Selective Dissolution Of Iron Research Articles

Morphological and Structural Transformations of Fe-Pd Powder Alloys Formed by Galvanic Replacement, Annealing and Acid Treatment.

In this article, we report the preparation and structural features of Fe-Pd powder alloys formed by galvanic replacement, annealing and selective dissolution of iron via acid treatment. The alloys were studied by the X-ray diffraction phase analysis, Mössbauer spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The Fe@Pd core–shell particles were obtained by a galvanic replacement reaction occurring upon treatment of a body-centered cubic (bcc) iron powder by a solution containing PdCl42− ions. It was found that the shells are a face-centered cubic (fcc) Pd(Fe) solid solution. HCl acid treatment of the Fe@Pd core–shell particles resulted in the formation of hollow Pd-based particles, as the bcc phase was selectively dissolved from the cores. Annealing of the Fe@Pd core–shell particles at 800 °C led to the formation of fcc Fe-Pd solid solution. Acid treatment of the Fe-Pd alloys formed by annealing of the core–shell particles allowed selectively dissolving iron from the bcc Fe-based phase (Fe(Pd) solid solution), while the fcc Fe-rich Fe-Pd solid solution remained stable (resistant to acid corrosion). It was demonstrated that the phase composition and the Fe/Pd ratio in the alloys (phases) can be tailored by applying annealing and/or acid treatment to the as-synthesized Fe@Pd core–shell particles.

Influence of the transient conditions on release of corrosion products and oxidation of alloy 690 tubes during pressurized water reactor restart after steam generators replacement

The radioactivity of the Reactor Coolant System (RCS) of Pressurized Water Reactors (PWRs) mainly comes from the release of corrosion products of Steam Generator (SG) tubes made of Ni-base alloys. In order to reduce this activity and thus the radiation exposure of PWR operators during maintenance operations, it is necessary to minimize the release. That requires prior understanding of the various mechanisms involved. EDF R&D constructed a loop, BOREAL, to specifically measure rates of release of SG tubes in various conditions of primary environment. Tubes were usually tested at high temperature, under constant conditions of primary chemistry. So, it is necessary to carefully investigate the impact of transient conditions during a PWR restart after SG replacement. Tests were performed on the industrial material with curvature, roughness, defects and heterogeneities, regularly observed on this type of component. Characterisations of the inner surface were done on as-received and corroded specimens of SG tubes and were correlated with the obtained release kinetics.The native oxide layer is formed of a very thin layer (1-2 nm) of oxidised matrix, without specific enrichment. During the restart, the most critical step for the release phenomenon is revealed from 170°C to 297°C. The majority of the metal is indeed released into the fluid during this step. The characterisations after release tests have made it possible to propose oxidation and release mechanism during a PWR restart after SG replacement. Up to 170°C, a thin layer of amorphous chromium oxide is formed by selective dissolution of iron and nickel. When the temperature rises, this chromium oxide layer is not stable enough to be protective and the diffusion phenomena are activated. At 325°C, the oxide does not exhibit any particular enrichment and corresponds to an oxidised metal layer; an equilibrium is established and the rate of release reaches a pseudo-stationary regime.

Copper Enrichment in Solid with Selective Reverse Leaching with Oxalic Acid

As the first part of a novel process proposed for copper extraction, selective dissolution of iron from chalcopyrite in oxalic acid solutions with hydrogen peroxide was studied. Despite the fact that both iron and copper are readily soluble in acidic conditions, it was found that oxalic acid provides altered solubility behaviors for these metals. It should be used to form iron (II) oxalate which has a higher solubility (0.008 g/100 g H2O at 293 K) than copper oxalate (2.16 × 10–10 g/100 g H2O at 293 K), while hydrogen peroxide was used to provide an oxidative leaching environment to break the chalcopyrite structure. Under the examined leaching conditions where 100 g/L of H2C2O4, 3 M of H2O2, 318 K of leaching temperature, 120 min of leaching time, 25 mL/g of liquid–solid ratio, and 400 rpm of stirring speed were applied, extraction rates of copper and iron from the chalcopyrite concentrate were almost 1.5% and 70%, respectively. Copper was found as a solid oxalate form in the residue obtained after the leaching process in these conditions.

Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active–passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H 2SO 4. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm −2. A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm 2. Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.

A more general method for ranking the enrichment of alloying elements in passivation films

AbstractThe corrosion of alloys is often characterized by an enrichment of one or other of the alloying elements within the surface oxide or even within the underlying metallic phase. For some three decades the measurement of such surface enrichment has been one of the most important contributions made by XPS to corrosion science. X‐ray photoelectron spectroscopy permits the composition of the oxide film to be distinguished from that of the metallic surface underlying the oxide so that both can be determined independently. In the mid‐1970s these measurements were crucial in showing that the selective enrichment of chromium in the electrochemical passivation of stainless steel occurred by selective dissolution of iron, and not by selective oxidation of chromium. Measurements of this type are important also in active corrosion, where there is no stable oxide film, as in studies of dealloying phenomena.The literature now contains numerous cases, many published by one of us (K.A.) based on studies of novel alloys produced in the Institute for Materials Research at Sendai. Typically, measurements are made for a series of binary compositions AxBy ranging from A‐rich to B‐rich alloys, in media that reflect the intended use of the alloy. The results are normally produced in the form of plots of cation composition An+/(An+ + Bm+) against bulk composition A/(A + B) or, in the case of dealloying, as Asurface/(Asurface + Bsurface) against the bulk composition. Although graphical representation is useful in giving the full picture of the alloy's behaviour, it is not so useful in cataloguing or indexing performance.In this paper we suggest a means to give a rank or performance index as a single number that will characterize the behaviour of the alloy over a wide range of composition for a given medium or exposure condition. The rank number does not imply that any particular mechanism of enrichment is in operation; in various cases of corrosion it might occur by selective precipitation (FeOOH on steels, CuO on cupronickels), by selective dissolution (dezincification of brass) or by preferential incorporation of ions in a passivating oxide or oxyhydroxide (Cr3+ on stainless steels). Although the rank has no mechanistic implications, it is useful within a known class of mechanistic behaviour for indicating the magnitude of the effect and thus for enabling an XPS measurement to be indicative of wanted, or unwanted, behaviour. For example, the rank number could be useful in indicating, perhaps from a single alloy composition, whether a degree of passivation is likely to be achieved for that mix of metallic elements. Copyright © 2004 John Wiley & Sons, Ltd.

Cyclic response-electrochemical interaction in mono- and polycrystalline AISI 316L stainless steel in H 2SO 4 solution—II. Potential dependence of the transient dissolution behavior during corrosion fatigue

The transient current behaviors of an AISI 316L stainless steel, in both mono- and polycrystalline forms, have been studied in corrosion fatigue tests performed in a 1N H 2SO 4 solution, under different potentials. Extreme care in experimental technique and instrumentation have disclosed minute current variations which provide information about mechanisms. The effects of both the cyclic process and the applied potentials on the current behavior have been investigated, in following up a study of the influence of strain on the transient current behavior (Part I). Selective dissolution of iron from slip steps has been demonstrated to have a major influence on the transient current, during the early stage and the cracking stage of cycling, and does not depend on applied potentials. A significant potential dependence of the transient current was found, however, in the stage of stable current (saturation), which is correlated with different electrochemical reactions in the slip bands. Corrosion fatigue lives were measured and analyzed in relation to the current behavior.

Cyclic response-electrochemical interaction in mono- and polycrystalline AISI 316L stainless steel in H 2SO 4 solution — I. The influence of mechanical strain on the transient dissolution behavior during corrosion fatigue

Mono-γ and polycrystalline specimens of an AISI stainless steel were cycled in a 1N H 2SO 4 solution under different potentials. The correspondence between strain wave form and transient current curve has been studied. Transient current, which has been measured with out-of-the-ordinary resolution, has been demonstrated to have two components: the current associated with plastic strain (CAPS) and the current associated with elastic strain (CAES), caused exclusively by plastic deformation and elastic deformation respectively. The strong interaction between the environment and the fresh area at slip steps is the main cause of the CAPS. The wave forms of the CAPS is affected by selective dissolution of iron, tension-compression asymmetry, and the fatigue cracking process. The CAES is produced by the variation in the energetic status of the surface, and possibly by the charging effect of the electric double layer.

