The effects of various pre-oxidation treatments on the Type II (i.e., 700°C) hot corrosion resistance of novel Pt+Hf-modified ¿'-Ni3Al + ¿-Ni alloys and coatings were studied and compared to a state-of-theart t-modified ß-NiAl coating. The pre-oxidation treatments were carried out in air and in Ar atmospheres at temperatures of 700, 1080, and 1100°C and for times of 1, 6, and 24h. It was found that pre-oxidation in air at higher temperatures and for a shorter period was quite beneficial in improving hot corrosion resistance of the Pt+Hf-modified ¿' + ¿ alloys and coatings. In fact, the results suggested that the oxide scale formed with such a treatment was much more protective than the Ar and longer-term air pre-oxidation treatments. It was also found that with an optimum pre-oxidation treatment, a Pt+Hfmodified ¿.+¿ coating exhibited superior hot corrosion resistance compared to a Pt-modified ß coating.
Advanced gas-turbine components face various harsh operating conditions such that surface degradation often occurs by high temperature oxidation (>1000°C) and/or hot corrosion (600-1000°C) [1]. The high-temperature components in gas turbines are often protected against oxidation and hot corrosion by either a diffusion or an overlay metallic coating that is able to form a continuous, adherent, and slow-growing oxide scale. Of particular concern in this study is Type II hot corrosion (600-750°C), which is an accelerated degradation that involves deposition of a salt from the surrounding environment. This type of corrosion may occur below the melting temperature of the salt deposit, in which case reaction with the component surface eventually results in some amount of liquid product [2-4]. The formation of a liquid is then followed by destruction of the oxide scale.
Gleeson et al. [5] reported that a wide range of Pt+Hf-modified ¿'-Ni3Al + ¿-Ni alloy compositions form a thin, planar, and adherent alumina scale and hence offer a viable alternative to current state-of-the-art Pt-modified ß-NiAl coating. This led to the development of novel Pt+Hfmodified ¿'+¿ coatings that exhibit excellent cyclic oxidation resistance [6].
To maximize the lifetimes of gas turbine components, it is critical to control the microstructure, composition, and growth rate of the thermally grown oxide (TGO) that develops on the component surface. This can be at least partly achieved with an optimum pre-oxidation treatment that is arrived at by adjusting various critical pre-oxidation parameters such as oxidation atmosphere, time, temperature, and oxygen partial pressure [7-9]. It was recently reported by our group that pre-oxidized Pt+Hfmodified ¿.+¿ coatings can exhibit superior Type I (i.e., 900°C) hot corrosion resistance and comparable Type II (i.e., 700°C) hot corrosion resistance compared to Pt-modified ß coating [10].
The present work was performed to obtain an optimum pre-oxidation treatment that can be applied to improve Type II hot corrosion resistance of Pt+Hf-modified ¿'+¿ coatings. A bulk ¿.+¿ alloy having a composition similar to the Pt+Hf-modified ¿.+¿ coatings was selected for this particular study. In an attempt to simulate hot corrosion in actual marine gas turbine service conditions, a laboratorybased furnace rig was used in which the test samples are subjected to a continuous deposition of Na2SO4 salt in an O2: SOx atmosphere.
