Oxidation resistance of stainless steels
Oxidation is the chemical reaction of a metal with oxygen, usually at its surface, to form a new compound known as an oxide. Thick oxides are also known as scales. Oxide scales, once formed, can slow down further oxidation if they are protective, i.e. are adherent, dense and defect-free. Defects/cracks can also be introduced into the oxide/scale if the steel subsequently deforms under load. In stainless steels, used at elevated temperatures, (up to 1100oC for heat resisting types), this is used to advantage, the scale formed being predominantly chromium rich which tends to be of the protective type. Thus the scale layer will prevent further oxidation, but the metal lost in the formation of oxide will reduce the effective strength of the steel section.
Oxidation resistance depends mainly on temperature, gas composition and moisture level and steel grade, (mainly chromium level).
Austenitic stainless steels are the best choice as they also have better elevated temperature strength than the ferritic family. The higher thermal expansion rates of the austenitics can however result in problems such as distortion and may lead to scale loss, (spalling), during thermal cycling.
Conditions for stable oxide formation
Oxidation is dependent mainly on the oxygen level available in the atmosphere. Gas mixtures involving air, carbon dioxide and steam all ‘support’ oxidation. Oxidation resistance is due to the formation of chromium rich oxides, (Cr2O3), on the steel surface. Once formed this only grows at a slow rate, thus protecting the underlying steel from further oxidation. Oxidising atmosphere conditions support the resistance to further oxidation. Water vapour can adversely affect the oxidation resistance of stainless steels.
This is probably the result of a decrease in the plasticity of the protective oxide scale. As a general rule the maximum service temperatures for service in moist air, compared to dry air, should be lowered by around 40-65oC. High temperature steam should be considered as a special case.
If the oxide layer cracks under cyclic temperatures conditions, then the overall rate of oxidation increases. This can be a problem for the austenitic family and is reflected in lower maximum service temperatures in ‘intermittent’ temperature conditions, than for ‘continuous’ service conditions. In contrast, the ferritic and martensitic stainless steels generally have higher intermittent than continuous service temperatures.
Effects of steel grade (composition) on oxidation resistance
Chromium content is most important for providing oxidation resistance. Although the 18% Cr levels of the ferritic 430, (1.4016), and austenitic 304, (1.4301), 316, (1.4401), and 321, (1.4541), provide ‘good’ oxidation resistance, steels specifically designed to resist oxidation generally have higher chromium levels in the range of 20-25%, such as grade 310, (1.4845).
Nickel also helps improve oxidation resistance. This is probably due to improved oxide layer adhesion.
Silicon and aluminium are also added to improve oxidation resistance and are present in certain grades, but usually in limited amounts as they can also adversely affect the formability and oxidation performance of heat resisting stainless steels. However, there is sub-class of special ferritic stainless steels with higher aluminium contents, e.g. ~3-5%, and their oxide, which forms at elevated temperatures, is Alumina rich, i.e. Al2O3, and this oxide grows far more slowly than than the normal Chromia, Cr2O3, high temperature oxide on stainless steels. Thus their oxidation resistance is higher.
Calcium added in smaller amounts can also benefit oxidation resistance.
Rare earth elements including cerium and yttrium are also added to make some of the specialised heat resisting grades. These additions have an effect similar to nickel, by assisting the adhesion of the chromium rich oxide layer to the metal surface, but their exact mode of action is disputed.
One example of a heat-resisting grade that uses a combination of these compositional benefits is ‘253MA’, (1.4835). This steel does not have a specified aluminium range, but nitrogen is added for enhanced strength.
|BS EN 10095, 1.4835 composition, weight %
Embrittlement at service temperature
As well as their lower elevated temperature strength, the ferritic steels can form brittle constituents after holding in certain temperature ranges.
The temperature ranges of 370-540oC should be avoided. In the higher chromium ferritic grades embrittlement at higher temperatures can also occur due to ‘sigma’ phase formation. This is also a problem if the 25% chromium, 1.4845, (310), type is used in temperatures BELOW about 900oC. Cracking on cooling to ambient temperatures for maintenance can occur after service at these temperatures.
The ‘standard’ austenitics, 1.4878 / 14541, (321), 1.4401, (316), or 1.4301, (304), may be better choices for these ‘lower’ temperatures, up to around 870oC, which is their maximum service temperature in air.
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