Influence of surface oxide characteristics and speciation on corrosion, electrochemical properties and metal release of atomized 316L stainless steel powders

Surface oxide characteristics of powder particles are important to consider for any toxicological risk assessment based on in-vitro or in-vivo tests. This study focuses on a multi-analytical approach (X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning- and transmission electron microscopy, and different electrochemical techniques) for in-depth characterization of surface oxides of inert-gas-atomized (GA) AISI 316L stainless steel powder, compared with massive sheet and a water-atomized (WA) 316L powder. Implications of differences in surface oxide phases and their surface distribution on corrosion, electrochemical properties and metal release are systematically discussed. Cr was enriched in an inner surface layer for both GA powders, with Mn and S enriched in the outermost surface oxide. The surface oxide was 2-5 nm thick for both GA powder size fractions, amorphous for the GA powder size

Influence of surface oxide characteristics and speciation on corrosion, electrochemical properties and metal release of atomized 316L stainless steel powders

Surface oxide characteristics of powder particles are important to consider for any toxicological risk assessment based on in-vitro or in-vivo tests. This study focuses on a multi-analytical approach (X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning- and transmission electron microscopy, and different electrochemical techniques) for in-depth characterization of surface oxides of inert-gas-atomized (GA) AISI 316L stainless steel powder, compared with massive sheet and a water-atomized (WA) 316L powder. Implications of differences in surface oxide phases and their surface distribution on corrosion, electrochemical properties and metal release are systematically discussed. Cr was enriched in an inner surface layer for both GA powders, with Mn and S enriched in the outermost surface oxide. The surface oxide was 2-5 nm thick for both GA powder size fractions, amorphous for the GA powder size

Effect of Running-In (Load and Speed) on Surface Characteristics of Honed Gears

The initial running-in cycles alter the surface integrity characteristics and influence gear performance. This article shows how the surface characteristics of honed spur gears evolved due to the combined effect of running-in load (0.9 or 1.7 GPa) and speed (0.5 or 8.3 m/s) in Forschungsstelle fur Zahnr\ue4der und Getriebebau tests. Running-in affected the surface layers to a depth of 5 \ub5m. High running-in load promoted plastic deformation of asperities, created microstructural changes associated with surface cracks, and relaxed residual stresses. It also enhanced the amount of phosphorous from extreme pressure (EP) additives at the surface. The surface contact fatigue failure—that is, micropitting—was promoted by running-in speed rather than load

A Microstructural and Kinetic Investigation of the KCl-Induced Corrosion of an FeCrAl Alloy at 600 A degrees C

The corrosion behaviour of a FeCrAl alloy was investigated at 600 A degrees C in O-2 + H2O with solid KCl applied. A kinetics and microstructural investigation showed that KCl accelerates corrosion and that potassium chromate formation depletes the protective scale in Cr, thus triggering the formation of a fast-growing iron-rich scale. Iron oxide was found to grow both inward and outward, on either side of the initial oxide. A chromia layer is formed with time underneath the iron oxide. It was found that although the alloy does not form a continuous pure alumina scale at the investigated temperature, aluminium is, however, always enriched at the oxide/alloy interface

Influence of Segregants on Creep Fracture in Powder Metallurgical Martensitic Stainless Steel

