Materials Science Forum Vol. 844

Abstract: Anodizing is one of the most promising surface treatments for lightweight metals as titanium because it can increase wear and corrosion-resistance, as well as provide aesthetic appearance and electrical insulation.Three different types of anodizing can be performed on titanium alloys: type I for elevated temperature forming, type II as anti-galling application and type III for coloured surfaces. The type II anodization, called also grey anodizing, is often requested in aerospace or biomedical applications. It is characterized by the use of alkaline electrolyte and it is standardized according to the SAE AMS 2488D norm. However, in literature it is difficult to find information about the process parameters of grey anodizing.In this work, different parameters of the grey anodizing process on a grade 5 titanium alloy were investigated and optimized, in order to obtain an anodized layer with the desired properties, in terms of corrosion resistance, thickness of the coating and wear properties. In particular, the effect of current and voltage applied, treatment time, temperature and electrolyte composition on the characteristics of the anodized layer was studied.The thickness, the composition, the morphology and the adhesion of the protective layers were characterized by SEM/EDS analysis. The chemical and phase composition were analyzed by XRF and XRD techniques. The corrosion resistance of the samples was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy tests. The best results were obtained using as electrolyte a solution containing sodium hydroxide, titanium dioxide, sodium silicate and activated charcoal, with the formation of an anodized layer mainly constituted by titanium oxides and silicates. Intermediate treatment times and ambient temperature were the best conditions to produce the anodized layer. The sample with the best performances showed a homogeneous protective layer about 5 μm thick and was characterized by a lower corrosion current density, higher corrosion potential and polarization resistance, compared with the other anodized samples.

Abstract: Features of the anode electrolytic plasma processing of commercial pure titanium and its alloys in aqueous solutions of ammonium chloride and ammonia additives are studied. It is identified that structure of modified layer contains an external TiO₂ or TiO layer with micropores of up to 100 nm and a diffusion sub-layer after nitriding in the solution with the ammonia addition. Some increase in the surface microhardness is found. The plasma electrolytic treatment of titanium makes it possible to enhance its corrosion resistance by short-term (5 min) saturation with nitrogen at 750 °C in an electrolyte containing 5% ammonia and 10% ammonium chloride. The oxide coating formed during the anodic treatment has a positive effect on the corrosion resistance of titanium and results in reduce of the corrosion rate by two orders under continuous tests. Saturation of titanium samples with nitrogen leads to an increase in their strength properties after corrosion tests with a slight decrease in ductility. An additional advantage of this coating is to reduce of leaching of alloying elements from samples in corrosive environments.

Abstract: Microhardness, friction coefficient, and wear rate of carburized titanium alloy VT 20 are considered. An X-ray diffractometer, a scanning electron microscopy (SEM) used to characterize the phase composition of the modified layer and its surface morphology. A pin-on-disc tribometer was occupied to evaluate wear behavior of the treated titanium alloys. It is established that the friction coefficient decreases from 0.46 (untreated sample) to 0.15 for the sample carburized at 750 °C during 5 min. Therefore, the anode carburizing of titanium alloys results in the reducing of the wear rate by 2 orders.

Abstract: Application of protective aluminum-based coatings is one of the ways to increase the oxidation resistance of low-carbon steel. The electrolytic deposition of aluminum in the NaF₃₃-KF_15.2-(AlF₃)_51.8 wt.% melt at 920°C and the current density of 0.8-1.0 A·cm^-2 (0.25 A·h·cm^-2) have provided a continuous aluminide coating based on Fe₂Al₅ with a good adhesion to the steel substrate due to dissolution of oxide film from the surface of the treated products in fluoride melt. The resistance of steel samples to sulfide corrosion was investigated in a lab-scale three-electrode cell at 900°C in contact with a “Soderberg” carbon anode, which was obtained by carbonization of the coal tar pitch containing 2.0-2.5 wt.% of sulphur. The IR_C (ohmic voltage drop in contact layer) growth rate was 3.3 times higher for uncoated steel due to formation of the oxide-sulfide layer based on FeS. The electrolytically aluminized steel with preliminarily formed α-Al₂O₃ layer possessed more stable value of the IR_C in comparison with the uncoated steel because of the higher chemical resistance.

