Stanisław Mrowec

The sulphidation and oxidation behaviour of sputter-deposited amorphous AlMo alloys at high temperatures

Abstract The sulphidation behaviour of sputter-deposited amorphous AlMo alloys has been studied as a function of temperature (973–1273 K) and alloy composition (34–46 at%Mo) in pure sulphur vapour at 10 3 Pa pressure. It has been shown that under these conditions the sulphidation process follows parabolic kinetics, being diffusion controlled. No influence of the alloy composition on the reaction rate has been observed. The scales were heterogeneous and composed of two layers. The outer scale layer was built of aluminium sulphide, while the inner layer was heterogeneous and composed of a mixture of Al 0.5 Mo 2 S 4 , Al 2 S 3 and MoS 2 phases, the latter being the major component. It is believed that the slowest step, determining the overall reaction rate, is the inward diffusion of sulphur through the inner barrier layer of the scale. Over the whole temperature range studied the alloys showed excellent resistance to sulphide corrosion, their sulphidation rates being comparable with, or even lower than the oxidation rates of chromia forming materials. The better protective properties of the sulphide scale on AlMo alloys in comparison with those of the MoS 2 scale on pure molybdenum result probably from lower defect concentration in aluminium-doped MoS 2 phase, constituting the major part of the inner barrier layer. Preliminary results indicate that the alloys under investigation show satisfactory oxidation resistance in the temperature range not exceeding about 1123 K. Under these conditions a protective oxide scale is formed composed virtually only of Al 2 O 3 and the oxidation rates are lower than those of chromia formers. At higher temperatures aluminium activity in the alloy is too low to suppress the formation and evaporation of molybdenum oxides.

The sulfidation and oxidation behavior of sputter-deposited Al-Ta alloys at high temperatures

As a part of a systematic study to elucidate oxidation and sulfidation resistance of Al-refractory metal alloys at high temperatures, the behavior of sputter-deposited Al-(33–80) at.%Ta alloys has been examined at temperatures ranging from 1073 K to 1273 K in He-S2 atmosphere and in Ar-O2 atmosphere. The sulfidation kinetics of these alloys follow a parabolic rate law in an early sulfidation stage, although, in some cases, the sulfidation rates are decreased after prolonged sulfidation. The sulfidation resistance of these alloys is comparable to that of high purity tantalum and remarkably higher than those of typical high temperature alloys. The sulfide scales on these alloys comprise an outer aluminum-rich layer and an inner tantalum-rich layer. The formation of a protective inner tantalum sulfide layer is responsible for the excellent resistance to high temperature sulfidation. The oxidation kinetics of the Al-Ta alloys change with alloy composition and temperature. The oxidation of Al-33Ta initially follows a parabolic rate law, but after a particular period of oxidation, rapid oxidation is observed at high temperatures above 1173 K. In contrast, the oxidation rates of higher tantalum alloys decrease with oxidation time, although the oxidation rates in the early stage are higher than those of Al-33Ta. At the temperatures below 1123 K a rapid weight loss during oxidation was observed for the Al-Ta alloys. This seems to result from disintegration of these alloys due to the pest phenomenon.

On the growth mechanism of the sulphide scale on amorphous AlMo alloys

Abstract The growth mechanism of the sulphide scale on amorphous Al-Mo alloys has been studied using a marker technique. A very thin (5 nm) gold marker film was deposited onto the surface of the Al-46Mo alloy and after sulphidation for 5 hours at 1073 K the gold marker distribution in the scale has been determined by RBS. It has been found that the main part of the marker remained at the scale surface indicating that the double-layer sulphide scale on the discussed alloy is growing by the inward diffusion of sulphur.

