Electrochemistry of active chromium: Part II. Three hydrogen evolution reactions on chromium in sulfuric acid

It was shown that chromium in deaerated sulfuric acid exhibits two stable corrosion potentials, depending whether the metal had previously been in contact with air or subjected to activation by cathodically evolving hydrogen. Electrochemical polarization measurements, as well as measurements of the actual metal dissolution rate at the corrosion potentials, anodic or cathodic polarization, using the analytical determination of Cr ions in the solution, weight-loss of metal, or volumes of hydrogen evolved, showed that hydrogen can evolve on chromium by three different reaction mechanisms. The first one is the electrochemical hydrogen evolution reaction from H⁺ ions at the bare chromium surface obtained by cathodic activation. This reaction and the active anodic dissolution of chromium determine one stable corrosion potential. The second reaction is the reaction of H⁺ ions on the oxidized chromium surface which, coupled with the anodic dissolution of passivated chromium determines the other stable corrosion potential. The third one is the “anomalous” or chemical reaction of chromium with water molecules and hydrogen ions whereby hydrogen is liberated. This is a potential independent reaction, occurring on the bare metal surface, and which is at pH <2 several times faster at the corrosion potential than the electrochemical hydrogen evolution reaction. The consequence is that the overall corrosion rate is several times faster than that determined by the usual electrochemical methods. The applicability of the different methods of measuring electrochemical corrosion rates and cathodic current efficiencies in chromium plating is discussed. Also, a possible role of the “anomalous” chromium dissolution in corrosion fatigue and stress corrosion cracking of stainless steels is considered.     

Chemical composition and structural transformations of amorphous chromium coatings electrodeposited from Cr(III) electrolytes

Amorphous chromium coatings were electrodeposited from Cr(III)-based solutions containing organic (HCOONa) or phosphorus-containing (NaH₂PO₂) additives. Their structure was studied by a combination of X-ray diffraction (XRD), valence-to-core X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) at the Cr K-edge. Metalloid atoms (C or P) incorporated in electroplates structure are chemically bonded to chromium (i.e. are located in the first coordination shell). Upon annealing at elevated temperatures in vacuum, these amorphous coatings crystallize into a mixture of phases containing metallic chromium and chromium carbides or chromium phosphides. Quantitative analysis of valence-to-core XES data demonstrates that the average local structure of chromium in the amorphous coatings does not change significantly during crystallization.     

Corrosion Investigation of Chromium Nitride and Chromium Carbide Coatings for PEM Fuel Cell Bipolar Plates in Simulated Cathode Condition

Transition metals nitrides and carbides are used as coatings on bipolar plates for proton exchange membrane fuel cells (PEMFCs) due to their suitable electrical conductivity and corrosion resistance. Chromium electroplated AISI 316L stainless steel bipolar plates were treated by plasma nitriding and solid carburizing to form chromium nitride and chromium carbide, respectively. The presence of CrN/Cr₂N and Cr₇C₃/Cr₂₃C₆ was verified by X‐ray diffractometry (XRD) in chromium nitride and chromium carbide coatings, respectively. The corrosion behavior of coatings was investigated by potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) in simulated cathode condition. Coated samples showed better corrosion behavior than untreated bare sample. EIS results indicated decrease in corrosion current density after 500 hours, however coatings acted as barrier against solution access to substrate. Corrosion current densities of coatings were close to targets of United Stated department of energy (DOE).     

Passivity of OC404 steel modified electrochemically with CeO2-Ce2O3 layers in sulfuric acid media

The effect of cerium oxides film, formed electrochemically on OC404 stainless steel (SS), upon the corrosion behavior of steel in 0.1N H₂SO₄ was investigated. The modification of the steel surface by deposition of cerium oxides films was found to improve the steel corrosion resistance. A linear dependence between the stationary corrosion potential of the cerium oxides/SS system and the cerium concentration in the oxide film was established. The shift of the corrosion potential in the positive direction was found to depend on the proceeding of a depolarizing cathode reaction of CeO₂ reduction (instead of the hydrogen depolarizing reaction) occurring on the cathodic zones, formed by this oxide. On the basis of XPS analyses of the samples, subjected to real corrosion under the conditions of self-dissolution, a pronounced drop of the surface concentration of CeO₂ was established. This is a proof of the occurrence of an effective cathode process of CeO₂ reduction to Ce₂O₃, which was then dissolved in H₂SO₄. Data were obtained (XPS) on the composition and structure of the surface film (SEM) after electrodeposition of cerium oxides and after corrosion in the sulfuric acid medium under consideration for time intervals ranging from 50 up to 1000 h. The ICP-AES studies acquired data on the quantity of dissolved elements, forming the passive layer. After exposure to the corrosive medium, the deposited layer showed enrichment in oxides of chromium and aluminium. The passive film on stainless steel, modified in this way, proved to be more stable to the effect of aggressive sulfuric acid medium, compared to the case of natural passive film.     