This thesis deals with the behaviour of segregants in Powder Metallurgical (PM) 10%Cr steel and their impact on creep fracture. Particular interest is taken in determining the reaction products involved in creep cavitation and how these are connected to the powder surface reactions during the gas atomisation and consolidation (hot isostatic pressing). The techniques applied include primarily Auger electron spectroscopy (AES) as well as Electron Spectroscopy for Chemical Analysis (ESCA), Secondary Ion Mass Spectroscopy (SIMS), Scanning Electron Microscopy (SEM) and optical microscopy. To facilitate thickness determination of reaction products on non-planar surfaces such as powder, a method for AES depth profiling was established based on the knowledge of the etch rate dependence on the angle of ion incidence and a stereophotogrametric technique. Three different PM 10%Cr steels are studied; two 10.5Cr1W1MoVNb steels, with and without 0.002wt% B addition, and one 11Cr2WVNb steel. The alloys are included in a national program aimed at developing new steam turbine materials. The goal is to enable use at temperatures above 600 \ub0C (compared to presently used 550-565 \ub0C), whereby improved energy efficiency should result. The analyses demonstrate the relation between the surface reactions during atomisation and consolidation, leading to products on the prior particle boundaries (PPBs), and the creep cavity formation. During atomisation, surface enrichment of impurities (e.g. Sn, Zn, Cu, S and possibly Ca) and minor alloying constituents (e.g. B in the B-alloyed steel) occurs in addition to the principal surface products; Fe, Cr and Mn oxides. The enrichment of several elements (Mn, Sn, Zn, Cu and S) can be attributed to the associated lowering of the surface energy of the liquid metal droplet. Powder finer than about 40 um is oxidised to much lesser degree and has a higher surface content of Mn and Cr oxides. The higher surface content of surface active elements (e.g. Mn and S) on smaller particles is suggested to result from these particles rapidly being highly undercooled before solidifying, whereby diffusion to the surface can dominate over evaporation. The decoration of PPBs in the compacted materials can generally not be visualised by means of optical microscopy. However, this was possible using imaging SIMS, which revealed PPB oxides containing Cr, Mn, Si and V together with the impurities Ca, Na, Al, Ti and, in B-alloyed material, B. In the B-alloyed material, coarse PPB inclusions are found. These inclusions consist of mixed B nitride and oxide containing Mn, Cr and Si. It is suggested that the B on the powder surfaces enhances the coarsening of the PPB products. Correlation between MnS inclusions and PPBs is also indicated. The creep rupture characteristics differ between the alloys with and without B. In the former alloy, creep cavity coalescence and creep fracture occurs at the former austenite grain boundaries (FAGBs), irrespective of creep test conditions. In the latter alloy, creep cavities coalesce much less, and with extended creep test duration, creep fracture appear to an increasing amount at PPBs. Only about 10% of the FAGBs are contiguous with the PPBs in both alloys. However, the FAGB/PPB contiguity is a crucial factor leading to creep cavity formation. In the B-alloyed material, creep cavity formation is further facilitated by the coarse PPB inclusions present. Irrespective of alloy composition, Cr-containing carbide/nitride appear together with segregated S in creep cavities. Furthermore, Sb and Sn are enriched in creep cavities. The enrichment of Sb increases with the cavity size, whereas also Sn is encountered in larger developed cavities. It is supposed that the presence of these elements in creep cavities enhances the cavity growth. Phosphorus is only found at limited levels in smaller cavities

Scale growth on austenitic alloys under KCl deposits at 500 deg 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 deg 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 mum thick and only few chromates were seen. Crystals of KCl formed in areas with thick porous and Fe rich oxides on both alloys

Field test of superheater corrosion in a CFB waste boiler: Part II - Scale formation characteristics

This study concerns the scales formed on the steels T22, Alloy 310, Alloy 28 and the nickel-based Alloy 65 in a superheater test coil at 460-540\ub0C in a CFB waste boiler. The methods used for the characterisation of the scales included SEM, EDX, Auger spectroscopy and XRD. The deposits on the tubes consisted mainly of alkali chlorides and calcium sulphate. The scales formed consisted of Fe 2 O 3 and Fe 3 O 4 on the T22 steel, NiFe 2 O 3 and Cr 2 O 3 on Alloy 310 and Alloy 28, and Cr 2 O 3 and NiO on Alloy 65. Rapid corrosion on the steel T22 was associated with the growth of an open columnar iron oxide below a thick porous chlorine-containing scale. Pitting corrosion on Alloy 310 occurred and it may be associated with selective corrosion, first following the grain boundaries then uniformly attacking the metal. The only protective oxide was observed on Alloy 28 that formed an inner chromium oxide separating the chlorides from the metal. Dense thin chromium oxides were observed in the scale on Alloy 28, but no major cracks were found perpendicular to the tube. Alloy 65 suffered from grain boundary attack and was locally attacked under thick porous chromium oxide with nickel chlorides in the advancing front. Molybdenum was enriched at the interface to the metal on both Alloy 28 and Alloy 65. \ua9 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Corrosion at the urea injection in SCR-system during component test

The corrosion behaviour of the austenitic stainless steels 304L and 904L was investigated after component testing in diesel exhaust urea environment. Cross-sectional analyses and depth profiling of corrosion products were performed with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The samples of both materials displayed high amounts of silicon oxide on their surfaces, though this did not appear to have an active role in the corrosion. The higher alloyed 904L exhibited only small signs of corrosion and the inner oxide below the silicon oxide was chromium rich. Substantial nitrogen uptake was detected, presumably due to the presence of NH3, cyanates and similar species. The lower alloyed 304L showed more severe corrosion. The oxide on the 304L samples mainly consisted of iron oxide apart from the silicon oxide present. The nitrogen uptake was less pronounced and possibly connected to presence of nitride precipitates. The most pronounced corrosion was detected at sites where deposits frequently form and decompose. Those deposits are a probable nitrogen source and may also form compounds enabling the breakdown of the oxide

Scale growth on austenitic alloys under KCl deposits at 500 deg 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 deg 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 mum thick and only few chromates were seen. Crystals of KCl formed in areas with thick porous and Fe rich oxides on both alloys