Scientists Improve Corrosion Resistance of Ceramics in Nitric Acid

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Apr 5 2022

In an article recently published in the open-access journal Materials, researchers reported the optimization of corrosion resistance of alumina ceramics in nitric acid.

Study: Optimization of Alumina Ceramics Corrosion Resistance in Nitric Acid. Image Credit: chemical industry/Shutterstock.com

Background

Alumina (Al₂O₃) is a type of ceramic with great hardness, wear resistance, and strength, as well as chemical stability. Ceramic corrosion is, nevertheless, examined and investigated in a variety of domains. Impurities and additives in small proportions have a significant impact on the manufacture and final qualities of alumina-based ceramics.

The desire for further knowledge of advanced ceramic chemical resistance in strong acid environments has expanded dramatically as a result of their rapid development, and hence, new application possibilities have emerged.

The "one-factor-at-a-time" technique (OFAT) can be used to investigate the characteristics that influence ceramic corrosion; however, it is a time-consuming method. Response surface methodology (RSM), on the other hand, is a useful technique for assessing the interactions between the process variables and, as a result, optimizing them.

The development of a model that can predict the development of corrosion procedures within experimental areas, determine interactions between factors, and define the conditions for minimal corrosion is the need of the hour. Within the given conditions, this type of model could also greatly reduce maintenance costs and improve the life expectancy of ceramic materials, such as alumina.

XRD pattern of the Al₂O₃ granules. 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^-3, 1.25 mol dm^-3, and 2.00 mol dm^-3 and exposure durations ranging from 1 to 10 days. Temperature effects at 25, 40, and 55 °C were also studied.

Powder X-ray diffraction (PXRD) was used to assess the phase composition of Al₂O₃ granules. The produced sintered samples' morphology was determined using a standard ceramographic technique. The sintered samples' hardness 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 assessed the corrosion resistance of Al₂O₃ ceramics by using inductively coupled plasma atomic emission spectrometry (ICP-AES) concentration measurements of eluted Fe³⁺, Mg²⁺, Na⁺, Al³⁺, Ca²⁺, and Si⁴⁺ ions, as well as the density measurements of the examined alumina ceramics.

Within the experimental "sample-corrosive media" domain, the response surface technique (RSM) was employed to optimize the corresponding parameters. According to the Box–Behnken design, alumina ceramics were exposed to aqueous HNO₃ solutions. Following the definition of the regression functions, the conditions for achieving the highest corrosion resistance of sintered ceramics were found through optimization within 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 eluted ions from the samples. Lower alumina ceramics density values were measured under the aforementioned conditions.

SEM images of the sintered Al₂O₃ ceramics with the magnification of (A) 2500× and (B) 6000×. Image Credit: Ropuš, I et al., Materials

Observations

The best circumstances for achieving the least amount of ion elution and the highest density of alumina ceramics were achieved at the start of the experiment, i.e., 0.50 mol dm^-3 concentration of HNO₃ at 25 °C, and 24 h exposure with the desirability of 93%. More than 98% of the total variation in the amount of all eluted ions was described by the regression models, as it was more than 83% of the density variation. The bulk density was found to be 3.864 ± 0.018 g cm^-3, with a relative porosity of 3.1 ± 0.5%.

With time, regression models revealed a higher elution of ions from alumina ceramics at lower HNO₃ concentrations and higher temperatures. After a minimum exposure duration of 24 h to 0.50 mol dm^-3 HNO₃ at 25 °C, optimum conditions for achieving the maximum corrosion resistance, i.e., the lowest number of eluted ions and the highest alumina ceramics density, were achieved inside the experimental "sample-corrosive media" area.

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 dissolving of segregated impurities such as Fe₂O₃, Na₂O, CaO, and SiO₂, as well as sintering aid, i.e., MgO, at the grain borders of alumina ceramics.

Normal plot of response residuals—amount of eluted Al³⁺ ions from Al₂O₃ ceramics after exposure to HNO₃. Image Credit: Ropuš, I et al., Materials

Conclusions

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 for up to 240 hours.

The authors emphasized that the experiment was designed using the Box–Behnken method such that the conditions in which the corrosion resistance was maximum could be achieved.

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