Kinetics and mechanism of chromium electrodeposition from formate and oxalate solutions of Cr(III) compounds

Kinetics of multistep reaction of Cr(III) ions discharge to metal was studied on a stationary electrode and on a rotating disk electrode from the solutions containing formic acid or oxalic acid. The electroreduction of Cr(III) complex ions in aqueous solutions is shown to proceed via the formation of relatively stable intermediates—Cr(II) compounds which are partially removed into bulk solution. The effect of pH, organic ligand concentration and disk rotation velocity on the partial current density of chromium electroplating was demonstrated. The kinetic equations of the studied process were derived and compared with the experimental data. Kinetic parameters for the discharge of Cr(II) ions were calculated. The mechanism of chromium electrodeposition reaction was proposed. The electrodeposition of chromium from formate bath is suggested to proceed with the participation of hydroxocomplexes of bivalent chromium. The oxalate complexes of bivalent chromium directly discharge in the electrolytes containing oxalic acid. The partial polarization curves of chromium electrodeposition exhibit a current peak which may be caused by blocking the electrode surface with poorly soluble Cr(III) hydroxide.     

Assessment of Cr doping on TiO2 thin films deposited by a wet chemical method

Undoped and Cr-doped TiO₂ thin films were synthesized by the dip-coating sol-gel process with titanium isopropoxide and chromium (III) chloride hexahydrate being used as the precursors. The chromium concentrations changed for different molar ratios, namely, 2, 4, and 8 wt.%. The samples, coated on glass substrates, were later annealed in air at 450 °C for 60 min. The influence of Cr doping on the structural, surface chemical, and optical properties of the samples was studied by several techniques, including EDS, Raman, UV–Vis, RT-PL, and XPS spectroscopies, as well as SEM and XRD. The diffraction patterns indicated that all the films displayed the anatase structure with the crystallite size decreasing with chromium doping. The same structure was confirmed by Raman spectroscopy measurements. UV–Vis absorption spectra of the samples showed a red shift of the fundamental absorption edge in the visible range following the increase of Cr doping concentration. In addition, the RT-PL study revealed that the dopant incorporation causes a decrease in the PL intensity. The EDS analysis revealed the presence of Ti, O, and Cr in the materials. Moreover, from high-resolution XPS Ti 2p spectra, titanium was found to be in the Ti⁴⁺ oxidation state evidencing the formation of TiO₂, while the Cr 2p fitting analysis showed that chromium is present in the Cr (III) and Cr-metal states.     

Chemical composition and corrosion protection of silane films modified with CeO2 nanoparticles

The present work aims at understanding the role of CeO₂ nanoparticles (with and without activation in cerium(III) solutions) used as fillers for hybrid silane coatings applied on galvanized steel substrates.The work reports the improved corrosion protection performance of the modified silane films and discusses the chemistry of the cerium-activated nanoparticles, the mechanisms involved in the formation of the surface coatings and its corrosion inhibition ability.The anti-corrosion performance was investigated using electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and d.c. potentiodynamic polarization. The chemical composition of silanised nanoparticles and the chemical changes of the silane solutions due to the presence of additives were studied using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance spectroscopy (NMR), respectively.The NMR and XPS data revealed that the modified silane solutions and respective coatings have enhanced cross-linking and that silane-cerium bonds are likely to occur.Electrochemical impedance spectroscopy showed that the modified coatings have improved barrier properties and the SVET measurements highlight the corrosion inhibition effect of ceria nanoparticles activated with Ce(III) ions. Potentiodynamic polarization curves demonstrate an enhanced passive domain for zinc, in the presence of nanoparticles, in solutions simulating the cathodic environment.     

CO2 corrosion of carbon steel in the presence of acetic acid at higher temperatures

The corrosion behaviour of X 65 carbon steel in the presence of acetic acid in N₂- and CO₂-saturated systems has been investigated using electrochemical techniques. The presence of acetic acid does not influence the anodic reaction but strongly accelerates the cathodic reaction. The cathodic reaction and consequently the corrosion rate of mild steel in the CO₂-saturated system increase with increase in acetic acid concentration and temperature. From the values of the apparent activation energies, the corrosion reaction in the absence of acetic acid was found to be under mixed interfacial reaction/diffusion control while interfacial reaction control dominates in the presence of acetic acid. The reduction of adsorbed undissociated acetic acid on the metal surface is proposed as the key species primarily responsible for accelerated corrosion rate at all temperatures.     

