Mats Norell

Influence of Water Vapor and Flow Rate on the High-Temperature Oxidation of 304L; Effect of Chromium Oxide Hydroxide Evaporation

The effect of roman PH2O and flow rate on the oxidation of 304Lat 873 K in oxygen is reported. High concentrations of water vapor and highflow rates result in breakaway corrosion. The mass gain after 168 hrincreased by four to five times, compared to oxidation in dry O2. Inthe presence of H2O, the corrosion products consisted of arelatively thin (Cr,Fe)2O3 oxide plus large oxide islandsconsisting mainly of Fe2O3. A mechanism explaining theeffect of water vapor on marginal chromia formers is proposed.

Indication of Chromium Oxide Hydroxide Evaporation During Oxidation of 304L at 873 K in the Presence of 10% Water Vapor

The oxidation of type 304L stainless steel wasinvestigated at 873 K in the presence of O2and O2 + 10% H2O. Oxidation timevaried between 1 and 672 hr. The oxidized samples wereinvestigated by a number of surface-analytical techniques includinggrazing-angle XRD, SEM/EDX, auger spectroscopy, SIMS andXPS. Oxidation in dry oxygen results in the formation acorundum-type oxide (Me2O3) withadditional formation of spinel oxides after prolonged exposure. Theoxide layer contained mainly chromium, with smalleramounts of Fe and Mn. Oxidation in the presence of watervapor results in an oxide that contains more Fe and less Cr, the outer part of the oxide beingdepleted in Cr. In the presence of water vapor, a massloss is detected after prolonged exposure. We show thatthe mass loss is caused by chromium evaporation. The volatile species is suggested to beCrO2(OH)2.

Surface characterisation of fine inert-gas- and water-atomised stainless steel 316L powders - formation of thermodynamically unstable surface oxide phases

Abstract New insights are presented on the speciation of surface oxide phases on fine inert gas atomised (GA, <45 and <4 μm) and water atomised (WA, <45 μm) stainless steel AISI 316L powders. X-ray photoelectron and Auger electron spectroscopy, scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry were applied for the characterisation. Oxidised manganese was strongly enriched in the outermost surface oxide of the GA powders (13 and 47 wt-%), an effect increasing with reduced particle size. Manganese and sulphur were enriched in oxide nanoparticles on the surface. Oxidised silicon (59 wt-%) was enriched on the WA powder surface. Tri- or tetravalent manganese oxides were observed on the GA particles in addition to α-Fe2O3, and Cr2O3. The oxide of the WA powder revealed in addition the likely presence of a silicate rich phase, mainly consisting of tetravalent Si, di- and/or trivalent Fe, and hexavalent Cr, which was confirmed not present as chromate.

Influence of KCl Deposit Morphology on Corrosion of Austenitic Alloys at 500°C

In biofuel combustion corrosion of the superheater tubes induced by alkali chlorides in the deposits limits the efficiency in electricity production. The most severe corrosion generally occurs at the edge of the deposits. This location may be governed by the transport through the deposit. While most of the literature is focused on the effect of the deposit composition this study examined how the morphology of solid KCl deposits affects the attack. Coupons of two austenitic alloys (Alloy 310 and Sanicro 28) inside tablets of pressed KCl with different density and thickness were exposed to N25%O210%H2O at 500°C for 168h. Prior to the exposure tablets were shaped to examine the effect of thickness gradients, edges and cracks. Potassium chromate and iron-chromium oxides formed for all deposit morphologies and chlorine was frequently observed at the interface to the metal. The thicknesses of the deposit clearly affected that of the reaction products, especially for Alloy 310. The thickest products formed at intermediate deposit thickness. This behaviour is similar to that observed for these alloys in a field test. Cracks in the deposits enhanced the attack. At least for Sanicro 28, the chromate formation was observed to break down the protective chromia and thus accelerate the attack. Both alloys were preferentially attacked at metal grain boundaries.

Study of Al∕Nb interface by spectroscopy of reflected electrons

We have studied Al/Nb interface with help of spectroscopy of reflected electrons. Ion sputtering combined with monitoring of Auger peaks has been used to approach the interface. The developed method of quantitative interpretation of spectra allowed us to characterize the distribution of Al and Nb in the interface region with a nanometer depth resolution. The Al/Nb interface was found to have an intermediate layer of about 10 nm thickness, which is more likely due to the interface roughness, rather than diffusion smearing. We discuss the possibility to distinguish by means of spectroscopy of reflected electrons either material under analysis is a single phase or a mixture of two phases.

