Thread Content
This post was last edited by Shaobin Fluid on 2026-4-16 at 14:58. Strong acids, particularly nitrogenous acids, sulfuric acid, and hydrochloric acid at high temperatures up to their boiling points, are used in many chemical processes. Only a few metal materials are suitable for use under such harsh conditions. Tantalum is the most resistant to boiling strong acids, followed by zirconium, niobium, and a special nickel-based alloy—Hastelloy. The materials listed are expensive, especially tantalum. Furthermore, tantalum is not convenient to use in some cases due to its high specific gravity. As the temperature of the medium increases, the resistance of the traditionally used, cheaper corrosion-resistant materials (titanium and stainless steel) drops sharply. They are not suitable for cooking sour dishes. As one of the options to overcome the difficulties associated with finding corrosion-resistant materials, a composite material based on titanium plates with a coating thickness of about 2 mm is proposed, consisting of alloys from the Ti-Ta and Ti-Ta-Nb systems. This material is produced by depositing a mixture of initial powders on a titanium substrate. Its cost is relatively low compared to the cost of the alloy components contained in the protective layer. In this study, zirconium was introduced into the deposition mixture. Figure 1 shows the so-called equal corrosion curves in the concentration-temperature coordinates for sulfuric acid (a) and hydrochloric acid (b). According to the name of the curve, all points on the curve indicate the same level of corrosion resistance, namely a corrosion rate of 0.1 millimeters per gram. The points above these curves were obtained at pressures higher than atmospheric pressure. It can be seen that the resistance of Zr is significantly higher than that of Nb, while the resistance of stainless steel and titanium remains incomparably low. Figure 1. Equivalent corrosion curves in sulfuric acid (a) and hydrochloric acid (b) corresponding to 0.1 mm/g. For the experiment, plates made of pure titanium VT1-0 with dimensions of 12´50´100 millimeters were prepared. The chemical composition of the powder metal components is given in Table 1. The ratio of various components in the powder is shown in Table 2. Table 1. Chemical composition of surface powder, weight %. Table 2. Concentrations of alloying elements in the surface alloy layer. To determine corrosion resistance, plates 1 millimeter thick and 20×30 millimeters in size were cut from the applied coating. The plate was cut from the middle part of the deposited layer using electric spark cutting. Figure 2 shows the cross-section of the alloy layer obtained under low magnification using an optical microscope. Figure 2. Cross-sections of the alloy layers on the surfaces of tantalum and zirconium at low magnification. The plate suspended from the fluoroplastic wire was immersed in a flask containing acid of a given concentration. To equalize the temperature of the acid inside the flasks and make the temperatures between the flasks the same, the flasks are immersed in a metal container filled with mineral oil, which is placed on an electric heating plate (Figure 3). The oil temperature is under control. Figure 3. Corrosion testing equipment. The boiling acid vapor rises from the flask into a water-cooled reflux condenser that is sealed to the flask, where it condenses and flows back into the flask. The tests were conducted in 65% nitric acid, 5%, 10%, 20%, and 30% sulfuric acid, and 5%, 10%, 20%, and 30% hydrochloric acid. The testing time depends on the degree of erosion of the medium. It is divided into 5 phases, each with an equal duration. Before the start of the test and after each stage, the samples are taken out, washed with distilled water, then cleaned with bǐng tóng, dried in a stream of hot air, and weighed on an analytical balance and a GR-300, with a precision of 0.0001 g. New acid is used to replace it after each stage. Composition and structure of the deposit: The proportion of titanium in the deposit depends on the extent to which titanium penetrates from the top layer of the substrate into the molten pool during the coating formation process. Table 2 shows the concentrations of the mixture components and the average concentration of alloying elements in the deposit layer. In the backscattered electron mode, the clear contrast in the images obtained from the unetched thin film surface (Figure 4) indicates the measurement of the chemical composition of local microvolumes; the lighter areas corresponding to dendrite branches are richer in tantalum, while an increase in the content of titanium and zirconium is observed in the spaces between dendrites. Table 3 shows the measured concentration values of the microsite shown in Figure 4. Figure 4. Scanning electron microscope images of the alloy layers: a-31Ta-12Zr, b-48Ta-20Zr. Table 3. Local chemical composition of tantalum and zirconium alloyed