Electrochemical corrosion characteristics of type 316 stainless steel in simulated anode environment for PEMFC

The corrosion behavior of type 316 stainless steel in simulated anode environment for proton exchange membrane fuel cell (PEMFC), i.e., dilute hydrochloric acid solutions bubbled with pure hydrogen gas at 80 °C, was investigated by using electrochemical measurement techniques. The main purpose is to offer some fundamental information for the use of stainless steels as bipolar plate material for PEMFC. Both polarization curve and electrochemical impedance spectroscopy (EIS) measurements illustrate that 316 stainless steel cannot passivate spontaneously in the simulated environments. The absorbed (and/or adsorbed) hydrogen atoms from cathodic corrosion reactions on the steel surface may deteriorate the passivity and corrosion resistance. The oxidation of these hydrogen atoms gives rise to a second current peak in the anodic polarization curve, and the current increases with immersion time. EIS spectra also reveal that a porous corrosion product layer formed on the steel surface during the active dissolution in the test solutions. 316 stainless steel exhibits the similar corrosion behavior in sulfate ions containing dilute hydrochloric acid solution.     

Catalytic gas-phase fluorination of 1,1,1-trifluoro-2-chloroethane over chromium (III) oxide: Preparation of hydrofluoroalkanes

The transformation of CF₃CH₂Cl over bulk chromium(III) oxide, fluorinated or not, leads mainly to the fluorination product CF₃CH₂F and to the alkene CF₂=CHCl. In the presence of HF, CF₃CH₂F is the main product. In absence of HF this product is also obtained with HF formed in the elimination reaction of CF₃CH₂Cl to CF₂=CHCl. At the same time a part of HF is used for the fluorination of chromium oxide. The XPS analysis of fluorinated catalysts with HF gives surface fluorine atomic contents which correspond to a F/Cr ratio of 0.36. A survey of the effect of partial pressures of HF and of CF₃CH₂Cl shows a zero order for HF and a 0.6 order for CF₃CH₂Cl. The fluorination reaction could occur on a partially fluorinated chromium(III) oxide involving HF oligomer species previously postulated by Rowley et al. [L. Rowley, J. Thomson, G. Webb and J.M. Winfield, Appl. Catal. A, 79 (1991) 89–103].     

Electrochemical study of carbon steel corrosion in buffered acetic acid solutions with chlorides and H2S

In this work, the corrosion behavior of SAE 1018 carbon steel in buffered acetic acid (HAc) solutions containing chlorides, with and without H₂S, was studied. Polarization curves obtained by different electrochemical techniques, indicate negligible modification of anodic slopes when adding H₂S; however, the cathodic branch is more sensitive showing an accelerated reduction reaction in the presence of H₂S. Interface characterization was performed by electrochemical impedance technique (EIS) in the absence and presence of H₂S and near to the corrosion potential (E_corr). Analysis of results shows no film of corrosion products, since the impedance spectra characteristics indicate a great activity of steel in the solutions studied, with differences only at low frequencies. The adsorbed complexes formed in the solution containing HAc, acetate and chlorides control the corrosion process and prevent passive film formation, even in the presence of H₂S.     

不锈钢电解着色工艺及电化学性能

对１Ｃｒ１７铁素体不锈钢电解着黑色工艺和膜层的电化学性能进行探讨。本工艺具有发黑速度快,色泽均匀,较好的抗蚀性能。电解着色不锈钢表面形成铬的复合氧化膜,增强了钝性,使得自然电位和阳极极化电位正移,提高了膜层的电化学稳定性能。     

Pilot plant investigation on petrochemical wastewater treatmentfor the removal of copper and chromium with the objective of reuse

