Characteristics of Zinc Corrosion and Formation of Conversion Films on the Zinc Surface in Acidic Solutions of Cr(III) Compounds

The influence of the components of an acidic solution: Cr(III) nitrate-malonic acid-Co(II) salt and treatment conditions on zinc dissolution and formation of Chromate films as well as on their decorative and protective properties have been studied using the analytical, XPS, structural and accelerated corrosion test methods. An organic acid is the main component, which has an essential influence on zinc dissolution and formation of Chromate films as well as their decorative and protective properties. The influence of organic acid is directly related with the state of the Cr³⁺ ions in chromating solution. When the Cr³⁺ ions are in the form of hexaaquaions, the organic acid increases the quantities of the zinc dissolved and the Cr(III) deposited on the zinc surface (especially at 60°C). It also predetermines the formation of a thick, porous Chromate film with large cracks at 60°C. Its decorative and protective properties are rather poor. When Cr³⁺ ions are in the form of a complex with organic acid, the quantities of the zinc dissolved and the Cr(III) deposited on the zinc surface significantly decrease and thinner Chromate films with an even surface, good decorative appearance and high corrosion resistance are formed Decorative blue-bright Cr(VI)-free films with a slight iridescent tint, obtained in solution, containing Cr(III) nitrate (0.2), malonic acid (0.3) and Co(II) nitrate (0.02) mol dm^?3, at p H 1.6-2.0 at room temperature over 30–60 s, possess corrosion resistance (192–240 h in a salt spray chamber) similar to that of iridescent Chromate films, obtained in acidic Cr(VI) solution.     

Effect of Mg2+ on the microstructure and corrosion resistance of the phosphate conversion coating on hot-dip galvanized sheet steel

The effect of Mg²⁺ in the phosphate solution on the microstructural evolution and corrosion resistance of the coating on hot-dip galvanized steel has been explored. Surface morphology observations reveal that increasing the solution Mg²⁺ concentration increases the population density and refines the grain size of the phosphate grains. In the presence of Mg²⁺, the phosphate coating is composed of mixed Zn phosphate hydrate and (Zn, Mg) phosphate hydrate. Furthermore, elevating Mg²⁺ in the solution reduces the porosity of the phosphate coating. As a result, the corrosion resistance of the phosphate coating is improved.     

Effect of additives on the properties of phosphate conversion coating on electrogalvanized steel sheet

Ni²⁺ and Mn²⁺ ions have been added separately to the phosphate solution to minimize the porosity of the phosphate conversion coating on electrogalvanized steels. Results showed that the Ni²⁺ or Mn²⁺ in the solution reduced the grain size and porosity of the phosphate coating; thereby, the corrosion resistance was enhanced. However, Ni²⁺ and Mn²⁺ played a different role in the coating formation. Ni²⁺ in the solution was reduced by Zn to form Ni, which enhanced the dissolution of Zn to promote the nucleation of hopeite grains. Mn²⁺ in the solution facilitated the nucleation by increasing the impingement of reacting species.     

On the Measurement of Strain in Coatings Formed on a Wrinkled Elastic Substrate

Highly stressed coatings, such as those formedby oxidation can, on occasion, wrinkle. Such wrinklinghas been suggested as a mode of deformation by which theoverall strain energy in a compressively stressed coating can be reduced. One of the consequencesof wrinkling is that the strain in the coating does notremain independent of position, but rather varies overthe wavelength of the wrinkling. The strain variation caused by sinusoidal wrinkling iscalculated using finite-element methods and the effectson both photostimulated Cr³⁺ luminescencepiezospectroscopy measurements and X-ray measurementscalculated. Wrinkling is shown to decrease theelastic-strain-energy density in the coating. A directmeasure of the decrease is the shift in the R2Cr³⁺ luminescence line and the X-raydiffraction peaks. Wrinkling of a compressive coating also causes stressesto be created perpendicular to the coating-substrateinterface, tensile at the crests in the wrinkles, andcompressive stress at the troughs.     

