The structure of external surfaces of scale forming on crofer 22APU steel in atmosphere containing H 2/H 2S budowa

Abstract

The paper presents results of structural tests carried out for scale formed during high-temperature sulphurisation of Crofer 22APU steel within temperature range 600-900°C, in atmosphere containing H 2/H 2S, at partial pressure of sulphur vapours: p S210 -7 up to 10 -1 Pa. Non-preoxidated and preoxidated (preliminarily oxidated in air atmosphere) samples were put through sulphurisation process. The SEM, EDX and XRD methods were employed to examine scale surface morphology, including its chemical constitution and phase composition. The SEM tests allowed to find that scale structure depends on atmosphere content and temperature. In conditions of high sulphur vapour pressure values (exceeding iron sulphide dissociation pressure), scale has two layers and consists of a mixture of iron sulphides (Fe nS m) and chromium sulphides (Cr xS y). Outer scale layer is built of large, column-shaped grains with composition corresponding to (Fe nS m), whereas its inner layer-fine-grained and porous-is a mixture of chromium sulphides, iron sulphides and sulphospinels. Occurrence of internal oxidation zone has been observed at the temperatures of 800°C and 900°C. Scale has single layer at low sulphur vapour pressure values, that is under FeS dissociation pressure. Preoxidated steel samples undergo sulphurisation process also after incubation period. This period is changeable and depends on sulphur vapour temperature and pressure. The structure and chemical constitution of scale formed on pre-oxidated samples depends on sulphurisation process parameters. Morphology of outer scale surface on preoxidated samples is complicated, because sulphides of all steel constituents are forming on the surface of oxides.