There has been great demand for development of technologies that remove toxic heavy metal ions from wastewater. Chemical precipitation operation is known to remove heavy metal ions from water. In this study applicability of alkaline reagents such as Ca(OH)₂ (lime) and NaOH (caustic soda) in removing copper and chromium ions were evaluated. Separation of heavy metals such as chromium compounds from petrochemical industries' cooling water wastes was achieved by conversion of hexavalent chromium, Cr(VI), to trivalent chromium, Cr(III). Maximum conversion occurred in the pH range of 2.0 and 2.3, adjusted by ferrous sulfate and sulfuric acid. Maximum precipitation of Cr(III) occurred at pH 8.7 with addition of Ca(OH)₂, followed by mixing and 2-h sedimentation. At the end, the concentration of chromate was reduced from 30 ppm to 0.01 ppm. In the case of copper, which is found in the form of cupro-ammonia in ammonia plant wastes, it was observed that the presence of ammonia in wastewater prevents effective chemical precipitation. Therefore, the quantity of ammonia was reduced by aeration. The optimum aeration rate was determined to be 70 L/min, and it was found that ammonia concentration reached equilibrium after 5.0 h o f aeration. Furthermore, hydroxide and carbonate methods were evaluated with respect to precipitation of heavy metals at bench scale, and the former was selected as the method of choice. The results obtained in the Jar test were then applied to pilot scale, and it was determined that the optimum pH for maximum copper precipitation was about 12.0 for both lime and caustic soda used in the hydroxide precipitation method. Lime was preferred due to economics and its high speed of precipitation. Finally, using established methods described here, the concentration of copper followed by coagulation with lime, mixing, 2-h sedimentation and filtration through Whatman 0.45 Am filters was reduced from 48.51 mg/L to 0.694 mg/L, which is below the environmental standards for water resources.     

Electrochemical corrosion behavior of chromium-phosphorus coatings electrodeposited from trivalent chromium baths

Chromium-phosphorus (Cr-P) coatings are electrodeposited from trivalent Cr (Cr(III)) baths containing hypophosphite. The electrochemical corrosion behavior of Cr-P coatings, traditional Cr coatings deposited in hexavalent Cr (Cr(VI)) baths, and chromium-carbon (Cr-C) coatings deposited in Cr(III) baths containing formate are studied by measuring potentiodynamic polarization curves in a 10 wt% HCl solution. The composition and morphology of the coating surface layers are investigated by X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM), respectively. The results of electrochemical tests show that Cr-P coatings exhibit better corrosion resistance than traditional Cr and Cr-C coatings, which is characterized by a lower critical current density, lower passive current density, and lager passive potential range. XPS and SEM analyses confirm that the excellent corrosion resistance of Cr-P coatings is attributed to the formation of a phosphide passive film, which has high stability and self-repairing ability, and can act as a “buffer” to reject the penetration of chloride ions.     

Behaviour of molybdate-passivated zinc coated steel exposed to corrosive chloride environments

The behaviour of molybdate conversion coatings on zinc coated mild steel in corrosive chloride environments was investigated using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and neutral salt fog exposure. It was found that the presence of a simple molybdate coating initially increased the rate of corrosion of zinc. Molybdenum species were initially present in the conversion coating in either the +V or +VI oxidation states. Exposure to neutral salt fog reduced molybdenum to either Mo(IV) or Mo(III). This reduction of molybdenum, an additional cathodic process, may result in the activation of zinc observed in these studies. For molybdate-passivated surfaces in the early stages of exposure to neutral salt fog, corrosion products were found to be less voluminous than those observed on untreated surfaces. This may be due to the presence of inhibiting Mo(IV) or Mo(III) species in the corrosion product layers. However, after 24 h exposure to salt fog, no molybdenum could be detected. This implies that the lower oxidation state molybdenum species formed are soluble. However, surfaces passivated from molybdate solutions appear to forestall the onset of red rust, during immersion in chloride solutions and exposure to salt fog, by approximately 12 to 24 h. This behaviour may be attributable to corrosion inhibition by Mo(III) and Mo(IV) species while they are present at the surface.     

Electrochemical,SEM and XPS investigations on phosphoric acid treated surgical grade type 316L SS for biomedical applications

A simple surface pre-treatment method was attempted to establish a stable passive layer on the surface of surgical grade stainless steel (SS) of type 316L for biomedical applications. Surgical grade type 316L SS specimens were subjected to H₃PO₄ treatment for 1 h by completely immersing them in the acid solutions to develop a passive barrier film. The effect of various concentrations of phosphoric acid on the localized corrosion resistance behavior of type 316L SS was investigated through electrochemical techniques using cyclic polarization studies and electrochemical impedance spectroscopy (EIS). X-ray photoelectron spectroscopy (XPS) was used to evaluate the nature and composition of the passive films. The surface morphology and relative elemental composition of the untreated and acid treated surfaces subjected to anodic polarization was studied by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) techniques, respectively. Compared with untreated (pristine) 316L SS, the 40% acid treated surface formed a stable passive layer that had superior corrosion resistance.     

