Page 709 - Corrosion Engineering Principles and Practice

CHAPTER 15

High-Temperature

Corrosion

15.1 Introduction
High-temperature corrosion is a form of corrosion that does not
require the presence of a liquid electrolyte. Sometimes, this type of
damage is called dry corrosion or scaling. The first quantitative
approach to oxidation behavior was made in the early 1920s with the
postulation of the parabolic rate theory of oxidation by Tammann
and, independently, by Pilling and Bedworth.
All materials have their limitations and the solution to high-
temperature problems is often a compromise between careful materials
selection when the cause of a problem is known, process control in
order to impose a safe limit for temperature or gas composition, for
example, and better design specifications to recognize mechanical
constraints at elevated temperature or resulting from thermal cycling.
The ultimate choice will be a compromise based on what is available
and how much it costs. In some cases it is rational to accept a short life
expectancy with a high reliability factor where the component is
replaced on a planned time schedule [1].
Alloys generally rely upon an oxidation reaction for the formation
of a protective scale that will improve the corrosion resistance to
sulfidation, carburization, and the other forms of high-temperature
attack. The properties of high-temperature oxide films, such as their
thermodynamic stability, ionic defect structure, and detailed
morphology, therefore play a crucial role in determining the oxidation
resistance of a metal/alloy in a specific environment.
In general, the names of the corrosion mechanisms are determined
by the most abundant dominant corrosion products. For example:
oxidation implies oxides, sulfidation implies sulfides, sulfidation/
oxidation implies sulfides plus oxides, and carburization implies
carbides [2]. Oxidizing environments refer to high oxygen activities,
with excess oxygen. Reducing environments are characterized by low
oxygen activities, with no excess oxygen available, a situation that
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