The Influence of Surface Preparation on The Kinetics of Film Growth on Mild Steel in Chromate Solutions

Abstract

Film growth on mild steel in chromate solutions was found to be logarithmic with respect to time for all types of surface preparation tested; these included polished, abraded, pickled and grit-blasted surfaces. Growth constants, and also the monolayer uptake of‘labelled’ chromium, Cr^*, before logarithmic film growth started, varied in a similar fashion as different types of surface preparation were used.

Relative values of surface roughness factor (S.R.F.) calculated from relative values ofgrowth constants agreed with published values of S.R.F. for comparable suffaces, where these were available.

It is suggested that measurements of Cr^* uptake with respect to time made instandardised conditions could be used as a method of determining relative S.R.F.     

Electrochemical assessment of chromium in passivation films on tinplate

Abstract

When tinplate, passivated in chromate solutions, is anodically polarised either with current measurement at steadily increasing potential or with potential measurement at constant current, an indication of the oxidation of Cr^III to Cr^VI in the surface film is obtained. The indication can be made quantitative for the estimation of Cr^III. The constant-current method is preferred as permitting an easy measure of the charge passed during the oxidation which is indicated as an arrest on the potential/time curve. The preferred conditions are a chloride-free neutral phosphate solution and a current density of 25 μA/cm².

For most commercial tinplates the estimated Cr^III is similar to the total chromium but the few exceptions and others giving unusual potential/time curves may help in relating treatment td performance in laboratory cathodic-anodic passivation treatments, the chromium was wholly oxidised to Cr^VI by passage of a sufficient charge in the anodic period.     

Inhibition of mild steel by strontium chromate in artificial acid rain solution

Abstract

The inhibition of mild steel in an artificial acid rain solution (pH 4·5) in low concentrations of strontium chromate pigment has been evaluated using a range of methods: potentiodynamic polarisation, electrochemical impedance, X-ray photoelectron spectroscopy and solution analysis. The individual effects of strontium and chromate have been evaluated and have been used to estimate inhibitor efficiency and film composition, and to determine the possible inhibition mechanism. The results show that, at low concentration, strontium chromate affects the cathodic reaction, with reduction of Cr⁶⁺ to Cr³⁺, and the surface film was composed of magnetite and hydrated chromium hydroxide. The presence of strontium was not found significantly to affect the inhibitory performance.     

Nature of the Oxide Film Present on Iron after Inhibition in 0·05 M Chromate Solutions

Abstract

Chemical and microprobe analyses have been used to study the composition of the oxide films formed on iron by 0·05 M potassium chromate, pH 4–8.

Chemical analyses have shown that the air-formed oxide film was thinned and evenly reinforced with a normal iron chromium spinel having a composition in the range Fe²⁺ (Fe³⁺_0·5 Cr³⁺_1·5)O₄ — Fe³⁺ Cr³⁺ O₃

Microprobe analyses indicated enrichment of chromium at scratch lines, but the effect was small compared with the overall thickening of the films.     

A comparative study of the corrosion protective properties of chromium and chromium free passivation methods

Commercially available passivation methods for white-rust protection of hot-dip galvanized steel have been investigated. The passivations were either based on trivalent chromium or chromium free. A chromate based conversion coating was used for reference. The treated panels were tested with regard to white rust protection and paintability. The surface chemistry of the conversion coatings was monitored with scanning Auger electron spectroscopy and X-ray photoelectron spectroscopy. Coating thicknesses were measured using Auger electron sputter depth profiling.The passivations were applied with a thickness recommended by the supplier and thus showed large variation. The thickness of the chromium free passivation (Cr-free) is approximately 75 nm. The coating contains the active ions; H₃O⁺, Ti⁴⁺, Mn²⁺, Zn²⁺, PO₄³⁻. The passivation based on trivalent chromium (Cr-III) is approximately 30 nm thick and contains the active ions; H₃O⁺ Cr³⁺, PO₄³⁻, F. The chromate based passivation (Cr-VI) is approximately 5 nm thick and contains the active ions Cr⁶⁺/Cr³⁺, F⁻.The Cr-free and the Cr-III passivations showed similar white rust protection in the corrosion tests. The corrosion resistance was good although it did not fully reach the level of the Cr-VI passivation. The results from the tests of the painted panels showed that the powder paint worked well on all three passivations. The solvent born paint worked best on the passivation based on trivalent chromium. The water born paint showed poor resistance to blistering in the Cleveland humidity test for all three passivations. In this test the passivation with hexavalent chromium showed slightly better results than the chromate free passivations.     

