Chromium Enrichment Research Articles

The effect of chromium enrichment in the film formed by surface treatments on the corrosion resistance of type 430 stainless steel

The effect of chromium concentration in the surface film formed by surface treatments on the corrosion resistance of Type 430 stainless steel was studied. It was found that the chromium content depends on the surface treatment method. HNO3 treatments result in a significant enrichment of chromium in the surface film on the Type 430 stainless steel, and the chromium enrichment increases with increasing HNO3 concentration. The pitting potential (Ep), corrosion potential (Ec) and polarization resistance (Rp) are changed by the chromium enrichment (SCr) in the surface film formed by treatment. The higher the SCr value, the higher the Ep, Ec and Rp values.

The influences of Mo addition and air exposure on the corrosion behavior of amorphous Fe8Cr13P7C alloy in de-aerated 1 M HCl

The change in the corrosion behavior of amorphous Fe8Cr xMo13P7C ( x = 0, 2, 4, 6) alloys in deaerated 1 M HCl with molybdenum content and pretreatment has been studied. Amorphous Fe8Cr13P7C alloy shows active dissolution in de-aerated 1 M HCl. This alloy becomes corrosion-resistant with the addition of molybdenum. X-ray photo-electron spectroscopic analysis reveals that the chromium content of the films decreases continuously with immersion time despite an initial sharp increase in chromium concentration in the surface film. No enrichment of chromium is detected in the surface films on molybdenum-free and molybdenum-bearing alloys except for 6 at % molybdenum alloy after prolonged immersion. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys throughout the immersion experiment for 20 h. Molybdenum oxyhydroxide, rather than chromium oxyhydroxide, is considered to be responsible for the formation of the passive film on molybdenum-bearing alloys immersed in de-aerated 1 M HCl. The addition of 6 at% molybdenum is effective in maintaining metallic luster for prolonged immersion due to the formation of the passive film in which molybdenum and to some extent chromium are concentrated. Phosphorus is little involved in the airformed films on all alloys but accumulates by immersion in the passive films on molybdenum-containing alloys up to its bulk composition, whereas the phosphorus content of the surface film on the molybdenum-free alloy is low. Air exposure for 24 h prior to polarization or immersion raises the initial open circuit potential of specimens and retards the enrichment of molybdenum in the passive film on the 6 at% molybdenum alloy in de-aerated 1 M HCl.

Statistical comparisons of heavy-metal concentrations in river sediments

Statisticalt tests were used to determine lead, copper, and chromium enrichment in sediments from the Lower Branch of the Rouge River in southeast Michigan, USA. Both absolute metal concentrations and ratios of trace metal to conservative metal concentrations were used to compare sampled sites along the Lower Branch of the Rouge River to background sites in the headwaters region. Concentration ratios were used to reduce the effects of certain chemical and physical characteristics on the level of metal contained in a given sediment. Results from the comparison of sample sites to the background reveal metal enrichment at several sites, particularly along the highly urbanized, downstream section of the river. This section of the Lower Branch of the Rouge River exhibits significant lead and copper contamination, as well as measurable chromium enrichment when using either concentrations alone or ratios as methods of comparison. The areas of metal enrichment appear to coincide closely with areas of known anthropogenic activities. Of particular interest, however, is the enrichment of lead and copper at two upstream sites where the statistical tests suggest an anthropogenic source for the enrichment, but where no previously known cultural activities existed. These data prompted a historical search of records, which discovered several abandoned landfills immediately upstream of the metal enrichment sites.

Statistical comparisons of heavy-metal concentrations in river sediments

Statisticalt tests were used to determine lead, copper, and chromium enrichment in sediments from the Lower Branch of the Rouge River in southeast Michigan, USA. Both absolute metal concentrations and ratios of trace metal to conservative metal concentrations were used to compare sampled sites along the Lower Branch of the Rouge River to background sites in the headwaters region. Concentration ratios were used to reduce the effects of certain chemical and physical characteristics on the level of metal contained in a given sediment. Results from the comparison of sample sites to the background reveal metal enrichment at several sites, particularly along the highly urbanized, downstream section of the river. This section of the Lower Branch of the Rouge River exhibits significant lead and copper contamination, as well as measurable chromium enrichment when using either concentrations alone or ratios as methods of comparison. The areas of metal enrichment appear to coincide closely with areas of known anthropogenic activities. Of particular interest, however, is the enrichment of lead and copper at two upstream sites where the statistical tests suggest an anthropogenic source for the enrichment, but where no previously known cultural activities existed. These data prompted a historical search of records, which discovered several abandoned landfills immediately upstream of the metal enrichment sites.

