Investigation of metal dusting corrosion process over UNS N08800 alloy

Abstract

Metal dusting corrosion is a catastrophic degradation phenomenon that initiates on metallic surfaces of chemical process equipment at elevated temperature and under carburizing atmospheres. It proceeds by a gradual breakdown of the metallic matrix into fine particles. The work is performed to correlate the surface and bulk composition and structure of application relevant alloys to the initial carbon formation and the progress towards metal dusting. UNS N08800 alloy coupons were prepared, polished and oxidized in diluted steam at 540 °C. Formation of carbon on the surface of the pre-oxidized samples was investigated by exposure to reducing, highly carburizing atmosphere (CO in Ar) at 20 bar and varying temperature (550- 750 °C). The resulting surfaces were examined by scanning electron microscopy (SEM), Raman spectroscopy, depth profile analysis by Auger electron spectroscopy (AES) under ion- sputtering. Given that very high carbon activity ac >> 1, the conditions represent an accelerated regime relative to the industrial environment, and hence significant amounts of carbon are formed. The amount and the type of carbon are found to vary strongly with the exposure temperature and the interaction with the metallic matrix. At the highest temperature, also the alloy undergoes considerable change in the elemental distribution as a function of depth. INTRODUCTION Metal dusting corrosion is a high temperature degradation phenomenon affecting Fe-, Ni- and Co- based alloys when exposed to strongly carburizing gases (carbon activity ac > 1) at elevated temperatures (> 400°C).1-3 The alloy disintegrates into a powdery mixture of metallic, carbidic and carbonaceous dusts. The industrial process conditions cause transfer of carbon to the alloy surface by one of more of the (1) CO reduction, (2) Boudouard, and (3) alkane thermal cracking reactions. For Fe-based alloys,1-2, 4 metal dusting corrosion begins with the formation of Fe3C or Fe5C2 carbides, the volume expansion creates defects on the alloy surface layer.5-6 Carbon atoms diffuses through the carbides and then precipitates as carbon at defects. Accumulation of carbonaceous deposits then separates the carbide particles from the metallic matrix that can be transported away from the surface of the alloy to leave a pit on the surface.