ABSTRACT
Carburization and oxidation of Fe-Cr-Ni alloys are common issues in petrochemical industry processes. This study investigates the transition between carburization and oxidation behaviors of three different kinds of commercial alloys: austenitic stainless steel UNS S34700, ferritic stainless steel UNS S44600, and Ni based alloy UNS N06025. Equilibrium stable corrosion products, the alloy phases, and their relative amounts for these three alloys were calculated at 1800 °F (982 °C) in CH4-H2O mixed gas atmosphere. The effects of gas composition, temperature and alloy composition upon phase evolution were investigated.
BACKGROUND
In carburizing and oxidizing atmospheres, carbon/oxygen are transferred from the environment into metal components. This process is called carburization or oxidation, which are high temperature corrosion phenomena1-2. This can change mechanical properties of metals and alloys mostly at temperature >800 °C when carbon or oxygen diffuses into alloys and react with metals to form internal carbide or oxide precipitates.
A direct result of carburization is an increase in hardness of the carburized materials. Low alloy steels are often purposely exposed to carbonaceous environments to produce hard surfaces to increase wear resistance. So with the proper heat treatment in a gas atmosphere, the consequence of carburization could be beneficial. Carburizing is often used in industry to improve the surface characteristics of metals, such as wear resistance, corrosion resistance and abrasion resistance.
However, carburization can also be viewed as a corrosion process that affects high temperature alloys, especially in petrochemical industry processes3. It is a common cause of failure of ethylene pyrolysis tubes and can deteriorate the corrosion resistance and mechanical properties of various metals and alloys. In some cases, coke deposition occurs at the inner walls of tubes during hydrocarbon cracking. The decoking process is necessary to remove the coke deposits. Generally, steam or steam-air mixtures are used to remove coke periodically. The removal processes occurs by oxidation of the coke to CO and CO2 and also by thermal shocking/spalling of the deposits. The decoking process also introduces oxidation as a corrosion concern.
Oxidation can deteriorate materials integrity in high temperature service, with formation of potentially external or internal oxide corrosion products. During internal oxide precipitation, the thickness of useful metal components is reduced, due to the changing mechanical properties in the internally corroded surface layer of the metal components.
This paper investigates the transition between high temperature carburization and oxidation of several commercial alloys for ranges of compositions, based upon CH4-H2O atmospheres. Equilibrium corrosion products and alloy phases were calculated using thermochemical equilibrium methods embodied in ASSET4 (Alloy Selection System for Elevated Temperatures), which is a commercial software developed by Shell Global Solutions (US) Inc. and further advanced by the US Department of Energy and a group of collaborating companies. The technology stores corrosion measurements and exposure conditions according to several different corrosion mechanisms in a database to generate correlations to predict corrosion as functions of exposure conditions and also archives a large compilation of thermochemical data, solution models and Chem - SAGE software to perform equilibrium calculations to predict the most stable combinations of corrosion products and alloy phases.
