40Cr钢液相等离子体电解氮碳共渗层的组织与性能

在NaCl溶液和甲酰胺组成的电解液中,应用液相等离子体电解氮碳共渗技术对调质态40Cr钢进行处理,表面得到氮碳共渗层,研究了其组织与性能。结果表明,经液相等离子体电解氮碳共渗处理后,试样表面为多孔形貌,处理10 min后渗层厚度可达38μm,渗层由两层白亮层和过渡层组成。XRD分析表明外白亮层由ε-Fe2-3N、Fe5C2、Fe3C和α-Fe(N)马氏体组成,SAED分析证明内白亮层为α-Fe(N)马氏体。渗层的显微硬度最高可达650 HV0.05,经氮碳共渗处理后试样的腐蚀速率远小于40Cr钢基体的腐蚀速率。     

Anodic plasma electrolytic nitrocarburising of VT22 titanium alloy in carbamide and ammonium chloride electrolyte

The structure of alpha- and beta-titanium alloy VT22, its microhardness, surface roughness, wear and corrosion resistance after anodic plasma electrolytic nitrocarburising in an electrolyte containing carbamide and ammonium chloride were investigated. An X-ray diffractometer and a scanning electron microscope were used to characterize the phase composition and structure of the modified surface. Tribological properties of the treated titanium alloy were evaluated using a ball-on-disc tribometer under lubricated testing conditions. The effect of processing temperature on corrosion resistance of the plasma electrolytic nitrocarburising samples was examined by means of potentiodynamic polarization in Ringer’s solution. It was shown that the anodic nitrocarburising provides saturation of the titanium alloy with oxygen, nitrogen and carbon and the formation of TiO₂ with a rutile structure and a nitrogen/carbon solid solution in titanium. The anodic plasma electrolytic nitrocarburising at all treatment temperatures diminishes the wear rate of the titanium alloy samples and the surface roughness. Friction coefficient of treated samples can be reduced by 4.7 times. The anodic nitrocarburising results in an enhanced corrosion resistance since the corrosion potential increases by an order of magnitude.     

甲酰胺浓度对38CrMoAl钢液相等离子电解渗氮过程的影响

采用液相等离子电解渗氮技术,研究了不同甲酰胺浓度对38CrMoAl钢渗氮层组织及性能的影响。利用OM、SEM、XRD观察分析渗氮层的微观组织结构和物相构成,RF-GDOES和Parstat2273电化学工作站分别表征渗氮层的元素分布和耐蚀性,并测试分析了渗层截面的显微硬度。结果表明,随着电解液中甲酰胺浓度的升高,渗氮致密层、白亮层、扩散层厚度呈先增加后降低的趋势,渗氮层最大显微硬度值呈增加的趋势;当甲酰胺浓度升高至70%时渗氮层达到最大,为125μm,白亮层为51μm;渗氮后扩散层硬度值较高,而心部组织硬度较未处理试样提升约2倍;渗氮层表层主要含Fe2N和Fe3N相,扩散层以Fe16N2、FeN相为主,过渡层主要为α-Fe、FeN0.097相;渗氮层的耐蚀性优于未经处理试样。     

Localised corrosion of highly alloyed stainless steels in an ammonium chloride and diethylamine chloride aqueous solution

This paper studies the pitting corrosion of highly alloyed stainless steels (SS) immersed in an ammonium chloride and diethylamine chloride aqueous solution at temperatures of 25, 40, 60 and 80°C. Four materials were tested: UNS S 31726 SS, UNS S 31254 SS, UNS S 32550 SS and titanium, for comparative purposes. Gravimetric, after immersion up to 103 days, polarisation curves, impedance, SEM, EDX and optical microscopy were the techniques used. UNS S 31726 SS shows the lowest corrosion resistance. UNS S 31254 and UNS S 32550 SSs show similar behaviour. Titanium presents impedance data three orders higher than the other SS materials. The SS materials present a pseudoinductive behaviour on impedance data, associated with an adsorption process of the amine compound which decreases as the temperature increases.     

