Characterisation of electrodeposited Co-W-P amorphous coatings on carbon steel

Electrochemistry and nucleation mechanism of induced co-deposition of cobalt, tungsten and phosphorus from a citrate bath was investigated using voltammetry and chronoamperometry techniques. It was found that the induced co-deposition of the species occurred under diffusion control and followed instantaneous nucleation mechanism. Co-W-P coatings were electrodeposited potentiostatically on plain carbon steels from a citrate bath containing CoSO₄, Na₂WO₄ and NaH₂PO₂. X-ray diffraction studies of the coatings revealed that the as-deposited Co₈₁W₁₀P₉ coatings had amorphous structure. The formation of some stoichiometric compounds like Co₃W, Co₂P and WP₂, however, was observed upon annealing at 600 °C. The hardness of the Co-W-P coatings increased with annealing temperature which was possibly due to the formation of these inter-elemental compounds. The corrosion resistance of the Co-W-P coatings increased with increase in annealing temperature which might also be due to the formation of the stoichiometric compounds at elevated temperatures. A comparison between coating characteristics of Co-W-P and chromium showed that the Co-W-P coatings exhibited nobler corrosion potential than chromium coatings.     

镍钨磷非晶态合金电刷镀工艺研究

在Ni-P电刷镀工艺的基础上,研究和开发了Ni-W-P电刷镀工艺,镀液中加入钨酸盐。通过试验确定各组分的含量及工艺条件。镀层测试结果表明：Ni-W-P镀层为非晶态镀层,硬度高、耐腐蚀性能好,并具有良好的结合力。可用于工、模具的修复。     

Morphological, structural, microhardness and electrochemical characterisations of electrodeposited Cr and Ni-W coatings

This study examines the corrosion of electrodeposited Cr and of two electrodeposited Ni-W coatings in 0.1 mol L⁻¹ NaCl solution, as well as the influence of heat treatment on the crystallographic structure and microhardness properties of these coatings. Physical characterisation is carried out using scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. Electrochemical characterisation is carried out using both the potentiodynamic linear polarization technique and open circuit measurements during long-term immersion tests. The corrosion products on the coating surfaces are characterised by ex situ Raman spectroscopy. As-electrodeposited Ni-W samples do not present defects, and the surface evolves from fine globular grains to rough polycrystalline morphology with decreasing electrodeposition current density. All the studied coatings corrode in the chloride medium and the corrosion is non-uniform for the Ni-W coatings. Raman analyses carried out after the immersion tests reveal Cr₂O₃ and Cr(OH)₂ corrosion products on the Cr coating surface, and Ni(OH)₂, NiO and WO₃ corrosion products on the Ni-W coating surfaces. Ni, Ni₄W and Ni-W phases are formed after heat treatment of the Ni₈₆W₁₄ coating at 600 °C. Although all the annealed Ni-W layers are cracked, their microhardness increases as the annealing temperature increases, suggesting that Ni-W coatings are potential substitutes for chromium in industrial applications in which good microhardness properties and stability at temperatures higher than 100 °C are required.     

Ni-W-P合金镀层性能研究

通过对Ni–W–P合金镀层在低温热处理前后的XRD和SEM检测得知,无论镀态还是热处理态,镀层结构均为非晶态,晶粒均匀,晶界清楚,并且热处理后晶粒长大。在硫酸和盐酸介质中的耐蚀性测试表明,除了低体积分数(5%左右)和高体积分数(20%左右)盐酸之外,热处理态的耐蚀性均高于镀态的耐蚀性。通过析氢性能测试,得知镍基合金的电催化活性依次为Ni–W–P>Ni–P>Ni–W>Ni。     

Corrosion behavior of amorphous carbon deposit in 0.89% NaCl by electrochemical impedance spectroscopy

Amorphous carbon films were deposited on SS316L substrates using a DC magnetron sputtering system, aiming at the application of the coated SS316L for biomedical implants. The biocompatibility and chemical stability of the carbon layers have been previously demonstrated. The films were deposited on top of sputtered titanium coatings introduced as a buffer layer to enhance film-substrate adhesion. The corrosion resistance of the a-C/Ti/SS316L systems was investigated by electrochemical techniques. The electrolyte used in this work was 0.89 wt.% NaCl at pH 7.4, which simulates body fluid ionic concentrations. The coated samples displayed corrosion resistance values in the saline solution much higher than the stainless steel substrates and the role of the Ti coating thickness was analysed in order to determine the optimal system for biological applications.     

