section epub:type=”chapter”> This chapter discusses case studies in dry oxidation. The chapter focuses on an important class of alloys designed to resist corrosion: the stainless steels. The chapter examines a complicated problem of protecting the most advanced gas turbine blades from gas attack. The basic principle applicable to both cases is to coat the steel or the blade with a stable ceramic—usually Cr2O3 or Al2O3. But the ways this is done differs widely. One might imagine that it is always a good thing to have a protective oxide film on a material; however, if one wishes to join materials by brazing or soldering, the protective oxide film can be a problem. It is this that makes stainless steel hard to braze and almost impossible to solder; even spot-welding and diffusion bonding become difficult. Protective films create poor electrical contacts—that is why aluminum is not more widely used as a conductor. And production of components by powder metallurgy, which involves compaction and sintering (diffusion bonding) of powdered material to the desired shape, is made difficult by protective surface films. In this chapter we look first at an important class of alloys designed to resist corrosion: the stainless steels. We then examine a more complicated problem: that of protecting the most advanced gas turbine blades from gas attack. The basic principle applicable to both cases is to coat the steel or the blade with a stable ceramic: usually Cr2O3 or Al2O3. But the ways this is done differ widely. Finally, we look at a process in which oxide films must not form on the surface—joining metals by soldering and brazing. Mild steel is an excellent structural material—cheap, easily formed, and strong. But at low temperatures it rusts, and at high temperatures, it oxidizes. There is a large demand, for high-temperature applications ranging from chemical reactors to superheater tubes, for oxidation-resistant steel. In response to this demand, a range of stainless irons and steels has been developed. When mild steel is exposed to hot air, it oxidizes quickly to form FeO (or higher oxides). But if one of the elements near the top of Table 25.1 with a large energy of oxidation is dissolved in the steel, then this element oxidizes preferentially (because it is more stable than FeO), forming a layer of its oxide on the surface. And if this oxide is a protective one, like Cr2O3, Al2O3, SiO2, or BeO, it stifles further growth, and protects the steel. A considerable quantity of this foreign element is needed to give adequate protection. The best is chromium, 18% of which gives a very protective oxide film: it cuts down the rate of attack at 900°C, for instance, by more than 100 times. Other elements, when dissolved in steel, cut down the rate of oxidation, too. Al2O3 and SiO2 both form in preference to FeO (refer to Table 25.1) and form protective films (refer to Table 25.2). Thus 5% Al dissolved in steel decreases the oxidation rate by 30 times, and 5% Si by 20 times. The same principle can be used to impart oxidation resistance to other metals. We shall discuss nickel and cobalt in the next Case Study—they can be alloyed in this way. So, too, can copper; although it will not dissolve enough chromium to give a good Cr2O3 film, it will dissolve enough aluminum, giving a range of stainless alloys called “aluminum bronzes.” Even silver can be prevented from tarnishing (reaction with sulfur) by alloying it with aluminum or silicon, giving protective Al2O3 or SiO2 surface films. Ceramics themselves are sometimes protected in this way. Silicon carbide, SiC, and silicon nitride, Si3N4, both have large negative energies of oxidation (meaning that they oxidize easily). But when they do, the silicon in them turns to SiO2 which quickly forms a protective skin and prevents further attack. Protection by alloying has one great advantage over protection by surface coating (like chromium plating or gold plating): it repairs itself when damaged. If the protective film is scored or abraded, fresh metal is exposed, and the chromium (or aluminum or silicon) it contains immediately oxidizes, healing the break in the film. As we saw in Chapter 24, the materials at present used for turbine blades consist chiefly of nickel, with various foreign elements added to get the creep properties right. With the advent of DS and SX blades, such alloys will normally operate around 950°C, which is close to 0.7TM for Ni (1208 K, 935°C). If we look at Table 25.2 we can see that at this temperature, nickel loses 0.1 mm of metal from its surface by oxidation in 600 h. The thickness of the metal between the outside of the blade and the integral cooling ports is about 1 mm, so in 600 h a blade would lose about 10% of its cross-section in service. This represents a serious loss in mechanical integrity and makes no allowance for statistical variations in oxidation rate—which can be quite large—or for preferential oxidation (at grain boundaries, for example) leading to pitting. Because of the large cost of replacing a set of blades, they are expected to last for more than 5000 h. Nickel oxidizes with parabolic kinetics (Equation (25.4)) so after a time t2, the loss in section x2 is given by substituting the data into: giving Obviously this sort of loss is not OK, but how do we stop it? As we saw in Chapter 24, the alloys used for turbine blades contain large amounts of chromium, dissolved in solid solution in the nickel matrix. If we look at Table 25.1, which gives the energies released when oxides are formed from materials, we see that the formation of Cr2O3 releases much more energy (701 kJ mol–1) than NiO (439 kJ mol–1). This means that Cr2O3 will form in preference to NiO on the surface of the alloy. Obviously, the more Cr there is in the alloy, the greater is the preference for Cr2O3. At the 20% level, enough Cr2O3 forms on the surface of the turbine blade to make the material act a bit as though it were chromium. Suppose for a moment that our material is chromium. Table 25.2 shows that Cr would lose 0.1 mm in 1600 h at 0.7TM. Of course, we have forgotten about one thing. 0.7TM for Cr is 1504 K (1231°C), whereas for Ni, it is 1208 K (935°C). We should probably consider how Cr2O3 would act as a barrier to oxidation at 1208 K rather than at 1504 K (Figure 26.1). The oxidation of Cr follows parabolic kinetics with an activation energy of 330 kJ mol–1. Then the ratio of the times required to remove 0.1 mm (from Equation 25.3) is
Case Studies in Dry Oxidation
Publisher Summary
26.1 Introduction
26.2 Case Study 1: Making Stainless Alloys
26.3 Case Study 2: Protecting Turbine Blades
