Oxidation
When stainless steels are exposed to atmospheric oxygen, an oxide film is formed on the surface. At low
temperatures this film takes the form of a thin, protective passive film but at high temperatures the oxide thickness
increases considerably. Above the so-called scaling temperature the oxide growth rate becomes unacceptably high.
Chromium increases the resistance of stainless steels to high temperature oxidation by the formation of a chromia
(Cr2O3) scale on the metal surface. If the oxide forms a contiuous layer on the surface it will stop or slow down
the oxidation process and protect the metal from further. Chromium contents above about 18% is needed in order
to obtain a continuous protective chromia layer. The addition of silicon will appreciably increase the oxidation
resistance, as will additions of small amounts of the rare earth metals such as cerium. The latter also increase the
adhesion between the oxide and the underlying substrate and thus have a beneficial effect in thermal cycling i.e. in
cases in which the material is subject to large, more or less regular, variations in temperature. This is, at least
partly, due to the fact that the addition of Ce promotes a rapid intial growth of the oxide. This leads to a rapidly
formed thin and tenacious protective oxide. The scale is then thin and the chromium depleated zone below is also
thin which makes reformation of the oxide rapid if cracks form in it during thermal cycling. High nickel contents
also have a benefical effect on the oxidation resistance. The scaling temperatures for various stainless steels are
shown in Table 4. It is worth noting that the ranking in resistance to localized corrosion is not applicable at high
temperatures and that an increase in molybdenum content does not lead to an increased scaling temperature.
Compare, for example, 304L - 316 - 317L
Under certain conditions heat resisting steels can suffer very rapid oxidation rtes at relatively low temperatures.
This is referred to as catastrophic oxidation and is associated with the formation of liquid oxides. If a liquid oxide
is formed it will penetrate and disrupt the protective oxide scale and expose the metal to rapid oxidation.
Catastrophic oxidation generally occurs in the temperature range 640 - 950 oC in the presence of elements whose
oxides either melt or form eutectics with the chromium oxide (Cr2O3) scale. For this reason molybdenum, which
forms low-melting-point oxides and oxide-oxide eutectics, should be avoided in steels designed for high
temperature applications. The presence of some other metals in the environment may cause catastrophic oxidation.
Vanadium, which is a common contaminat in heavy fuel oils, can easily cause rapid or catastrophic oxidation due
to its low melting point oxide,V2O5, which melts at 690 oC. Some other metals, such as lead and tungsten, may
also act in this way.
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