Most metals are thermodynamically unstable and tend to change to more stable phases by reaction with their service environment. These oxidation or corrosion processes are usually controlled by kinetic factors, and the rate often increases with increasing temperature. Degradation by these processes is an important, often life-limiting, factor for critical components in thermal-power systems and other high-temperature process plants, and it can have considerable economic and technical consequences. Corrosion can be particularly severe, for example, in

Gas-turbine blades and vanes, Gas coolers in coal-gasification systems, Superheaters and reheaters in coaland oil-fired boilers, In-bed tubes in fluidized-bed coal combustion systems, Components in high-temperature fuel cells, Components in steam-reforming plants, Components in incinerator plants, and Exhaust systems (e.g., automobiles).

Although there is considerable awareness of such problems and much attention is given to the materials requirements for high-temperature systems, failures still occur and can be catastrophic (fire, explosion, burst tube, etc). Plant conditions and control are particularly important since damage can occur due to overheating and overstressing when a material is taken beyond its physical, mechanical, and chemical limits or due to so-called dew-point corrosion when the system is cooled and acidic vapors condense to electrolytically corrode unprotected surfaces.

The oxidation of metals and alloys in oxygen at high temperatures is now reasonably well-understood. Indeed, alloys have been successfully developed to resist environmental attack in such atmospheres. The oxidation resistance of these materials is due to the presence of alloying elements—such as chromium, aluminum, and silicon—which can react with oxygen to form stable, protective, external oxide scales on the component surface, thereby inhibiting further high-temperature degradation.

The first stainless steels, mainly low carbon chromium-iron alloys, have been known since the beginning of this century. These steels show good resistance against wet corrosion and high-temperature corrosion. This article focuses on high-temperature corrosion, with emphasis on gaseous sulfidizing and oxidizing environments. The discussion is limited to these two gases since corrosion involving halogen-and/or carbon-containing gases involves other specific processes. The behavior of binary and ternary alloys will be successively examined, then the role of minor elements will be considered.

Fundamental Mechanisms of High-Temperature Corrosion of Stainless Steel

Usually, a dry corrosion process results in the formation of corrosion products, giving a simple or complex oxide or sulfide scale on a metallic substrate, separating it from the aggressive gaseous environment and, consequently, acting as a protective barrier. Scale growth is controlled by the conductivity of the reaction products which are solid electrolytes. Generally, the mechanism of scale growth is governed by outward cation or inward anion diffusion processes. This is the basis of the model originally put forward by Wagner for a single metal and subsequently developed for alloys, and particularly, for stainless steels. This one-way point-defect diffusion process is responsible for the observed parabolic scaling kinetics characterized by a parabolic rate constant kp. This model is well described in the literature.

In the case of stainless steels, formation of a protective scale is required; this is possible if the oxide or sulfide products have a low diffusivity to cations or anions due to a low density of point defects in the crystal lattice. The protective characteristics of the corrosion products may be experimentally determined by measurement of their electrical conductivity, although the scales should also be effective against short-circuit transport of ions, atoms, or molecules. The best barriers consist of oxides, such as Al2O3, SiO2, and Cr2O3.

For more than 50 years, scientists have studied the “magic dust” of high-temperature oxidation—certain oxygen active or “reactive” elements which, when added to alloys in small quantities, effect profound improvements in their oxidation resistance. In general, high-temperature oxidation resistance is achieved by the oxidation of one or more alloy components to form a dense, stable, slow-growing, external oxide layer, or ’scale” such as α-Cr2O3, α-Al2O3, or SiO2. When added properly, reactive elements have a beneficial effect on the formation and growth of both α-Cr2O3 and α-Al2O3 scales. A standard list of reactive element (RE) effects would include: (1) an improvement in scale adhesion or resistance to spallation, (2) a change in the scale growth mechanism, (3) a reduction in the oxidation rate, related to the change in mechanism, (4) a modification in the scale microstructure, and (5) in the case of alloys that form Cr2O3 scales, an improvement in selective oxidation, meaning that a lower Cr concentration in the alloy is required to form and maintain an external Cr2O3 scale.

The intermetallic compound, TiAl, and materials based on it have been receiving considerable attention because of their high specific strengths at elevated temperatures. Their mechanical properties are being extensively studied, with a view to application in aircraft or automobile engines. Before using these materials at elevated temperatures, their interactions with the environment should be investigated and suitable measures taken to overcome any degradation. It has, however, been reported that protective Al2O3 scales do not form when TiAl is exposed to an oxidizing atmosphere in spite of its high aluminum content. The formation of an A12O3 scale, or at least a scale containing a continuous A12O3 layer, is a prerequisite for providing sufficient protection. Therefore, much effort has been expended in choosing suitable alloying additions. However, relatively less effort has been devoted to surface coatings on such materials. The purpose of the alloying addition is two-fold: to improve mechanical properties, particularly ductility, and to improve oxidation resistance. However, compatible alloying elements are rare. For instance, some elements which improve the ductility, e.g., vanadium, chromium, and manganese, lower the oxidation resistance. Accordingly, a reasonable concept for the designer is to add effective alloying elements to improve the mechanical properties and to apply a coating to provide protection against the environment. For improved reliability, elements that improve the oxidation resistance can also be added to increase the protection unless they reduce the required mechanical properties.

