Symposium A Poster Session

Tuesday, May 1, 2001 5:00 PM in Room Atlas Foyer

AP-1-1 Corrosion of CrN and CrN/TiN Coated Heat-Resistant Steels in Molten A356 Aluminum Alloy
Chao-Sung Lin, Chung-Shin Ke (Da-Yeh University, Taiwan, ROC); Hsuan Peng (Industrial Technology Research Institute, Taiwan, ROC)
The feed screw and barrel of an injection molding machine for processing Al melts into solid parts can be markedly corroded by the molten Al. A 4µm CrN coating and a CrN/TiN multilayer coating that provide the physical and chemical barriers between the molten reactive Al and the steel substrate were deposited by Cathodic Arc Evaporation onto 10-mm-thick heat-resistant steel plates. Dipping tests were conducted in a 700°C A356 melt for a duration ranging from 1 to 21 hr at intervals of 3 hr. Coating damage was evaluated by cross-sectional scanning electron microscopy (SEM) and TEM. For both CrN and CrN/TiN coated steels, no coating damage was observed after 1 hr of dipping. However, CrN coated steel was locally corroded after 3 hr of dipping, resulting in local hemispherical corrosion pits. Vertical cracks were observed in the CrN coating overlaying the corrosion pit, although the CrN coating did not react with the molten Al. Local corrosion was observed on the CrN/TiN coated steel after 9 hr of dipping. Once local corrosion occurred, the corrosion areas of coated steels increased with dipping time, regardless of the type of coating. After 21 hr of dipping, the CrN coating that remained adhering to the steel substrate showed no reaction with the molten Al. Whereas, microcracks parallel to the interface between the coating and steel substrate were observed in the CrN/TiN coating of which the thickness markedly decreased after 21 hr of dipping. The different coating damage of the distinct coatings can be explained by their different coating microstructures. That is, the CrN coating exhibited a mixture of coarse columnar and granular structure; whereas the CrN in the CrN/TiN coating displayed a fine-grained structure. The structural refinement resulted from the multilayer coating process reduced the density of the pinholes in the CrN/TiN coating. Consequently, the CrN/TiN coating has better corrosion resistance than CrN coating as dipped in A356 melts.
AP-1-2 A Comparative Study Among Coatings Used in the Processing of the Metallic Uranium
G. Vasconcelos, de (Instituto de Estudos Avancados, Brazil); N.A.S. Rodrigues (Centro Tecnico Aeroespacial); V.H Baggio_Scheid (Centro Técnico Aeroespacial IEAv-CTA); J.R. Martinelli (Instituto de Pesquisas Energéticas e Nucleares)
The metallic uraniun is very reactive at high temperatures, then, in its processing, special refractory crucibles should be employed. The choice of these crucibles, is based on its chemical inertia to the uraniun. Minimizing these thus, the amount of impurities in the molten material and maximizing its durability. This work presents, a comparative study amount three types of coverings, used in the processing of the metallic uraniun, in temperatures above of its fusion point. In this work, samples of metallic uraniun with dimensions of 2x2x2mm, without its oxide layers, (removed by acid attack), had been supported on coverings of alumina, titanium nitretum and vitreous carbon, inside a vacuum furnace. It was closed, evacuated and heated, remaining in the temperature of interest by a definite time and after that, it was cooled. Undesirable reactions of the sample or coverings with the residual gases had been minimized, by heating the furnace only when pressures nearest to 10-6 mbar approached. @paragraf@ Finally, the samples were removed from the furnace and prepared for characterization. @paragraf@ This stage consisted in burnishing the interest faces and milling of the products of the reaction, originated in the interfaces. The possibility of contamination environment and the proper operator with the uraniun, (toxic and radiative element), was minimized by the mixture of uranium powder with polyester resin. The size and the products of the reaction in function of the temperature, had been characterized by using optic and electronic microscopy and X-rays difratometry, respectively. .
AP-1-3 Growth of Oxide Scales Upon Isothermal Oxidation of CVD-FBR Aluminide Coated Stainless Steel
F.J. Pérez, F. Pedraza, M.P. Hierro (Universidad Complutense de Madrid, Spain); J. Balmain (Université de La Rochelle, France); G. Bonnet (Université de La Rochelle)
The resistance of different alloys exposed at high temperature environments depends upon their mechanical resistance as well as their corrosion/oxidation properties. When the mechanical requirements are not in compromise, austenitic stainless steels may play a role in substituting the more expensive Ni and Co base alloys. However, at temperatures close to 950@super o@C, the chromia scale usually grown to protect the alloy may be further oxidised into CrO@sub3@, which is a volatile oxide and thus, the naked material may undergo a catastrophic oxidation. Fe-Cr-Al (Kanthal-like) alloys have been shown to be oxidation resistant at high temperatures (up to about 1000@super o@C). These alloys rely on the formation of alumina scales to protect the alloy, having a chromium reservoir so as to reduce the aluminium amount needed to maintain the oxide scale. @paragraph@ Typically, aluminide coatings have been produced on different substrates by the pack cementation method normally leading to an improvement of the oxidation resistance of the coated alloys. In this case, we are trying to develop aluminium diffusion coatings at shorter times and lower temperatures than even in the high activity/low temperature pack cementation processes. We have recently presented how to grow these coatings on an AISI 304 stainless steel and it is the purpose of this work to keep on the same track by studying the properties of the oxide layers grown on the same substrate steel. Thus, aluminium diffusion coatings obtained by Chemical Vapour Deposition in Fluidised Bed Reactors (CVD-FBR) at 525@super o@C for 1.5 h on Fe-18wt%Cr-8wt%Ni austenitic stainless steel plates have been exposed to isothermal experiments at 950°@super o@C for up to 200 h at both discontinous and continuous intervals under atmospheric pressure. Muffle furnaces and thermogravimetry apparatus were used to oxidise both kinds of specimens whereas XRD and SEM/EDX were employed to investigate the morphologies and compositions of scales grown after exposure at high temperature.@paragraph@ The results will show quite different kinetics between the oxidised parent material and the Al-coated one. Loss of protective behaviour on the uncoated substrate will be attained, due to Fe incorporation into the oxide scale, initially grown as Cr@sub 2@O@sub 3@. However, the coated specimens will present lower kinetics, which may be divided into different steps related to the @alpha@Al@sub 2@O@sub 3@ formation, its transition into @theta@-Al@sub 2@O@sub 3@ and final step with a certain contribution of Cr and/or Fe from the substrate. Overall, the Al-coated alloy will be considered as a potential candidate for high temperature applications.