Microstructure modifications and corrosion behaviors of Cr4Mo4V steel treated by high current pulsed electron beam

In this work, Cr4Mo4V steel was irradiated by high energy current pulsed electron beam (HCPEB) with energy density of 6 J/cm². Morphology and phase composition of the surface layer were analyzed using scanning electron microscopy (SEM) and glancing angle X-ray diffraction (GXRD). The crater-like morphology was observed on surface after HCPEB treatment, and the thickness of melted layer was ∼7 μm. Results from GXRD revealed that HCPEB treatment could suppress martensite transition and the content of retained austenite in the melted layer increased with irradiation number. The corrosion resistance was evaluated by electrochemical polarization tests in neutral 3.5% NaCl solution. Compared with the untreated Cr4Mo4V steel, corrosion potential of the samples treated by HCPEB improved and the corrosion current density decreased. The improved corrosion resistance is attributed to the absence of the carbide, formation of retained austenite and dissolution of alloy elements, particularly of Cr and Mo, into the matrix.     

AM50镁合金强流脉冲电子束表面改性的研究

采用强流脉冲电子束（HCPEB）对含稀土的AM50镁合金进行表面改性处理。利用金相显微镜、扫描电镜和显微硬度计等对试样的截面和表面进行观察,研究了强流脉冲电子束表面改性对其摩擦磨损性能及耐腐蚀性能的影响。结果表明：处理层的截面组织经脉冲电子柬轰击后发生了很大的变化;从表面形貌可以看出存在许多弥散分布的微米尺度的熔坑;显微硬度测试结果表明,处理层的硬度比基体的硬度明显提高;耐磨性及耐腐蚀性能均较基体提高,同时在不同处理参数下耐磨性及耐腐蚀性能有所不同。     

The properties of gas tungsten arc deposited SiCP and Al surface coating on magnesium alloy AZ31

SiC particles and aluminum powders were used to deposit on the surface of magnesium alloy AZ31 by pulse square-wave alternating current gas tungsten arc (GTA) processing. This method is an effective technique in producing a high performance surface modified composite layer. The microstructure, microhardness, wear resistance and corrosion behavior of the GTA surface modified composite layer were evaluated. It was proved that no reaction products were formed at the SiC-matrix interface and no melting or dissolution of the SiC particle occurred during GTA surface modification. The microhardness of GTA surface modified composite layer was between 100 and 150 HV according to the variation of the GTA processing parameters. The microhardness, wear resistance and corrosion behavior of the GTA surface modified layer were superior to that of the as-received AZ31. The optimum processing parameters of the GTA surface modification of magnesium alloy AZ31 with SiC + Al for the formation of a homogeneous crack/defect-free and grain refinement microstructure were established.     

Phosphating treatment and corrosion properties of Mg–Mn–Zn alloy for biomedical application

A phosphating treatment was applied to Mg–Mn–Zn alloy in order to improve the corrosion resistance. Surface morphology and phase constitute were observed and identified by SEM, EDS, SAXS, XRD and XPS. SEM observation showed that a rough and crystalline reaction layer was formed on the surface of Mg alloy. With the increasing of phosphating time, the layer became thicker and denser. It has been showed that the reaction layer was mainly composed of brushite (CaHPO₄ · 2H₂O). Small amount of Zn²⁺ was also detected by XPS and EDS. The corrosion resistance of the phosphated samples was measured by the electrochemical polarization and the immersion test in comparison with the bare alloy. The results manifested that the corrosion resistance of Mg alloy was improved by the phosphating treatment, and the corrosion resistance increased with the increase of the phosphating time within 50 min. Immersion tests showed that the phosphate layer could protect magnesium alloy from fast corrosion. The brushite layer has been transformed into hydroxyapatite (HA) during the immersion in the simulated body fluid (SBF) solution, which suggested the brushite layer could provide good biocompatibility.     

Influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel

The influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel, of commercial name “DC53”, in 3.5% NaCl solution is reported. The specimens were nitrided at 520 °C for different treatment times using a constant [N₂ + H₂] gaseous mixture by a DC-pulsed plasma system. The microstructure of the nitrided layers was investigated by optical microscopy and X-ray diffraction. The corrosion behavior was evaluated by potentiodynamic polarization experiments. The plasma nitriding process considerably improves the corrosion resistance of material in NaCl environment as compared to the unnitrided DC53 steel. The modified surface layer consisting mainly of ε-nitride (Fe_2–3N) and a small amount of γ′-nitride (Fe₄N) confers this outstanding behavior. The corrosion resistance dependence on specific nitriding processes is reported and the role of the ε-nitride is discussed. In particular, the correlation of pitting current density, density of pits, and volume fraction of ε-nitride with nitriding time is analyzed. The results denote that the most important parameter for controlling the corrosion resistance of the material is the volume fraction of ε-nitride and the nitrided layer thickness. It is expected that a nitrided layer would be thicker and rich in ε-nitride phase to achieve a high corrosion resistance.     

Study of oxidized layer formed on aluminium alloy by plasma oxidation

This paper presents results of plasma oxidizing of aluminium alloy which was carried out in a low-frequency pulse excited plasma. The processing temperature was about 450 °C and the processing time was 3 h. The modified layer was characterized by X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy. Atomic force microscopy and scanning electron microscopy observations indicated that no evident defects appeared on the surface of the oxidation layer. Immersion experiments in 3.5% NaCl solution were carried out to test the effect of plasma oxidation on the corrosion resistance of surface oxidized aluminium alloys. After immersion for 200 h, the severe corrosion occurred on the untreated sample surface while no evident corrosion was observed for the treated sample.     

Improvement of wear and corrosion resistances of 17-4PH stainless steel by plasma nitrocarburizing

17-4PH stainless steel was plasma nitrocarburized at 460 °C for improving its mechanical properties without compromising its desirable corrosion resistance. The plasma nitrocarburized layers were studied by optical microscope, X-ray diffractometer, microhardness tester, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The experimental results show that the nitrocarburized layer depths increase with increasing duration time and the layers growth conform approximately to the parabolic law. The phases in the nitrocarburized layer are mainly of γ′-Fe₄N and α′-Fe with traces of CrN phase. The surface hardness of the modified specimen is more than 1200 HV, which is three times higher than that of untreated one. The friction coefficient and corrosion resistance of the specimen can be apparently improved by plasma nitrocarburizing. With the increase of duration time, the surface hardness slightly decreases whereas the friction coefficient and corrosion resistance of the modified specimen are first increase and then decrease. The 8 h treated specimen has the lowest friction coefficient and the best corrosion resistance in the present test conditions.     

Electrolytic deposition of hydroxyapatite coating on thermal treated Ti-40Zr

In this study, hydroxyapatite (HA) was coated on both thermal treated and untreated Ti-40Zr substrates by means of electrolytic deposition. It was predicted that the HA layer would increase the bioactivity and osteoconductivity of the Ti-40Zr substrate, and a thermal treatment would improve the bonding strength between the HA layer and Ti-40Zr substrate, and prevent the corrosion of the Ti-40Zr substrate. First, the Ti-40Zr samples were annealed at various temperatures (200, 300, 400, 500 and 600°C respectively). After annealing, samples were immersed in a Ca(NO₃)₂ · 4H₂O and (NH₄)₃PO₄ · 3H₂O solution for the electrolytic deposition of the HA coating. Various analyses of the coating were conducted, including surface morphology, phase structure, corrosion resistance, biocompatibility, and bond strength between HA and Ti-40Zr. Experimental results indicated that the bonding strength of the HA coating on the thermal treated Ti-40Zr was markedly improved when compared to that of the HA coating on an untreated Ti-40Zr alloy. The corrosion resistance of Ti-40Zr was also improved by the use of the thermal treatment, as shown by a potentiodynamic polarization test. Finally, osteoblast-like cells cultured on the HA coating surface were found to have proliferated on all samples.     