Mechanical and Tribological Properties of HVOF-Sprayed (Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-NiCr+Ni) Composite Coating on Ductile Cast Iron

The aim of the investigations was to compare the microstructure, mechanical, and wear properties of Cr₃C₂-NiCr+Ni and Cr₃C₂-NiCr coatings deposited by HVOF technique (the high-velocity oxygen fuel spray process) on ductile cast iron. The effect of nickel particles added to the chromium carbide coating on mechanical and wear behavior in the system of Cr ₃ C ₂ -NiCr+Ni/ductile cast iron was analyzed in order to improve the lifetime of coated materials. The structure with particular emphasis of characteristic of the interface in the system of composite coating (Cr ₃ C ₂ -NiCr+Ni)/ductile cast iron was studied using the optical, scanning, and transmission electron microscopes, as well as the analysis of chemical and phase composition in microareas. Experimental results show that HVOF-sprayed Cr₃C₂-NiCr+Ni composite coating exhibits low porosity, high hardness, dense structure with large, partially molten Ni particles and very fine Cr₃C₂ and Cr₇C₃ particles embedded in NiCr alloy matrix, coming to the size of nanocrystalline. The results were discussed in reference to examination of bending strength considering cracking and delamination in the system of composite coating (Cr ₃ C ₂ -NiCr+Ni)/ductile cast iron as well as hardness and wear resistance of the coating. The composite structure of the coating provides the relatively good plasticity of the coating, which in turn has a positive effect on the adhesion of coating to the substrate and cohesion of the composite coating (Cr₃C₂-NiCr+Ni) in wear conditions.     

The Corrosion Behaviour of Electrochemical Zn Coated with Conversion Chromate and Phosphate Films in Concrete

The corrosion behaviour of electrochemical Zn coatings with and without chromate (VI, III) and phosphate (amorphous and crystalline) films in aerated 0.1 M NaOH + 0.1 M NaCl solution and concrete, immersed into water has been studied. The estimated corrosion rate in 0.1 M NaOH + 0.1 M NaCl solution is minimal cf. Zn chromatised in Cr³⁺ solution. The value of j_cor of Zn chromatised in Cr⁶⁺ solution is 1.5 times as high as that in Cr³⁺. The experiments carried out in concrete immersed into water (pH 12.5) have shown that all the chromate films studied do not extend the operating time of the sample. The best corrosion protection is provided by crystalline phosphate film.     

Influence of Water Vapor on the Cyclic-Oxidation Behavior of a Low-Pressure Plasma-Sprayed NiCrAlY Coating

The oxidation of a low-pressure plasma-sprayed (LPPS) NiCrAlY coating on a nickel-base superalloy was studied at 1050 °C in flows of O₂, and mixture of O₂ and 5% H₂O under atmospheric pressure. Water vapor has an obvious effect on the cyclic oxidation of the NiCrAlY coating. There is more decrease in weight gain when exposure to O₂ is replaced by exposure to O₂ + 5% H₂O. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in pure oxygen is composed mainly of Cr₂O₃, and a thin Al₂O₃-rich layer is formed at the interface between the Cr₂O₃-rich layer and the coating. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in a mixture of O₂ + H₂O is composed of NiCr₂O₄, NiO and Cr₂O₃. The effect of water vapor on the oxidation of the NiCrAlY coating may be attributed to an increase in Ni and Cr cation transport, stress-corrosion cracking of Al₂O₃ and moisture-enhanced volatility of the Cr₂O₃ scale.     

Structure,mechanical and thermal properties of Y-doped CrAlN coatings

CrAlYN coatings with different Y contents (0, 5 and 12 at.%) were deposited by cathodic arc evaporation to investigate the influence of Y-addition on the structure, mechanical and thermal properties of CrAlN coatings by using X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, thermal gravimetric analysis and nanoindentation. The structural transformation of single phase cubic Cr_0.42Al_0.58N and Cr_0.39Al_0.56Y_0.05N coatings to cubic–wurtzite mixed Cr_0.32Al_0.56Y_0.12N coating leads to a drop in hardness from (30.2±0.7) GPa of Cr_0.42Al_0.58N and (32.0±1.0) GPa of Cr_0.39Al_0.56Y_0.05N to (25.2±0.7) GPa of Cr_0.32Al_0.56Y_0.12N. The incorporation of 5 at.% Y retards the thermal decomposition of CrAlN, verified by the postponed precipitation of w-AlN and N-loss upon annealing. Correspondingly, Cr_0.39Al_0.56Y_0.05N coating consistently exhibits the highest hardness value during thermal annealing. Nevertheless, alloying with Y exerts an adverse effect on the oxidation resistance of CrAlN.     