Microstructure Evolution of Selected Ferriticmartensitic Steels under Dual-Beam Irradiation

AbstractWe present experimental results on the microstructure evolution of dual-beam irradiated (300 keV heavy ions plus 15 keV helium ions) ferritic-martensitic steels (Manet, DIN 1.4926, F82H mod). The helium bubble morphology as well as microchemistry of the alloys are investigated by means of transmission electron microscopy and field-ion microscopy with atom probe. The alloys were irradiated to fluences up to 50 dpa and implanted with helium up to a concentration of 1 at.% at the temperatures of 723 K and 773 K. The damage and implantation rates variied from 2.5 · 10−3 dpa/s to 2.5·10−2 dpals and from 0.5 appmls to 5 appm/s, respectively. Size and number density of helium bubbles is found to be rate dependent. Smaller implantation rates produce larger helium bubbles and smaller bubble number densities. Regions of local enrichment of alloy elements, typically 5 nm in size, containing chromium (up to 40 at.%), silicon, and nickel are detected. Number densities of helium bubbles and of regions of chromium enrichments are comparable and lie between 1023/m3 and 1024/m3. Possible extrapolation of the present ion irradiations to spallation source and fusion reactor conditions is shortly adressed.

The effect of air exposure on the corrosion behavior of amorphous Fe-8Cr-Mo-13P-7C alloys in 1 M HCl

Amorphous Fe-8Cr-13P-7C alloy is corroded severely in 1 M HCl open to air. The addition of more than 2 at% molybdenum is effective in promoting spontaneous passivation. According to XPS analysis, chromium accumulates continuously in the surface film on molybdenum-containing alloys throughout the immersion experiment for 20 h, whereas for the molybdenum-free alloy an initial slight increase in chromium concentration in the surface film is followed by an almost continuous decrease. Molybdenum and phosphorus are deficient in the air-formed film but become concentrated in the film with immersion time. The Mo 4+ species which is the constituent of the passive film on molybdenum metal is increased. Air exposure for 30 min prior to the polarization or immersion experiment in 1 M HCl raises the open circuit potential of specimens and retards the enrichment of chromium in the passive film on molybdenum-bearing alloys.

Passivity of high-nitrogen stainless alloys: the role of metal oxyanions and salt films

Abstract Surface-analytical studies of high-nitrogen austenitic stainless steels exposed to de-aerated 0.1 M HCl have revealed that nitrogen alloying additions influence the composition of salt layers and the passive film/alloy interface. It was shown that nitrogen, nickel and molybdenum additions stimulate selective dissolution of iron, resulting in a significant enrichment of chromium beneath the passive film. The build-up of a protective ferrous molybdate layer was seen to be most strongly enhanced with additions of nickel and, to a lesser extent, nitrogen. While the primary kinetic barrier to anodic dissolution of high-nitrogen stainless steels is a chromium-oxide-based passive film, it appears that a mixed nitride surface layer and an ultra-thin layer of ferrous molybdate act as secondary kinetic barriers.

Effects of tensile and compressive stresses on the passive layers formed on a type 302 stainless steel in a normal sulphuric acid bath

Type 302 stainless steel samples were subjected to a U-bend test in a normal sulphuric acid bath at room temperature. The passive layers which subsequently grew on both sides of the sample, were studied by Auger electron spectroscopy. Each strained sample was immersed and passivated in a sulphuric acid solution under a voltage of +450 mV/SCE and the anodic current density was studied as a function of the immersion time. The results obtained show that the anodic current density is higher for the strained specimens than for the unstrained specimen. Thus, passivation occurs more rapidly for an unstrained sample than for strained samples. There is also clear evidence that the decrease in the anodic current density is more rapid for an elastically strained sample than for a plastically strained one. The influence of the tensile and compressive stresses on the composition of the passive films was studied by comparing the elemental Auger depth profiles. The results obtained show that oxidation is more important on the tensile stressed side than on the compressive stressed one, whether the samples were elastically or plastically strained. Similarly, the width of the region affected by the chromium enrichment was enhanced by tensile straining and reduced by compressive straining, whether the samples were elastically or plastically strained.