HCPEB表面处理的研究现状

强流脉冲电子束(HCPEB)表面处理是一种新兴的高能束表面处理技术.综述了脉冲电子束处理,表面熔坑的形成机制及熔坑、弥散颗粒、裂纹、波状起伏、条形纹理、胞状晶等典型形貌影响因素和演变规律方面的研究成果,总结了表层结构变化与表面显微硬度、耐磨性、耐蚀性能等改性工艺的研究现状,并认为扩大强流脉冲电子束应用范围和系统的探究是目前研究的重点.     

Surface characterization of multiple coated H11 hot work tool steel by plasma nitriding and hard chromium electroplating processes

Formation of multi-layer coating by plasma nitriding and hard chromium electroplating on the surface of H11 hot work tool steel was investigated. Specimens were coated via a triple process containing plasma nitriding, hard chromium electroplating and plasma nitriding. Surface composition has been studied by X-ray diffraction analysis. The surface morphology and elemental analysis was examined by using scanning electron microscopy. Wear tests were conducted by the use of pin-on-disk method, a cemented tungsten carbide pin and 1000 gF load. Polarization corrosion tests were carried out in distilled water solution containing 3% NaCl. The improvement in hardness distribution after third step is discussed in considering the forward and backward diffusion of nitrogen in the chromium interlayer. Also, the formed phases in the hybrid coating were determined to be CrN + Cr₂N + Cr + Fe_2-3N + Fe₄N. Wear results showed that although the multi-layer coated specimens have higher wear resistance in comparison with the reference specimen, their wear resistance is less than that of two and one layer coated specimens due to micro-ploughing and removal of hard surface nitrides from the surface. By increasing the third step time and temperature, the wear resistance of specimens increases due to higher diffusion of nitrogen in the chromium layer. But polarization results showed that triple coated specimens have the lowest corrosion rate.     

Corrosion resistance of the layers formed on the surface of plasma-nitrided AISI 304L steel

Austenitic stainless steels have good corrosion resistance, but their low hardness and low wear resistance limit their use whenever surface hardness is required. Nitriding treatments have been successfully applied to stainless steels to improve their mechanical and tribological properties; however, at temperatures above 723 K, gas or salt bath nitriding processes decrease the corrosion resistance due to the formation of CrN and other phases within the modified layer. Chromium compounds draw chromium and nitrogen from the adjacent regions, degrading the corrosion resistance. The plasma nitriding technique permits the use of treatment temperatures as low as 623 K without promoting degradation in the corrosion resistance of stainless steel. In this work, the pulsed glow discharge (PGD) technique was used for nitriding steel (AISI304L) in order to investigate the effect of the temperature of this treatment in the morphology and, as a consequence, in the anodic behavior of the formed layers, in solution with and without chloride ions. Four different temperatures were employed (623, 673, 723, and 773 K). The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), microhardness measurements, and electrochemical tests with potentiodynamic anodic polarization curves. The nitriding temperature alters the anodic behavior due to a displacement of the polarization curve towards higher currents, in a solution free of chloride ions. In a chloride solution, the nitriding temperature increases the pitting potential up to the oxygen evolution region.     

Mössbauer spectroscopy study on the corrosion resistance of plasma nitrided ASTM F138 stainless steel in chloride solution

Plasma nitriding of ASTM F138 stainless steel samples has been carried out using dc glow discharge under 80% H₂–20% N₂ gas mixture, at 673 K, and 2, 4, and 7 h time intervals, in order to investigate the influence of treatment time on the microstructure and the corrosion resistance properties. The samples were characterized by scanning electron microscopy, glancing angle X-ray diffraction and conversion electron Mössbauer spectroscopy, besides electrochemical tests in NaCl aerated solution. A modified layer of about 6 μm was observed for all the nitrided samples, independent of nitriding time. The X-ray diffraction analysis shows broad γ_N phase peaks, signifying a great degree of nitrogen supersaturation. Besides γ_N, the Mössbauer spectroscopy results indicated the occurrence of γ′ and ε phases, as well as some other less important phases. Corrosion measurements demonstrate that the plasma nitriding time affects the corrosion resistance and the best performance is reached at 4 h treatment. It seems that the ε/γ′ fraction ratio plays an important role on the resistance corrosion. Additionally, the Mössbauer spectroscopy was decisive in this study, since it was able to identify and quantify the iron phases that influence the corrosion resistance of plasma nitrided ASTM F138 samples.     