甘油体系中不锈钢表面液相等离子体电解渗碳工艺

采用液相等离子体电解渗方法,在80%甘油水溶液中实现了304不锈钢表面快速渗碳。研究施加电压和放电时间对渗碳过程的影响,分析了渗碳层的显微组织,并比较了不同工艺条件下渗碳层显微硬度分布。结果表明:经过5 min的快速放电处理,渗碳层厚度接近100μm,硬度达到880 HV。在350 V和3 min渗透条件下渗碳层质量较好。     

离子氮碳共渗后的氧化处理对40Cr钢耐蚀性能的影响

为了进一步提高离子氮碳共渗后40Cr钢的耐蚀性能,对离子氮碳共渗后的试样进行了后氧化处理.氧化处理分别采用不同氧化气氛的炉内氧化处理和盐浴氧化、高温发黑、水蒸汽氧化等.试样的耐蚀性能分别采用极化曲线和盐雾腐蚀试验法测定.试验结果表明,用w(H2):W(02)=4:1的混合气体进行炉内氧化处理的试样耐蚀性能最好.     

Influence of electrolyte on corrosion properties of plasma electrolytic conversion coated magnesium alloys

The synthesis of oxides in a low-temperature electrolytic plasma allows to cover surfaces of magnesium and its alloys with multifunctional protective oxide-ceramic coatings. The corrosion properties of these layers are strongly dependent on their porosity. In order to minimize the porosity and to optimize the corrosion properties of the layers, the electrolyte concentration and composition (addition of CrO₃ as corrosion inhibitor) were varied, and the influences on layer structure, composition, and properties with a main focus on corrosion behaviour were studied.The corrosion properties of various layers thus generated were studied in 5% NaCl solution by measuring electrochemical polarization curves and by electrochemical impedance spectroscopy (EIS) at pH 3 and 6. Using XRD, LM, SEM and EDX to evaluate the composition and microstructure of the modified surfaces, the corrosion results were related to the microstructure and composition of the specific layer. The better results were obtained for layers produced at higher electrolyte concentration, whereas the addition of CrO₃ had no significant beneficial effect.     

On the formation of magnetite coatings on low-carbon steel in ammonium nitrate solution

Initial stages of the formation of magnetite coating (MC) on low-carbon steel in ammonium nitrate solution at a temperature of 98°C is studied by atomic force microscopy. Analyzing the results obtained by flicker-noise spectroscopy allows the degree of continuity and structuring of MC to be estimated. It is shown that the formation of magnetite nuclei in the course of direct electrochemical reaction may take place in the first minutes of oxidation; continuous coating is formed in 10–15 min. Original Russian Text ? D.B. Vershok, P.I. Misurkin, V.A. Timofeeva, A.B. Solov’eva, Yu.I. Kuznetsov, S.F. Timashev, 2008, published in Korroziya: Materialy, Zashchita, 2007, No. 10, pp. 28–35. This work was supported in part by the program “New Approaches to Corrosion and Radiation Resistance of Materials, Radioecological Safety” of the Department of Chemistry and Materials Science, Russian Academy of Sciences, and by the subprogram “Creating New Constructional and Functional Materials based on Nanotechnologies” of the Presidium of the Russian Academy of Sciences program “Developing Methods for Obtaining Chemical Substances and Creating of New Materials”.     

Characterization of plasma electrolytic oxide formed on AZ91 Mg alloy in KMnO4 electrolyte

The aim of this work is to investigate microstructure, corrosion resistance characteristics and nanohardness of the oxide layer on AZ91 Mg alloy by applying different voltage with KMnO₄ contained solution. There are lots of closed pores that are filled with another oxide compound compared with the typical surface morphology with pore coated until 350 V of coating voltage. The thickness of oxide layer increases with increasing coating voltage. The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium permanganate consists of MgO and Mn₂O₃. Corrosion potential of the oxide layer on AZ91 Mg alloy obtained at different plasma electrolytic oxidation(PEO) reaction stages increases with increasing coating voltage. The corrosion resistance of AZ91 Mg alloy depends on the existence of the manganese oxide in the oxide layer. The inner barrier layer composed of the MgO and Mn₂O₃ may serve as diffusion barrier to enhance the corrosion resistance and may partially explain the excellent anti-corrosion performance in corrosion test. Nanohardness values increase with increasing coating voltage. The increase in the nanohardness may be due to the effect of manganese oxide in the oxide layer on AZ91 Mg alloy coated from electrolyte containing KMnO₄.     