Enhanced corrosion performance of magnesium phosphate conversion coating on AZ31 magnesium alloy

Magnesium phosphate conversion coating (MPCC) was fabricated on AZ31 magnesium alloy for corrosion protection by immersion treatment in a simple MPCC solution containing Mg²⁺ and PO^3?₄ ions. The MPCC on AZ31 Mg alloy showed micro-cracks structure and a uniform thickness with the thickness of about 2.5 µm after 20 min of phosphating treatment. The composition analyzed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the coating consisted of magnesium phosphate and magnesium hydroxide/oxide compounds. The MPCC showed a significant protective effect on AZ31 Mg alloy. The corrosion current of MPCC was reduced to about 3% of that of the uncoated surface and the time for the deterioration process during immersion in 0.5 mol/L NaCl solution improved from about 10 min to about 24 h.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

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.     

Effect of TiO2 nanoparticles on anticorrosion property in amorphous Ni-P-Cr composite coating in artificial seawater and microbial environment

To obtain the composite with a function of corrosion resistance in seawater and microbial environment, we apply low toxic electrolyte which contained Cr³⁺ instead of Cr⁶⁺ as a chromium source to produce Ni-P-Cr/TiO₂, Ni-P-Cr/ZrO₂ nanocomposite coatings, and Ni-P-Cr/TiO₂ microcomposite coating using direct current (DC) electrodeposits technique. The surface morphology, texture, and composition of the composite coatings are characterized by SEM, XRD, and EDX measurements, respectively. The corrosion behavior of the composite coatings is evaluated by polarization curves and electrochemical impedance spectroscopy (EIS). The results with more positive-shifting E_corr, lowest i_corr, and relatively high value of polarization resistance (R_p) proved that compared with other composite coatings, Ni-P-Cr/TiO₂ nanocomposite coating with its better passive film and unique antibacterial activity caused by TiO₂ nanoparticles incorporated, displayed excellent antibacterial and corrosion resistance property, consistent to the phenomenon that corrosion of this nanocomposite coating in SRB culture medium is not observed.     

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.     

铜离子对银电解的影响

采用电导率仪测定不同Cu2+浓度下AgNO3溶液的电导率,应用扫描伏安法研究Cu2+对阴极极化的影响,通过恒电流电解精炼研究Cu2+浓度对电解银粉含铜量的影响。结果表明:AgNO3溶液的电导率随着Cu2+浓度的增加而逐渐升高;Cu2+对银的析出具有抑制作用,随着其浓度的增加银的析出电位负移;电解液Cu2+浓度超过21.3 g/L时,银粉铜含量便会超标;银电解液中的Cu2+浓度在10~20 g/L范围时,对银粉中其他杂质元素Bi、Fe、Pb、Te的析出具有抑制作用。     

Corrosion properties of materials coated with Cr2O3/NiCr and with Cr2O3+NiCr functionally gradient materials

This paper studied the corrosion properties of five kinds of Cr₂O₃ coated materials: the SUS316L austenitic stainless steels respectively coated with the Cr₂O₃ layer (Cr₂O₃/316), Cr₂O₃ and 80Ni‐ 20Cr layers (Cr₂O₃/80NiCr/316), Cr₂O₃ and 50Ni‐ 50Cr layers (Cr₂O₃/50NiCr/316), Cr₂O₃ + 80Ni‐ 20Cr functionally gradient materials (Cr₂O₃ + 80NiCr FGM), and Cr₂O₃ + 50Ni‐ 50Cr FGM (Cr₂O₃ + 50NiCr FGM). All the coatings were made by atmospheric pressure plasma spraying method (APPS). The corrosion resistance of the coated materials was analyzed by immersion tests and electrochemical evaluations. A mechanism of the corrosion failures for these kinds of coated structures was proposed. The different coating structures for improving the adhesion between ceramics and substrates were assessed, and the effect of Cr content in the NiCr coatings on the corrosion property was discussed. The ceramic coatings with the 50Ni‐ 50Cr intermediate layer possessed a better corrosion resistance than that with 80Ni‐ 20Cr. The FGM structures appeared to offer weaker resistance to corrosion attack than that with the intermediate layer in general. Under corrosion test conditions, the corrosion‐proof abilities of the coated structures were, respectively: the Cr₂O₃/50NiCr/316 in the best rank; the Cr₂O₃/316, Cr₂O₃/80NiCr/316, and Cr₂O₃ + 50NiCr FGM in the second rank; and the Cr₂O₃ + 80NiCr FGM in the last rank. This means that applying the 50NiCr intermediate layer under the Cr₂O₃ ceramic coating can further improve the corrosion resistance of Cr₂O₃/316. Porosity analysis was used to explain the difference of corrosion resistance between the Cr₂O₃/50NiCr coated material and the Cr₂O₃ + 50NiCr FGM. The porosity in each layer of the Cr₂O₃ + 50NiCr FGM was higher than that in the Cr₂O₃/50NiCr coating, and as a result the corrosion resistance of Cr₂O₃/50Ni Cr/316 is better than Cr₂O₃ + 50NiCr FGM.     

