Abstract
The high temperature oxidation of the ferritic alloy Fe78Cr22 has been investigated in the present work. The effect of small alloying additions of cerium and/or silicon was also investigated. The alloys were oxidized at 973, 1173 and 1373 K in either air or a hydrogen/argon mixture. The various reaction atmospheres contained between 0.02 and 50% water vapour.
The oxide scales formed on the various alloys at 973 K consisted of thin chromia layers.
The oxide scales grown on the alloys at 1173 K also consisted of a chromia layer. The microstructure of the chromia scales was found to depend on the reaction atmosphere. The chromia scales grown in hydrogen/argon atmospheres formed oxide whiskers and oxide ridges at the surface of the scales, while the chromia scales grown in air formed larger oxide grains near the surface. This difference in oxide microstructure was due to the vaporization of chromium species from the chromia scales grown in air. Two different growth mechanisms are proposed for the growth of oxide whiskers. The growth rate of the chromia scales was independent of the oxygen activity. This is explained by a growth mechanism of the chromia scales, where the growth is governed by the diffusion of interstitial chromium.
The addition of silicon to the iron-chromium alloy resulted in the formation of silica particles beneath the chromia scale. The presence of silicon in the alloy was found to decrease the growth rate of the chromia scale. This is explained by a blocking mechanism, where the silica particles beneath the chromia scale partly block the outwards diffusion of chromium from the alloy to the chromia scale.
The addition of cerium to the iron-chromium alloy improved the adhesion of the chromia scale to the alloy and decreased the growth rate of chromia. It was observed that the minimum concentration of cerium in the alloy should be 0.3 at.% in order to observe an effect of the cerium addition. The effect of cerium is explained by the “reactive element effect”.
The oxide scales grown on the various alloys at 1373 K exhibited the presence of thick iron rich oxides in the scales. The presence of iron rich scales is described as a result of failure of the chromia scale and subsequent breakaway oxidation of the alloy. The breakaway oxidation was initiated at the sample edges. Furthermore, the breakaway oxidation was affected by the oxygen activity in the reaction atmosphere, where a low oxygen activity protected the alloys from breakaway oxidation. A mechanism for the breakaway oxidation is put forward, which explains the observed oxidation behaviour of the alloys.
The effect of water vapour on the chromia growth rate was investigated. It was found that the presence of water vapour in the reaction atmosphere increased the growth rate of chromia. It is proposed that hydrogen dissolves in the chromia scale and thereby increases the transport of reactants through the chromia scale. Furthermore, the growth rate of chromia was observed to decrease, when the alloys were oxidized in air containing large amounts of water vapour. This is a result of vaporization of chromium containing species from the chromia scale during the oxidation. The effect of water vapour was studied through thermogravimetric measurements in various reaction atmospheres. The measurements were partly performed on an equipment developed during this Ph.D.-study. The development was performed through a collaboration with Netzsch. The development of the novel equipment is described in this thesis.
The oxide scales formed on the various alloys at 973 K consisted of thin chromia layers.
The oxide scales grown on the alloys at 1173 K also consisted of a chromia layer. The microstructure of the chromia scales was found to depend on the reaction atmosphere. The chromia scales grown in hydrogen/argon atmospheres formed oxide whiskers and oxide ridges at the surface of the scales, while the chromia scales grown in air formed larger oxide grains near the surface. This difference in oxide microstructure was due to the vaporization of chromium species from the chromia scales grown in air. Two different growth mechanisms are proposed for the growth of oxide whiskers. The growth rate of the chromia scales was independent of the oxygen activity. This is explained by a growth mechanism of the chromia scales, where the growth is governed by the diffusion of interstitial chromium.
The addition of silicon to the iron-chromium alloy resulted in the formation of silica particles beneath the chromia scale. The presence of silicon in the alloy was found to decrease the growth rate of the chromia scale. This is explained by a blocking mechanism, where the silica particles beneath the chromia scale partly block the outwards diffusion of chromium from the alloy to the chromia scale.
The addition of cerium to the iron-chromium alloy improved the adhesion of the chromia scale to the alloy and decreased the growth rate of chromia. It was observed that the minimum concentration of cerium in the alloy should be 0.3 at.% in order to observe an effect of the cerium addition. The effect of cerium is explained by the “reactive element effect”.
The oxide scales grown on the various alloys at 1373 K exhibited the presence of thick iron rich oxides in the scales. The presence of iron rich scales is described as a result of failure of the chromia scale and subsequent breakaway oxidation of the alloy. The breakaway oxidation was initiated at the sample edges. Furthermore, the breakaway oxidation was affected by the oxygen activity in the reaction atmosphere, where a low oxygen activity protected the alloys from breakaway oxidation. A mechanism for the breakaway oxidation is put forward, which explains the observed oxidation behaviour of the alloys.
The effect of water vapour on the chromia growth rate was investigated. It was found that the presence of water vapour in the reaction atmosphere increased the growth rate of chromia. It is proposed that hydrogen dissolves in the chromia scale and thereby increases the transport of reactants through the chromia scale. Furthermore, the growth rate of chromia was observed to decrease, when the alloys were oxidized in air containing large amounts of water vapour. This is a result of vaporization of chromium containing species from the chromia scale during the oxidation. The effect of water vapour was studied through thermogravimetric measurements in various reaction atmospheres. The measurements were partly performed on an equipment developed during this Ph.D.-study. The development was performed through a collaboration with Netzsch. The development of the novel equipment is described in this thesis.
| Original language | English |
|---|
| Place of Publication | Roskilde |
|---|---|
| Publisher | Risø National Laboratory |
| Number of pages | 170 |
| ISBN (Print) | 87-550-3353-9 |
| Publication status | Published - 2003 |
| Series | Risø-PhD |
|---|---|
| Number | 2(EN) |
Keywords
- Risø-PhD-2(EN)
- Risø-PhD-2
- Risø-PhD-002