Thermodynamic properties and phase equilibria in the Cr-P system

A Knudsen effusion method with mass-spectrometric analysis of gaseous phase has been applied to investigate the thermodynamic properties of the chromium phosphides (1341 to 1704 K) and Cr-P liquid alloys (1664 to 1819 K). Simultaneously, DSC has been used to measure heat capacities of chromium phosphides Cr₃P and Cr₁₂P₇ in the temperature range of113 to 873 K. The entropies of formation of chromium phosphides calculated according to the second and third laws of thermodynamics agree within the limits of experimental error. The Gibbs energies of formation of the phosphides from solid Cr and P₂ gas have been approximated with the following equations (in J/mol): A_fG⁰(Cr₃P) = −(244 112 ±2800) + (70.95 ±1.80)T ΔG⁰(Cr₁₂2P₇) = −(1563 678 ±15 350) + (440.6 ±9.90)T Thermodynamic properties of liquid solutions have been described with the ideal associated-solution model assuming that CrP, Cr₂P, Cr₃P, and Cr₃P₂ complexes exist in the melt. The phase diagram computed with the help of the thermodynamic data agrees with the published information.     

Plasma processing for surface modification of trivalent chromium as alternative to hexavalent chromium layer

Plasma source ion implantation (PSII) treatment was undertaken to improve the mechanical properties of electrodeposited trivalent chromium layers. Nitrogen ions were implanted, with energies of − 15 to − 25 keV and doses of 1, 5 and 10 × 10¹⁷ atoms cm^− 2, to modify the surface properties of Cr plating layer. The surface properties of the films were characterized by XRD, SEM, ruby-ball on disk type tribometer and nanoindenter. Polycrystalline CrN films with (200), (220) and (222) orientations were preferentially grown and numbers of surface cracks were increased by N⁺-PSII onto trivalent chromium layers. The surface hardness of the Cr³⁺ plating layer was increased from 16 to 25 GPa by N⁺-PSII. Severe wear and higher friction was observed on N⁺-PSII treated trivalent Cr plating. It seemed that the wear debris from hardened and cracked surface of the N⁺-PSII treated specimen prompted abrasive wear in the wear test. Roughness of the Cr³⁺ plating layers was smoothed with increasing implantation doses.     

Effect of chromium substitution on structural,electrical and magnetic properties of NiZn ferrites

Ni_0.5Zn_0.5Fe_2−xCr_xO₄ (0≤x≤0.5) ferrites were successfully prepared by conventional solid state reaction method to investigate the effect of chromium substitution on the structural, electrical and magnetic properties. X-ray powder diffraction results demonstrate that all the prepared samples are well crystallized single-phase spinel structures without secondary phase. As chromium concentration increases, the lattice parameter and crystallite size gradually decrease. The magnetic measurement indicates that saturation magnetization is substantially suppressed by Cr³⁺ doping, changing from 73.5 A·m²/kg at x=0 to 46.3 A·m²/kg at x=0.5. While the room-temperature electrical resistivity is more than four orders of magnitude enhanced by Cr³⁺ substitution, reaching up to 1.1×10⁸ Ω·cm at x=0.5. The dielectric constant monotonously decreases with rising frequency for these ferrites, showing a normal dielectric dispersion behavior. The compositional dependence of dielectric constant is inverse with that of electrical resistivity, which originates from the reduced Fe²⁺/Fe³⁺ electric dipole number by doping, indicating inherent correlation between polarization and conduction mechanism in ferrite.     