Formation of a chromium-carbide conversion coating on silicon carbide

The formation of a chromium-carbide conversion coating on SiC was achieved using the pack-cementation technique. The conversion coating is intended to improve the corrosion resistance of SiC and its derivatives, such as SiC-base continuous fibers and composites, by forming a protective Cr₂O₃ scale upon exposure to high-temperature corrosive environments. Different pack chemistry and processing parameters were evaluated in the laboratory. Results indicated that the coating morphologies and compositions achieved were significantly affected by variation of these processing factors. In this paper, the conversion coating obtained from one of the systems investigated is reported. The coating consists of a multilayered structure with each of the sublayers containing a high-Cr concentration. In addition, the coating surface is relatively dense and pore free compared to the underlying SiC substrate material. A dense and pore-free morphology is highly desirable for coating applications, especially on porous substrates. The multilayered coating structure consists of the following sublayers: Cr₂₃C₆/Cr₇C₃/Cr₇C₃+Cr₃Si/Cr₅Si₃C_x/SiC substrate.     

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.     

Mechanical and tribological properties of plasma-sprayed Cr3C2-NiCr, WC-Co, and Cr2O3 coatings

Mechanical properties such as Young’s moduli and fracture toughness of plasma-sprayed Cr₃C₂-NiCr, WC-Co and Cr₂O₃ coatings were measured. The tribological properties of the three kinds of coatings were investigated with a block-on-ring self-mated arrangement under water-lubricated sliding. Furthermore, the influences of the mechanical properties on the tribological properties of the coatings were also examined. It was found that the Young’s moduli, bend strengths and fracture toughness of the coatings were lower than the corresponding bulk materials, which may be attributed to the existence of pores and microcracks in the coatings. Among the three kinds of coatings, the magnitude of wear coefficients, in decreasing order, is Cr₃C₂-NiCr, WC-Co and Cr₂O₃, and the wear coefficient of Cr₂O₃ coating was less than 1 × 10⁻⁶mm³N⁻¹m⁻¹. The wear mechanisms of the coatings were explained in terms of microcracking and fracturing, and water deteriorated wear performance of the coatings. The higher the fracture toughness and the lower the porosity and length of microcracking of the coating, the more the wear-resistance of the coating.     

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.

     

Improvements in oxidation resistance and conductivity of Fe-Cr metallic interconnector by (La0.8Ca0.2)(Cr0.9Co0.1)O3 coating

In this study, the effect of perovskite (Ca,Co)-doped LaCrO₃ (LCCC) coating on a commercial ferritic stainless steel (SS430) as metallic interconnector in intermediate-temperature solid oxide fuel cell has been investigated in terms of oxidation kinetics and area specific resistance (ASR) at 800 °C in air environment. The LCCC coating was deposited on SS430 by atmospheric plasma spraying from a spray-dried (La_0.8Ca_0.2)(Cr_0.9Co_0.1)O₃ powder of 10–30 um. The plasma-sprayed LCCC coating had a low porosity of ～1.0 area% and a good electrical conductivity of 33 S/cm at 800 °C. The protective LCCC coating significantly lowered the growth rate of thermally grown oxide scale of SS430 by inhibiting oxygen inward diffusion. The oxidation of the LCCC coated SS430 for 1200 h led to the formation of duplex Cr₂O₃-LaCrO₃ scale layer of ～1 um in thickness. A high conductive LaCrO₃ phase in the oxide scale was formed by the chemical interactions between the Cr₂O₃ scale and the LCCC coating. The ASR for the LCCC coated sample was limited to 9.2 mΩ·cm² after oxidation for 1200 h at 800 °C, which was much lower than ASR for the uncoated sample.