Raman microscopy of chromate interactions with corroding aluminum alloy 2024-T3

Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K₂Cr₂O₇. Raman spectra acquired in and near corrosion pits allowed identification of two products of Cr^VI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an Al^III-Cr^VI mixed oxide formed in formerly active or metastable pits. This mixed oxide is similar to a covalent compound which can be made synthetically by adding NaOH to solutions containing Al³⁺ and CrO₄²⁻. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a Cr^III-Cr^VI mixed oxide, formed by partial reduction of Cr^VI to Cr^III by exposed Al metal or by H₂. At the pitting potential, a mixture of Al^III and Cr^III mixed oxides was observed, with the Al^III species located primarily within pits. A model for the formation mechanism of the mixed oxides is proposed, and the consequences of the findings to corrosion inhibition by Cr^VI are considered. In particular, the concentration of Cr^VI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion.     

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.     

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.     

Nature of Chromium on Passivated Tinplate

Abstract

Samples of tinplate passivated by a cathodic dichromate treatment have been analysed by photo-electron spectroscopy to determine the nature of the chromium on the surface. This has confirmed the presence of chromium metal which appears to be present in discrete regions rather than as a continuous film. The entire surface composed of exposed tin and regions of chromium metal is shown to be covered with trivalent chromium compounds. The X-ray Photoelectron Spectroscopy results are consistent with other models postulated from anodic polarisation work which suggests that a Cr(OH)₃ overlayer is present with some Cr₂O₃ underneath formed by air oxidation of chromium metal.     

Novel Alkaline Method for the Preparation of Low-Chromium Magnesia

The typical method for the preparation of chromium-magnesia is by energy-consuming grinding, but the pollution of Cr⁶⁺ ions is a problem. However, the chromium-magnesia refractory is still irreplaceable due to its outstanding performance. In this regard, low-chromium magnesia was prepared by the alkaline chromium precipitation method from cheap light-burned magnesia. The impurities in the light-burned magnesia was removed in order to avoid the formation of Cr⁶⁺. The chromium was precipitated directly on the light-burned magnesia surface without the grinding process. The low-chromium magnesia was obtained after the calcination. The reaction temperature, solid content, reaction time and stirring speed were examined and low-chromium magnesia with Cr³⁺ 0.4%, MgO 98.5% was obtained. The effect of temperature on the surface tomography of low-chromium magnesia was also discussed. The present novel alkaline method should be a promising way for the preparation of low-chromium magnesia.

     

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.     

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.     

Reports of Meetings

Abstract

Selected commercial and experimental nickel/chromium and 37% nickel/chromium/iron alloys have been exposed, in industrial heat-treatment furnaces, to controlledatmospheres of various types, and the nature and degree of the corrosion attack has been assessed by measurement of weight-change and by metallographic examination.

Specimens of the same materials, and of a more extended range of alloys, were exposed,in laboratory tests, to an atmosphere of carbon monoxide, and additional tests were made in atmospheres of the water gas type contaminated by lead.

Except in lead-contaminated atmospheres, high-chromium content conferred protection against severe attack by carburisation and oxidation. In alloys of insufficient chromiumcontent, beneficial effects were produced by additions of niobium and silicon, and sometimes by tungsten and titanium. In 37% nickelf chromium/iron alloys, vanadium proved detrimental.

In a lead-contaminated atmosphere chromium was detrimental, but, as in the other atmospheres, niobium was found to be the most effective addition.

Separation of the oxide-containing and carbon-containing zones on specimens attacked by ‘green rot’ corrosion indicates that the mechanism of attack is not necessarily by direct oxidation of carbide particles, but by internal precipitation of oxides from a solid–solution band behind the carbide.     

Oxide films formed during chemical polishing of steels

Abstract

Some mild steels when chemically polished in oxalic acid/hydrogen peroxide solutions developed oxide films, containing chromium, which were unusually resistant to removal. The same steels developed similar films during electrochemical passivation in sulphuric acid. Susceptibility to chromium enrichment seems to be not solely influenced by chromium content but is affected by heat-treatment and possibly depends on some chromium remaining in solution in the ferrite. For mild steels, chromium enrichment does not prevent brightening, but for steels deliberately alloyed with chromium it may.

Chromium enrichment of the surface of a stainless steel was also found after electrochemical polishing. The use of anodic oxidation at constant current for the examination of stainless steel surfaces is discussed.     

The effects of yttrium ion implantation on the oxidation of nickel-chromium alloys. II. Oxidation of yttrium implanted Ni-20Cr

Yttrium ions were implanted into an alloy of nickel containing 20 wt.% chromium. The oxidation weight gains of the implanted alloy, as well as unimplanted Ni-20Cr, were measured in 1 atm. of oxygen at temperatures between 815°C and 1100°C. Though a yttrium fluence of 2×10¹⁵ ions/cm² increased the weight gained at a given time for the implanted alloy compared to the unimplanted alloy, a fluence of 7.5×10¹⁶ ions/cm² considerably retarded the oxidation process. This retardation was caused both by a reduction in the duration of transient oxidation and by a reduction in the rate of parabolic oxidation that follows the transient stage. Examination of oxidized specimens revealed that the higher fluence yttrium implantation caused an earlier formation of Cr₂O₃, thus shortening the time during which NiO forms and contributing to the reduced time and weight gain during transient oxidation. This accelerated formation of Cr₂O₃ is explained by reference to the yttrium-implantation-induced amorphous phase present on the alloys, the formation of small grains of nickel-chromium solid solution within this amorphous phase, and the presence of NiCr₂O₄ at the oxide/gas interface. Potential causes of the reduced rates of parabolic oxidation are also discussed.     