An electrochemical and mechanical approach to the corrosion resistance of cathodic electrocoatings under combined cyclic and DC polarization conditions

A modified AC/DC/AC test has been introduced. It consists of a combination of EIS test, cathodic polarizations, wet and dry cycles. This test method was applied to cathodically electrocoated mild steel substrates baked at different conditions. This study aims at characterizing protective behavior of under-, normal- and over-baked electrocoatings under cyclic corrosion conditions. It was found that the resistive failure is the dominant failure mechanism of all the coatings in this test. It was also found that a low crosslink density coating withstands the mechanical stresses due to dry–wet cycles better than a higher crosslink density one.     

Electrodeposition of amorphous/nanocrystalline and polycrystalline Ni-Mo alloys from pyrophosphate baths

Pyrophosphate plating bath was found to be a good alternative to citrate bath for deposition of Ni-Mo amorphous alloys. The addition of wetting agents such as 2-butyne-1,4-diol and rokafenol N-10 to the pyrophosphate bath resulted in the removal of bumps, spheres and cracks from the Ni-Mo alloy surface. The plated alloy layers adhered well to Cu-Zn brass and steel, were of thickness from a fraction to tens of micrometers and the molybdenum content was independent of the distance from the support. An increase in the concentration of the molybdate ion in the bath leads to an increase in the amount of Mo in the alloys up to 33-35 at.% and to a decrease in the deposition rate. These changes and the influence of pH are discussed in the paper. The atom arrangement in the alloys changes from (2 2 0) preferred for pure-nickel deposition to (1 1 1) for content of Mo higher than 15 at.%. For 20 and more at.% of Mo the structure of the alloy is amorphous like. An analysis of SEM and STM micrographs obtained indicates that contrary to the Ni-W alloy the “amorphous” phase is made of circa 10-50 nm in diameter objects and not by long needles perpendicular to the substrate.     

Influence of the size of spraying powders on the microstructure and corrosion resistance of Fe-based amorphous coating

The Fe-based amorphous coatings with the composition of Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ were successfully sprayed on mild steel substrate by the high velocity oxygen fuel (HVOF) spraying process with different feedstock powder sizes (i.e., powder A: −33 + 20 μm, powder B: −45 + 33 μm, powder C: −55 + 45 μm). The coatings were characterized for its morphology, microstructure and thermal stability by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The corrosion behavior of the coatings in 3.5 wt% NaCl solution was studied with potentiodynamic and potentiostatic polarization test. It was found that the particle size of the feedstock powders had a significant influence on microstructure and corrosion resistance of the resultant coatings. The coatings sprayed with the finest powders show the most compact structure; while the coating with the coarser powders exhibits a better corrosion resistance. It is found that the corrosion resistance of the coatings is closely related to the wetting behavior which is affected by the oxygen content and the roughness of coatings. The coatings with hydrophobicity exhibit a better corrosion. The present result demonstrates that the amorphous coatings with hydrophobicity and excellent corrosion resistant are promising for industrial application in marine environment.     

氧化铬加入量对出钢口填充料性能的影响

在填充料的常规配方中,分别添加质量分数为0、1％、2％、4％氧化铬,研究了不同氧化铬加入量对材料性能的影响。结果表明：氧化铬的引入明显提高了材料抗渣性能,但同时导致材料体积密度和强度略微降低,加入2％的氧化铬时,材料综合性能最好。     

A comparative study of the available electrochemical methods for the investigation of epoxy and fluoropolymer coatings deterioration

The corrosion behaviour of phosphatized galvanized steel coated with both epoxy films of different thickness and fluoropolymer films has been studied by means of a.c. impedance spectroscopy (EIS), break-point frequency, potentiodynamic measurements and faradaic distortion methods as well as by the salt spray test. It was observed that the degradation of protective films appears after a long-lasting initial period but once the process starts, the area of defects increases with exposure time. The rate of degradation depends both on type of polymer and of film thickness for the same type of polymer. The same behaviour can be observed from the decrease in pore resistance and charge-transfer resistance (EIS) and increase in double-layer capacitance (a.c. impedance measurements) and corrosion current (potentiodynamic measurements and harmonic analysis).     