Silicon-based ceramics are leading candidate materials for high-temperature structural applications such as heat exchangers, advanced gas turbine engines, and advanced internal combustion engines. They have excellent oxidation resistance in clean oxidizing environments due to the formation of a slow-growing silica scale (SiO2). However, durability in high-temperature environments containing molten salts, water vapor, or a reducing atmosphere can limit their applications. Molten salts react with silica scale to form liquid silicates. Oxygen readily diffuses through liquid silicates and rapidly oxidizes the substrate. High water vapor levels lead to hydrated silica species, such as Si(OH)4(g) and subsequent evaporation of protective scale. Complex combustion atmospheres containing oxidizing (CO2, H2O) and reducing (CO, H2) gases form SiO2 and then reduce it to SiO(g). In situations with extremely low partial pressures of oxidant, direct formation of SiO(g) occurs. All these reactions can potentially limit the formation of a protective silica scale and thus lead to an accelerated or a catastrophic degradation.

One approach overcoming these potential environmental limitations is to apply a barrier coating which is environmentally stable in molten salts, water vapor, and/or reducing atmospheres. Refractory oxides such as mullite (3Al2O3 · 2SiO2), yttria-stabilized zirconia (ZrO2-Y2O3), or alumina (Al2O3) are promising candidate coating materials because of their excellent environmental stability in these severe conditions. Refractory oxide coatings can also serve as thermal barrier coatings because of their low thermal conductivity. Key requirements for an adherent and durable barrier coating include coefficient of thermal expansion (CTE) match and chemical compatibility with the substrate. Mullite in general meets all the requirements and thus appears most promising.

Environmental degradation poses a serious limitation on the utility of engineering materials such as metals, plastics, glasses, and structural ceramics. However, at least water would seem to be a harmless environment for ceramics, which are considered to be the most corrosion-resistant of all materials. When we drink water from a glass or a ceramic cup, we never think about an aqueous alteration of these materials. We cannot imagine that our ceramic mug may be dissolved in water or fail due to stress corrosion during breakfast. Even hot water which we use to make coffee or tea does not seem to be a hazardous environment for our china and earthenware. Why then are we going to discuss the effect of water on structural ceramics? The answer is simple: Under conditions of normal use of structural ceramics, which include high temperatures, high pressures, and mechanical stresses (Figure 1), even the most stable and corrosion-resistant materials may interact with water and water may become a hostile environment. It can be adsorbed on the surface of ceramics and act as a solvent for oxide ceramics or as an oxidant for non-oxide ceramics (Figure 2). However, water can cause a degradation of glasses and oxide ceramics even at room temperature if mechanical stresses are simultaneously applied (so-called stress corrosion). Water or humid air can decrease significantly the mechanical properties of certain zirconia ceramics at only slightly elevated temperatures (~200°C).

Thermal-barrier coatings are finding increasing use in engineering applications, particularly in gas turbines. Such coatings, consisting of ceramic insulating layers bonded to the superalloy substrate by oxidation-resistant alloy coatings, are deposited onto components to reduce heat flow through the cooled substrate and to limit operating temperature. They have been used effectively on static components such as combustor cans, flare heads, hot gas seal segments, fuel evaporators, and deflector plates, giving considerable improvements in component life. They have been used successfully on vane platforms. In recent years, the emphasis has shifted toward the development of coatings for high-risk areas, such as turbine blades.

A ceramic thermal-barrier coating needs to be refractory and chemically inert, and to have low thermal conductivity. However, it also needs to possess a high thermal expansion coefficient of ~11 × 10−6 K−1, to match the nickel-base superalloy substrate. The latter specification has focused attention on ZrO2. However, ZrO2 is polymorphic and undergoes two phase changes, cubic to tetragonal at 2350°C and tetragonal to monoclinic at 1170°C. The latter transformation is accompanied by a 5% volume increase which means that ZrO2 has to be alloyed to stabilize one of the high-temperature phases. Early systems in the 1970s consisted of ZrO2 stabilized with MgO, but this has been shown to be a metastable system. Present-day commercial thermal-barrier coatings consist of a plasma-sprayed yttria- or magnesiastabilized zirconia layer on top of an M-Cr-Al-Y bond coat. The latter plays a very important role by helping to key the ceramic to the alloy substrate and to accommodate the mechanical strains arising because of differences in thermal expansion coefficients and elastic moduli between the ceramic and the substrate.