Search Results (15)

Chromium salt fillers commonly used in current anti-corrosion coatings are highly toxic. However, due to the unique high–low valence transformation and passivation mechanisms of chromium-based functional fillers and their wide applicability, chromium-free coatings find it challenging to achieve the same performance and industry acceptance. This study introduces an innovative approach that uses zinc to reduce molybdate (MoO₄²⁻) in an acidic solution, thereby forming a multivalent MoO₄²⁻ system (PMZ system), and applies it to chromium-free insulating coating for oriented silicon steel. The effects of reductant dosage on the valence composition of molybdenum in the PMZ system and the corrosion resistance of the coating were investigated. Additionally, the difference in the valence composition of molybdenum between the PMZ system and the multivalent phosphomolybdate system (PMNZ system) and its impact on corrosion resistance were studied. The results indicate that the PMZ system contains trivalent molybdenum and hexavalent molybdenum, while the PMNZ system contains pentavalent molybdenum and hexavalent molybdenum. The systems leverage the reactivity of lower-valence molybdenum to delay the corrosion by reacting with oxygen while maintaining the original mechanism of molybdenum salt fillers and forming sediment with iron ions to form a passivation layer. As the content of trivalent molybdenum in the PMZ system increases, the corrosion resistance of the insulating coating improves. When the amount of zinc added in the PMZ system is 0.006 g, the relative proportion of trivalent molybdenum reaches 20.52%, and the salt spray resistance of the coating developed with the PMZ system reaches 248 h with a corrosion area of less than 5%. When the contents of the main components and sodium molybdate in the PMZ coating and the PMNZ coating are the same, the corrosion resistance of the PMZ coating, which contains trivalent molybdenum, is better than that of the PMNZ coating, and the salt spray resistance exceeds 192 h. Full article

Aluminum alloys with low-weight property are promising structure materials for sports equipment. Alloying element-rich second-phase particles create the risk of localized corrosion and result in failure of sports equipment. Chromate conversion coatings as conventional and successful surface treatments were employed to provide a thin but compact film against corrosion. However, chromate species were toxic and carcinogenic for human beings and this process has been highly restricted. In this sense, alternative processes such as trivalent chromium conversion coating with low environmental risk require better corrosion-resistant performance compared to chromate conversion coating. In addition, the closed-loop system of the chromate electroplating process has been used in Europe and the United States. This is also a sustainable process for surface treatment of aluminum alloys applied in sports equipment. The present paper aims to summarize the methods and types of different aluminum alloy surface treatments and compiles the effects of various surface treatments on the corrosion resistance of aluminum alloys. The eco-friendly application of aluminum alloys in the field of sports equipment may be facilitated in the future. Full article

A literature review was conducted to examine the current understanding of the kinetics and mechanism of electrochemical reactions occurring during the electrodeposition of chromium coatings from electrolytes based on trivalent chromium compounds. The research in this scientific field is crucial, as it addresses the pressing need for an alternative to chromium plating processes that rely on solutions containing highly toxic and harmful hexavalent chromium compounds. Numerous literature data on the kinetics and mechanism of the stepwise reduction process of Cr(III) complex ions were analyzed. The influence of various additives and surfactants on the reaction kinetics of the stepwise reduction of trivalent chromium ions was considered. Special attention was given to the kinetics of the stepwise discharge of trivalent chromium ions in ionic liquids and deep eutectic solvents. Full article

Due to new environmental regulations, hexavalent chromium electrolytes can no longer be used for thick, hard chromium plating. In response to this industrial and environmental challenge, trivalent chromium electrolyte plating has been developed. In this paper, we propose a study of the adhesion of Cr^III coatings based on the implementation of numerical models in comparison with an identified experimental scenario. The aim is to dissociate the influence of coating and substrate behaviours from the adhesion work by describing the intrinsic damage of the chromium layer and the coating–substrate interface. Two types of cracking were studied: transverse cracking and delamination. For the former, the crack density was higher for Cr^III than for Cr^VI and increased with deformation and coating thickness. Microtensile tests with scanning electron microscopy (SEM) observations allowed us to highlight the cracking process in the coating (transverse cracking) and at the coating–substrate interface (delamination). The numerical simulation of the test allowed us to estimate a damage-initiation threshold normal stress of 1900 MPa, which occurred at an average applied strain of 2.5%. Delamination of the coating was complete at an average strain of 13.6% and an interfacial normal stress of 2600 MPa. Full article