Effect of Iron sulfate as electrolyte additive on plasma electrolytic oxidation of aluminum alloy

This study has been carried out to investigate the incorporation of Iron(II) sulfate as an additive of electrolyte on formed AA1010 aluminum alloy, using plasma electrolytic oxidation method in silicate-based electrolytes containing Iron(II) sulfate. In order to fabricate nanocomposite coating, silicon nitride nanopowder was added to electrolyte. The effects of iron(II) sulfate additive on the voltage-time trend, microstructure, compositions, wear, and corrosion resistances of PEO coatings were investigated. In addition, current density and concentration of additive were studied as parameters that were effective on coating. Results showed that although FeSO₄ enters to the coating structure, but it does not develop a new phase. The corrosion and wear behavior of coated samples with FeSO₄ indicate an improvement as compared to those without additive.     

Effect of electrolyte on mechanical properties of AZ31B Mg alloy in electrolytic plasma processing

Coatings on Mg alloys were prepared using NaOH + Na₂SiO₃ as basic electrolyte containing electrolyte of Na₂SiF₆ or NaF. EPP treatment was carried out on AZ31 Mg alloys matrix under a hybrid voltage of AC of 200 V combined with DC of 260 V for 30 min. Structural and morphological analyses of ceramic coatings were analyzed by XRD and SEM. Wear and hardness of coatings were measured by pin-on disk test and Vickers hardness test. The coatings formed in Na₂SiF₆ and NaF electrolytes were mainly composed of MgO and Mg₂SiO₄. The measured micro-hardness of coating formed in Na₂SiF₆ electrolyte was found to be over HV 1100, while, coating formed in NaF electrolyte possessed micro-hardness of HV ～900. These results show that the mechanical properties of AZ31B Mg alloys can be enhanced by the proper selection of electrolyte agent.     

Electrochemical features of plasma electrolytic thermochemical processing of steel and titanium alloys

This review is devoted to an analysis of plasma electrolytic saturation of steel and titanium alloys with light elements (nitrogen, carbon, and oxygen). These technologies have a number of advantages because of the high rates of processes and the low cost of equipment. Electrochemical aspects of the process, which include the formation of a gas–vapor shell, its composition, processes of anodic dissolution, and oxidation and adsorption of a saturating element on the surface processed, are considered. Data of investigations of the mechanism of the release and transport of nitrogen and carbon from the electrolyte into metal are given.     

低温活性屏等离子氮碳共渗对3Cr13不锈钢组织和耐蚀性的影响

利用XRD、OM和SEM等试验方法,研究了氮碳共渗温度对3Cr13不锈钢渗层的组织结构及耐腐蚀性的影响。结果表明:当氮碳共渗温度为450℃时,渗层主要由α'N相、少量ε相和γ'相组成;处理温度高于480℃时,渗层中开始有Cr N相析出。氮碳共渗处理样品表面的显微硬度较未处理样品明显升高,且随处理温度升高,显微硬度逐渐增大;试样表面耐腐蚀性在处理温度为450℃时显著提高,随处理温度升高耐腐蚀性降低。     

Generation mechanism of microdischarges during plasma electrolytic oxidation of Al in aqueous solutions

The generation mechanism of microdischarges during plasma electrolytic oxidation (PEO) of aluminium was investigated at constant current densities in aqueous alkaline solutions. Microdischarges were generated only in weakly alkaline solutions under high applied voltages. The breakdown voltage of the anodic oxide film was not dependent on the applied anodic current density while it was strongly dependent on the OH⁻ concentration in the solution. The size and density of microdischarges became larger and lower, respectively, with increasing PEO treatment time. The mechanism of microdischarge generation could be explained based on the movement of ions in the oxide film and resistive property of the oxide film.     

