Materials Science Forum Vols. 530-531

Abstract: This work presents aspects related to new sintered materials for valve seat inserts application. Two types of materials were evaluated. The materials were made using powder metallurgy technique from a basic mixture of high-speed steel (AISI M3/2), iron and carbide powders. The microstructures of these materials before and after heat treatment are presented. Under the heat treatment condition, the activation of the diffusion mechanism among phases was promoted and a better distribution of the Cu phases along the matrix was achieved. The results indicate that the materials under development have a potential for commercial application as valve seat inserts.

Abstract: The characterization of poly-{trans-[RuCl2(vpy)4]}films, deposited on Nd-Fe-B magnets and on Pt, Au and Glassy Carbon (GC) electrodes, produced in the present work were carried out by several in situ and extra situ techniques, aiming by the understanding of film structure improve its use as corrosion protection coating and/or electrocatalyst. Cyclic Voltammetry (CV) was used to estimate the surface coverage, film stability and the surface redox potential of films produced at different conditions. By Chronocoulometry films diffusion coefficient (Dct) were evaluated. Film morphology was studied by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Chemical composition analyses were carried out by Energy Dispersive Spectroscopy (EDS), X-ray Photonelectron Spectroscopy (XPS) and Raman Spectroscopy. The results indicated that the film structure is similar to the Ru complex in solution. Films Dct is limited by the electron transfer rate between Ru metallic centers. Films on Nd-Fe-B were composed by Ru3+ whereas on Pt by Ru2+.

Abstract: The use of generated coated polymeric films by electrochemistry means in the sintered metallic surfaces has been being the focus of our research aimed at the corrosion protection of small mechanical components obtained from powder metallurgy. The sintered alloys Fe- 2%Ni, Fe-5%Ni and 10%Ni with polymeric coating were tested using electrochemistry methods in corrosive solutions of NaCl 3%, KNO3 1.25M, H2SO4 0.5M, HNO3 0.5M, acetic acid 1%, oxalic acid 1% and lactic acid 1%. The electrochemistry corrosion tests were performed using two techniques, Eocp vs. Time and Potentiodynamic and they proved the efficiency of the polymeric coating on the corrosion protection of sintered alloys.

Abstract: The present work describes the studies on corrosion of powder injection molded 316L stainless steel potentiostatically coated by poly{trans[RuCl2(vpy)4]} where vpy (4- vinylpyridine) acts as ligand. Scanning Electron Microscopy (SEM) coupled to an Energy Dispersive X-Ray Analysis (EDAX) characterized the coated electrodes. Anodic polarization tests were performed in 3% NaCl to estimate the anodic dissolution current density of the electrode coated by the polymeric material. In addition, the specimens were exposed to salt spray and acid rain simulated environment. The microestrutural analysis indicated that the films were approximately 58 μm thick. Cyclic voltammetry experiments were carried out in 0.1M HTBA/methyl isobutyl ketone solution. The results revealed a scan rate dependent wave corresponding to a Ru2+/Ru3+ redox reaction, thus confirming the presence of ruthenium as metallic center in the polymer backbone. Pitting corrosion was observed in coated specimens only after thirty days of exposure to salt spray and after two days of exposure to acid rain simulated environment.

Abstract: New P/M lubricant formulations implement micron-sized (“micronized”) cellulose particles (MCP) to create MCP-enhanced lubricants. Surprising increases in green strength, up to 70% for iron and even higher for stainless steel are obtained with standard transverse rupture bars shapes made with 0.75 wt-% additive concentrations in iron (MPIF F-0000) and 316L stainless steel powders in comparison with the same parts made under the same conditions with industry standard lubricantbinders of zinc stearate, ethylene-bis-stearamide (EBS), and oxidized polyolefin (OP) wax. Two specific forms of new MCP materials are used in these tests: micronized cotton fibers (MCF), and micronized starch particles (MSP). The findings support expectations of significantly increased process yields for green parts and related production cost savings.

Abstract: Rare earth elements are often added to chromium dioxide forming alloys to improve its high temperature oxidation resistance. The rare earths can be also added as oxide dispersions. Significant cost reductions are possible if rare earth oxide concentrates can be used instead of pure rare earth oxides, the former being the precursor to obtaining pure rare earth oxide. In this study the effect of adding pure and concentrates of rare earth oxides to AISI 304L on its oxidation behavior has been evaluated. AISI 304L stainless steel powder compacts containing 2 vol% of pure lanthanum and yttrium oxides or their concentrate were prepared by milling followed by pressing. The compacts were vacuum sintered and isothermally oxidized in air for up to 200h at 900°C. The parabolic rate constants were determined and the reaction products examined using a scanning electron microscope. X-ray diffraction analysis of the reaction products was also carried out. The compacts with pure rare earth oxides and the concentrates exhibited similar oxidation behavior.

