High temperature heat exchangers require the use of oxidation and creep-resistant alloys. Austenitic stainless steels are often specified for reasons of cost, availability, and ease of fabrication. Water vapor, present in the exhaust gas as a by-product of combustion, has been shown to be detrimental to the elevated temperature oxidation resistance of common stainless steels. Increasing the amount of chromium and nickel in heat-resistant alloys appears to alleviate the risk of breakaway oxidation in air containing water vapor. This paper gives results and analysis from extended high-temperature oxidation testing in air with and without added levels of water vapor of two common wrought nickel-base superalloys (UNS N06626 and UNS N06002) and an experimental austenitic stainless steel (Fe-20Cr-25Ni+Nb). The alloys were tested as thin foils, as the primary focus of this study was materials for primary surface recuperator-type heat exchangers. Alloy 625 exhibited significant weight losses due to oxide scale evaporation in humidified air. The other two alloys in the test program were resistant to this form of attack due to the formation of an external manganese chromate spinel oxide scale layer. Both nickel-base alloys proved resistant to breakaway oxidation, but the stainless steel exhibited mixed oxide nodule formation during the most severe exposures. Conclusions relevant to the use of such materials in water vapor-bearing environments are provided.
Water vapor is encountered as a minor component in ambient air and in larger concentrations as a byproduct of various industrial processes such as combustion, and as such is commonly encountered in gases in high temperature heat exchangers. 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. When present in oxygen-bearing atmospheres or as the primary oxidant, water vapor appears to hasten the onset of rapid oxidation of both austenitic (Fe-Cr-Ni) and ferritic (Fe-Cr) stainless steels at elevated temperatures. The presence of water vapor typically accelerates the rate of oxidation, leads to the formation of rapidly growing oxide nodules and layered scales, increases the amount of chromium depletion during oxidation, and increases the starting chromium alloying content necessary to form and maintain a protective oxide film. Studies involving austenitic stainless steels noted that a thin 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 large enough to result in significant depletion of the underlying metal. Therefore, the occurrence of breakaway oxidation in this case does not appear to be due solely to chromium depletion of the substrate.
Accelerated oxidation for heat-resistant stainless steels is caused by the formation and growth of mixed oxide nodules. These nucleate at isolated locations after an incubation time and then spread, consuming the initially-formed protective oxide layer. This observation is supported by the fact 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 water vapor plays a role in inhibiting healing of flaws, cracks, and spalled regions by the formation of new chromium oxide. Work focused specifically on Fe-Ni-Cr alloys suggests that higher chromium and nickel contents (or conversely, low iron content) are beneficial in preventing nodule formation.
The damaging effect of water vapor on austenitic stainless steels has led to the substitution of highe