The influence of sulphate,chloride and nitrate anions on the cyclic strain-enhanced dissolution behaviour of mild steel

The transient electrochemical behaviour of mild steel during cyclic plastic deformation in 0.5 M aqueous solutions of sulphate, chloride and nitrate ions was studied in the pH range 3–14. In all solutions dissolution current transients were observed during both tensile and compressive strain cycles. The magnitude of the dissolution effects, however, depended upon the solution composition, pH, potential and the extent of strain cycling. Analysis of the current transient characteristics revealed that for the anion concentration employed in this study, sulphate appeared to exhibit more aggressiveness than chloride and both these exerted a greater disruptive influence on surface films than nitrate ions. Tests in 0.25 M Na₂SO₄ revealed that the effect of sulphate could not be attributed simply to the increased ionic strength of the solution.     

Corrosion properties of HVOF-coated steel in simulated concrete pore electrolyte and concentrated chloride environments

The corrosion susceptibility of steel and HVOF-coated steel in solutions simulating the alkaline concrete pore environment and with the addition of chloride was investigated using potentiodynamic polarization and potential step techniques. The surface characterization was performed using SEM and the surface elemental analysis was determined by EDS. The concentration of chloride was 2.8 M to simulate the concentration of chloride spread in many local regions of Saudi Arabia and called Sabkha. It was found that, in the case of the simulated concrete pore electrolytes, the HVOF coating resulted in an anodic shift of the corrosion potential with marginal effect on the corrosion current. However, upon addition of 2.8 M chloride solution, the corrosion rate of the HVOF-coated steel was found to increase by a factor of two. SEM showed a network of pores within the coating which provides a path for the electrolyte. This would result in preferential corrosion around splat boundaries and confirmed by EDS which showed that the corroded splats have higher oxide contents. Potential step experiments at 400 mV above open circuit potential showed a suppressed current of the HVOF-coated steel compared to the steel substrate alone. The corrosion potential versus time experiments resulted in a more anodic Ecorr which decreased with time and became equal to the Ecorr of the bare steel after 34 h. After that, the corrosion potential of the HVOF-coated steel decreased due to the increase in galvanic coupling between the steel and the HVOF coating.     

Corrosion behavior of a superduplex stainless steel in chloride aqueous solution

Super duplex stainless steels (SDSS) have been widely used as structural materials for chemical plants (especially in those engaged in phosphoric acid production), in the hydrometallurgy industries, and as materials for offshore applications due to their excellent corrosion resistance in chloride environments, compared with other commercial types of ferritic stainless steels. These alloys also possess superior weldability and better mechanical properties than austenitic stainless steels. However, due to their two-phase structure, the nature of which is very dependent on their composition and thermal history, the behavior of SDSS regarding localized corrosion appears difficult to predict, especially in chloride environments. To improve their final properties, the effect of the partition of the alloying elements between the two phases, and the composition and microstructure of each phase are the key to understanding the localized corrosion phenomena of SDSS. This paper concerns the effects of the SDSS microstructure and heat treatment on the SDSS corrosion resistance in aqueous solutions, containing different amounts of NaCl at room temperature.     

Microstructure and corrosion resistance of ceramic coating on carbon steel prepared by plasma electrolytic oxidation

Ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation (PEO). The microstructure of the coating including phase composition, surface and cross-section morphology were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR) and scanning electron microscopy (SEM). The corrosion behavior of the coating was evaluated in 3.5% NaCl solution through electrochemical impedance spectra (EIS), potentiodynamic polarization and open-circuit potential (OCP) techniques. The bonding strength between Q235 carbon steel substrate and the ceramic coating was also tested. The results indicated that PEO coating is a composite coating composed of FeAl₂O₄ and Fe₃O_4. The coating surface is porous and the thickness is about 100 μm. The bonding strength of the coating is about 19 MPa. The corrosion tests showed that the corrosion resistance of Q235 carbon steel could be greatly improved with FeAl₂O₄-Fe₃O₄ composite coating on its surface.