Hot-salt corrosion

Oxidation tests on Nimonic 90A, in which sodium chloride was introduced into the atmosphere, showed that this constituent produces a significant deterioration in the protective nature of the normally adherent film. Although under certain service conditions the presence of sodium chloride is likely, this is not always so, and thus the general applicability of the results of laboratory tests in sodium sulphate and mixtures involving sodium chloride may be questioned. Test procedures for hot-salt corrosion have been reviewed by Saunders and Nicholls who concluded that burner rig testing is the most appropriate procedure provided contaminant flux rates similar to those found in an operating turbine are used in the rig. 

A considerable body of data on the oxidation of TiAl is available, albeit for relatively short times at elevated temperatures, 800-1400°C [5-19], Oxidation data at potential operating temperatures within the turbine (650-750°C) is much more scarce, [17], while data on the hot salt corrosion of y-TiAl does not exist. 

This paper therefore concentrates on a study of the hot salt corrosion of 7-TiAl over the temperature range 500-700°C. Limited oxidation data at 700°C is also presented by way of comparison with the hot salt corrosion data, and published high temperature oxidation data. 

The potential of using y-TiAl to replace nickel based alloys within the high pressure compressor and fourth stage turbine requires that the -y-TiAl be resistant, not only to oxidation but hot salt corrosion as well. The level of salt expected at these location is low, typically 0.3-0.5 ig/cm2/h, but nevertheless may have a significant effect on oxidation processes. 

Table 1 and Figure 5, summarise the thermogravimetric studies of hot salt corrosion for y-TiAl over the temperature range 500-700 °C. 

Table 1. A summary of the results for discontinuous and continuous hot salt corrosion thcmogravimctric tests...

Fig. 5. Thermomicrobalancc studies, under hot salt corrosion conditions, a) Data over the temperature range 500-700"C, b) Data at 500 and 550°C...

Fig. 7. Logarithmic corrosion kinetics for the early stage of hot salt corrosion attack of -y-TiAl, at 600 °C...

EPMA and XRD of the thicker hot salt corrosion scales, shows that the majority of the scale is rutile. Alumina is present as a discrete phase within the scale, but does not form a continuous layer. Further alumina is present as whiskers on the scale surface. Aluminium is also detected within the rutile rich regions, at a low level, inferring that it is either in solution or distributed as micro particles below the resolution of the SEM. 

One must ask, therefore Are these levels of salt to be expected in service for if they are hot salt corrosion will be a significant factor in the adoption of y-TiAl for turbine applications and secondly What is the mechanism by which small salt deposits accelerate the oxidation rale of y-TiAl . 

A result of these hot salt corrosion and hot salt stress corrosion studies of titanium alloys was the importance of alloy composition in determining a titanium alloy susceptibility to attack. It was shown that alloys high in aluminium were preferentially attacked by hot salt [24, 25], with both A1 and Zr being incorporated in the internal and external oxide scales [26]. This early work raised the importance of aluminium in the hot salt corrosion of titanium alloys. 

NaCl has been shown to have a major influence on the early stages of scale formation. Every corrosion test, when salt coated, suffered a high initial rate of mass gain, over the first 5-10 h of exposure.This behaviour can be seen in Figure 6, where at 700°C the initial mass gain is increased by an order of magnitude in the presence of salt and is evident in the hot salt corrosion tests at 500 and 550°C which have mass gains similar to that observed at 700 °C under isothermal oxidation conditions. 

During stage 1 growth, the localised salt deposits react with the thin rutile surface scale, formed on the alloy surface. Drawing on the experience of hot salt corrosion of titanium alloys, then sodium litanate [27] and sodium hydroxide [30] should be expected to form. Possible reactions include ... 

Ti-based alloys have long been known to be susceptible to hot-salt stress corrosion (e.g. Gray, 1973). Recent work (Nicholls et al., 1997) indicates that TiAl-based alloys suffer from accelerated hot-salt corrosion attack in the temperature range 650 to... 

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