Gunnar Hultquist

The influence of minor alloying elements (Nb, Ti and Cu) on the corrosion resistivity of ferritic stainless steel in sulfuric acid solution

Abstract The effect of the minor alloying elements (Nb, Ti and Cu) and of Moon the corrosion resistivity of ferritic stainless steel (Fe-26Cr alloy) was investigated in 1.0 mol dm −3 sulfuric acid solution with or without 0.5 mol sodium chloride, based on potentiodynamic polarization, potential decay and reactivation potential measurements. Surface characterization of the ferritic stainless steel subjected to active dissolution or passivated was performed with simultaneous use of Auger electron spectroscopy and an argon-ion sputter-etching technique. Copper and molybdenum suppressed the anodic dissolution of the steel. Copper and niobium were significantly enriched on the steel surface during active dissolution. A chromium enrichment was always found in the passive film of all the investigated steels. No enrichment of the other alloying elements was observed in the passive film and the substrate. Niobium, titanium and molybdenum stabilized the passive film, whereas copper had a detrimental effect on the passivity. The influence of the alloying elements on the corrosion resistivity of the steel is discussed in relation to the surface analytical results.

Selective dissolution and surface enrichment of alloy components of passivated Fe18Cr and Fe18Cr3Mo single crystals

Abstract The possible surface enrichment of chromium and molybdenum during dissolution of Fe18Cr (110) and Fe18Cr3Mo (110) alloys at constant potentials in the passive region is elucidated by taking into account quantitative information on partial dissolution rates of alloy components as measured by γ-spectrometry and on chemical composition of passivating films as measured by ESCA and AES. Combining results from all methods it is found that chromium under all conditions is enriched in the passivating films. An accumulation of chromium in the alloy is also indicated. For molybdenum, an enrichment in the passive film is observed at − 0.2 V (SCE), whereas in the potential interval 0.1–0.9 V, the molybdenum enrichment as determined by ESCA and AES is hardly significant. On the other hand, γ-spectrometry gives a clear indication of molybdenum enrichment in both potential regions.

Hydrogen and Yttria in Chromium: Influence on the High-Temperature Oxidation Kinetics in O2, Oxide-Growth Mechanisms and Scale Adherence

Oxidation of chromium in 20 mbar isotopicallylabeled molecular oxygen was studied at 900°C. Thesamples comprised specimens with as-received contents ofhydrogen and specimens where this hydrogen had been eliminated by vacuum annealing. Two-stageoxidation was performed by exposure of the samples to16,16O2 and 18,18O2. Reaction rates were studied bymeasuring the change in oxygen pressure in a closedreaction-chamber with a mass spectrometer during oxidation. Theoxide formed was characterized by secondaryion massspectrometry (SIMS) and scanning electron microscopy(SEM). The adherence of the oxide scale to theunderlying metal substrate is poor and the oxidation rateis initially slightly higher when the metal containshydrogen prior to oxidation. It is shown by 18O depthprofiles that chromium diffusion within theCr2O3 scale is increased and oxygen diffusion isdecreased for metal containing hydrogen. It is believedthat the poor adherence of the oxide scale, in thiscase, is due to low anion diffusion relative to cation diffusion, which leads to an insufficientportion of oxide growth at the metal-oxideinterface.

Surface composition studies of the (100) and (110) faces of monocrystalline Fe0·84Cr0·16

Abstract Surface compositions of the (100) and (110) faces of monocrystalline Fe0.84Cr0.16 have been determined using Auger Electron Spectroscopy (AES) and monocrystalline Cr (100), Fe (100), Cr (110) and Fe (110) surfaces as standards. When subjected to different annealing procedures with and without oxygen gas present, drastic changes in Cr enrichment occurred in the investigated surface region, the thickness of which is of the order of 10 A. Under thermal equilibrium conditions the Cr Fe ratio is mainly influenced by differences between Cr and Fe as to surface free energy and affinity to oxygen whereas far from equilibrium, the difference in diffusion rate between Cr and Fe also plays an important role.

