Clara Schlereth

Hydrogen is a key factor in energy storage and transportation, which will inevitably lead to a substantial increase in demand. Ammonia has emerged as a promising carrier molecule due to its high hydrogen density and the possibility to use existing infrastructures to import hydrogen/ammonia produced from renewable sources (green hydrogen). However, utilizing catalytic ammonia cracking to recover hydrogen from ammonia requires better process understanding and new material solutions as current Ni‐catalyst–based reactors are too inefficient and unsuitable for the dynamic requirements of future energy systems. To improve the knowledge base, this work provides a literature overview of the nitridation and degradation of structural materials under ammonia cracking conditions. The dependence of the corrosion rate on a wide range of parameters and the missing database for conditions beyond those similar to the Haber–Bosch process are discussed. Closing these critical knowledge gaps will enable scalable green hydrogen production, supporting the global energy transition and addressing the challenges of climate change.

Metal dusting is a critical corrosion phenomenon in industrial processes using carbonaceous gases and high temperatures. Cu can suppress metal dusting when added in sufficient amounts to pure Ni. In this study, metal dusting attack on Monel® 400 alloy (Ni with 29–30 wt.% Cu, 1 Mn wt.%, and 1–2.5 wt.% Fe) processed via additive manufacturing (AM) was investigated in detail. To determine the role of Mn and Ti in the metal dusting attack, AM Monel® 400 without Mn, with higher Mn content (2 wt.% Mn), and with TiN nanoparticles were examined and compared to Ni70Cu30 and Ni69Cu30Mn1 model alloys. Increasing Mn content or adding TiN promoted mass gain mainly through carbon uptake and graphite precipitation. The catalytic roles of Mn and Ti oxide in carbon uptake are discussed in detail. In this metal dusting attack, oxidation, catalysis, carbon dissolution, carbon diffusion, and carbon precipitation play a significant role and drive the degradation depending on the alloy composition.

Chemical plants which process highly carbonaceous gases at elevated temperatures are prone to catastrophic corrosion by metal dusting. Typically, commercial alloys with high amounts of protective oxide scale formers (Cr, Al, and Si) are used in these environments. However, scale failure is still frequently observed after an incubation time initiating pits. In this study, the microstructure and subsequent metal dusting-induced failure of the oxide scale on the commercial Ni-based alloy 601 was analyzed. Samples were exposed in different aggressive metal dusting gases and characterized using metallographic cross sections, electron beam microanalysis (EPMA), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). A thin and protective chromia scale formed in some regions with a continuous silica layer below. Across most of the alloy 601 surfaces, internal oxidation of Al could be linked to metallic particles in the outer scale. Additionally, MnCr₂O₄ was observed in the outer scale. Together with pores in the chromia, the spinel and metallic particles in the outer scale combined to provide pathways for carbon ingress. After exposure in a gas with a higher driving force for carbon deposition, a higher amount of carbon was incorporated in the growing oxide scale, resulting in earlier scale failure and metal dusting pit initiation. Graphical Abstract

Previous studies showed some transition metal high-entropy alloy (HEA) compositions can have good oxidation resistance in air up to 800 °C. Four equiatomic HEAs have been developed based on FeCoCrNi with additions of Mn, Cu, Al or Al+Cu. The oxidation behavior of these HEAs was compared in humid (10 vol.% H₂O) air at 800 °C for 100–500 h to investigate the influence of water vapor on the oxidation mechanisms. The Cu- and Al-containing alloys exhibited improved oxidation resistance over the Mn composition. For the Cu-containing alloy, a local attack of the Cu-rich phase was observed, which formed an Fe/Ni/Co/Cr spinel that was surrounded by Cr₂O₃. This oxide was thicker for the humid air atmosphere when compared to dry air, and the transition of the Cu oxide to the spinel was accelerated. The Al-containing HEA formed a thin Al₂O₃ scale with humidity suppressing AlN formation and forming a smoother oxide layer. The Al+Cu composition had the highest overall oxidation resistance (minimal local attack, no nitridation) and also showed a smooth oxide scale topography under humid air oxidation as opposed to a plate-like, rougher scale under dry air.

Welds of the commercial Ni‐based alloys 602 CA, 699 XA, 601 and 690 were exposed under harsh metal dusting conditions. The pit formation was compared for as‐welded, brushed, ground, glass bead blasted, steel shot peened and dry cathodic pickled surfaces. Surface treatments were applied by industrial partners. When (stainless) steels were involved in the surface treatment, Fe contaminations acted as catalyst sites for pit initiation. A fast metal dusting attack was also observed on dry cathodic pickled samples. A beneficial effect was attributed to deformation of the sub‐surface zone, as it provides diffusion paths for oxide formers (by grinding and glass bead blasting). In the same region, formation of α‐Cr (BCC) precipitates was observed. The best performance was found for samples where the weld bead was fully flattened by grinding to P40 grit. In addition, it was demonstrated that on‐site slurry aluminization further enhances the resistance against metal dusting attack.