layers. The average distribution of alloying elements across the depth of the deposited layer is sufficiently uniform, as shown in Figure 5, which depicts the element distribution in selected rectangular regions at different depths within the 31Ta-12Zr sample (Table 4). Table 4.31 Depth distribution of alloying elements in Ta-12Zr samples. Figure 5. Average concentration of alloying elements in the selected rectangular area at different depths within the deposited layer. According to the X-ray diffraction results (Figure 6), the coating in a single deposited sample consists of three phases: martensite α, α, and the equilibrium β phase. Two conditions can facilitate a complete transition to the β phase: the β-stabilizing effect of Ta and Zr at high concentrations, which accelerates the cooling of the deposited layer from the high-temperature region. According to the equilibrium Ti-Ta phase diagram, a tantalum concentration of 85% is required to form the β phase at room temperature. According to the Ti-Zr diagram, zirconium is a very weak β-stabilizer, and its presence seems to be negligible. Considering the second condition—rapid cooling. Due to the four-fold overlayer, a layer with a tantalum concentration of 54%±3% was obtained throughout the entire depth of the fourth layer. As can be seen from the X-ray diffraction pattern of this layer (Figure 6), it is primarily composed of the β phase, with a small amount of the α’’ phase as well. Since the cooling conditions of this sample are similar to those of 48Ta-20Zr, and no complete transition to the β phase occurred even at 54% Ta, we can conclude that the alloy layer showing a transition to the β phase in the 48Ta-20Zr sample cannot be attributed to a high cooling rate. In this ternary system, Zr with a high concentration of Ta in the titanium matrix acts as a significant β-stabilizer in this system. Figure 6. X-ray phase analysis results of the corrosion-resistant layer. The corrosion resistance of the molten layer: The test results for nitric acid and sulfuric acid are shown in Table 5, while those for hydrochloric acid are shown in Table 6. The nitric acid test lasts for 120 hours. Since sulfuric acid and hydrochloric acid are more corrosive, the testing time of the coatings in these acids is significantly reduced. Unstable VT1-0 titanium and 31Ta-12Zr single-layer coatings were tested for 50 minutes, while the more resistant 48Ta-20Zr, zirconium, and tantalum were tested for 5 hours. During the testing period, the weight loss of all samples followed a roughly linear pattern over time, indicating no observation of a passivation effect in the surface layer. Table 5. Corrosion rates of the surface alloy layer and reference sample in boiling solutions of nitric and sulfuric acids at different concentrations, mm/year. *The testing time in sulfuric acid was 5 hours. Table 6 shows the corrosion rates of the surface alloy layer and the reference sample in boiling hydrochloric acid solutions at different concentrations, in mm/year. *The testing time in sulfuric acid was 5 hours. The resistance of the Ti-Ta-Zr system to nitric acid is generally lower than that of the Ti-Ta-Nb system, whereas the highly alloyed 48Ta-20Zr layer has a resistance of 6 µm/year in nitric acid. Figure 7 shows the surface corroded by hydrochloric acid, which has the strongest corrosivity. In Figure 7a, due to the low acid concentration of 5%, the tantalum surface is hardly affected. The risk of handling samples with sandpaper remains unchanged. In Figure 7b, after tantalum was exposed to 20% acid, some roundness of the relief became evident. A comparison of Figure 7c and Figure 7d shows that under the same conditions in 10% acid, with a single-layer coating, the risk almost disappears after 50 minutes of testing, whereas with a two-layer coating, the risk remains present after 5 hours of testing. When immersed in 20% acid, significant corrosion occurred in both coating layers (Figures 7e and 7f); therefore, we believe their performance is limited to a hydrochloric acid concentration of 10%. Figure 7. Surface of the alloy layer after corrosion resistance testing in boiling hydrochloric acid solution and reference sample: a-tantalum in 5% hydrochloric acid ； c-31Ta-12Zr in 20% hydrochloric acid and d-48Ta-20Zr tantalum in 10% hydrochloric acid. ； e, f-48Ta-20Zr in 20% hydrochloric acid. Conclusion The deposited composite material is resistant to corrosion in three types of boiling strong acids: nitric acid at a concentration of up to 65%, sulfuric acid at a concentration of up to 40%, and hydrochloric acid at a concentration of up to 10%. The estimated duration of its operation in these acids could be: nitric acid – several decades, sulfuric acid and hydrochloric acid – a few years. Statement: This article was first published on the WeChat official account; the original title is “Titanium-tantalum-zirconium-based composite materials used in boiling sulfuric and hydrochloric acids”.