Thermal decomposition of N-expanded austenite in 304L and 904L steels

In this study, expanded austenite was prepared by an industrial low-temperature plasma nitriding process in 304L and 904L austenitic stainless steels. The current investigation focuses on the assessment of the thermal stability and related phase evolution of the expanded austenite layer during isothermal annealing in protective argon atmosphere at temperatures ranging from 450 to 600°C for 24 and 168 h. Characterisation of the original expanded austenite and the decomposed surface layers was performed. Denitriding, inward N-diffusion and Cr-compounds precipitation occurred at different extent, depending on annealing conditions and alloy composition. Expanded austenite in 304L exhibited a near complete eutectoid decomposition after a short annealing time, while 904L showed significantly better thermal stability. A fine dispersion of small CrN precipitates resulting from expanded austenite decomposition at relatively low annealing temperature or short duration could further positively affect the surface hardness of both materials. Precipitate growth reduces hardness at higher annealing temperatures/times.

Effect of temperature gradient and sulfur dioxide addition on erosion – corrosion of iron- and nickelbased alloys

Abstract In this study, high temperature erosion – corrosion with trace amounts of added SO2 and the role of isothermal or thermal gradient conditions are studied. The studies are focused on 304L as the model material, but results for other alloys (Alloy 625 and 9Cr1Mo – steel) are also included to support the results and trends observed. The experiments include 550°C exposures in the erosion – corrosion rig under isothermal and thermal gradient conditions. The internal cooling used to create the thermal gradient meant that the 550°C material temperature could be maintained, despite of the higher bed temperature of 750°C. Laboratory oxidation experiments in a similar environment are also included as reference tests. The evaluation of material degradation was done by using a combination of different techniques such as scanning electron microscopy (SEM) including X-ray microanalyses, Auger and X-ray electron spectroscopy. The results demonstrate three important aspects related to erosion – corrosion attack in fluidized bed combustion. First, provided oxide growth is rapid enough leading to thickness in the micrometer range, low material wastage is obtained as demonstrated for the 9Cr1Mo– steel and also for the 304L and Alloy 625 materials when exposed to air plus 50 ppm added SO2 in isothermal conditions. Because of the large catalytic surface in the test rig for the thermodynamically favored oxidation of SO2 in air, significant presence of SO3 is expected. The material wastage is then also associated with a shift in erosion pattern with maximum material wastage at the impact of erodent (called type B). Detailed microstructure investigation of oxide in question on the 304L showed that the inner part of the oxide then is dense and possibly isolated from the environment and also Cr-rich. Adding a temperature gradient then returns the wastage pattern to type A for both 304L and Alloy 625 and leads to increased wastage for the former material. The resulting oxide thickness is also smaller and always less than 0.2 µm. The laboratory reference tests showed a similar range of average oxide thickness for air and air plus SO2. However, in air plus 50 ppm SO2 the tendency for break away corrosion on 304L was less than for air exposed samples.

Scale Growth on Austenitic Alloys under KCl Deposits at 500°C

The scale growth on two austenitic alloys, Alloy 310 and Sanicro 28, under KCl deposits was examined. This is relevant to the long term corrosion of superheater tubes in biofuel combustion. Coupons were encapsulated in tablets so that 1 mm of KCl with a relative density of 91% covered the metal. Samples were tested at 500°C in 5%O2-10%H2O-N2 for 24, 168 and 672 h. After exposure the salt was broken off and the scale was characterised by using SEM-EDX and AES. After 24 h a 50 nm thick oxide surrounded 500 nm thick chromates on the surface. No oxide layer was detected under the chromates and no Cl was found under either layer. The chromate growth requires lateral transportation of Cr along the surface. This reduces the protectiveness of the oxide and accelerates the formation of less protective Fe rich oxides. The formation of chromates also releases HCl inside the KCl tablet. The chromates did not grow significantly between 24 and 168 h, but the oxide grew equally thick beneath and between them and Cl was enriched around the metal oxide interface. After 672 h the oxides were about 5 μm thick and only few chromates were seen. Crystals of KCl formed in areas with thick porous and Fe rich oxides on both alloys.

Study of Nb/Al Interface Combining Spectroscopy of Reflected Electrons with Ion Sputtering

The study of the interface between Nb and Al thin films is motivated by the fact that electrical characteristics of Nb/Al-AlOx/Nb superconductive tunnel junction are very sensitive to the structure of the interfaces in the trilayer. We present a method of interpretation of energy spectra of electrons reflected from layered samples, with the help of which we can determine depth profiles and morphologies of interfaces inside Nb/Al-AlOx/Nb structure with a nanometre resolution. Methodological specifics of the method is accounting for the whole spectrum recorded in a wide range of energy losses, rather than limited to interpretation of certain peaks as in REELS. We reconstruct depth profile data by fitting calculated spectra to recorded ones. The calculations are based on solution of boundary problem of electron transport equation in multi-layered slice-uniform media, as well as on Monte-Carlo modelling. The Nb/Al interface was found to have an intermediate layer of about 3 nm thick as-deposited which developed into about 6 nm thick layer as a result of annealing at 180oC for 20 minutes.