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 H 2O and 0.5 M NaCl) and acid solutions (1 M and 9.2 M of HNO 3 and H 2SO 4) 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 MH 2SO 4. 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.

Mechanical and chemical properties of tantalum-implanted steels

The surface mechanical and chemical properties of tantalum-implanted AISI 52100, AISI M50 and AISI 9310 steels and pure iron were investigated. Sputter Auger profiles of pure iron indicate significant carbon incorporation during implantation. For AISI 52100 steel the unlubricated kinetic coefficient of friction is reduced from 0.6 to 0.38, the load-carrying capacity is increased and the pitting potential in a 0.01 M NaCl solution is increased by 510 mV. The corrosion resistance of tantalum-implanted AISI M50 steel in 0.5 M H 2SO 4 is equal to that of high dose chromium implantation. The rolling contact fatigue life is significantly improved for tantalum-implanted AISI M50 steel and a 24% increase in load-carrying capacity is measured for Ryder gear scuffing tests on tantalum-implanted AISI 9310 steel. The mechanism producing the improvements in corrosion resistance is thought to be selective dissolution of iron with the formation of a tantalum-rich passive oxide film on the surface, while friction reduction is at least partly responsible for improving the wear properties.

Quantitative Measurements of Passive Dissolution of Chromium, Iron, and Molybdenum from a Stainless Steel

Selective dissolution behavior and concomitant changes in surface composition have been elucidated by combined γ‐spectrometry and ESCA studies of a passive stainless steel immersed in an artificial saliva solution. Quantitative γ‐spectrometry data show that there is a selective dissolution of iron during all times of exposure. Quantitative ESCA data suggest that all expected surface accumulation of chromium occurs in the passive film formed. Under present conditions, the chromium content of the film increases With time of immersion, having its highest values at the outermost surface region and exhibiting a decrease when approaching the film/alloy interface.

Ion beam analyses of surface films formed on amorphous Fe12Mo18C alloy in 1 N HCl

The Rutherford backscattering with 2 MeV He + and the 16O(d,p) 17O ∗ nuclear reaction with 1.8 MeV D 2 + were utilized to determine the concentration profiles of iron, molybdenum and oxygen in the surfaces of an amorphous Fe12Mo18C alloy polarized in 1 N HCl at potentials from − 0.17 to 1.6 V(SCE). The thickness of the surface film was estimated as 20–200 nm. In the primary active region, selective dissolution of iron and carbon causes enrichment of molybdenum ions in the corrosion product film and of metallic molybdenum in the topmost part of the underlying alloy. A further potential increase led to a decrease in the molybdenum content in the film by transpassive dissolution. Molybdenum ions were not concentrated in the film formed in the stable passive region of 0.5–1.5 V(SCE). The high passivating ability of the amorphous alloy even in the aggressive HCl was ascribed to the homogeneity of the alloy structure as well as to the effect of molybdenum.

ChemInform Abstract: EFFECT OF HELIUM, IRON, AND PLATINUM IMPLANTATION ON THE ABSORPTION OF HYDROGEN BY IRON

AbstractPermeationsmessungen an Ferrovac‐E‐Eisenmembranen zeigen, daß die Implantation von Pt die Kinetik der H2‐Entwicklungsreaktion und somit auch die Wasserstoffabsorption beeinflußt. Sowohl in 0.1 N NaOH als auch in 0.1 N H2SO4 nimmt die Geschwindigkeit der Absorption mit steigender Pt‐Konzentration an der Oberfläche zu, wofür ein katalytischer Mechanismus verantwortlich gemacht wird.

