Preparation And Characterization Of IrO_xPH Sensor

The corrosion of steel bars is one of the most common reasons of the damage of reinforced concrete structures.The high alkaline environment in concrete causes a passivation film on the surface of reinforcing steel bars to prevent the corrosion of steel bars.CO₂ diffuse into the concrete then react with concrete pore solution and form CaCO₃.This react cause the decrease of the pH value of concrete pore solution,when the pH value of concrete pore solution is lower than 9,the passivation film of steel bar will be damaged and cause the steel corrosion.At present,the most commoly used methods to test the pH value of concrete pore solution are leaching mehod and expression method.However,these methods are destructive methods and can not monitor the pH value of concrete in real-time.Thereforce,the development of embedded concrete pH sensor has great significance for monitoring the durability of concrete.In this paper,iridium oxide pH electrode was prepared by constant current deposition method and high temperature carbonate oxidation method.The effect of deposion current density,oxidation temperature and carbonate types on the electrode performance was investigated.SEM?EDS?AFM and XPS were performed to investigate the morphology and chemical composition of the electrode.The self-made iridium oxide pH electrode and manganese/manganese dioxide sensor were embedded in the mortar to monitor the change of pH values.The main results of this paper include:?1?Electrode prepared by constant current deposition method uses 0.16 m A/cm²,1 mA/cm²,2 mA/cm²,3 mA/cm² four kinds of deposition current density,the electrode surface prepared by this method showed"mud crack"surface morphology.The surface roughness of the electrode prepared by 1mA/cm² deposition current density is larger,and the electrode made at 1mA/cm² has the shortest response time of 67s and nernst calibration sensitivity and intercept were 74.91mV/pH,1001.43mV,the correlation is better.However,the degree of hydration of the electrode prepared by constant current deposition method is high and the deposition layer is easy to fall off.?2?High temperature sodium carbonate oxidation method using700?,750?,800?,850?four kinds of oxidation temperature to prepare electrode,SEM photographs show that the electrode surface particles become more larger with the increase of oxidation temperature.The oxide film prepared at high temperature appeared overlap and other defects.The response time of the electrode prepared by high temperature sodium carbonate oxidation method is longer,and most of the response time is over 5min.?3?High temperature lithium carbonate and sodium peroxide oxidation method were used to prepare electrodes at 750?,800?and 850?.The results showed that the surface particles increased and the uniformity decreased with the increase of oxidation temperature.The response time of electrode made at 750?was about 5s,and the sensitivity and intercept of the electrode decreased with the increase of oxidation temperature.The electrode sensitivity at 750°C was 56.52 mV/pH,641.76 mV.Electrode stability is better after aging process.?4?High temperature lithium carbonate oxidation method using 750?,800?,850?three kinds of oxidation temperature electrode,electrode microstructure showed dense granular structure,the electrode oxide layer particles become more compact and uniform with the increase of oxidation temperature.The adhesion of the oxide film and matrix become more strong with the increase of oxidation temperature.The electrode response time at 850?was about 19s,the electrode sensitivity was 54.6 mV/pH,and the monthly potential was reduced by 13.4 mV/pH in the long-term stability test.The electrode prepared at 850?has the best performance.?5?The electrode prepared by high temperature lithium carbonate oxidation method after half a year of aging,the calibration results tend to a uniform line.The electrode calibration results are close after the electrode applied-200mV,constant voltage 30min.?6?Considering the apparent morphology,response time,stability and Nernst response of the electrode,the electrode prepared by 850?high temperature lithium carbonate oxidation method is suitable as the pH electrode for embedded concrete.The pH sensor with the self-made manganese/manganese dioxide reference electrode have a good linear correlation for the E-pH calibration.The test in the mortar shows that the sensor can be used to monitor the pH of mortar.