DECHEMA

About: DECHEMA is a nonprofit organization based out in Frankfurt am Main, Germany. It is known for research contribution in the topics: Corrosion & Oxide. The organization has 756 authors who have published 1307 publications receiving 25693 citations.

Metallographische Untersuchungen über den Aufbau des Zunders nach Oxydation von Eisen zwischen 700 und 900 °C in CO/CO2‐Gemischen mit kleinen Gehalten an COS, H2S oder SO2

Abstract: Die Ergebnisse der metallographischen Untersuchungen stehen im Einklang mit den kurzlich veroffentlichten (Werkstoffe und Korrosion 21 [1970] 925) kinetischen Messungen und analytischen Untersuchungen des Zunders. Ist aufgrund des CO/C02-Verhaltnisses nur eine Reaktion mit der Schwefelverbindung moglich, so entstehen in COS- und H2 Saurehaltigen Gasen reine Sulfidschichten. Ist dagegen neben der Reaktion mit der Schwefelverbindung auch eine Reaktion mit CO2 moglich, so entsteht bei linearem zeitlichem Verlauf der Verzunderung ein eigenes Gemenge von FeO und FeS. Bei hohem Oxidgehalt des Zunders ist FeS zeilenformig in eine FeO-Matrix eingelagert. Mit mittleren Oxid- und Sulfidgehalten des Zunders entsteht eine perlitartige Struktur von parallel angeordneten FeO- und FeS- Lamellen, deren Lage sich von Korn zu Korn andert. Mit dem Ubergang zu geschwindigkeitsbestimmender Diffusion von Eisenionen und Elektronen durch die Zunderschicht, wird nur noch das thermodynamisch stabile FeS gebildet. Unter bestimmten Bedingungen wachsen aus der kompakten Zunderschicht Nadeln aus uberwiegend reinem FeS. Diese Nadeln erreichen bei Durchmessern zwischen 5 und 20 um Langen bis 600 um. Mit dem Ubergang zum parabolischen Zeitgesetz wachsen sie wahrscheinlich in die Breite und bilden schlieslich eine geschlossene FeS-Schicht. In SO ,haltigen CO-CO2-Gemischen entstehen im wesentlichen die gleichen Strukturen, wobei zu beachten ist, das SO2 neben Schwefel auch Sauerstoff abgeben kann. Dadurch entstehen auch FeO/FeS-Lamellen in solchen Gasgemischen, in denen C02 keinen Sauerstoff abgeben kann. Bei hoheren SO2- und CO2,-Gehalten der Gase tritt mit dem Wachsen der Zunderschicht ein „Entarten” der FeO/FeS-Lamellen zu einem groberen Gemenge von Oxid und Sulfid ein. Metallographic investigations into the structure of the scale after oxidation of iron between 700 and 900°C in CO/Co2 mixtures with minor additions of COS, H2S or SO2 The results of metallographic investigations are in agreement with kinetic measurements published recently (Werkstoffe u. Korrosion 21 [1970] 925) and with analytical investigations of the scale. When because of the CO/CO2 ratio only a reaction with the sulfur compound is possible, pure sulfide layers are formed in gases containing COS and H. S. When, however, in addition to the reaction with the sulfur compound. A reaction with CO2 is feasible, an inti-mate mixture of FeO and FeS is formed according to a linear scaling law. When the scale is high in oxide, the FeS particles are embedded in linear shape in a FeO matrix. When the scale has medium contents of oxides and sulfides a perlitic structure is formed consisting of FeO and FeS lamellae in parallel arrangement, the location changing from one grain to the other. With the transition to a parabolic law, i.e. with the transition to rate controlling diffusion of iron ions and electrons through the scale layer, only thermodynamically stable FeS is formed. Under certain conditions needles, predominantly of pure FeS, grow out from the compact scale layer. These needles have diameters between 5 and 20 lm, and may attain lengths up to 600 pn. With the transition to the parabolic law they probably grow in thickness and finally form a coherent FeS layer. In CO/CO2 mixtures containing SO2 essentially the same structures are formed, in this context it must be noted, that SO2 may supply not only sulfur but also oxygen. This is why such FeO/FeS lamellae are formed in such gas mixtures where CO2, cannot supply oxygen. Higher SO2 and CO2, contents in the gas the FeO/FeS lamellae “degenerate” form a coarser mixture of sulfide and oxide.

The origin of the hysteresis in cyclic voltammetric response of alkaline methanol electrooxidation

Abstract: The mechanism of the alkaline methanol electrooxidation reaction on platinum is complex and not fully understood However, a better understanding will facilitate reaching the theoretical performance of an alkaline methanol fuel cell Cyclic voltammetry is an often used method to investigate the mechanism of electrochemical reactions The cyclic voltammogram of methanol electrooxidation shows a hysteresis in potential between the oxidation peaks in the forward and backward scans The origin of this hysteresis has not been fully clarified By means of parameter variations, such as the upper potential or the starting point of the CV measurements, and by physicochemical modeling, we investigate the formation of platinum oxides as a possible cause of this behaviour The experiments show that the formation of platinum oxides is more likely to cause the hysteresis than the strongly adsorbed intermediates, which are formed during the forward scan The simulation includes the formation of platinum oxide species and supports the experimental results that the hysteresis is due to the formation and reduction of platinum oxides Besides, the simulation reveals that in the investigated potential area, different oxide forms are present

Fireside Corrosion of Chromium- and Aluminum-Coated Ferritic–Martensitic Steels

Abstract: In modern fossil power plants, biomass is used more and more as secondary fuel in addition to coal. This leads to a significant decrease of the carbon footprint of such power plants. However, the demands on the corrosion resistance of the materials in the boilers increase because of chlorine in the atmosphere and salt-containing sulfides and chlorides. Heat-resistant ferritic–martensitic steels such as P91 are of great interest as superheater material. However, their corrosion resistance has to be improved for an application in modern fossil power plants with biomass combustion. For this purpose, chromium and aluminum diffusion coatings were developed and applied on P91 steel. The uncoated and coated material was investigated in a simulated biomass–brown coal ash with CaSO4, Na2SO4, K2SO4, KCl, and Al2O3 deposits and an atmosphere containing nitrogen with H2O, CO2, O2, SO2, and HCl. The improvement of the corrosion resistance is illustrated using metallographic methods such as electron probe micro-analysis.