Abstract: The use of AISI 316L stainless steels for biomedical applications as implants is widespread due to a combination of low cost and easy formability. However, wrought 316L steel is prone to localized corrosion. Coating deposition is commonly used to overcome this problem. Ceramic hard coatings, like titanium nitride, are used to improve both corrosion and wear resistance of stainless steels. Powder injection moulding (PIM) is an attractive method to manufacture complex, near net-shape components. Stainless steels obtained from this route have shown mechanical and corrosion properties similar to wrought materials. The literature on the use of PIM 316L steel, either coated or not, as implants is still very scarce. The aim of the present work was to study the corrosion behaviour of PIM 316L in two conditions: TiN-coated and bare. Electrochemical investigations were performed using EIS and potentiodynamic polarization techniques.

Abstract: Nd-Fe-B magnets present outstanding magnetic properties. However, due to their low corrosion resistance, their applications are limited to non-corrosive environments. Nowadays, significant efforts are underway to increase the corrosion resistance of these materials, through the use of coatings. Herein are presented the results of a study on the corrosion resistance of Nd-Fe-B magnets coated with polypyrrole (PPY). The electrochemical behavior of coated and uncoated magnets has been studied by Electrochemical Impedance spectroscopy (EIS) in synthetic saliva. The results were compared to previous investigations, which were carried out under similar conditions, in Na2SO4 and NaCl solutions. In sulphate solution, the corrosion resistance of the PPY-coated magnet was 3 times larger (1600
.cm2) than that of uncoated magnet (500
.cm2). In NaCl solution, however, the corrosion resistance of coated and uncoated magnets were very similar (250
.cm2). In synthetic saliva, both the uncoated and coated magnets presented good corrosion performance (1940
.cm2). Such behavior can be attributed to the phosphate ions in saliva, which play a role as corrosion inhibitor, producing phosphating, at least partially, of the magnet surface. The PPY-coated magnets presented a strong diffusional control from moderate to low frequencies, caused by the polypyrrole film. The thicker PPY film increased the corrosion resistance of the magnet in synthetic saliva.

Abstract: The erosion-oxidation (E-O) behavior of high velocity oxy fuel (HVOF) sprayed Ni20Cr alloy as well as WC and Cr3C2 cermet coatings on a steel substrate were studied. The E-O tests were carried out in a rig with specimen assemblies that were rotated through a fluidized bed of erodent particles in the temperature range 500-850°C and with erodent impact velocities of 2.5-19.5 ms-1. Alumina powder (~200Am) was used as the erodent. The E-O resistance of the coatings was determined as wastage, as a function of temperature. The three coatings did not exhibit any significant change in E-O at temperatures up to 500-600°C. At higher temperatures, wastage increased with temperature, reached a maximum at 700°C and then decreased with further increase in temperature. Different E-O regimes were identified. The specimen surfaces were examined in a scanning electron microscope and their roughness determined. E-O maps have been drawn that define conditions under which the coatings undergo low, moderate and severe wastage.

Abstract: Al-Si hypereutectic alloys produced by spray forming represent an important technological development for application in the aeronautical and automotive industries. Due to some special properties these alloys find application as cylinder liners, mainly due to their high strength and wear resistance. Much research has been carried out to evaluate the mechanical properties of these alloys but the literature on their corrosion performance is still very scarce. In this study, the corrosion behaviour of a spray formed Al-Si-Cu hypereutectic alloy was investigated by electrochemical impedance spectroscopy (EIS) in acid (pH 3.3), neutral (pH 7) and alkaline (pH 11) solutions. Intense localized attack occurred in the acid electrolyte and the impedance decreased with time of immersion. In the neutral medium, the alloy was passivated but localized attack also occurred on weak areas of the passive film. In the alkaline medium, precipitation of corrosion products occurred during the first hours of immersion causing the increase of impedance but thickening of this layer led to cracks growth with time and, consequently, to the exposure of metallic substrate, resulting in the decrease in impedance for longer periods.