Abstract: Results of plasma electrolytic hardening and nitrohardening of medium carbon steels are discussed. The hardness of medium carbon steels after its plasma electrolytic quenching is HRC 56–62. The electrolytic heat-treatment in aqueous solution under pulse conditions can achieve different heating rates by varying the pulse periods and the ON:OFF ratios. The nitrogen diffusion decreases the austenitization temperature and results in formation of martensite after the sample cooling in the electrolyte. The aqueous solution that contained 15 wt.% NH₄Cl allows one to obtain the layer microhardness up to 1060 HV during 5 min at 750 °C. In this work, we investigate the corrosion resistance of the medium carbon steel which is obtained as a result of plasma electrolytic nitriding with following hardening in the same electrolyte. The cross-sectional microstructure, composition and phase constituents of modified layer under different processing conditions were characterized. It is shown that external oxide layer and nitrided layer promote the increase in the corrosion resistance. The maximal corrosion potential and minimal corrosion current density are observed after the nitriding of steels in the aqueous solution (11 wt.% of ammonium chloride and 11 wt.% of ammonium nitrate) at 750 °C during 5 min followed by the steel cooling in air.

Abstract: Rice husks (RH) are characterized by a high content of silicon dioxide up to 23 wt. %. Silica in the form of nanoparticles creates surface layers formed in various plant parts which ensure protective properties and mechanical stability. These nanoparticles with a dimension in the range of tens of nanometers, are formed during biochemical processes and photosynthesis. Individual nanoparticles are interconnected between themselves and between layers with organic phase via cellulose fibres. Accompanying ions mainly potassium, calcium, sodium, magnesium and aluminium extremely important for plant growth have also been identified in rice husks. In this research paper we investigated mechanical properties of composite epoxy resin material, which was composed of ChS Epoxy 520 filled with silica obtained from rice husks. Nanoparticles of silicon dioxide with the size in dozen of nanometers were prepared by calcination of raw plant parts. We found that the 0.1 phr of filling (0.01 g of filler + 10 g of epoxy) demonstrated a significant increase of wear resistance and decrease of coefficient of friction. An excellent adhesion between epoxy resin and silica nanoparticles was also observed. The silicon dioxide in epoxy resin plays the role of the hard phase, which transfers part of the load and protects the surface of polymer against wear. The presence of this filler does not change the mechanical properties of the original resin.

Abstract: The aim of the research was the indication of the causes of a difference in thickness of the zinc coating on the surface shaped by plasma cutting. The research was focused on fittings for overhead power lines - a double eyes links made of steel S355J2. The prepared materials were cut by plasma. Then, links were subjected to mechanical treatment (shot-blasting) and chemical treatment (pickling, rinsing and fluxing). The hot-dip galvanizing process was performed in industrial conditions by applying the constant temperature equal to 457°C, and a dipping time equal to 150 s. The research was conducted on the link’s flat surface (after rolling) and the side surface (after cutting). The (Zn) - coating morphologies and sub-layer thicknesses were evaluated on the basis of metallographic analysis and corrosion tests (acc. to EN ISO 9227). The correlation between the results of corrosion test and coatings morphology was determined.The observed changes are closely related to the outer layer steel structure (HAZ) which subsequently influences the mechanism and zinc coating growth kinetics

Abstract: Ni-W alloys were prepared by electrodeposition at diverse processing conditions. The Ni-W alloys were studied by SEM, EDX and XRD analysis to determine composition and morphology of the surface in dependence on electrodeposition conditions. Focus was put on surface with electroactive sites for hydrogen evolution. Stability of the alloys in chloride medium was determined applying chronopotentiometry and potentiodynamic polarization. Electrochemical behavior of the alloys was tested in alkaline solution by cyclic voltammetry. It was found that processing conditions directly influence quality of the Ni-W alloys concerning phase, morphology and composition. Prevailing amorphous phase of Ni-W alloys supports corrosion rate growth.

Abstract: The paper presents results of microstructural analysis of Hf-modified aluminide coatings. The coating was obtained using chemical vapour deposition (CVD) method at 1040°C using BPX-Pro 325 S equipment (Iond Bond). The deposition process time was 960 mintutes. The IN-718, IN-100 as well as CMSX-4 single-crystal nickel superalloys were the substrate material. The observation of coating was carried out using scanning electron microscopy. Chemical composition was analyzed using EDS method. The results showed that hafnium accumulates mainly on diffusion/additive layer interface and forms a „chain” of small precipitations. Hafnium was found in the additive NiAl layer of aluminide coating deposited on IN-100 superalloy. Its amount did not exceed 0.3 at %.

Abstract: In the article the hafnium modified aluminide coatings deposited using chemical vapour deposition (CVD) method were analyzed. The influence of surface treatment (grinding, sandblasting with different pressures) on microstructure of coatings were described. The Re 80 and M-247 nickel superalloys were used as substrate. Thickness of the obtained aluminide coating was in the range 32-45 mm on Re 80 and 40-45 mm on M-247 respectively. The average amount of Al in the additive layer was 22-24 wt% on Re 80 and about 21 wt % on M-247 base alloy. The total amount of hafnium in coatings did not exceed 2.5 wt % - usuallly below 0.5 wt %. The conducted research has shown that there is no strong influence of surface preparation methodology on microstructure of aluminide coatings obtained by CVD method.