Water vapor can be detrimental to the elevated temperature oxidation resistance of alloys which rely on the formation of a protective chromium oxide layer. The resulting degradation can be significant, particularly when such alloys are at light gauge. Long term test results will be presented for both stainless steels and nickel-base superalloys exposed at elevated temperatures in environments containing various levels of water vapor.
Basic 18Cr?8Ni austenitic stainless steels are often specified for use at elevated temperatures because they retain significant mechanical strength and exhibit resistance to creep deformation, particularly when alloyed with carbide?forming elements such as niobium. They are relatively resistant to oxidation but are not truly heat resistant alloys?the risk of breakaway oxidation must be considered when selecting them for certain elevated temperature applications, particularly when they are used in the form of thin sections. Breakaway oxidation can occur when chromium is depleted from the substrate to the extent that damage to the protective oxide layer will not heal. The amount of chromium removed from the metal and sequestered in the oxide scale is a function of the rate of oxidation and the amount of surface exposed. Thin foils have a chromium during oxidation when compared to thicker material. The result is that a disruption in the initially formed chromia scale can result in rapid oxidation and subsequent degradation of the foil.
Water vapor is encountered as a minor component in ambient air and in larger concentrations as a by? product of combustion processes. It has been known for some time that the presence of water vapor in oxidizing environments can alter the degradation process for many different metals.1 When present in oxygen?bearing atmospheres or as the primary oxidant, water vapor appears to hasten the onset of rapid oxidation at elevated temperatures for Fe?Cr and Fe?Ni?Cr alloys.2?22 The results vary from study to study, but general trends are that the presence of water vapor accelerates the rate of oxidation, leads to the formation of layered scales, and increases the amount of chromium required to form a protective oxide film.
Numerous mechanisms have been proposed for the actions of water vapor. One of the most promising is the fact that water vapor tends to increase the rate of formation of volatile oxide species for several systems, leading to evaporation.23,24 Chromium oxide evaporates in the absence of water vapor as gaseous chromium oxides, an effect most pronounced at elevated temperatures (>1000ºC) Recent work on austenitic stainless steels has shown that water vapor can increase the evaporative rate loss to levels where it is significant at lower temperatures, particularly in rapidly flowing gas streams.16?19\\
Chromium oxide layer is established on the sample surface during initial exposure to air containing water vapor. The amount of chromium incorporated into this scale is not significant enough to result in breakaway oxidation due solely to chromium depletion of the substrate. Rapid weight gain occurs due to the formation and growth of mixed oxide nodules. These nucleate after an incubation time and then spread, consuming the initially formed protective oxide layer. This is supported by the finding that remnants of the initial chromium oxide layer survive on heavily degraded samples. It is unclear if the nodules form as a direct result of the action of water vapor or if it plays a role in inhibiting healing of flaws, cracks, and spalled regions by the formation of new chromium oxide.21,22 This work aims to further studies on the oxidation of thin metal foils in air containing water vapor.