Info-pack #4: Nitridation of Metals

1 INFO-PACK #4 • NITRIDATION OF METALS High temperature, pressure and wet ammonia: testing alloy corrosion limits Introduction Nitridation of Metals The ammonia-to-hydrogen PCER-system developed in SINGLE will comprise several balance-of-plant components that will be exposed to wet ammonia-containing atmosphere at high temperatures and elevated pressure. The alloys used to fabricate these components should have sufficient resistance against nitridation to allow for safe and reliable long-term operation. To aid with the materials selection, SINGLE has evaluated seven alloys with varying contents of Fe, Ni and Cr for resistance against nitridation in the temperature range of 170-750 °C and gas mixtures containing 6-85 vol.% NH₃ at 20 bar. This info-pack summarizes key learnings from the results of this experimental work. Ni-based alloy 625 has the overall greatest resistance against nitridation among the tested materials, while less expensive alloys such as 316L display acceptable nitridation resistance only at temperatures below 300 °C. ABSTRACT The ammonia-to-hydrogen PCER-based system developed in SINGLE will comprise several components for handling process gases and recuperating heat, e.g., the reactor housing, boiler, condenser and heat exchangers. These components will be exposed to ammonia (NH₃) - containing gas mixtures at various temperatures up to 750 °C. Safe and reliable long-term operation of the system relies on selecting alloys that can endure the exposure conditions in terms of pressure, temperature and NH₃ concentration. While there is a lot of knowledge on alloy stability in dry NH₃-containing atmospheres originating from experience with ammonia synthesis, the behavior in wet NH₃-containing atmospheres is less known and requires more research. SINGLE aims to address this knowledge gap by investigating the effect NH₃ has on different alloys under wet conditions in the temperature region of 170-750 °C. Key points • Increasing temperature and ammonia concentration both lead to more severe degradation of alloys in wet ammonia-containing atmospheres. • Overall, alloys with high Ni content and low Fe content show the greatest resistance against nitridation across a wide range of exposure conditions, but AISI 444 has good resistance in humified ammonia at 750 °C. Related KERs or Info-packs • Info-pack 2: Pilot • KER 2: PCER module design • KER 3: 10 kg/day pilot INFO-PACK #4 Experimental set-up Seven different Ni-based alloys and austenitic and ferritic stainless steels were selected to cover a range of Ni, Fe and Cr contents, as shown in Table 1. A dedicated corrosion test rig was construct- 2 INFO-PACK #4 • NITRIDATION OF METALS Degradation caused by ammonia Exposure to ammonia-containing gas mixtures at high temperature led to nitridation, which could be quantified by comparing the sample weight before and after exposure. The depth of nitridation was found to correlate well with the mass gain after exposure, thus, mass gain measurements provide an efficient way of comparing the nitridation resistance of alloys at the different exposure conditions, as shown in Figure 1. In the most severe cases (e.g. exposing Alloy 444 to 550 °C and > 23% NH₃), we observed complete disintegration of the 0.5 mm thick alloy sheet after only 200 h. Figure 1: Mass gain after 200 h of exposure to 20 bar wet-ammonia-containing gas mixture as a function of temperature (left) and ammonia concentration (right. ed to allow for exposure of the alloys in NH₃ atmosphere up to 20 bar and 750 °C humidified with 2 vol.% H₂O. The exposures were carried out for 200 hours at each test condition and the extent of nitridation was evaluated by a combination of mass gain measurements and post-exposure analysis by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Table 1. Overview of alloys tested and their composition in weight percent. Grade EN Few Ni Cr Mn Si Other 625 2.4856 3.73 60.6 22.38 0.09 0.16 8.87 Mo, 3.43 Nb, 0.27 Ti, 0.07 Co 600 2.4816 9.6 72.8 16.6 0.4 0.2 0.14 Ti, 0.03 Co 800HT 1.4959 47.6 30.4 20.1 0.8 0.3 0.05 Cu, 0.26 Al, 0.25 Ti, 0.16 Co 310S 1.4845 Bal. 19.3 25 1.26 0.58 0.2 Mo 316L 1.4404 Bal. 10.1 16.8 1.2 0.4 2.1 Mo 444 1.4521 Bal. 0.2 17.7 0.30 0.50 1.85 Mo, 0.30 Nb, 0.15 Ti EF101 N/A Bal. - 12.4 0.1 1.25 3.7 Al The observed impact of ammonia at high temperature is exemplified in the SEM cross sectional images in Figure 2, which shows one affected (316L) and one non-affected (625) sample. Nitrogen (from