When a material is exposed to an oxidizing atmosphere at high temperatures, an oxide layer is usually formed on the surface. This layer will retard further oxidation. If the temperature of the material increases, the oxide growth rate will increase and the layer will finally crack and spall off, thus losing its protective effect - the scaling temperature has been reached.
Although oxidation is seldom the main cause of high temperature corrosion failures, the oxidation performance is of primary interest, because the properties of any formed oxide layer will determine the resistance to other aggressive elements in the environment.
The toughness and adherence of the oxide layer also determine the erosion resistance of the alloy.
The oxidation resistance of all HT grades rely on the formation of a protective oxide layer, rich in chromium, aluminium, and/or silicon. Additional alloying elements may improve the properties further.
The REM addition and increased Si content of the MA grades reduce the isothermal oxidation rates of these alloys.
However, since these alloying additions also improve the adherence of the oxide, the MA grades also show an improved cyclic oxidation resistance.
Historically, the oxidation resistance of an alloy has been specified as the "scaling temperature", i.e. the temperature, at which the oxidation rate becomes unacceptably high. Since this temperature is of little technical importance, we have abandoned the "Scaling temperature" concept, for "Maximum recommended service temperature", which is based on service experience together with long- and short-time, isothermal and cyclic laboratory tests.
Outokumpu
Steel Grade |
Maximum service temperature in dry air (°C) |
EN |
ASTM |
| 4984 |
800 |
1.4984 |
304H |
| 4878 |
800 |
1.4878 |
321H |
| 153 MA™ |
1000 |
1.4818 |
S30415 |
| 4828 |
1000 |
1.4828 |
|
| 4833 |
1000 |
1.4833 |
309S |
| 253 MA™ |
1100 |
1.4835 |
S30815 |
| 4845 |
1100 |
1.4845 |
310S |
| 4841 |
1125 |
1.4841 |
314 |
| 353 MA™ |
1150 |
1.4854 |
S35315 |
Water vapour
Most flue gases (except from coal combustion) have an increased water vapour content. Its presence will make the formed oxides more porous and therefore less protective. The maximum recommended service temperature can be reduced by 50 - 150 °C, depending on steam content.