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.     

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.     

Hard chromium substitution using HVOF coatings

Abstract

Electroplated hard chromium coatings have long been used to improve the wear and/or corrosion resistance of components. However, concerns over the health problems associated with hexavalent chromium solutions have led to increasing regulation of this process route. One promising substitute for hard chromium is 'cermet' layers (e.g. Cr₃C₂–NiCr and WC–CoCr) which may be deposited by the high velocity oxyfuel process. High velocity oxyfuel coatings were characterised by microscopy, X-ray spectroscopy, density and a microabrasive wear test. The WC–CoCr coating provided the best overall performance and is an excellent candidate for hard chromium substitution.     

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.     

Microstructure and wear behaviour of pulse electrodeposited alumina nanoparticle reinforced Co–W nanocomposite coatings

Abstract

The microstructure and wear behaviour of alumina nanoparticle reinforced Co–W alloy coatings have been investigated for potential replacement of hard chrome coatings. The composite coatings were pulse electrodeposited on steel substrates using a citrate bath. The effects of current density, in the range of 1–9 A dm^?2, on the particle reinforcement, phase/microstructure, microhardness, and wear properties of the coating have been studied. The coatings codeposited with current density of 5 A dm^?2 at 333 Hz pulse frequency and 33% duty cycle exhibited microhardness comparable to hard chromium coatings.     

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.     

A fundamental study of Fe-Cr binary alloy-oxide film interfaces at 288 °C by computational chemistry calculations

A quantum chemical molecular dynamics method was used in order to understand the oxide film degradation mechanism at metal/metal-oxide interfaces. The present study shows that oxygen diffusivity in the metal is significantly higher at a Fe-Cr/Fe₂O₃ interface compared to a Fe-Cr/Cr₂O₃ interface. This indicates that Cr₂O₃ enables protection of the surface for a longer period of time than Fe₂O₃ in a high temperature environment. Applied tensile strain enhances the oxygen mobility towards the metal surface. This process helps to increase the oxidation of the metal surface by forming metal oxygen bonds. Atomic charge analysis reveals that the oxygen atoms are negatively charged and the chromium atoms are more highly positively charged than iron ones. The negatively charged oxygen atoms are able to make covalent bonds with the positive metal atoms. This charge transfer process facilitates the formation of metal-oxygen bonds and weakens the metallic bonds.     

NaCl-induced accelerated oxidation of chromium

This paper describes new phenomena about chloride-induced accelerated oxidation of chromium. Thermal analysis was adopted to examine the oxidation, which was studied particularly in the case of NaCl. The presence of NaCl remarkably accelerates the oxidation of chromium. The process occurs below the melting point of NaCl, and the main reaction product is Cr₂O₃. In the accelerated oxidation NaCl plays a catalytic role because it is not consumed significantly in the process. DTA analysis reveals that the heat of reaction also accelerates the rate of oxidation, especially at an early stage of the reaction. The accelerated oxidation takes place similarly under the presence of chlorides other than NaCl, but the oxidation rate depends on the kind of salt. Therefore the Cl^– anion plays an important role in the process, while the nature of the cation affects the rate of acceleration.     

Structure of Surface Oxides Formed on A 20% Cr/25% Ni/Nb-Stabilised Steel in Carbon Dioxide at High Temperatures

Abstract

Electron and X-ray diffraction studies of oxide growth on a 20% Cr/25% Ni/Nb-stabilised austenitic steel in carbon dioxide at 750° and 850° are described. The results are interpreted in terms of a duplex structure comprising an inner rhombohedral Cr₂O₃-layer and an outer spinel oxide of the M²⁺(M³⁺)₂O₄ type. The influence of minor alloying elements, including rare earth metal additions, on the constitution and distribution of the surface oxide phases formed on the same basic steel is also discussed as a means of improving oxidation resistance.     

The role of molybdenum in the crevice corrosion of stainless steels

The paper starts with a literature review of Mo as an alloying element in stainless steels, with particular reference to crevice corrosion. Relevant aspects of the chemistry and electrochemistry of Mo are also summarized and used to discuss the possible role of soluble Mo species in corrosion inhibition. The need to examine solutions of low-valent Mo is indicated. Experiments are reported on stainless steels in the active range of corrosion potentials and the influence of soluble Mo^III and Mo^IV. No inhibitive effects are found; but inhibition is produced by films of insoluble Mo^IV oxide deposited on stainless steels. Such films can be produced by reduction of Mo^VI solutions, but not by oxidation of Mo^III. It is concluded that, in the corrosion of alloys, Mo probably passes directly from the metal into the protective film, and that soluble Mo compounds play no part in this process.