decomposed ammonia) is dissolved into the near-surface 3 INFO-PACK #4 • NITRIDATION OF METALS Figure 2: SEM images of alloy sample cross sections after 200 h of exposure to 20 bar 6%NH₃69%H₂-23%N₂-2%H₂O at 750 °C. Impact of alloy composition Impact of ammonia concentration Overall, the extent of nitridation correlated well with the Ni and Fe contents in the diff erent alloys; alloys with high Ni and low Fe content, such as Alloy 625, were most resistant against nitridation, while alloys with low Ni and high Fe content, such as 316L, displayed lower resistance to nitridation. The ferritic steels AISI444 and EF101 deviated from this trend for temperatures > 650 °C, becoming more resistant against nitridation by forming a protective oxide-scale in wet conditions. Increasing the NH₃ concentration (during exposure at 550 °C) led to more severe reactions for all alloys, both in terms of how deep into the alloy nitrogen diff used and in terms of oxidation and/or phase separation taking place within in the nitrided area. After exposure to 85% NH₃ at 550 °C, the alloys AISI444 and EF101 were completely disintegrated while 316L was nitrided across the whole thickness of the alloy sheet (0.5 mm) and had started to crack up. Cracking within the nitridated area was also observed when alloys 800HT and 600 were exposed to 85 % NH₃. Alloy 625 remained stable, showing essentially no signs of nitridation even after exposure to 85% NH₃. region of the 316L alloy and reacts with the alloying elements to form Cr-nitride precipitates. The nitrided area can be clearly distinguished in the SEM image by its darker contrast compared to the un-aff ected alloy in the centre of the sample. Nitridation increases the brittleness of the alloy and may thus compromise the mechanical integrity of the component. Impact of temperature Increasing the exposure temperature while keeping the NH₃ concentration constant led to more severe nitridation of Ni-based alloys and austenitic stainless steels, evident by a greater depth of nitridation and formation of larger/more distinct metal nitrides 4 INFO-PACK #4 • NITRIDATION OF METALS Conclusions General trends in nitridation resistance Based on corrosion testing of seven diff erent alloys in a range of varying gas compositions and temperatures, the general trends in nitridation resistance that were observed can be summarized as follows: • Overall, alloys with high Ni and low Fe content are most resistant against nitridation in NH₃, while alloys with low Ni and high Fe content are more prone to nitridation; • Increasing the ammonia concentration increases the depth of nitridation and the severity of the attack in terms of cracking; • Increasing temperature leads to more severe nitridation of Ni-based alloys and austenitic stainless steels, while the ferritic steels show the opposite trend. Recommendations for the PCER system Results of the corrosion testing in SINGLE clearly showed that the Ni-based alloy 625 displays the greatest resistance against nitridation across a wide range of temperatures and ammonia concentrations. Although the ferritic stainless steel AISI 444 showed no reaction with wet ammonia at 750 °C, the complete disintegration of this material at lower temperature and higher ammonia concentrations prevents its application. Alloy 625 is therefore the safest choice, especially for components of the PCER system that will be exposed to the most demanding conditions. For components with service temperature < 300 °C, the less expensive and more widely available stainless steel grade 316L can be utilized. within the nitrided zone. The ferritic steels AISI444 and EF101 showed the opposite eff ect of temperature, and, notably, AISI444 showed no signs of nitridation after exposure to 750 °C and wet ammonia-containing conditions. The sample surface was covered by a dual-layer oxide comprised of CrMn-rich oxide on the top and Si-rich oxide beneath this, possibly acting as barrier against nitrogen ingress. Author Belma Talic, Senior Research Scientist at SINTEF Industry [email protected] 4 INFO-PACK #4 • NITRIDATION OF METALS Co-funded by the European Union, the Clean Hydrogen Partnership and its members Hydrogen Europe and Hydrogen Europe Research. Views and opinions expressed are those of the author(s) only and do not necessarily reflect those of the European Union or Clean Hydrogen Partnership. Neither the European Union nor the granting authority can be held responsible for them.  singleh2.eu  [email protected]  SINGLE H2 PROJECT