The passivation of zinc alloy coating was achieved through the utilization of both silicate and trivalent chromium passivation systems, employing a specific process formula consisting of Co(NO₃)₂ at a concentration of 2.5 g/L, C₇₆H₅₂O₄₆ at 3 mL/L, Na₂SiO₃ at 25 g/L, C₆H₅Na₃O₇ at 15 g/L, and an appropriate amount of organic accelerator. The composite passivation of silicate and tannic acid was found to be more effective than the trivalent chromium passivation film, as it successfully eliminated the dendrite structure on the coating surface and reduced surface defects. The coordination between negatively charged SiO²⁻ or SiO₂ micelles and Zn²⁺ results in the formation of a passivation film that exhibits lower corrosion current and higher corrosion potential compared to the trivalent chromium passivation film. Additionally, the impedance test fitting results indicate that the silicate passivation film possesses a higher resistance value. Overall, the proposed silicate passivation system presents a viable alternative to the toxic chromate passivation system, offering non-toxicity and superior protective performance relative to the trivalent chromium passivation system. Full article

Hard chromium coatings have demonstrated their performance for several decades, particularly for their wear and corrosion resistance properties. However, the traditional process using chromium trioxide (CrO₃) is very toxic and is intended to disappear in the European Union by 2024, thanks to the REACH regulation. This study aimed to determine the residual stress of amorphous chromium deposits elaborated from baths containing trivalent chromium ions. However, the amorphous structure of these deposits does not allow the determination of residual stresses by conventional means. In this study, we propose adapting a recent method called “FIB-DIC”, which is not limited by the material’s crystalline structure compared to the classical X-ray diffraction (XRD) analysis. The method is based on the measurement of relaxation-induced displacement fields following the ablation of material on a very local scale. The results obtained by the classical (XRD) and the FIB-DIC method on crystallized heat-treated Cr(III) deposits are 296 ± 45 and 377 ± 275 MPa. Then, the FIB-DIC technique allowed us to evaluate a residual stress level of 479 ± 359 MPa for amorphous trivalent chromium coating, which has never been performed before. Full article

The present study employed a tungsten pH microelectrode to study the formation of trivalent chromium conversion (TCC) coatings on Al, AA 2024-T3 aluminum alloy and AZ91D magnesium alloy in SurTec ChromitAL solutions. The tungsten microelectrode had a sensitivity of –60 mV/pH in the SurTec ChromitAL solutions. The peak and final pH values for pre-treated Al, AA 2024 alloy and AZ91D alloy were 4.9 and 3.5, 4.3 and 4.1, 4.7 and 3.5, respectively. The initial pH value is of great importance in influencing coating deposition-driven force and final coating thickness. In addition, the time to attain the peak values was 10, 100 and 20 s for Al, AA 2024 alloy and AZ91D alloy, respectively. The interfacial pH evolution is associated with the hydrolysis process of the weak acid nature of zirconium and chromium salts as the main components of TCC coating solutions. Raman spectra revealed the presence of chromium and zirconium oxides and sulphate in the solution deposits from in-house Cr/Zr and SurTec ChromitAL solutions. Full article

With plasma electrolytic oxidation (PEO), one can easily obtain thick (tens of microns), mechanically resilient and chemically stable oxide coating on aluminum and other valve metal alloys. The study of luminescent PEO coatings is a relatively new subfield of the already well-established coating preparation methods. In recent years, many new luminescence-based approaches have been developed, one of which is the detection of ionizing radiation of carbon-doped PEO alumina coating. This study presents an improved approach by doping the alumina coating with chromium using citric acid as an additive in the electrolyte. Trivalent chromium ions replacing aluminum in the crystalline lattice of the coating exhibit characteristic sharp lines in the luminescence spectrum. The effectiveness of different DC voltages, process times and citric acid concentrations in electrolyte were examined. The use of citric acid in the electrolyte also provides the conditions required for the formation of an energy trap in the bandgap of the material, thus opening up the possibility for the coating to be used as an ionizing radiation detector by measuring its thermoluminescence. Chromium atoms are incorporated in the coating from the Al6082 aluminum alloy itself and are not added in the electrolyte, therefore making the process much more reliable, repeatable, and environmentally friendly. Full article