Tribological and electrochemical behavior of thick Cr-C alloy coatings electrodeposited in trivalent chromium bath as an alternative to conventional Cr coatings

Alternative process to hexavalent chromium plating, substitute materials and new designs are urgently needed owing to the requirement of “clean” manufacture. This comparative study was conducted to systematically investigate the tribological and electrochemical behavior of the Cr-C alloy coatings electrodeposited from a trivalent chromium bath and the hard Cr coatings electrodeposited from conventional hexavalent chromium bath, using reciprocating ball-on-disc tribometer and electrochemical analyzer. The electroplated Cr-C alloy coatings with thickness of 50 μm and acceptable quality that can be used for wear resistance as well as corrosion resistance purposes were produced successfully. The results show that the as-deposited Cr-C alloy coatings exhibited crack-free surface and amorphous/microcrystalline structure. The following heat treatment resulted in the cracked surface and the increase in hardness for the electroplated Cr-C alloy coatings. In contrast, the conventional Cr coatings exhibited cracked surface and their hardness decreased with the increase in annealing temperature. The electroplated Cr-C alloy coatings after heat treatment at 200 °C for 1 h exhibited better wear resistance than the conventional Cr coatings. In regard to the electrochemical behavior, the as-deposited Cr-C alloy coatings exhibited better corrosion resistance than the conventional Cr coatings. Therefore, the electroplated Cr-C alloy coatings are environmentally acceptable candidates to replace the conventional Cr coatings.     

Improving the formation and protective properties of silane films by the combined use of electrodeposition and nanoparticles incorporation

The electrodeposition and nano-silica incorporation were used together to prepare the novel composite dodecyltrimethoxysilane/SiO₂ thin films from silane sol-gel system on aluminum substrate. The results showed that both the two techniques can improve the films formation and their protectiveness. The influences of the deposition potential and the silica content in silane solution were investigated. A “critical deposition potential” and a “critical silica content” were both observed, under which the obtained silane films had the highest protective properties. The enhancement in film thickness has been detected by these two techniques from the elemental depth profiles of silane films as measured by secondary-ion mass spectroscopy (SIMS). Current-time curves were on-line recorded on aluminum electrodes in silane solutions. As compared with that in blank solution, the current response was found to be larger in silica-contained precursor, probably suggesting that the silica particles participate in the film deposition.     

Characterisation and corrosion protection properties of polypyrrole electropolymerised onto aluminium in the presence of molybdate and nitrate

This paper describes the first attempts for the electrosynthesis of polypyrrole films containing molybdate onto aluminium electrodes. Electrogeneration was carried out in an alkaline solution in the presence of molybdate, nitrate and the monomer. The optimum concentrations for electropolymerisation were chosen in order to improve the corrosion protection of the substrate. The coatings were characterised by SEM/EDX and IR spectroscopy and the cyclic voltammetric response of the film is discussed. Pitting corrosion resistance was studied by means of open circuit potential and potentiodynamic measurements. The results presented below show that molybdate remains entrapped into the polymer matrix, providing greater resistance to breakdown of passivity in chloride media.     

A comparative study on the electrochemical properties of ring‐substituted polyanilines

The redox behavior of polyanilines with ring‐substituted groups synthesized by chemical polymerization, poly‐2,5‐dimethoxyaniline (PDMAn), poly‐m‐chloroaniline (PmClAn) and poly‐o‐toluidine (POT), was studied and the morphology and crystal orientation of platinum particles deposited on these polymer membranes was compared. The oxidation of isopropanol on platinized polyaniline‐modified electrodes as a model reaction was also investigated to examine the electrocatalytic properties of the polymers. The results show that the first oxidation potential of the polymers increases in the following sequence: PDMAn, POT and then PmClAn, which can be explained in terms of the electronic and steric effects of ring‐substituted groups. The growth of Pt particles electrodeposited on doping‐state POT and PDMAn polymer membranes takes place by a ‘progressive nucleation mechanism’, but by an ‘instantaneous nucleation mechanism’ on PmClAn. Platinum crystallites trend towards preferred‐orientation and Pt (200) is the preferred face, but the degree of preferred orientation depends on the polymer. The oxidation potentials of isopropanol are located near 0.3 V, and the oxidation currents increase on platinized polymer‐modified electrodes, indicating that the interaction of polymer with Pt particles might improve the catalytic activity of Pt. Polyanilines act not only as dispersion media but also change the electronic properties of Pt crystalline grains. Copyright © 2005 Society of Chemical Industry     

换热器专用钢管08Cr2AlMo的性能及应用

着重介绍了换热器专用钢管 0 8Cr2AlMo的耐腐蚀性能、力学性能 ,以及 0 8Cr2AlMo钢管与 16Mn管板的焊接工艺以及在炼油厂的应用     

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.     

