Scientists Improve Corrosion Resistance of Ceramics in Nitric Acid
In an article recently published in the open access journal Materialsresearchers have reported the optimization of the corrosion resistance of alumina ceramics in nitric acid.
Study: Optimization of the corrosion resistance of alumina ceramics in nitric acid. Image Credit: Chemical Industry/Shutterstock.com
Alumina (Al2O3) is a type of ceramic with high hardness, wear resistance and strength, as well as chemical stability. Nevertheless, the corrosion of ceramics is studied and studied in many fields. Impurities and additives in low proportions have a significant impact on the manufacture and final qualities of alumina-based ceramics.
The desire to deepen our knowledge of the advanced chemical resistance of ceramics in strong acidic environments has greatly expanded due to their rapid development, and as a result, new application possibilities have emerged.
The “one factor at a time” technique (OFAT) can be used to study the characteristics that influence the corrosion of ceramics; however, it is a time-consuming method. Response Surface Methodology (RSM), on the other hand, is a useful technique for evaluating interactions between process variables and, therefore, optimizing them.
The development of a model capable of predicting the evolution of corrosion procedures in the experimental areas, of determining the interactions between the factors and of defining the conditions for minimal corrosion is the need of the hour. Under the given conditions, this type of design could also significantly reduce maintenance costs and improve the service life of ceramic materials, such as alumina.
Al XRD model2O3 pellets. Image credit: Ropuš, I et al., Materials
About the study
In this study, the authors investigated the corrosion resistance of alumina ceramics in aqueous nitric acid (HNO3) solutions with concentrations of 0.50 mol dm-31.25 mol dm-3and 2.00 mol dm-3 and exposure times ranging from 1 to 10 days. The effects of temperature at 25, 40 and 55°C were also studied.
X-ray powder diffraction (PXRD) was used to evaluate the phase composition of Al2O3 pellets. The morphology of the sintered samples produced was determined using a standard ceramographic technique. The hardness of the sintered samples was determined using a hardness tester. After unloading, the diagonals were measured using an optical microscope. The Vickers hardness of each sample was determined ten times.
The researchers evaluated the corrosion resistance of Al2O3 ceramics using inductively coupled plasma atomic emission spectrometry (ICP-AES) concentration measurements of eluted Fe3+mg2+N / A+Al3+California2+and if4+ ions, as well as the density measurements of the alumina ceramics examined.
In the experimental field “samples-corrosive media”, the response surface technique (RSM) was used to optimize the corresponding parameters. According to the Box–Behnken design, the alumina ceramics were exposed to aqueous HNO3 solutions. Following the definition of the regression functions, the conditions to achieve the highest corrosion resistance of sintered ceramics were found through optimization in the experimental region.
During the static corrosion test, the team used the Box-Behnken design to investigate the effect of immersion time, temperature, and HNO3 concentration on the chemical stability of sintered alumina samples by monitoring their density and the number of ions eluted from the samples. Lower alumina ceramic density values were measured under the above conditions.
SEM Images of Sintered Al2O3 ceramic with the magnification of (A) 2500× and (B) 6000×. Image credit: Ropuš, I et al., Materials
The best circumstances to obtain the least amount of ion elution and the highest density of alumina ceramics were obtained at the beginning of the experiment, i.e. 0.50 mol dm-3 HNO concentration3 at 25°C and 24 h exposure with a desirability of 93%. Over 98% of the total variation in the amount of all ions eluted was described by the regression models, as it accounted for over 83% of the density variation. The bulk density was 3.864 ± 0.018 g cm-3with a relative porosity of 3.1 ± 0.5%.
Over time, regression models revealed higher ion elution from alumina ceramics at lower HNO3 higher concentrations and temperatures. After a minimum exposure time of 24 h at 0.50 mol dm-3 HNO3 at 25°C, the optimum conditions for achieving the maximum corrosion resistance, i.e. the lowest number of eluted ions and the highest alumina ceramic density, were obtained at inside the “sample corrosive medium” experimental zone.
A second optimum was also found at 2.00 mol dm-3 HNO3 at 40°C and 24 h exposure. Lower HNO3 concentrations at higher temperatures were found to have a greater impact on the dissolution of segregated impurities such as Fe2O3N / A2O, CaO and SiO2as well as a sintering aid, i.e. MgO, at the grain boundaries of alumina ceramics.
Normal plot of response residues – amount of Al eluted3+ Al ions2O3 ceramic after exposure to HNO3. Image credit: Ropuš, I et al., Materials
In conclusion, this study elucidated the chemical stability of alumina at temperatures of 25, 40 and 55°C with HNO3 concentrations of 0.50, 1.25 and 2.00 mol dm-3 up to 240 hours.
The authors pointed out that the experiment was designed using the Box-Behnken method such that the conditions under which corrosion resistance was maximal could be achieved.
Ropuš, I., Curkovic, L., Cajner, H., et al. Optimization of the corrosion resistance of alumina ceramics in nitric acid. Papers 15(7) 2579 (2022). https://www.mdpi.com/1996-1944/15/7/2579.