Size fractionated impactor sampling of aerosol particles over the Atlantic Ocean from Europe to Antarctica as a methodology for source identification of Cd, Pb, Tl, Ni, Cr, and Fe

Four different samples of aerosol particles over the North Atlantic, South Atlantic, an area near the equator influenced by Saharan mineral dust, and the Antarctic Ocean were collected on board of the German research vessel “Polarstern” by a six stage cascade impactor system. A continental sample, typical in its size distribution pattern and heavy metal enrichment factors (relative to crust material) for industrialized areas, was used for comparison. To analyse the elements of interest, isotope dilution mass spectrometry (IDMS) using the thermal ionization technique was applied. The samples were digested with nitric acid followed by an electrodeposition of the heavy metals to be analysed in alkaline solution. Source identification could be carried out by the distribution of the heavy metals and enrichment factors on the different impactor stages using iron as a reference element for crustal origin. Two opposite types of size distribution patterns were obtained over the Atlantic Ocean. On the one hand, the main heavy metal fraction was found to be associated with the smallest particles collected on the last two impactor stages and the back-up filter with aerodynamic diameters (AD) of less than 0.95 μm. This pattern together with the high enrichment factors of up to several thousand indicates combustion processes and biogenic emissions as possible sources and were typical for cadmium and lead. On the other hand, chromium and iron were preferably associated with the larger particles of >1.5 μm AD. This and the low chromium enrichment factor demonstrate that the earth crust is the major source for these two elements in marine aerosol particles. Thallium and nickel could not be classified by one of these two size distribution patterns, which indicates that at least two different primary sources contribute to the content of these heavy metals in marine aerosol particles depending on the region investigated. Contrary to that, the sample collected over the Antarctic Ocean showed some significant differences. Here, a substantial amount of the total cadmium and lead was associated with the larger particles. However, relatively high enrichment factors found for cadmium, nickel, lead, and thallium in the smallest particles suggest a natural source, probably biogenic activities, in the Antarctic Ocean.

Surface structure of nitrogen ion-implanted 304 stainless steel

Modification of the structure and the creation of nitrides in the surface layer of 304 stainless steel due to nitrogen ion implantation with doses of 1 × 10 17 , 3 × 10 17 and 5 × 10 17 N cm -2 at an accelerating potential of 90 kV were investigated by X-ray photoelectron spectroscopy combined with argon ion sputter etching. In a thin surface layer of unimplanted steel there is a structure which consists of a chromium enriched oxide layer and an adjacent inner layer enriched by nickel. After the ion implantation, the double-layer surface structure remained, but chromium enrichment was enhanced and the thickness of the chromium-rich oxide layer increased with an increase in the implantation dose. This modification can be explained qualitatively by radiation-enhanced migration of metal atoms to the surface and their oxidation in a sub-surface reaction zone. The creation of Cr 2 N and Fe 4 N in the main area of nitrogen distribution and CrN in the chromium enriched layer near the surface was observed.

Chemical Composition, Chemical States, and Resistance to Localized Corrosion of Passive Films on an Fe‐17%Cr Alloy

The resistance to pitting of an Fe‐17%Cr alloy in neutral chloride solution was studied, as a function of chloride concentration and of polarization time (aging), using combined electrochemical measurements and XPS analyses. Potentiodynamic experiments (at 1 V/h) show that pitting occurs at chloride concentrations above . Potentiostatic polarizations of the alloy in the passive region were performed in a chloride solution (pH 6), at +390 mV/SHE (in the passive region), for different times (5 min, 1 h, and 27 h). Prolonged polarization induces better protective properties of this film (lower residual current) and better resistance to pitting (higher pitting potential). XPS analyses of the passive films show that these properties are related to transformations of the passive layer with time. The increased pitting resistance observed is attributed to chromium enrichment in the passive layer and a thickening of the inner oxide layer of the passive film. Chloride ions are not incorporated in the oxide layer, but are present in the outer layer of hydroxide.

Chemical and Structural Effects of Phosphorus on the Corrosion Behavior of Ion Beam Mixed Fe‐Cr‐P Alloys

Ion beam mixing has been used to produce a variety of thin films of Fe‐Cr‐P metallic glasses to study the relative importance of alloy chemistry and amorphicity on the corrosion resistance of those alloys. Films of were produced by multilayer evaporation of iron, chromium, and iron phosphide, and ion beam mixed to homogenize the layers and induce the amorphous phase transformation. Films with 25 and 35 a/o phosphorus were entirely amorphous, films with 15 a/o were mixtures of crystalline and amorphous phases, and films with 0 and 8 a/o were entirely crystalline (P in solid solution bcc Fe‐Cr). The amorphous films were devitrified by heat‐treating. The alloy films produced in this study exhibited remarkable corrosion resistance in sulfuric acid solutions, with and without chloride additions. The addition of large amounts of phosphorus to Fe‐10Cr lowered the passive current density by nearly two orders of magnitude. The active dissolution regime was eliminated by the addition of phosphorus, lowering the open‐circuit corrosion rate by four orders of magnitude, primarily by suppressing the anodic dissolution reaction. Devitrification of the amorphous phase had no significant effect on the corrosion behavior of the resistant alloys. Examination of the passive oxides by x‐ray photoelectron spectroscopy indicated that the presence of phosphorus increased chromium enrichment in the passive oxide, but that the major mechanism of protection was the development of a phosphate‐rich oxide which may act as a chemical shield to prevent anodic dissolution of the underlying metal.