A stochastic analysis of the effect of hydrostatic pressure on the pit corrosion of Fe-20Cr alloy

The effect of hydrostatic pressure on the pit corrosion behavior of Fe-20Cr alloy was investigated in 3.5% NaCl solution by means of potentiodynamic polarization and potentiostatic technology, and the experiment data was analyzed based on stochastic theory. With the increase of hydrostatic pressure, the pit corrosion resistance of Fe-20Cr alloy was deteriorated, which was distinguished by the decrease of critical pit potential (E_cirt) and the increase of passive current density. The results also demonstrated that there exist two effects of hydrostatic pressure on the corrosion behavior of Fe-20Cr alloy: (1) the pit generation rate was evidently increased compared to that under lower hydrostatic pressure, and the metastable pits become faster and larger. However, it seemed that pit generation mechanism shows no hydrostatic pressure dependence; (2) the probability of pit growth increased with the increase of hydrostatic pressure, which implied that the metastable pit on Fe-20Cr alloy exhibited higher probability to become larger pit cavity during shorter time interval than that under lower hydrostatic pressure.     

Effect of organoclay structure on morphology and properties of nanocomposites based on an amorphous polyamide

An amorphous polyamide (a-PA) and three organoclays, M₃(HT)₁, M₂(HT)₂ and (HE)₂M₁T₁, were melt processed to explore the effect of the organoclay structure on the extent of exfoliation and properties of these nanocomposites. Wide angle X-ray scattering, transmission electron microscopy, and stress-strain behavior were used to determine the degree of exfoliation of the nanocomposites. For quantitative assessment of the structure of the nanocomposites, a detailed particle analysis was made to provide various averages of the clay dimensions and aspect ratio. The results evaluated from different methods were generally consistent with each other. Nanocomposites based on the organoclays with one alkyl tail and hydroxyl ethyl groups gave well-exfoliated structures and high matrix reinforcement while nanocomposites from two-tailed organoclay contain a considerable concentration of intercalated stacks. Nanocomposites from the organoclays with one alkyl tail showed slightly better exfoliation and matrix reinforcement than those from the organoclays with hydroxyl ethyl groups. The organoclay structure trends for a-PA are analogous to what has been observed for nylon 6; this suggests that a-PA, like nylon 6, has good affinity for the pristine silicate surface of the clay leading to better exfoliation and enhanced mechanical properties with one-tailed organoclay than multiple-tailed organoclay. Furthermore, heat distortion temperatures were predicted from the dynamic mechanical properties of nanocomposites.     

Electrochemical corrosion performance in 3.5% NaCl of the electrodeposited nanocrystalline Ni films with and without dispersions of Cr nanoparticles

Nanocrystalline (nc) Ni films with and without dispersions of Cr nanoparticles were electrodeposited from a nickel sulfate bath. The grain size of the nc-Ni films was reduced with increasing in the co-deposition content of Cr nanoparticles. Potentiodynamic polarization tests showed that increasing in the co-deposition content resulted in an enlarged passive region of the nc-Ni in 3.5% NaCl through reducing the corrosion potential and increasing the breakdown potential. Scanning electron microscopy (SEM) observation indicated that the polarized pure nc-Ni film exhibited numerous large and deep pits. However, they became smaller and shallower when 4.5 wt.% Cr nanoparticles were co-deposited, and almost disappeared when 10.9 wt.% Cr were co-deposited. X-ray photoelectron spectroscopy (XPS) analysis showed that the different electrochemical corrosion performance was associated with the ability of the Cr nanoparticles co-deposited nc-Ni film to form a continuous Cr-oxide passive film.     

Electro- and magneto-transport properties of amorphous carbon films doped with iron

Electro- and magneto-transport properties of amorphous carbon films doped with iron element have been systematically studied. The electro-transport mechanism of the films is dominated by thermal activation at T > 200 K, Mott-type variable range hopping (VRH) at 200 K > T > 60 K and Efros-Shklovskii type (ES-) VRH at T < 60 K. An anomalous giant positive magnetoresistance (MR) 6.40% is found at the ES-VRH range, which is attributed to the spin blockage effect. At high temperatures, an anomalous Hall effect (AHE) is also found with a large AHE coefficiency 49.6 μΩcm/T. Electron energy loss spectroscopy (EELS) reveals that iron atoms chemically bond with carbon matrix. These iron carbides exist as amorphous nanoparticles with a diameter of 6-12 nm, which is regarded as the origin of the MR and AHE. Besides, the films are p-type conductive at high temperature, which might be related with the iron doping. These properties make iron doped amorphous carbon films applicable in carbon-based solar cells, magnetic sensors or some other multifunctional devices.