Das Korrosionsverhalten von Eisen and Stahl in flüssigem Ammoniak

Abstract

An Eisen in fliissigem Ammoniak wurden bei -40°C und bei + 20 "C Stromspannungskurven in stark und schwach sauren Elektrolytlosungen gemessen. Die Auflosungsgeschwindigkeit des aktiven Eisens zu Eisen(I1) ist unabhangig von der Elektrolytzusammensetzung und folgt einer Tafel-Geraden mit der Neigung RTI2 . 0.4 F.Bei positiven Elektrodenpotentialen wird das Eisen passiv. Die stationaren Stromdichten sind nahezu potentialunabhangig und nehmen mit der Saurekonzenrration und der Temperatur zu. Bei sehr positiven Elektrodenpotentialen wachsen die Stromdichten wieder exponentiell mit dem Potential. Die Stromausbeuten fur die Entwicklung von Stickstoff und die Eisen (111)-auflosung sind ahnlich groB. Luftpassives Eisen unterscheidet sich im elektrochemischen Verhalten deutlich von in Ammoniak passiviertem. Es entsteht sehr wahrscheinlich eine Eisennitridschicht mit einer Dicke der GroBenordnung nm. Passiver Stahl FG 32 korrodiert im weichgegliihten Zustand etwa ebenso schnell wie reines Eisen, aber im kaltgezogenen Zustand rund 8mal schneller.Polarization curves were measured with iron in liquid ammonia containing various electrolytes of different acidities at -40°C and + 20 "C. The dissolution rate of iron by formation of iron(I1) is independent of the electrolyte composition and follows a Tafel-line of the slope RT12.0.4 F. At positive potentials iron becomes passive. The steady state current densities are nearly independent of the electrode potential and grow with acid concentration and temperature. At very positive electrode potentials the current densities again increase exponentially with the potential. The current efficiencies for nitrogen evolution and iron(II1) dissolution are of equal magnitude. Iron oxidized in air exhibits an electrochemical behavior distinctly different from the one of iron passivated in liquid ammonia. Very probably the passivating layer is composed of iron nitride with a thickness of the order of nm. Passive steel FG 32 in the soft state after heat treatment corrodes at about the same rate as pure iron, but approximately 8 times faster in the cold worked state.

As Precious as Platinum: Iron Nitride for Electrocatalytic Oxidation of Liquid Ammonia

“…Instead it quickly levels off to values less than 5%. Essentially the same behavior was seen at 1.6 V. These results are in agreement with the suggestion by Ahrens et al that there are other chemical processes competing with Fe 3+ dissolution . At lower overpotentials, e.g., 1.2 V versus NHE, nearly all of the anodic current initially goes toward Fe 3+ dissolution.…”

“…The most obvious process that can compete with NH 3 (l) oxidation is the corrosion of the Fe electrode. A detailed study of the corrosion of Fe in NH 3 (l) was published by Ahrens et al Utilizing a distillation procedure similar to ours and Fe wire anodes, they identified two different corrosion processes for Fe in NH 3 (l): One process occurs at relatively low potentials (reported around 0 V versus Tl/TlCl, or approximately −0.34 V versus NHE) where the anode dissolves into Fe 2+ ions in solution. A second process is seen at relatively high potentials (reported around 1 V versus Tl/TlCl, which is approximately 0.66 V versus NHE) where the anode dissolves as yellow/brown Fe 3+ ions in solution.…”

“…Ahrens et al proposed the formation of Fe 4 N, which they suggested would inhibit further corrosion. However, they did not provide direct evidence . Indeed, after extended periods of positive applied potential, a black film forms on the Fe electrode surface, which could indicate a change in its composition (Figure S5 in Supporting Information).…”

“…By comparison, the corrosion behavior of various transition metals in NH 3 (l) have been better studied. ,− This work was largely motivated by the commercial interest in storing NH 3 (l) in stainless steel containers. Thus, corrosion studies have focused primarily on stainless steel components such as nickel, zinc, and iron.…”

“…Thus, corrosion studies have focused primarily on stainless steel components such as nickel, zinc, and iron. The Fe corrosion study by Ahrens et al demonstrated two separate potential regions for corrosion (evolving Fe 2+ and Fe 3+ , respectively) with a large, 0.5 V wide passive region between them . They also discussed the possibility of nitridation of the Fe anode surface, which could possibly serve as a catalyst for NH 3 oxidation.…”