Corrosion of Copper by Water

We have studied copper corrosion in a system comprised of deionized water, absolute pressure gauges, and a palladium membrane. A transition from O-2-consuming to H-2-evolving copper corrosion is ob ...

Initial oxidation stages on FeCr(100) and FeCr(110) surfaces

Abstract Simultaneous LEED and AES are used to follow early stages of oxidation of monocrystalline FeCr(100) and (110) between 700 and 900 K in the oxygen pressure range 10−9–10−6 Torr. A chromium-rich oxide region at the alloy/oxide interface is observed, which exhibits different surface structures on oxidized FeCr(100) and FeCr(110). The chromium concentration in this initially formed oxide film is found to be enhanced by low oxygen pressures or high temperatures. During further oxidation different behaviours are observed on FeCr(100) and FeCr(110), which are explained by assuming different ion permeabilities through the initial chromium rich oxide regions on the two surface planes. On FeCr(110) surfaces oxidation is initiated on chromium enriched (100) facets at 800 K or below. At 900 K a film consisting of rhombohedral Cr2O3 or (Fe, Cr)2O3 is epitaxially growing with its (001) plane parallel to the alloy (110) face. On FeCr(100) surfaces the chromium rich oxide region next to the substrate is of fcc type. As soon as the diffusion of iron from the alloy to the gas/oxide interface is observable, a spinel type oxide is formed and connected with the location of iron in tetrahedral lattice sites. Closer to the fcc lattice the spinel oxide consists of FeCr2O4 or a solid solution of FeCr2O4 and Fe3O4 whereas next to the gas phase the oxide is pure Fe3O4.

Hydrogen evolution in corrosion of copper in pure water

Abstract The corrosion of copper in pure water has been studied. An increase in hydrogen concentration has been found in the gas volume above the water level. When the abundance of oxygen is low the hydrogen evolution rate, monitored above the water, is similar to the corrosion rate, as measured by weight gain of the copper.

A LEED/AES study of the oxidation of Fe0.84Cr0.16 (100) and (110)

The interaction between single crystalline Fe0.84Cr0.16 (100) or (110) and oxygen gas in the pressure range 10−9 to 10−7 torr was studied at room temperature and at 800 K, using LEED and AES. The interaction starts with a chromium-oxygen reaction next to the alloy surface, followed by an iron—oxygen reaction outside the chromium-oxygen layer. At 800 K these reactions are connected with redistribution of cations between the interior of the alloy and the surface region, whereas at room temperature only a redistribution of cations within the surface region is observed. Different symmetries and lattice parameters of oxides which grow epitaxially on Fe0.84Cr0.16 (100) are compared with the corresponding surface compositions. It is found that the formation of spinel-like oxide layers is favoured by lower values of the Cr/Fe surface ratio.

The dissolution behaviour of iron, chromium, molybdenum and copper from pure metals and from ferritic stainless steels

Dissolution rates of iron, chromium, molybdenum and copper as pure metals and as components of three ferritic stainless steels in neutral solutions (distilled H2O and 0.5 M NaCl) and acid solutions (1 M and 9.2 M of HNO3 and H2SO4) are presented. The relative position of dissolution rates of the elements is in general the same in pure metal and alloy dissolution. Selective dissolution of iron and simultaneous surface enrichment of chromium is observed under all the investigated conditions. A slight surface enrichment of molybdenum is found upon exposure in the neutral and sulfuric acid solutions but not in nitric acid solution. Large amounts of copper are found in the surface of the copper-containing steel exposed to 1 MH2SO4. Evidence is presented which shows that an increase in electrolyte volume decreases the thickness and changes the composition of the passive film. It is believed that this hardly ever mentioned effect is due to an increased amount of dissolution of the film components upon increased volume of electrolyte.

Self-repairing metal oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O2 gas near 20 mbar. Information on the position of oxide growth has been gained from the 18O/SIMS technique. Rates of O2 dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O2. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O2. An increased rate of O2 dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.