Gradient micro-structured surface layer on aluminum alloy fabricated by in situ rolling friction stir welding

A gradient micro-structure was formed in the surface layer of 2219 aluminum alloy joint by means of in situ rolling friction stir welding (IRFSW). The micro-structured surface layer is about 200 μm deep, corresponding to a gradient change in microhardness from 86.8 to 59.4 HV in the coarse-grained weld nugget zone (WNZ). Compared with those of the base material, the friction coefficient values are evidently decreased and the wear resistance is obviously enhanced on the surface layer. The corrosion current was relatively low and corrosion potential value was positive with respect to that of the base material. The second-phase particles in the upper surface layer were much more and smaller than those of the base material.     

Metallurgical Analysis of Crack Initiation of Wire-Cut Electrical Discharge-Machined Spline Actuators Made of 17-4 PH Stainless Steel

Spline actuators made of investment cast 17-4 PH (precipitation hardening) stainless steel were found to contain micro-cracks. The cracked actuators were subjected to optical and scanning electron microscopy and hardness testing, which revealed that the failure occurred due to fatigue crack initiation and growth after electrical discharge machining (EDM). The rehardened layer produced by the EDM remained after machining, and the cracks and surface irregularities associated with this layer provided sites for crack initiation and growth, which ultimately caused rejection of parts. Close dimensional tolerances on actuators require post-heat treatment EDM. Thickness of the recast layer was measured to be about 38–55 μm, and precipitation in vicinity of the machined surface is a potential source for corrosion. Post-machining polishing by means of fluidized bed granules was employed to remove recast layer and associated precipitates. Test results proved that removal of surface layers improved the microstructure and the resistance to crack formation. The post-EDM polishing and subsequent annual inspections proved that problem was solved.     

Influence of the Al(Co,Ni) layer on the corrosion resistance of a cobalt based alloy (Mar‐M‐509®)

In the present work, the results of studies on the structure and corrosion resistance of Al(Co, Ni) layer are shown. The diffusion Al(Co, Ni) layer was created on the cobalt alloy Mar‐M‐509 substrate by chemical vapor deposition (CVD) method with aluminum trichloride (AlCl₃) under the hydrogen atmosphere. The scanning electron microscope (SEM) observations and microtomography measurements of layers were performed. Also an analysis of the chemical (energy‐dispersive X‐ray spectroscopy (EDS)) and phase (X‐ray diffraction (XRD)) composition was carried out. By the X‐ray diffraction method (sin² φ) also the residual stresses were calculated in the matrix of the material. The corrosion resistance was tested with impedance and potentiodynamic methods in 0.1 M Na₂SO₄, 0.1 M H₂SO₄ solutions and acidulous 0.1 M NaCl solution (pH = 4.2) at room temperature. The results indicate that the analyzed layer with a thickness of about 14 μm have a similar corrosion resistance compared to the base material – Mar‐M‐509^® cobalt alloy. Only in the strongly acidic environments, the corrosion resistance of the layer is remarkably decreased.     

Improving the hot corrosion resistance of plasma sprayed ceria–yttria stabilized zirconia thermal barrier coatings by laser surface treatment

Ceria–yttria stabilized zirconia (CYSZ) thermal barrier coatings (TBCs) were deposited by air plasma spraying on NiCoCrAlY-coated Inconel 738LC substrates. After that, the surface of plasma sprayed CYSZ TBCs were glazed using a pulsed Nd:YAG laser. The effects of laser glazing on hot corrosion resistance of the coatings were evaluated in presence of 45 wt%Na₂SO₄ + 55 wt%V₂O₅ corrosive molten salt at 1000 °C. The results revealed that the hot corrosion resistance of plasma sprayed CYSZ TBCs were enhanced more than twofold by laser surface glazing due to reducing specific reactive area of the dense glazed surface layer and consequently, decreasing the reaction between molten salt and zirconia stabilizers.     