In situ studies of passive film chemistry using X-ray absorption spectroscopy

The application of X-ray absorption spectroscopy to the study of passivation of thin metal films is discussed. Data are presented which demonstrate the effectiveness of the technique for monitoring both potential- and time-dependent data on valency and selective dissolution from passive films. It is shown that iron undergoes reductive dissolution from the passive oxide film on both pure iron and Fe-26Cr, in the latter case leading to enrichment of chromium in the passive film. Transpassive dissolution of chromium can take place on Al-Cr and Fe-Cr only after it has been enriched in the passive film by dissolution of the other alloying element. Rapid polarization of Al-Cr leads to trapping of electroactive chromium in the passive film.

The anodic dissolution and passivation of Ni[sbnd]Cr[sbnd]Fe alloys studied by ESCA

The surface compositions of two NiCrFe alloys [Ni21Cr8Fe (at%) single crystal and Ni17Cr10Fe] have been characterized by angle-resolved ESCA, after polarization in the passive state or in the active state in 0.05 M H2SO4. The passive layer formed on Ni21Cr8Fe(100) has a bilayer structure with an oxide film in the interior of the film and an hydroxide layer on the surface. The film is markedly enriched with chromium oxide and hydroxide. The inner oxide layer consists of 96% Cr2O3 and 4% Fe2O3. There is no detectable Ni2+. The outer hydroxide layer is ∼100% Cr(OH)3. At 300 mV(SHE), the thickness of the passive film is 11Å, and the thickness of the inner oxide layer is 6.5Å. At higher potentials the film is thicker: the thickness of the inner oxide layer is 9Åat 600 mV(SHE). On the Ni17Cr10Fe alloy, similar results were obtained for the passive film composition, but the film was found to be slightly thicker (17Å). An enrichment of chromium is observed in the metallic phase under the passive film. A chromium concentration of 43% was estimated for the first atomic plane of the alloy under the passive film. In the active state, the bilayer structure does not exist. After polarization at +80 mV(SHE), a thin (7Å) layer containing Ni2+ [as Ni(OH)2], Cr3+ [as Cr(OH)3 and Cr2O3], and Fe3+ (as FeOOH) was observed. The layer is enriched with Cr3+. A surface enrichment of chromium in the metallic state was also detected (∼75% chromium in the first atom plane of the alloy), which reveals that selective dissolution occurs on this alloy. On the basis of the experimental results, and using a simple model for the surface composition in the stationary state of dissolution, the surface chromium concentration CsCrin the active state can be predicted for NiCrFe alloys with different bulk chromium contents CbCr. A relation CSCr ∼ 12CbCr/(1 + 11CbCr) is predicted, which leads to a sharp increase in CsCr for CbCr in the range 0–15%.

Chemical and structural effects of phosphorus on the corrosion behavior of ion beam mixed Fe-Cr-P alloys

An experimental program was conducted to determine the mechanisms by which phosphorus affects the corrosion and passivation behavior of Fe-Cr-P alloys. To identify separately the effects of structure and chemistry on the corrosion behavior, thin films of Fe-10 Cr- xP (0≤ x≤ 35 at.%) were prepared by ion beam mixing. Films with a phosphorus content greater than approximately 20 at.% were found to be entirely amorphous. Devitrification of the amorphous phase was accomplished by heating the samples to 450 °C in an inert environment. Standard polarization tests of the alloys in sulfuric acid (with and without Cl -) indicated that the films containing phosphorus were more corrosion resistant than Fe-10Cr, at both active and passive potentials. There was a monotonic relationship between the amount of phosphorus in the alloy and the corrosion resistance, with the open-circuit corrosion rate of Fe-10Cr-35P nearly four orders of magnitude lower than that of Fe-10Cr. Devitrification of the alloys had no significant effect on the corrosion rate, indicating that the primary effect of phosphorus is chemical in nature, and not structural. The passive oxides were depth-profiled using X-ray photoelectron spectroscopy, which indicated that phosphorus was a primary constituent, as phosphate. The presence of phosphate in the passive oxides reduced the overall corrosion rate directly, by suppressing anodic dissolution. The presence of phosphorus did enhance chromium enrichment in the oxide, but that was not thought to be the primary mechanism by which phosphorus increased the corrosion resistance.