Effect of Helium, Iron, and Platinum Implantation on the Absorption of Hydrogen by Iron

A preliminary study was made of the effect of certain elements implanted in iron on the absorption of hydrogen by Ferrovac‐E iron. Using the permeation technique, it was found that the location of implanted Pt, as modified by selective dissolution of iron from the surface, affects the kinetics of the hydrogen evolution reaction and, hence, of the hydrogen absorption process. The rate of hydrogen absorption decreased with increase in Pt concentration on the surface in both and . A catalytic mechanism is proposed to explain the marked reduction in hydrogen permeation. There are no significant differences in the permeation behavior of unimplanted and helium‐ or iron‐implanted iron membranes in . The experimentally observed Tafel slope, the permeation‐(charging) potential relationship, and the permeation‐(charging) current relationship indicate a coupled discharge‐recombination mechanism of hydrogen evolution on He‐, Fe‐, or Pt‐implanted iron. At higher cathodic overpotentials in , corresponding to potentials more negative than −1.0V (SHE), another mechanism of hydrogen evolution is indicated. Selective dissolution of iron from the Pt‐implanted Fe surface layer may involve some platinum and iron interdiffusion according to Rutherford backscattering analyses.

The effect of chloride ions on passive layers on stainless steels

The effect of chromium on the formation and stability of the passive film formed on ferritic stainless steels (5.9 – 29.9% Cr) in 3% NaCl was studied by electron spectroscopy for chemical analysis. The film was formed by selective dissolution of iron with concomitant enrichment of chromium on the surface. The chromium enrichment depends on the alloy concentration and is proportional to the logarithm of the exposure time. In chloride-free solutions the same mechanism occurs but with a lower dissolution rate. The stability of the film depends on the chromium content in the steel and on the chloride activity. Low chromium steel does passivate in pure water but the film will break down in the presence of chloride ions. Even with high chromium steels there is a reversal of the chromium build-up after several hundreds of hours in salt water, which must be assumed to lead to eventual breakdown. When steels containing 11% chromium or more are pre-passivated in pure water and are then immersed in salt water, the enrichment of chromium proceeds at the rate originally established in pure water, i.e. much more slowly than when fresh surfaces are exposed to salt water. Pre-passivation of low chromium alloys (5.8% and 7.8% Cr) in pure water causes films to be formed which are soon destroyed in salt water.

The passive state of stainless steels

With the advent of surface-sensitive techniques it has become possible to determine the surface composition of alloys. Analyses using electron spectroscopy for chemical analysis of CrNiMo alloyed steels exposed in acids and neutral solutions show that the reaction products formed on the surface consist mainly of chromium oxide and hydroxide. Nickel is not present in the passive films. The molybdenum content in the oxide is of the same order of magnitude as in the alloy. It is suggested that the beneficial effect of molybdenum is not related to the molybdenum content in the oxide. It is shown that the alloy elements nickel, molybdenum and chromium are accumulated in the metal phase underneath the oxide and that the enrichment of chromium seems to be dependent on the molybdenum content of the alloy. The enrichment of these elements is explained by selective dissolution of iron during the active phase preceding passivation. It is postulated that increased surface concentrations of chromium and molybdenum in the metal synergistically lower the dissolution rate in the active phase and provoke the formation of passive films of high chromium content both at first passivation and during repassivation in conditions of pitting and crevice attack.

The electrochemical behaviour of 18Cr-8Ni stainless steel in hot concentrated aqueous magnesium chloride solution

The electrochemical behaviour of 18-8 stainless steel in concentrated aqueous magnesium chloride solution (bp 154°C) at 140°C has been examined. “Active” anodic dissolution of material carrying the original air-formed oxide film begins below −250 mV(she). As the potential is raised, passivation slowly sets in, possibly through the selective dissolution of iron and chromium. Active dissolution begins again at −170 mV(she) and increases up to ca. −130 mV, when passivation by oxide-film formation begins; this is substantially complete between −120 and −110 mV(she). Breakdown of the “oxide” passivity by chloride ion begins at −110 mV(she) and major pitting with large anodic c.d. is observed at ca. −80 mV(she) and above. These values, obtained by slow potentiodynamic scan (2·5 mV/min) in the positive direction, are confirmed by potentiostatic current/time observations. The very large variation of anodic dissolution rate over a very small potential range is noted as being significant in the mechanism of stress-corrosion crack propagation in these alloys exposed to chloride-containing environments.