The electrodeposition of chromium from chromium(III) solutions — a study using microelectrodes

Potential sweep and pulse experiments at a carbon microdisc electrode (radius 4 m) have been used to investigate chromium plating from aqueous solutions of chromic ions containing sulphate or thiocyanate ions and buffered at pH 3.3. While chromium deposition occurs in many media, it is shown that the plating reaction is always in competition with other cathodic reactions including hydrogen evolution. This leads to loss in current effciency and, in some circumstances, to precipitation of chromic hydroxide on the cathode and, hence, contamination of the metal. In addition, it is demonstrated that the chromium(III) exists in solution as mixtures of complexes, not all of which are electroactive within the accessible potential range. Hence the investigation confirms that the quality of electroplates will vary strongly with the choice of medium. A commercial chromium(III) plating bath, Envirochrome, has also been studied and it is concluded that its properties are superior to simple sulphate or thiocyanate solutions.     

Supported chromium oxide catalysts using metal carboxylate complexes: dehydrogenation of propane

The present paper constitutes the first stage of a systematic study of the influence of precursor nature and structure on the catalyst behavior in dehydrogenation of propane, by varying the nature of the ligands and the nuclearity of the starting compounds at fixed net surface potentials. We show results obtained by the use of different Cr carboxylates to design the Cr₂O₃/γ-Al₂O₃ catalyst at low loadings and these results are compared with those obtained for chromic acid. The following short chain chromium compounds were selected as precursors: citrate (Cr(III)), dimer monohydrate acetate (Cr(II)), acetyl acetonate (Cr(III)) and hydroxyacetate (Cr(III)). An exhaustive characterization by means of BET surface, XPS, XRD, X-ray fluorescence, laser Raman spectroscopy, EPR and catalytic test in propane dehydrogenation enabled us to draw relevant conclusions. Low metal–support interaction might be the major cause of polymerization in Carbox. The interaction between the chromia phase and the support surface, which stabilizes different oxidation stages and coordinations of the chromia species, defines the surface architecture. Part of the Cr³⁺ is incorporated in the vacant octahedral sites of the spinel surface. The larger extension of this phenomenon in Cral might be responsible for surface inactive species that could cause a loss of catalytic activity. Deactivation between cycles might depend on the catalyst stabilization by the incorporation of Cr³⁺ into the support structure. The surface array of monomer species in a polymer species environment stabilizes the catalyst architecture and confers characteristics of high stability between cycles. The above considerations permit to infer that chromium carboxylates are interesting precursors for the control of surface species in chromia/Al₂O₃ catalysts.     

Corrosion Protection of Mild Steel in Acid Solutions Using Red Cabbage Dye

Red cabbage dye (RCD) and its inhibitory effect on the corrosion of mild steel in 1 N HCl and 1 N H₃PO₄ was investigated by weight loss and electrochemical methods. RCD was a good inhibitor for mild steel corrosion in acid solutions, affecting anodic and cathodic reactions. The inhibition efficiency increased with RCD concentrations, with more pronounced effects observed in HCl. The effect of temperature on corrosion inhibition was studied and kinetic activation parameters were calculated and discussed. The inhibition mechanism of RCD on the mild steel surface was related to the dye make up and behavior in acid solutions.     

Influence of metal carbides on dissolution behavior of biomedical CoCrMo alloy: SEM, TEM and AFM studies

The influence of precipitate carbides on dissolution tendency and behavior of a biomedical CoCrMo alloy was investigated at microscopic scale. SEM/EDS, TEM/EDS and XRD were performed to characterize crystallographic structure and composition of different precipitate carbides. Scanning Kelvin probe force microscope (SKPFM) was used to evaluate relative nobility of the carbides. In addition to polarization curves, in situ electrochemical AFM (EC-AFM) measurements were performed to investigate the effect of the carbides on local dissolution processes. SEM/EDS, TEM/EDS and XRD characterizations showed non-uniform structure and composition of Cr and Mo carbides. SKPFM analysis suggested the carbide boundaries as preferential sites for corrosion/dissolution process. Cyclic polarization curves of the alloy in phosphate-buffered saline (PBS) solution showed a large current density increase above a certain potential, but only a small hysteresis loop during reverse scan. No noticeable pitting corrosion was observed by SEM after the experiments. In situ AFM images of the sample in PBS showed a stable surface at potentials in the passive region and around the potential corresponding to the current increase and slight etching-like dissolution around the carbides at higher potentials. Carbide boundaries are preferential sites for metal dissolution and carbides with non-uniform composition might exhibit different dissolution rates.