Chapter 11: High-Temperature Corrosion: Mechanisms & Materials

High-Temperature Corrosion: Mechanisms & Materials overview of high-temperature corrosion investigates the mechanisms, kinetics, and material behaviors associated with metal degradation in gaseous environments, a process often referred to as dry oxidation. The chapter introduces the Pilling-Bedworth ratio as a fundamental empirical criterion to predict whether an oxide scale will be protective or unprotective based on the volume ratio of the oxide formed relative to the metal consumed; generally, a ratio (lesser than) 1 indicates an insufficient volume to cover the surface, while significantly higher ratios can lead to internal stresses and cracking. Oxidation is described as an electrochemical process where metal ions and electrons must migrate through an oxide scale, which acts as both an electrolyte and an electronic conductor. The narrative details how oxide defect structures—categorized as n-type or p-type semiconductors—dictate the transport of species, and how the "doping" effect of alloying elements can either accelerate or retard corrosion by altering defect concentrations. Kinetic rate laws are a primary focus, distinguishing between the linear law for non-protective scales, the parabolic law for ideal diffusion-controlled growth, and logarithmic behaviors often seen in thin films at lower temperatures. The chapter also addresses severe degradation modes such as catastrophic oxidation, characterized by rapid exothermic reactions or the formation of low-melting liquid oxides, and internal oxidation, where stable oxides precipitate within the metal matrix due to high oxygen solubility. Beyond simple oxygen reactions, the text explores the effects of other gases, including hydrogen attack—which leads to decarburization and methane-induced cracking—and sulfidation in sulfur-rich environments. Mechanical integrity is highlighted through the study of creep and stress rupture, particularly concerning the development of superalloys for high-temperature applications like gas turbines. Finally, the complex phenomenon of hot corrosion is analyzed, where molten salt deposits on metal surfaces create a thin-film electrolyte that accelerates degradation through acidic or basic dissolution of protective oxide scales.