An RBS study of the sulphidation behaviour of niobium and NbAl alloys

Abstract The growth mechanisms of the sulphide scales on niobium and niobium-aluminium alloys at a temperature of 1073 K and sulphur pressure of 1 kPa have been studied by means of Rutherford backscattering (RBS) with the aid of a marker technique. A gold marker film of about 1 nm thickness was deposited on the surface of niobium as well as on Nb-0.6 at% Al, Nb-15.2 at% Al and Nb-63.4 at% Al alloys by vacuum evaporation. After sulphidation the gold marker distribution in the scale has been determined by RBS. It has been found that sulphidation of niobium occurs by the outward diffusion of niobium cations since the gold marker has been located at the metal/scale interface. The scale structure for the Nb-0.6Al alloy is almost the same as that for niobium because the alloy aluminium content is too low to form a separate phase of aluminium sulphide. Other niobium-aluminium alloys form a double layer scale with an outer aluminium sulphide layer and an inner niobium sulphide layer, both of which are formed by the outward diffusion of cations.

The sulfidation of sputter-deposited niobium-base aluminum alloys

Abstract The sulfidation behavior of sputter-deposited niobium base Nb-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (0.6–15.2 at%) and sulfur activity (40–4000 Pa) under isothermal-isobaric conditions. It has been found that under steady-state conditions the sulfidation process follows a parabolic rate law, the slowest step of the overall reaction rate being the outward diffusion of cations. The initial transient state of the reaction was longer when the aluminum content in the alloy and the temperature of sulfidation were higher. The sulfide scales on all the alloys consisted of the NbS 2 phase doped with aluminum and with a strongly developed growth texture, characterized by long columnar crystals situated perpendicular to the substrate surface. The e-axis of the hexagonal structure of niobium sulfide has been found to be parallel to the underlying alloy, i.e. the two-dimensional sulfide planes were situated perpendicular to the alloy surface. With increasing aluminum content in the alloy the structure of the scale changes from 3s-NbS 2 to 2s-NbS 2 . The surfaces of the sulfide scales on Nb-5.0A1 and Nb-15.2A1 alloys were transformed to Nb 2 O 5 and A1NbO 2 , respectively, as a result of interaction of the scale surface with trace amounts of oxygen.

The sulphidation behavior of MoAl alloys with low aluminum contents

Abstract The beneficial effect of alloying with aluminum to enhance the sulphidation resistance of molybdenum using sputter-deposited amorphous Mo-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (1.7, 6.2 and 10.7 at% Al) and sulphur vapor pressure (20–4000 Pa) in He-S 2 gas mixtures. It has been shown that the sulphidation process follows parabolic kinetics, being diffusion controlled. The reaction rate of Mo-Al alloys with low aluminum contents is lower than that of pure molybdenum and decreases with increasing aluminum content in the alloy. Because sulphide scales formed on Mo-Al alloys containing small amounts of aluminum are homogeneous and composed only of MoS 2 , the lower sulphidation rate of those alloys is explained in terms of the aluminum doping effect in the MoS 2 scale. It is believed that the better protective properties of the sulphide scale on Mo-Al alloys in comparison with those of the MoS 2 scale on pure molybdenum result from a lower defect concentration in aluminum-doped MoS 2 phase. The experimentally obtained dependence of the sulphidation rate of Mo-1.7Al alloy on the sulphur pressure at 1173 K is in accordance with that predicted from theoretical considerations.

