The addition of Si to Fe-based alloys as a means of enhancing sulphidation resistance in complex mixed atmospheres

Abstract

Of the three principal oxide-forming elements present in most high temperature engineering alloys (Cr, Al and Si), silicon has been the poor relation, largely as a result of its potentially adverse effect upon fabricability and weldability. However, several advanced heat resistant alloys containing between 2 and 3.5% Si have come onto the market in recent years with reported good resistance to corrosive atmospheres [1–8]. However, these alloys contain high levels of Cr, Ni and/or Co and are therefore relatively expensive.The present study explores the “Si-effect” using a cheaper 12%Cr ferritic, Type 410 class of steel. A commercial alloy, HCM 12, containing 0.2% Si has been selected as a base composition to which Si has been added in amounts ranging from nominally 1 to 4%. Samples of this commercial alloy have been exposed alongside similar specimens machined from specially cast higher-Si model alloys in highly reducing, non-equilibrated CO-based gas mixtures containing 4% CO2, 0–3% H2O and 0.1 and 0.8% H2S, balance H2. These atmospheres simulate those found in a dry-feed entrained slagging coal gasifier where corrosive degradation in the form of sulphidation attack limits operating temperature and therefore plant efficiency. Constructional costs remain an important issue, however, and comparisons are made with the performance of higher alloyed materials, HR 3C and Alloy 800H, exposed at the same time.Initially, experiments were carried out for up to 2,000 hours at 450°C in a 0.8%H2S containing atmosphere. Based upon the findings of these tests, a second series was carried out at a higher temperature, i.e. 550°C, for 1,000 hours in a 0.1%H2S gas mixture. Specimens were subjected to periodic interruptions for weight change measurements and visual inspection with selected samples also being discontinued for more in-depth surface and cross-sectional microstructural studies as well as metal loss measurements. The influence of Si upon corrosion kinetics and in modifying the nature and extent of attack has been established.