Synthesis and structure screening of nanostructured chromium oxide powders

The evolution with calcinations of chromium oxide (Cr₂O₃) nanocrystals of catalytic interest, prepared from reduction of K₂Cr₂O₇ with maleic acid has been studied. The characterization of the materials was confirmed by X-ray diffraction (XRD), Fourier transformation infrared (FT-IR), transmission electron microscope (TEM), N₂ adsorption–desorption isotherms, and thermo-analytical methods. The influence of the operating variables such as molar ratio, pH, gelation time and specific surface area was investigated and discussed. The results showed that the K₂Cr₂O₇/maleic acid of 1:1.7 molar ratio at pH 7 and 15 days gelation time were considered to be the best conditions for producing Cr₂O₃ of the pore diameter distribution in the range of 2–6 nm and specific surface area of 123 m²/g. The particle size and phases of chromium oxide were affected by the ratio of K₂Cr₂O₇ to maleic acid, temperature, and time of calcination. The amorphous phase appeared at room temperature, whereas it turned to crystalline phase when the calcination temperature increased to about 400 °C.     

Distinctions between the Activating Effects of Sulfuric and Phosphoric Acids on the Process of Zinc Chromating

Activating effect of anions of sulfuric and phosphoric acids (both in the range from 0.025 to 0.200 mol/l) on the process of zinc chromating in acid (pH 1.1) 0.2 M CrO₃ solution is studied by analytical methods. A general balance of oxidized zinc (Zn²⁺) and reduced chromium (Cr³⁺) and their distribution between the solution and chromate film for different solution compositions, as well as the elemental composition throughout the film's depth, are determined. It is found that the zinc oxidation and Cr⁶⁺ reduction reactions do not proceed in the absence of SO^2– ₄ ions (that is, when only PO^3– ₄ ions are present), so that the chromate film cannot form. However, the PO^3– ₄ ions combined with SO^2– ₄ ions increase the Zn²⁺ and Cr³⁺ ions concentration in the solution significantly, while their concentrations in the film correspondingly decrease. SO^2– ₄ ions activate the zinc surface because they form soluble complex compounds with Zn²⁺ and Cr³⁺ ions and increase the part of the surface, on which the cathodic reduction of Cr⁶⁺ to Cr³⁺ occurs. The activating action of phosphoric acid is caused by the increase in the total (analytical) concentration of H⁺ ion in the solution; hence, the deposition of the Cr³⁺ and Zn²⁺ hydroxide compounds onto zinc is retarded, due to the increased near-surface concentration of H⁺ ion.     

Comparative study of Cr3C2-NiCr coatings obtained by HVOF and hard chromium coatings

In the present work the corrosion resistance of micro-cracked hard chromium and Cr₃C₂-NiCr (HVOF) coatings applied on a steel substrate have been compared using open-circuit potential (E_OC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coatings surfaces and cross-section were characterized before and after corrosion tests using optical microscopy (OM) and scanning electron microscopy (SEM). After 18 h of immersion, the open-circuit potential values were around −0.50 and −0.25 V/(Ag∣AgCl∣KCl_sat) for hard chromium and Cr₃C₂-NiCr, respectively. The surface analysis done after 12 h of immersion showed iron on the hard chromium surface inside/near surface cracks, while iron was not detected on the Cr₃C₂-NiCr surface even after 18 h. For longer immersion time hard chromium was more degraded than thermal sprayed coating. For hard chromium coating a total resistance values between 50 and 80 kΩ cm² were measured and two well-defined time constants were observed, without significant change with the immersion time. For Cr₃C₂-NiCr coating the total impedance diminished from around 750 to 25 kΩ cm² as the immersion time increased from 17 up to 132 h and two overlapped time constants were also observed. Polarization curves recorded after 18 h of immersion showed a lower current and higher corrosion potential for Cr₃C₂-NiCr coating than other samples studied.     

The oxidation of Iron-Chromium-Manganese alloys at 900°C

The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900°C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)₂O₃, -Fe₂O₃, and Fe₃O₄. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr₂O₄ with Cr₂O₃ underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.     

