Hongying Yu

Effect of nano-SiC Particles on the Corrosion Resistance of NiP-SiC Composite Coatings

NiP-SiC (≈11wt% P) composite coatings were electroplated in a Brenner type plating bath. The coatings had amorphous nano-phase composite structure. Direct current and alternating current electrochemical tests were carried out on such coatings in a 3.5wt% solution of NaCl to evaluate their corrosion resistance. The potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) tests, and exposure experiments all show that the corrosion resistance of NiP-SiC coatings first increases and then decreases when the SiC content increases, but the corrosion resistance of NiP-SiC composite coating is better than that of amorphous NiP coatings.

Characterization and wear behavior of WC-0.8Co coating on cast steel rolls by electro-spark deposition

Abstract To prepare high wear resistance and high hardness coatings, electro-spark deposition was adopted for depositing an electrode of a mixture of 92wt%WC+8wt%Co on a cast steel roll substrate. The coating was characterized by classical X-ray diffractometer (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX). The results indicate that the coating shows nanosized particulate structure and dendritic structure including columnar structure and equiaxed structure. The primary phases of the coating contain Fe3W3C, Co3W3C, Fe2C and Si2W. The coating has a low friction coefficient of 0.13, its average wear-resistance is 3.3 times that of the cast steel roll substrate and the main mechanism is abrasive wear. The maximum microhardness value of the coating is about 1573.9 Hv0.3. The study reveals that the electro-spark deposition process has the characteristic of better coating quality and the coating has higher wear resistance and hardness.

Molybdenum based amorphous and nanocrystalline coatings prepared by high velocity oxy-fuel spraying

The high velocity oxy-fuel (HVOF) based thermal spray process has developed as a potential advantageous approach for fabricating various kinds of functional coatings. In this article, the coatings of Mo-based alloy were synthesized using the HVOF process. The microstructure and the mechanical properties of the HVOF-processed coatings were investigated using SEM, TEM, XRD, and hardness and wear tests. Annealing treatment was applied to the as-sprayed coatings to develop the microstructure and its effect on the microstructure and mechanical properties of the coatings was examined. It is found that the HVOF-processed Mo-based alloy coatings are comprised of an amorphous splat matrix embedded with nano-sized crystalline particles. Annealing at temperatures over 950 °C results into crystallization of the amorphous matrix. The mechanical properties of the as-sprayed coatings are enhanced with annealing temperature up to 750 °C and from 950 to 1050 °C, keeps constant between 750 and 950 °C, and reduce over 1050 °C. The change of the mechanical property with the microstructure was illustrated in the study.

Numerical Study on Fatigue Damage Properties of Cavitation Erosion for Rigid Metal Materials

Cavitation erosion is an especially destructive and complex phenomenon. To understand its basic mechanism, the fatigue process of materials during cavitation erosion was investigated by numerical simulation technology. The loading spectrum used was generated by a spark-discharged electrode. Initiation crack life and true stress amplitude was used to explain the cavitation failure period and damage mechanism. The computational results indicated that the components of different materials exhibited various fatigue lives under the same external conditions. When the groove depth was extended, the initiation crack life decreased rapidly, while the true stress amplitude was increased simultaneously. This gave an important explanation to the accelerating material loss rate during cavitation erosion. However, when the groove depth was fixed and the length varied, the fatigue life became complex, more fluctuant than that happened in depth. The results also indicate that the fatigue effect of cavitation plays an important role in contributing to the formation and propagation of characteristic pits.

Local Fine Structural Insight into Mechanism of Electrochemical Passivation of Titanium

Electrochemically formed passive film on titanium in 1.0 M H2SO4 solution and its thickness, composition, chemical state, and local fine structure are examined by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure. AES analysis reveals that the thickness and composition of oxide film are proportional to the reciprocal of current density in potentiodynamic polarization. XPS depth profiles of the chemical states of titanium exhibit the coexistence of various valences cations in the surface. Quantitative X-ray absorption near edge structure analysis of the local electronic structure of the topmost surface (∼5.0 nm) shows that the ratio of [TiO2]/[Ti2O3] is consistent with that of passivation/dissolution of electrochemical activity. Theoretical calculation and analysis of extended X-ray absorption fine structure spectra at Ti K-edge indicate that both the structures of passivation and dissolution are distorted caused by the appearance of two different sites of Ti-O and Ti-Ti. And the bound water in the topmost surface plays a vital role in structural disorder confirmed by XPS. Overall, the increase of average Ti-O coordination causes the electrochemical passivation, and the dissolution is due to the decrease of average Ti-Ti coordination. The structural variations of passivation in coordination number and interatomic distance are in good agreement with the prediction of point defect model.

Preparation of IrO2+MnO2 coating anodes and their application in NaClO production

Abstract To improve the durability as well as to reduce the cost of anodes, the IrO2+MnO2 composite coating anodes for NaClO production were prepared by thermal decomposition. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction analysis (XRD) were carried out to investigate the morphologies, element distribution, and microstructure. The anodic polarization curves were employed to study the effect of sintering temperature on the Cl2 evolution reaction (CER) of the electrodes. The accelerated life tests (ALT) and electrochemical impedance spectroscopy measurement (EIS) were utilized to investigate the stability. The rules of NaClO production were also studied by the static electrolysis experiment. The results indicate that sintering temperature has a significant influence on the CER properties as well as the ALT values of the electrodes. The electrode prepared at 400°C has the best CER properties and the longest ALT value.

Repassivation behaviour of 316L stainless steels using new type of abrading electrode technique

Abstract In this paper, a novel electrode abrading technique is designed to improve the conventional methods. By employing the new depassivation method, 316L stainless steel has been tested in 0.05M H2SO4 solution with and without chloride ion. The repassivation kinetics has been analysed in terms of the current density flowing through the abraded surface bare metal surfaces. The results are described by the linear relationship of log i(t) versus log t. At potentials within the passive region, the passive film grew according to the high field ion conduction model in which log i(t) is linearly proportional to 1/q(t). The repassivation process was analysed in three stages according to different kinetics. The effect of applied potential and chloride ion concentration on repassivation kinetics was also discussed, and the change of calculated film thickness is more sensitive to applied potential than chloride ion concentration.