Intermetallic diffusion coatings for enhanced hot-salt oxidation resistance of nitrogen-containing austenitic stainless
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27/4/04
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Intermetallic Diffusion Coatings for Enhanced Hot-Salt Oxidation Resistance of Nitrogen-Containing Austenitic Stainless Steels U. KAMACHI MUDALI, N. BHUVANESWARAN, P. SHANKAR, H.S. KHATAK, and B. RAJ This article presents the preparation, characterization, and hot-salt oxidation behavior of nitrogencontaining type 316L stainless steel (SS), surface modified with intermetallic coatings. Three different types of intermetallic coating systems, containing aluminum, titanium, and titanium/aluminum multilayers, were formed by diffusion annealing of type 316L austenitic SS containing 0.015, 0.1, 0.2, and 0.56 pct nitrogen. Analysis by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and secondary ion mass spectroscopy (SIMS) confirmed the formation of various intermetallic phases such as AIN, Al13Fe4, FeAl2, FeTi, Ti2N, and Ti3Al in the coatings. Hot salt oxidation behavior of the uncoated and surface-modified stainless steels was assessed by periodic monitoring of the weight changes of NaCl salt-applied alloys kept in an air furnace at 1023 K up to 250 hours. The oxide scales formed were examined by XRD and stereomicroscopy. Among the various surface modifications investigated in the present study, the results indicate that the titanium-modified alloys show the best hot-salt oxidation resistance with the formation of an adherent, protective, thin, and continuous oxide layer. Among the four N-containing alloys investigated, the titanium and Ti/Al multilayer modified 0.56 pct N alloy showed the best hot-salt oxidation resistance as compared to uncoated alloys. The slower corrosion kinetics and adherent scale morphology indicate that the surface-modified titanium intermetallic coatings could provide high-temperature service applications up to 1073 K, particularly in chloride containing atmospheres, for austenitic stainless steels. I.
INTRODUCTION
WITH rapid advances taking place in engineering technology, stringent property requirements and rigorous performance of the structural materials are demanded, particularly for materials used under higher temperatures and severe chemical environments in service. The structural materials in many industries, particularly in power generation and chemical/ petrochemicals, are subjected to severe conditions involving heat, pressure, and chemical environment. Austenitic stainless steels are commonly used in hightemperature applications, in order to exploit the economic advantages they offer together with their inherent properties to resist chemical and high-temperature attack.[1,2] They are suitable materials for power plant tubes, which have to operate at temperatures above 950 K, or for aero engines, and they generally show better corrosion resistance than martensitic and ferritic stainless steels. However, because of their low hardness or poor wear-resistance properties, the application is greatly limited. Hot-salt oxidation rates for austenitic stainless steel are generally not high until temperatures of 923 to 1023 K are reached.[3] Hot-
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