Sulfuric acid (H2SO4), the most widely used inorganic acid in industrial chemical processes, is involved in the most frequently encountered corrosion problems in the chemical process industry. Tantalum and tantalum-alloy refractory metals are used in many of these processes to control corrosion by H2SO4. Ta (UNS R05210) and Ta-2.5% W (R05252) alloy corrosion behavior is superior to other metals over the entire concentration range of H2SO4 (up to 98 wt%), especially at elevated temperatures, and can solve many engineering problems in applications where the advantages of a metallic material, such as ductility, strength, superior heat-transfer properties, weldability and other important service behavior, are required.
Ta and Ta-2.5W, with a corrosion rate of <0.01 mm/y (<0.4 mpy), are corrosion resistant in all applications up to 95 wt% H2SO4 at the boiling temperature at atmospheric pressure. Ta is the most corrosion-resistant metal in H2SO4 at all concentrations up to 98 wt% (Fig. 1). The superior corrosion resistance of Ta and Ta-2.5W is due to a passive layer consisting of tantalum oxide (Ta2O5), which has chemical properties comparable to glass or glass linings. The passive layer can be attacked by strong alkalies, hydrogen fluoride (HF), fluorine compounds and sulfur trioxide (SO3) in aqueous or acidic solutions [1], and the protective structure of the oxide film can be altered at temperatures from ~190 to 250 C (375 or 480 F) depending on the environment involved [2]. Where corrosion occurs, it is mostly general corrosion with a certain corrosion rate [3-5]. Hydrogen embrittlement (HE) also has been observed [1], especially if galvanic coupling with less noble corroding metals is not precluded in acidic environments.
Industrial applications
H2SO4 is used in the production of several chemicals, particularly organic compounds. One of the most important reactions is the nitration process, which typically takes place in sulfuric/nitric acid (H2SO4/HNO3) mixtures at temperatures that depend on the end product. H2SO4 is not consumed, but is discharged in a diluted, contaminated form. Ta and Ta-2.5W are sometimes chosen for use in certain equipment in processes requiring very high temperatures.
After the nitration process, the HNO3 concentration decreases to a few ppm as a residual in the H2SO4 in the range of 20 to 80 wt%. Several processes for the recovery/recycling of this waste have potential for the application of Ta and Ta-2.5W, especially where evaporation processes are applied. The main aspects of the recovery process are the reconcentration of H2SO4 for reuse and the removal of organic compounds, the latter of which is dependant on the applied temperature. The higher the temperature is in a heat exchanger, the lower is the amount of residual organic compounds, resulting in higher purity H2SO4.
Waste/spent-acid recovery processes include thermal decomposition of spent H2SO4 (Monsanto Enviro Chem), evaporation in several steps to a high H2SO4 concentration (Chematur Ecoplanning), thermal cracking oxidation (Rhone Poulenc) and an integrated concept (Bayer) involving preconcentration, multistage evaporation, crystallization and separation of metal sulfates by filtration, concentration of filtrates, thermal decomposition of metal sulfates and production of fuming H2SO4 from sulfur dioxide formed [6].
The temperature limit for Ta or Ta-2.5W heat exchangers handling 96 wt% H2SO4 are normally 170 to 190 C, or 340 to 375 F [3,4]. Published Ta or Ta-2.5W corrosion resistance data dictate this limitation, which also limits of process efficiency. Therefore, reliable data about precise corrosion-resistance limits (maximum temperatures and H2SO4 concentrations are necessary.
Most available corrosion data [7-9] for these materials are based on short-term corrosion tests, and less clearly defined corrosion behavior of these inherently corrosion-resistant materials in more severe conditions limit their use in certain applications. Determination of the corrosion behavior in more severe conditions requires long-term testing to obtain reliable data about corrosion-resistance limits (maximum temperatures and H2SO4 concentrations). Prediction of long-term corrosion behavior of reactive materials requires exposure times of at least 2,000 hours.
Long-term testing
Long-term tests of as-welded and nonwelded corrosion coupons exposed to H2SO4 (technical grade) in glass retorts up to 2,500 hours have defined more clearly the limiting capabilities of Ta and Ta-2.5W in the chemical processes mentioned above.
Test results show:
- Ta-2.5W has superior corrosion behavior to that of unalloyed Ta under all test conditions.
- Mostly uniform corrosion was observed in cases where the corrosion rate was >0.01 mm/y (>0.4 mpy). In some instances, a preferred grain-face etching occurred in the heat-affected zone (HAZ) or in the weld.
- Corrosion rates remained at constant low values up to 175 C (345 F) and increased only slightly at a temperature of 200 C (390 F), provided the H2SO4 concentration did not exceed 96 wt%.
- The higher the H2SO4concentration, the lower the temperature must be to meet acceptable corrosion behavior regardless of which Ta material is considered.
- Corrosion rates showed a time-dependent behavior, particularly approach-ing the limits of behavior.
Recovered spent acid from nitration processing
Figure 2 shows results of immersion tests conducted to determine the maximum allowable temperatures to optimize process efficiency for spent acid recovery. The recovered nitration spent acid led to lower corrosion rates because of residual nitrates. The impurities serve as inhibitors, as occurs for stainless steels and nickel-based alloys in diluted H2SO4in the presence of cupric or ferric ions. This result is in agreement with data mentioned in previous literature [10]. In spent acid containing oxidizing compounds, both Ta and Ta-2.5W were used in 96% H2SO4 up to 200 C. At this acid concentration, a temperature of 230 C (445 F) appears to be satisfactory, provided sufficient wall thickness was provided.
Conclusion
Ta-2.5W performs better than Ta, particularly in spent acids containing oxidizing impurities. A realistic maximum corrosion rate in applications of plate heat exchangers or tube heat exchangers with 0.6 mm wall thickness is 0.05 mm/y. Ta-2.5W can be used in H2SO4 up to 96% and temperatures of 200 C, and should be used at higher acid concentrations, such as 97.5%. Ta cannot be used in 98% H2SO4 up to 200 C, as opposed to published corrosion data [7].
In the recovery process of spent acids, Ta and Ta-2.5W tubing and plate having higher (wall) thickness can be used economically up to corrosion rates of 0.15 mm/y. In mineral acids containing H+ ions, Ta and its alloys are limited to corrosion rates of L0.01 mm/y because of potential degradation by HE. In contrast, in highly concentrated H2SO4, molecules cannot be reduced in cathodic reactions [11]. Hydrogen reduction is inhibited because of the high overvoltage of the reduction reaction of H+ [11,12]. Therefore, there is no risk of HE in the application of Ta or Ta-2.5W at elevated temperatures in the recovery operation for spent H2SO4 at corrosion rates up to 0.15 mm/y. Under these circumstances, the maximum application temperature for Ta-2.5W is 230 C (445 F) with thick-walled plates or tubes.
References
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