AES/GDS characterization of thin oxide films on FeCr sheet steels

AbstractThin oxide films on four kinds of commercial sheet were characterized with Auger electron spectroscopy (AES) and glow discharge optical emission spectroscopy (GDS). Enrichment of alloying elements was noticed in the thin oxide films formed in air. The depth profiles obtained by AES with ion sputtering showed that enrichment of chromium occurred at the interface between the oxide and the matrix. It was difficult with AES to obtain the profiles of silicon and manganese because of overlap of their peaks with the iron peaks on the spectrum. On the other hand, with GDS, silicon and manganese could be analysed quantitatively in thin films that are <0.05 μm thick.

A Comparison of U-bend and Slow Strain Rate Procedures for Assessing the SCC Resistance of Type 304 Stainless Steel in High-Temperature Water

Abstract The effects of temperature, chloride content, and dissolved oxygen content on the susceptibility to stress corrosion cracking (SCC) of type 304 stainless steel in high-temperature water have been investigated. Both U-bend and slow strain rate stress corrosion tests were performed, complemented by electrochemical polarization curve measurements and extensive oxide film analyses by Auger electron spectroscopy (AES). The U-bend data showed decreasing susceptibility to cracking with increasing temperature, which correlated with thicker oxide films and greater enrichment of chromium in the films. In contrast, the slow strain rate tests (SSRTs) indicated maximum susceptibility to cracking at 250°C in the temperature range 200 to 300°C and the susceptibility to cracking increased with increasing oxide thickness. The extent of chromium and nickel enhancement in the oxide layers were different for the two specimen types tested in equivalent environments. Greater enhancement of nickel occurred in films for...

The effect of solute content on grain boundary segregation in electron-irradiated Fe-Cr-Mn alloys

Solute distribution and microstructural development in the vicinity of grain boundaries in ferritic Fe-10 Cr- xMn-3 Al ( x = 5, 10 or 15) alloys were studied during electron irradiation to 10 dpa at 723 K. In addition, X-ray diffraction analysis was performed for determination of the volume size factor of solutes. The oversized solute atoms, manganese and aluminum, were depleted at grain boundaries, whereas the concentration of the oversized chromium rose sharply at the boundary. The amount of segregation of the solutes decreased with increasing atomic volume depending on manganese content. Segregation of aluminum, which had the greater volume size factor relative to that of manganese, was higher than that of manganese. The amount of radiation-induced segregation of manganese and aluminum at the grain boundary is consistent with arguments based on the atomic size effect, but the enrichment of chromium at the grain boundary is not and seems to related primarily to the formation of chromium-rich precipitates at the boundary.

Compatibility of a V-15Cr-5Ti alloy with SiC at high temperatures

To clarify the chemical compatibility between a V-15Cr-5Ti alloy and silicon carbide, the extent of reaction, the reaction products and the structure of reaction layer were studied at temperatures ranging from 1423 to 1523 K in an argon gas stream. The reaction layers formed in the interfacial reactions consisted of the suicides, V 5Si 3 and V 3Si, and minor amounts of TiC over the temperature range, and a remarkable internal titanium carbide was also observed. The enrichment of chromium in the alloy matrix adjacent to the reaction layer was caused by the preferential diffusion of vanadium toward the SiC side. The growth of reaction layers followed a parabolic rate law. The parabolic growth rate constant was expressed by k p ( m 2 s −1) = 1.8 × 10 −8 exp(−197 kJ mol −1/ RT).

The Effect of Tungsten on the Corrosion Behavior of Amorphous Fe‐Cr‐W‐P‐C Alloys in 1M HCl

The active current density of Fe‐8Cr‐7W‐13P‐7C alloy is almost three orders of magnitude lower than that of the Fe‐8Cr‐13P‐7C alloy. The passive current density in the low potential region also decreases with increasing alloy tungsten content. The surface analysis by XPS reveals that chromium and tungsten ions are concentrated in the surface film on a tungsten containing alloy in the active region. The thickness of the surface film formed on the low tungsten alloy in the active region is x‐ray photoelectron spectroscopically infinite, whereas the film thickness on the Fe‐8Cr‐7W‐13P‐7C alloy in the active region is of the same order of magnitude as the thickness of the passive film. The chromium enrichment in the passive film becomes more significant with increasing tungsten content. Passivation of low tungsten alloys seems to occur by the formation of the passive film with the least enrichment of chromium ions necessary for passivation due to different dissolution rates of iron and chromium during a large amount of alloy dissolution. Passivation of tungsten containing alloys takes place through transformation of the air‐formed film to the passive film as a result of preferential dissolution of a small amount of iron without dissolution of chromium ions.