XPS and AES studies of the high temperature corrosion mechanism of Fe-30Cr alloy

Fe-30Cr alloy specimens were pre-oxidized at two different oxygen partial pressures (10^?16 and 10^?19 atm) at 900°C and subsequently exposed to environments containing both oxygen and sulfur. The sulfur and oxygen partial pressures were maintained such that Cr₂O₃ was the stable phase. The Cr₂O₃ scales formed during pre-oxidation became rapidly unstable when exposed to an environment whose composition approached the chromium sulfide-chromium oxide phase boundary; but when exposed to a higher oxygen partial pressure with the same sulfur partial pressure, the preformed scales remained intact. For elucidating the sulfidation mechanisms, the reaction products on the surface were analyzed at different stages of sulfidation by X-ray photo-electron spectroscopy and Auger electron spectroscopy. Correlation of the reaction mechanisms with thermodynamic and transport parameters are discussed.     

High temperature electrochemistry of molten fluoride salt for measurement of dissolved chromium

Abstract

An in situ electrochemical probe, utilising anodic stripping voltammetry (ASV), has been developed to measure dissolved Cr concentration in the molten salt LiF–KF–NaF (46·5–11·5–42 mol.-%), commonly referred to as FLiNaK. Tests were performed in a glassy carbon crucible in an argon atmosphere glovebox at a molten salt temperature of 650°C. Cyclic voltammetry was performed to determine reversibility and mid peak potential. The Cr²⁺/Cr⁰ reaction occurred with a mid peak potential of ?0·51 V versus a 10 mol.-% Ni²⁺/Ni⁰ reference electrode. The integrated current from the anodic waves at this potential were used for Cr cation quantification. The integrated current from the ASV measurements correlated linearly with Cr concentration, measured independently using neutron activation analysis, over a range of 30–950 ppm (1·2–38 mmol L^?1). This study is significant for determining Cr concentrations in high temperature molten FLiNaK salts exposed to Cr containing structural materials.     

Thermodynamic analysis for separation of vanadium and chromium in V(IV)–Cr(III)–H2O system

To recycle vanadium and chromium from the V–Cr-bearing reducing slag, the thermodynamics of separating V(IV) and Cr(III) at 298 K was summarized in the form of potential–pH diagram and activity–pH diagram. The potential–pH diagrams of V–Mn–H₂O and Cr–Mn–H₂O systems show that the electrode potential of MnO₂/Mn²⁺ is higher than that of VO₂⁺/VO²⁺ but lower than that of Cr₂O₇^2–/Cr³⁺, which proves that it is feasible to selectively oxidize low valent vanadium using MnO₂. The activity–pH diagrams of V(V)–H₂O and Cr(III)–H₂O systems show that the precipitation pH of V(V) is far lower than that of Cr(III), and therefore V(V) and Cr(III) can be separated through precipitation method. Based on the thermodynamic analysis, the flowsheet of recovery of vanadium and chromium from the V–Cr-bearing reducing slag is designed.     

Chromium electrodeposition from Cr(III) aqueous solutions

The aim of this work is to study an alternative plating process to obtain chromium coatings through electrodeposition from baths containing trivalent chromium, as aqueous solutions of Cr (III) are significantly less dangerous, in terms of human health and environmental impact, as compared to the traditional Cr (VI) baths employed for this purpose. In order to overcome problems regarding the reduction of Cr (III) in aqueous solution, two approaches were followed: i) the possibility of co-depositing chromium along with a second metal, which could help the process of discharge of Cr³⁺ on the substrate; ii) the use of a specific ligand for the Cr³⁺ ion, which can generate easily reducible complexes at the metal-solution interphase. Both approaches led to interesting results: in particular, the co-deposition enabled us to obtain NiCr alloy with a high percentage of chromium, and the deposition using specific complexing agents allowed optimal bath compositions to be developed both for decorative and hard chromium plating.     

Chromium-nitride in situ composites with a compositional gradient formed by reactive DC plasma spraying