In this study, low-iron Zn–Fe alloy coatings and pure Zn coatings, with or without trivalent chromium passivation treatment, were electrodeposited onto a sintered NdFeB magnet from a weak acid chloride bath. The surface morphology and structure of the coatings were then examined using the X-ray diffraction, a scanning electron microscope and 3D white-light interfering surface analysis. Meanwhile, the electrodeposition behavior and anti-corrosive properties of the coatings were investigated using cyclic voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy, and natural salt spray tests. The results indicate that a passivated Zn–Fe alloy coating with a 0.9 wt.% Fe content provided much better corrosion resistance than a pure Zn coating and could provide both anodic protection and physical barrier function in the NdFeB substrates. The Fe element in Zn–Fe alloy coating was predominantly in solid solution in η-phase and small amounts in elemental form, which was beneficial to acquire a compact coating and passivation film. Finally, the passivated Zn–Fe alloy coating withstood 210 h against a neutral 3.5 wt.% NaCl salt spray without any white rust, which was 3–4 times longer than the pure Zn coating. Full article

In view of new environmental directives, hexavalent chromium baths can no longer be used to electroplate thick hard chromium deposits. To meet these industrial and environmental challenges, deposits are developed from trivalent chromium electrolytes. Cr(III) coatings are usually studied from the point of view of the use properties and hardness, but their intrinsic properties remain widely unknown. The novelty of this work consists in the mechanical characterisation of these coatings. Properties such as hardness, stiffness, yield strength, and toughness of trivalent chromium deposits are determined by combining instrumented hardness tests, in situ FEG–SEM observations, and finite element simulations. These are explained according to the microstructure of the deposits, which is determined by scanning electron microscopy and X-ray diffraction. Their composition was characterised by glow discharge spectrometry. The structure characterisation deposits showed a more severely fractured coating of trivalent chromium than in the case of hexavalent chromium. Non-post-treated trivalent chromium deposits have a higher hardness (13 ± 1.7 GPa) and yield strength (5 GPa) than hexavalent chromium deposits. However, their stiffness (191 ± 13 GPa) and toughness (1.37 ± 0.13 MPa√m) are lower. Its mechanical behaviour is elastofragile. These differences in mechanical properties can be explained by the amorphous structure of the deposits and their high carbon content. Full article

Background: This study aimed to prepare monoclonal antibodies (mAbs) with high immunoreactivity, sensitivity, and specificity for the chelate (Cr(III)-EDTA) of trivalent chromium ion (Cr(III)) and ethylenediamine tetraacetic acid (EDTA). Further, the study established an indirect competitive enzyme-linked immunosorbent assay (icELISA) for detecting the total chromium content in food, feed, and environmental samples. Methods: Hapten Cr(III)-iEDTA was synthesized by chelating Cr(III) with isothiocyanatebenzyl-EDTA (iEDTA). Immunogen Cr(III)-iEDTA-BSA formed by chelating Cr(III)-iEDTA with bovine serum albumin (BSA), and coating antigen Cr(III)-iEDTA-OVA formed by chelating Cr(III)-iEDTA with ovalbumin (OVA) were prepared using the isothiocyanate method and identified by ultraviolet spectra (UV) and inductively coupled plasma optical emission spectrometry (ICP-OES). Balb/c mice were immunized with the Cr(III)-iEDTA-BSA, and the anti Cr(III)-EDTA mAb cell lines were screened by cell fusion. The Cr(III)-EDTA mAbs were prepared by induced ascites in vivo, and their immunological characteristics were assessed. Results: The immunogen Cr(III)-iEDTA-BSA was successfully synthesized, and the molecular binding ratio of Cr(III) to BSA was 15.48:1. Three hybridoma cell lines 2A3, 2A11, and 3D9 were screened, among which 2A3 was the best cell line. The 2A3 secreted antibody was stable after six passages, the affinity constant (Ka) was 2.69 × 10⁹ L/mol, its 50% inhibition concentration (IC50) of Cr(III)-EDTA was 8.64 μg/L, and it had no cross-reactivity (CR%) with other heavy metal ion chelates except for a slight CR with Fe(III)-EDTA (1.12%). An icELISA detection method for Cr(III)-EDTA was established, with a limit of detection (LOD) of 1.0 μg/L and a working range of 1.13 to 66.30 μg/L. The average spiked recovery intra-assay rates were 90% to 109.5%, while the average recovery inter-assay rates were 90.4% to 97.2%. The intra-and inter-assay coefficient of variations (CVs) were 11.5% to 12.6% and 11.1% to 12.7%, respectively. The preliminary application of the icELISA and the comparison with ICP-OES showed that the coincidence rate of the two methods was 100%, and the correlation coefficient was 0.987. Conclusions: The study successfully established an icELISA method that meets the requirements for detecting the Cr(III)-EDTA chelate content in food, feed, and environmental samples, based on Cr(III)-EDTA mAb, and carried out its preliminary practical application. Full article