The effect of an interfacial tantalum oxide layer on interactions in the Ta/Sn system

Liquid Sn is a corrosive agent for tantalum at high temperatures. Oxidation of tantalum is often used to form a surface protective oxide layer in order to improve its chemical resistance in the liquid metal. In this study the stability of tantalum oxide layer, obtained by thermal oxidation and plasma anodization of tantalum, in contact with liquid Sn was examined in the 700–1000 °C temperature range. It was established that the stability of the oxide layer is controlled by its dissolution into the substrate, which takes place along with the formation of non-stable Ta sub-oxides. It was found that the thermal oxide layer may provide sufficient protection against tantalum corrosion by liquid Sn up to 700 °C, whereas the protective layer obtained by the plasma anodization gives an adequate corrosion resistance up to 1000 °C.     

Electrophoretic bilayer deposition of zirconia and reinforced bioglass system on Ti6Al4V for implant applications: An in vitro investigation

The physical, chemical and biological properties of the bioglass reinforced yttria-stabilized composite layer on Ti6Al4V titanium substrates were investigated. The Ti6Al4V substrate was deposited with yttria stabilized zirconia — YSZ as the base layer of thickness ≈ 4–5 μm, to inhibit metal ion leach out from the substrate and bioglass zirconia reinforced composite as the second layer of thickness ≈ 15 μm, which would react with surrounding bone tissue to enhance bone formation and implant fixation. The deposition of these two layers on the substrate was carried out using the most viable electrophoretic deposition (EPD) technique. Biocompatible yttria-stabilized zirconia (YSZ) in the form of nano-particles and sol gel derived bioglass in the form of micro-particles were chosen as precursors for coating. The coatings were vacuum sintered at 900 °C for 3 h. The biocompatibility and corrosion resistance property were studied in osteoblast cell culture and in simulated body fluid (SBF) respectively. Analysis showed that the zirconia reinforced bioglass bilayer system promoted significant bioactivity, and it exhibited a better corrosion resistance property and elevated mechanical strength under load bearing conditions in comparison with the monolayer YSZ coating on Ti6Al4V implant surface.     

Corrosion behavior of friction stir welded AZ31B magnesium alloy with plasma electrolytic oxidation coating formed in silicate electrolyte

A protective ceramic coating of about 50 μm thick on a friction stir welded (FSW) joint of AZ31B magnesium alloy was prepared by plasma electrolytic oxidation (PEO) in silicate electrolyte. Electrochemical corrosion behavior of uncoated and coated FSW joints was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The equivalent circuits of EIS plots for uncoated and coated FSW magnesium alloy were suggested. The corrosion resistance of FSW magnesium alloy depended on microstructure of the FSW joint. The heat-affected zone with severe grain growth was more susceptible to corrosion than the stir zone and base metal. The PEO coating consisted of a porous outer layer and a dense inner layer. The inner layer of PEO coating played a key role on corrosion protection of the FSW joint of magnesium alloy. Meanwhile, corrosion potential, corrosion current density and impedance at different zones of coated FSW joint were almost the same. The PEO surface treatment significantly improved the corrosion resistance of FSW joints of AZ31B magnesium alloy.     

Effect of tin on the corrosion behavior of sea-water corrosion-resisting steel

This paper investigated the effect of tin on the corrosion resistance of tin-containing steel and tin-free steel using electrochemical measurements in seawater. Results showed that tin-containing steel had lower corrosion current and higher impedance than tin-free steel. Surface analyses of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) indicated that tin could form SnO₂ and SnO in rust layer, and both of them could improve impedance and corrosion resistance of rust layer. Besides, the coprecipitation process of tin oxides with iron oxides could make the rust layer more uniform and compact, which could make the tin-containing steel have better corrosion resistance than tin-free steel. Secondary ion mass spectrometer (SIMS) showed that there was no obvious segregation of tin on substrate steel when tin addition was 0.038 wt.%, and tin could improve the oxidation resistance of substrate steel evenly by lowering the steel's Fermi energy from − 9.276 eV to − 14.445 eV.