ABSTRACT

The elevated temperature oxidation of 18Cr-8Ni austenitic stainless steels in air is normally characterized by the formation of a protective, external chromium oxide scale. The low growth rate of this layer protects the underlying metal from further oxidation. Laboratory testing of tlfin stainless steel foil specimens demonstrates that the presence of water vapor decreases the time required for breakaway oxidation to occur. Accelerated oxidation begins after the end of an incubation period, the length of which is affected by the amount of water vapor present. Changes in scale microstructure accompany the transition from normal to accelerated oxidation. Rapid oxidation is accompanied by the formation of a thick, duplex scale comprised of a complex inner layer and an iron oxide outer layer.

INTRODUCTION

Common 18Cr-8Ni austenitic stainless steels are often specified for use at elevated temperature because they retain significant mechanical strength and exhibit resistance to creep deformation. They are relatively resistant to thermal oxidation because they contain enough chromium to form an external protective oxide scale. However, these grades are not truly heat resistant alloys--the risk of breakaway oxidation must be considered when selecting them for certain elevated temperature applications. Breakaway oxidation can occur when chromium, the element responsible for protective oxide scale formation, is depleted from the substrate to the extent that damage to the protective oxide layer will not heal. The result is the formation of fast growing oxides of nickel and iron. Breakaway oxidation is of particular concern for thin stainless steel foils.

A thin foil has an intrinsically high surface area to volume ratio, resulting in rapid depletion of chromium during oxidation when compared to a hea W section. 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 of ambient air and in larger concentrations as a by- product of cornbustion processes. It has been known for some time that the presence of water vapor can alter the oxidation 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 at elevated temperatures for Fe-Cr and Fe-Ni-Cr alloys such as stainless steels. 2-16 The results vary from study to study, but general indications 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.

The reasons for the observed effects remain unclear. Water vapor is known to increase the rate of formation of volatile oxide species for several systems, leading to increased oxidation by evaporation of the protective film. iv Alternatively, the defect structure of oxides may be altered by the incorporation of charged species (protons or hydroxide ions), leading to an increase in the growth rate of the scale. TM Much of the work cited above was performed at relatively high temperatures. One recent study found that exposure of thick Type 304L stainless steel samples in air mixed with 10% water vapor at 600°C resulted in the evaporation of a chromium-bearing species, possibly chromium oxide hydroxide CrO2(OH)2 , as detected by analysis of condensed species in the exit gases. The end result was the formation of a scale rich in iron at its outer extent. 19

This paper describes experiments performed to clarify the effect of water vapor on the oxidation of thin austenitic stainless steel foils at relatively low tempe

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