prepared by the electrochemical method consists of Ir(III) and Ir(IV). A hydration treatment of 30 days is required to establish a super-Nernstian response to pH (67 mV/pH). IrO x film prepared by thermal oxidation has a stronger adhesion with the substrate and consists of Ir(III), Ir(IV) and Ir(VI). A more thorough hydration of 60 days is required to establish a Nernstian response to pH (59 mV/pH). The pH sensing mechanism of both electrodes is discussed and their respective application field is recommended.

Performance of IrOx pH Sensor Prepared by Electrochemical and Thermal Oxidation

pH measurement has received extensive attention from researchers in numerous fields including medical diagnostics agriculture, food industry, marine, and chemical engineering. Owing to their electrode stability, drift degree, and preparation process, iridium/iridium oxide electrodes are considered as the most promising candidate for pH detection under these conditions. In this study, iridium/iridium oxide electrodes were fabricated by thermal cycling oxidation method. The relationship between the electrode structure and performance was investigated and discussed. To study the influence of supercritical treatment, the as-prepared electrodes were treated at 35 MPa and 400 °C for 3 h. After examining their pH response, the supercritically treated electrodes exhibit better performance than the untreated iridium/iridium oxide electrodes. The structural changes of the treated electrodes were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammograms and scanning electron microscopy. The results revealed the important changes in the iridium oxide film of the electrode after supercritical treatment, such as its morphology, chemical bond, valence state, and defects, which collaborated to achieve its improved performance. 

Improved Iridium/Iridium Oxide pH Electrode through Supercritical Treatment

This letter investigates the use of external voltage resets to reduce drifting issues of iridium oxide (IrO x ) pH-sensing films. Two types of IrO x films prepared by sol–gel and electroplating techniques were examined comparing hydrous and anhydrous material characteristics. The thin-film IrO x allows inexpensive processing for flexible polyimide substrates enabling planar and monolithic integration for wearables and miniature pH sensors. Open-circuit potential drifts and external voltage resets were investigated for freshly prepared electrodes and retested after 14 days. External resets were examined at different voltage levels. Sol–gel electrodes showed 57% and 81% drift improvement in acid and alkaline test solutions, respectively, while electroplated ones showed 84% and 88% improvement. Sensitivity and hysteresis were investigated after external resets and showed promising results. A total of 15 sol–gel and 15 electroplated electrodes were characterized. Differences in pH characteristics are attributed to different types of IrO x compositions. The quicker response transitions and reduced potential drifts will enable reliable long-term pH monitoring and reduce frequent calibration for pH-sensing applications.

Improvement on Potential Drifting for Sol–Gel and Electroplated Iridium Oxide-Based pH-Sensing Films

Iridium oxide pH electrodes employing the carbonate melt oxidation method were fabricated with oxidation temperatures of 750 °C, 800 °C and 850 °C, respectively. Scanning electron microscope (SEM) and atomic force microscope (AFM) images showed that the oxide film regularized with the increase in oxidation temperatures. The pH response, response time and long-term stability of the electrodes indicated that the electrodes made at 850 °C had the best performance. X-ray photoelectron spectra (XPS) surveys investigated the change in the electrodes' chemical composition and element oxidation states at 850 °C, and the results showed that the relative content of Ir3+ had increased by 23.9%, and the Ir4+ and Ir6+ had decreased by 10.9% and 13%, respectively, in the surface oxide layer after one month of aging. However, the relative contents of Ir3+, Ir4+ and Ir6+ were almost constant for the inner oxide layer. Meanwhile, the XPS result also indicated that the outer oxide layer of the electrode had a higher hydration degree than the inner oxide layer.

https://www.mdpi.com/1424-8220/19/21/4756/pdf

Effect of Oxidization Temperatures and Aging on Performance of Carbonate Melt Oxidized Iridium Oxide pH Electrode.

In practice, almost all of the concrete structure exposed to environment are partial saturated except the concrete structure under water. So, the influence of W/C and moisture content on capillary absorption of concrete was investigated. Results indicated that the ultimate saturation increases with W/C linearly. The porosity and pore size distribution of concrete will be changed with W/C. Concrete with lower W/C with the same saturation absorbs less water and the rate of capillary absorption is also lower. The degree of saturation of the concrete plays a strong role on capillary adsorption. Concrete with lower saturation with the same W/C absorbs more water and the rate of capillary absorption is also quicker. The bigger of W/C, the bigger of the initial coefficient of capillary absorption and the bigger of saturation, the smaller of the initial coefficient of capillary absorption. The initial coefficient of capillary absorption increases with W/C linearly, while it decreases with saturation linearly within the accuracy of the measurements.

https://dpi-proceedings.com/index.php/dtmse/article/download/9928/9482

Influence of W/C and Moisture Content on Capillary Adsorption of Concrete

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Papers

Effect of Oxidization Temperatures and Aging on Performance of Carbonate Melt Oxidized Iridium Oxide pH Electrode.

Influence of W/C and Moisture Content on Capillary Adsorption of Concrete