Laser-based inspection of trivalent chromium conversion coatings on rough, cold-rolled aluminium substrates is studied from a basic physics perspective by means of angle and wavelength dependent measurements. As a result, we show that the correlation between the scattered laser light and the coating weight of the conversion layer is dominated by the phenomenon of interference. The combined experimental and numerical approach of our study is based on an appropriate layer model which was developed from a set of reference measurements of confocal microscopy, electron microscopy and X-ray photoelectron spectroscopy. The aluminium alloy AA3003 with a trivalent chromium conversion coating serves as an example. Our derived model is capable to reconstruct the reflectance of a laser beam at grazing incidence even for a pronounced surface roughness of $R_q\approx 300 \mathrm{n}\mathrm{m}$ , for different coating thicknesses less than 70 $\mathrm{n}$ $\mathrm{m}$ corresponding to coating weights between zero and $0.5$ $\mathrm{g}$ / $\mathrm{m}$ ${}^2$ and for laser wavelengths from 405 $\mathrm{n}$ $\mathrm{m}$ to 785 $\mathrm{n}$ $\mathrm{m}$ . In our discussion we outline the possibility to transfer the results to other aluminium alloys and/or other metallic substrates, as well as their potential for industrial applications such as 100% inline-capability, costs, velocity and ruggedness. Full article

The morphology, composition and corrosion properties of commercial hexafluoro-zirconate trivalent chromium coatings (SurTec^® 650) deposited on chemically cleaned aluminum alloy 3003 were studied. The coatings were deposited at room temperature using different concentrations of SurTec^® 650 (10, 25 and 50 vol.%) and different conversion times (90 s, 11 min and 18 min). Scanning electron microscopy with energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy and time-of-flight secondary ion spectrometry were employed to investigate the surface morphology, composition and thickness of uncoated and coated AA3003 samples. The morphology of the coating varied from uniform nodular to non-uniform and cracked; coatings were deposited at intermetallic particles and at the alloy matrix. The main constituents of conversion coatings were Zr(IV) and Cr(III) oxides; in addition to oxides, fluorides were also formed. The corrosion properties were investigated in two solutions: more aggressive sodium NaCl and less aggressive simulated acid rain. These commercial conversion coatings exhibited a good corrosion resistance but only after longer immersion in solution, i.e., 24 h. The results reveal an interesting behavior of zirconate-based coatings on aluminum-manganese alloy. Full article

In this work, the effect of treatment time on the performance of Cr(III) conversion coatings (TCC) on hot dip Zn–55Al–1.6Si (Zn55Al) coated steel sheet were investigated. The surface 3D morphology and roughness of TCCs were examined by a 3D topography instrument and the structure, chemical composition, and elemental depth distribution were studied by means of scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and auger electron spectroscopy (AES). The results indicated that during the formation of TCC, the macro-roughness of Zn55Al surface was reduced, but the micro-roughness increased, which are considered to be key factors in enhancing the adhesion strength of epoxy primers. The AES depth profiles showed a two-layer TCC for both dendritic and inter-dendritic regions and chemical composition analysis of XPS showed that the surface of TCC was mainly oxides, fluoride and a small number of hydroxides. Overall, Zn55Al specimen prepared in a diluted commercial Cr(III)-based solution for 180 s at 40 °C performed a better adhesion strength to epoxy primer and had the largest polarization resistance among all TCCs in this work. Additionally, longer Cr(III) passivation process (TCP) treatment time will increase the sensitivity of the TCC to micro-cracks. Full article

Metal-on-metal bearings for total hip replacements have been introduced as an alternative to polyethylene in young and more active patients. These have, however, been shown to be prone to implant malpositioning and have been limited by some specific design features. In that context, coatings present an option to increase wear resistance by keeping the high fracture strength of the metal substrate. A custom-made electroplating setup was designed for the coating of CoCr substrates using (a) an industrial standard chromium electrolyte; (b) a custom-made hexavalent chromium (Cr⁶⁺) electrolyte with a reduced chromium trioxide (CrO₃) content, both without solid additives and (c) with the addition of fullerene (C₆₀) nanoparticles; and (d) a trivalent chromium (Cr³⁺) electrolyte with C₆₀ addition. All coatings showed an increase in microhardness compared with the metal substrate. Trivalent coatings were thinner (10 µm) than the hexavalent coatings (23–40 µm) and resulted in increased roughness and crack density. Wear was found to be reduced for the hexavalent chromium coatings by 70–84% compared with the CoCr–CoCr reference bearing while the trivalent chromium coating even increased wear by more than 300%. The addition of fullerenes to the electrolyte did not show any further tribological effect. Full article