Nitrides of transition metals have good wear- and corrosion-resistant properties because of their high hardness and chemical stability. Chromium-nitride coatings can be deposited by ion plating; however, the thin thickness due to the slow deposition rate must be improved for severe wear-resistant applications. The main objective in this paper is to realize good structural control in the processing of chromiumnitride in situ composite coatings formed at a high deposition rate. They were synthesized by reactive low-pressure plasma spraying using elemental chromium powder as a spray material. The transferred arc between the gun electrode and the substrate was used to accelerate the nitriding reaction. The sprayed coatings consist of chromium, Cr₂N, and CrN, which have a composition gradient from the substrate interface to the surface. The volume fraction of Cr₂N increases with transferred arc current, and nonreacted chromium concurrently decreases, except close to the substrate. The CrN phase, however, only exists as a surface layer of 20 to 30 μm because it is decomposed to Cr₂N above 1420 K. The hardness of the composite coatings depends on the volume fraction of Cr₂N, and it increases to 1300 HV at a Cr₂N volume fraction of 0.98. The seizure stress with lubricant depends on the coating hardness. The maximum seizure stress of 24.9 MPa is obtained at a hardness of 1300 HV. The composite coatings also show a superior wear resistance. Hence, the Cr₂N in situ composite coatings synthesized by reactive plasma spraying with transferred arc are expected to be good candidates for wear-resistant applications.     

TEM and ToF-SIMS studies on the corrosion behavior of vanadium and chromium containing WC-Co hard metals in alkaline solutions

The corrosion behavior of hard metals with VC and Cr₃C₂ grain growth inhibitors was investigated in alkaline solutions by electrochemical methods. The two inhibitors have opposite effects on the corrosion behavior: Cr₃C₂ significantly improves the corrosion behavior, whereas VC-containing alloys show a poor resistance. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and analytical transmission electron microscopy (TEM) analyses of the distributions of Cr and V in the composite material, as well as in the surface layers formed during corrosion were employed to clarify the influence of these elements on the corrosion behavior. The measurements showed that VC is precipitated mostly along the WC/binder interface after the liquid-phase sintering process, while Cr₃C₂ is almost homogeneously dissolved in the binder. As VC is chemically instable in alkaline solutions, it completely dissolves out of the binder. In accordance with this observation no V was found in the corrosion product layer on the surface. As WC is more noble than Co, galvanic coupling between the two phases reinforces the Co dissolution, while the WC-phase is protected cathodically. Contrary to VC, chromium is stable in alkaline environments and forms a passivating Cr₂O₃ layer. Enrichment of Cr in the corrosion product layers was detected by TEM and ToF-SIMS. Due to surface passivation by Cr₂O₃, galvanic coupling effects between Co and WC play a much less important role in the corrosion process of the composite material.     

Pulse-reverse electroplating of chromium from Sargent baths: Influence of anodic time on physical and electrochemical properties of electroplated Cr

Herein, chromium film was successfully synthesized by pulse-reverse (PR) electrodeposition using various anodic current time in a Sargent bath which is mainly composed of hexavalent chromium (Cr⁶⁺) and sulfuric acid (H₂SO₄). The correlation between anodic time during electroplating and various physical properties was investigated. The crack density, hardness, and thickness of electrodeposited chromium was decreased with an increase in anodic time for PR electroplating. The chromium prepared by PR electroplating showed higher corrosion resistance than that prepared by direct current (DC) electroplating owing to low crack density. Consequently, the optimal anodic time for PR electroplating was found to be 0.001 s based on the crack density, hardness, current efficiency, and film thickness. The results obtained suggest that this optimized process is a promising route for electroplating chromium film with low crack density and high corrosion resistance.     

Effect of the oxidation state of gold on the complete oxidation of isobutane on Au/CeO2 catalysts

Catalysts of highly dispersed gold supported on ceria were prepared by deposition precipitation method. Au is dispersed as Au⁰, Au⁺ and Au³⁺ species on ceria. The content of Au⁺ and Au³⁺ was highest on catalyst prepared on uncalcined ceria, which possess least ordered surface. It is inferred that oxygen vacancy on disordered ceria surface is essential for the preparation of highly dispersed gold catalysts and in stabilizing monolayer surface Au⁺ clusters while cationic vacancies are sites for substitutional Au³⁺. Au/CeO₂ catalysts showed low-temperature isobutane oxidation activity with maximum conversion at 150–180°C. Ex-situ XPS results demonstrated that the low temperature isobutane oxidation activity was closely related to the content of Au⁺ which we interpreted as surface gold oxide formed under reaction conditions. Isobutane oxidation activity associated with ceria at temperature above 300°C was enhanced by substitutional Au³⁺.