Sulfidation- and oxidation-resistant alloys prepared by sputter deposition

Abstract In order to tailor novel alloys resistant to high temperature corrosion in multicomponent sulfidizing–oxidizing environments, amorphous or nanocrystalline Al-refractory metal alloys with and without silicon and Cr-refractory metal alloys have been prepared by sputter deposition. The sulfidation and oxidation behavior of the alloys has been studied as a function of temperature in sulfur vapor pressure of 10 3 Pa and in oxygen of 2×10 4 Pa, respectively. The sulfidation of these alloys generally follows a parabolic rate law, being thus diffusion controlled. The sulfidation rates of Al-refractory metal and Cr-refractory metal alloys are several orders of magnitude lower than those of conventional high temperature alloys and comparable to or even lower than those of the corresponding refractory metals. The sulfide scales formed on these alloys consist of two layers, comprising an outer Al 2 S 3 or Cr 2 S 3 layer and an inner refractory metal disulfide layer. The formation of the inner layer is attributed to the excellent sulfidation resistance of these alloys. The oxidation resistance of Al-refractory metal alloys is not sufficiently high, but the addition of silicon improves remarkably their oxidation resistance by synergistic effect of aluminum and silicon. Although the CrMo alloys possess poor oxidation resistance, due to the formation of volatile molybdenum oxide, the oxidation resistance of the CrNb and CrTa alloys is as high as that of typical chromia-forming alloys.

The sulfidation and oxidation behavior of sputter-deposited Cr-refractory metal alloys at high temperatures

Abstract Cr-Nb and Cr-Mo alloys have been sputter-deposited on to quartz substrate, and their sulfidation and oxidation behavior has been studied as a function of temperature and alloy composition in He-S 2 and Ar-O 2 atmospheres. The sulfidation of these alloys follows a parabolic rate law, being diffusion controlled. The sulfidation rates of Cr-Nb alloys decrease with increasing niobium content in the alloy, and the sulfidation resistance of the high niobium alloys is comparable with that of niobium. The sulfidation resistance of Cr-Mo alloys is independent of alloy composition, being comprable with that of molybdenum. The sulfide scales formed on these alloys consist of two layers, comprising an outer chromium sulfide layer and an inner layer composed mainly of refractory metal sulfides. The formation of the refractory metal sulfide scales is responsible for the high sulfidation resistance to sulfide corrosion. Under the oxidation condition the Cr-Mo alloys are rapidly oxidized due to the formation of volatile molybdenum oxide. The oxidation of Cr-Nb alloys proceeds accompanying partial breakdown and the restoration of the scales, but the average oxidation rates are almost the same as their sulfidation rates. Consequently, the Cr-Nb alloys possess high resistance to both sulfidation and oxidation at high temperatures.

Sulphide corrosion of pure and chromium-modified β-NiAl intermetallic compound at high temperatures

Abstract The kinetics and mechanism of high-temperature sulphidation of the intermetallic compound β-NiAl and of pseudobinary NiAlCr alloys have been studied as a function of temperature (1073–1273 K) and sulphur pressure (1.6−2 × 103 Pa) in flowing HeS2 gas mixtures. The sulphidation process for all the materials studied follows a parabolic rate law after a certain initial period. The rate-determining step of the overall scale growth is the outward diffusion of cations. The scale on pure β-NiAl was essentially homogeneous and composed of NiAl2S4 sulphospinel only, while that on the NiAlCr alloys also contained another spinel phase, (Cr,Al)3S4, the amount of which increased with the chromium content of the alloy. The sulphidation rate of the pseudobinary alloys was higher than that of pure β-NiAl and increased with chromium content because of the less protective properties of (Cr,Al)3S4 compared with NiAl2S4. The rate of sulphidation for all the materials studied has been found to exceed the oxidation rate of β-NiAl adn FeCrAl or CoCrAl alumina-forming alloys by at least six orders of magnitude.

The Influence of Yttrium on Kinetics and Mechanism of Chromia Scale Growth on Fe-Cr-Ni Base Steels

The influence of yttrium on the corrosion resistance of four valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) in combustion gases of fuel oil containing 10 wt. % of biocomponents has been studied at 1173 K under thermal shock conditions. These experiments have been carried out by rapid heating of the studied materials up to 1173 K and after two hours cooling down to room temperature. These thermal shocks have been repeated hundreds of times, determining after every shock the mass changes of the corroded samples. It has been found that small amount of yttrium deposited on the steel surface using electrochemical treatment considerably improves the scale adherence and consequently corrosion resistance in the case of X33CrNiMn23-8 steel, containing the highest chromium concentration.