In recent years a new group of ferritic-martensitic chromium steels for the use in fossil power stations has been developed with chromium contents between 9 and 12%. Typical representatives of these steels are P91, E911 and Nf616, which are nowadays widely used in the more advanced power plants. In the development phase the focus was on the mechanical properties of these steels but when taking them to practical operation conditions it turned out that much of the life-time of the materials and components is determined by their oxidation properties. Oxidation resistance is first of all a function of alloy composition. For the steels of this group it is chromium, silicon, manganese and molybdenum that decide their oxidation performance and since the contents especially of the four elements can be significantly different for the different steels there can be clear differences in oxidation behaviour. One of the most important issues from this point of view is how the concentrations of these elements change in the metal subsurface zone during operation/oxidation since if their level drops below a critical level oxidation resistance of the steels will be lost. In the work to be reported the influence of alloy composition and metal subsurface zone concentration as a function of oxidation time up to 10000 h was investigated in dry air and air up to 10% water vapour at 650 °C. The investigations comprised several of the advanced commercial 9% Cr steels including P91, E911, Nf616 and six laboratory melts of Nf616 with different amounts of silicon. As the results of the investigations show humidity, which is omnipresent in combustion environments, can dramatically accelerate oxidation. Silicon as an alloying element reduces the detrimental effect of water vapour significantly while molybdenum has a negative effect. The effects of the key alloying elements in these steels was quantified for conditions with and without water vapour in the environment including the role of mechanical load and recommendations were developed on how to guarantee the optimum oxidation resistance of these steels

In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidation leads to Cr subsurface zone depletion the question arises as to how the oxidation behaviour is affected by the decrease in Cr concentration with oxidation time. Four ferritic heat-resistant commercial steels containing 9-12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650°C in air t? determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for t1mes up to 3000 hours. In addition to isothermal tests, thermal cycling tests and creep tests were also performed. Surprisingly large differences in oxidation behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9Cr steels while the other one was best. Thermal cycling improved the oxidation behaviour of the steels which was worse under isothermal conditions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal subsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical value which had been determined by model calculations. The tendency observed under isothermal conditions is enhanced by superimposed creep deformation. It is concluded from the results that growth stresses in the oxide scales combined with the actual Cr-concentration in the metal subsurface zone play a major role in oxidation resistance.

Cyclic voltammetry and polarization measurements on nickel under reducing gas atmospheres were performed to investigate the corrosion behavior of nickel as a base metal for molten carbonate fuel-cell separator plate alloys. The anodic reactions observed are oxidation of hydrogen, oxidation of nickel to nickel oxide and to dissolved nickel ions, and oxidation of bivalent nickel to trivalent nickel incorporated in the oxide scale. The cathodic reactions are reduction of trivalent to bivalent nickel in the scale and reduction of bivalent nickel to metallic nickel. During the cathodic scan of the cyclic voltammogram a change in the morphology of the oxide takes place. This is probably due to recrystallization of the oxide. During the cathodic reduction of NiO the scale loses contact from the metal at approximately -1200 mV; a small crevice is formed between the metal and the scale. The stresses in the loose scale cause cracking. Consequently, the crevice is filled with carbonate. Then nickel oxide dissolves, forming complexes of nickel and carbonate ions. These complexes are reduced to nickel (and carbonate ions) in the crevice. Due to the basic nature of the melt in the crevice, the nickel is oxidized in the following anodic scan, at lower potential (--1200 mV) than observed for an oxide-free surface (--700 mV). ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.218.248.200 Downloaded on 2015-04-10 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.218.248.200 Downloaded on 2015-04-10 to IP

The corrosion behavior of chromium in molten carbonate was investigated with electrochemical techniques in combination with quenching after polarization at fixed potentials. Between -1700 and -1500 mV carbon deposition takes place on the surface. The stationary corrosion product formed on chromium after polarization at -1700 mV is probably chromium carbide. Between -1600 and -300 mV a LiCrO2-layer is present on the surface of the chromium. The layer is continuous between -1600 and -500 mV; at -300 mV the scale is nonadherent and porous. At potentials above approximately -500 mV chromate formation and dissolution take place. At cathodic potentials point defects (oxygen vacancies and bivalent chromium ions) are assumed to be present in the scale, causing a high ionic conductivity. The corrosion rate is expected to be determined by a combination of applied electrode potential and electrical transport properties of the oxide layer. When the potential increases, the oxidation rate of the chromium increases due to the larger driving force for oxidation. In the potential region where oxygen vacancies are filled and bivalent chromium ions are oxidized (-1100 to -1000 mV), the conductivity of the scale decreases and the oxidation rate is determined by the transport properties of the scale: the passive properties of the LiCrO2-scale have improved. At potentials above -500 mV chromate dissolution takes place. In the anodic scan of a cyclic voltammogram two peaks can be observed, corresponding with the oxidation of point defects (-950 mV), and the formation of instable intermediate chromium oxide (-700 mV). These reactions are accompanied by the formation of lithium chromite. While scanning cathodically, first chromate ions are reduced (-600 mV). This is probably followed by small changes in the oxide scale. At very cathodic potentials (-1300 mV) trivalent chromium ions are reduced to bivalent chromium ions and point defects (oxygen vacancies and bivalent chromium ions), which are incorporated in the LiCrO2-lattice, and water is reduced. These reactions may be accompanied by the reduction of the instable chromium oxide formed during the preceding anodic scan. Near -1700 mV carbonate decomposes, lithium chromite is reduced and possibly carbide formation also takes place.