High-temperature corrosion principles

The purpose of this review paper is to survey the principles of high temperature oxidation or high temperature corrosion. A typical situation is that of a metal exposed to a hot gas which can act as an oxidant. In many cases the oxidation product forms a layer which separates the reactants, the metal and the gas atmosphere. Under special conditions, the kinetics are diffusion controlled, i. e,, the rate of the reaction (the rate of oxide thickness growth) depends on the diffusion of species, ions and electrons, through the layer (sometimes called a tarnish layer). Actually when a metal or alloy is exposed to a corrosive gas, the reaction kinetics may be controlled by one or more of the following steps ... 

During low temperature oxidation, oxide films grow by high-field conduction (Section 8.1) rather than by solid-state diffusion because the value of the diffusion coefficients is too small. Under these conditions the thickness of the oxide layer does not exceed a few nanometers. In contrast, in high temperature corrosion, volume diffusion and grain-boundary diffusion are the principle transport mechanisms by which oxide layers grow. As a consequence their thickness can reach much larger values. 

Chapters 1 to 3) contains fundamental principles governing aqueous corrosion and high-temperature corrosion and covers the main environments causing corrosion such as atmospheric, natural waters, seawater, soils, concrete, as well as microbial and biofouling environments. 

The simplest form of a solid corrosion product on a metal surface is a continuous homogeneous surface scale consisting of one phase. Such a situation is encountered, e.g., in the oxidation of pure nickel where only nickel oxide is formed. Therefore, in many textbook examples, this type of reaction has been used to illustrate the principles of high-temperature corrosion. In the present chapter, the initial stages of the oxidation process are not addressed as this has been part of another chapter in this book. 

The principles pertaining to carbon blast furnace hearths apply as well to submerged-arc furnace hearths. In some processes, such as in d-c arc furnaces, the electrical conductance of carbon is a most important factor. The long life of carbon linings in these appHcations is attributable to carbon s exceptional resistance to corrosive slags and metals at very high temperatures. 

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C < T < 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T>374°C, p>218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

In another study [35], the electrochemical emission spectroscopy (electrochemical noise) was implemented at temperatures up to 390 °C. It is well known that the electrochemical systems demonstrate apparently random fluctuations in current and potential around their open-circuit values, and these current and potential noise signals contain valuable electrochemical kinetics information. The value of this technique lies in its simplicity and, therefore, it can be considered for high-temperature implementation. The approach requires no reference electrode but instead employs two identical electrodes of the metal or alloy under study. Also, in the same study electrochemical noise sensors have been shown in Ref. 35 to measure electrochemical kinetics and corrosion rates in subcritical and supercritical hydrothermal systems. Moreover, the instrument shown in Fig. 5 has been tested in flowing aqueous solutions at temperatures ranging from 150 to 390 °C and pressure of 25 M Pa. It turns out that the rate of the electrochemical reaction, in principle, can be estimated in hydrothermal systems by simultaneously measuring the coupled electrochemical noise potential and current. Although the electrochemical noise analysis has yet to be rendered quantitative, in the sense that a determination relationship between the experimentally measured noise and the rate of the electrochemical reaction has not been finally established, the results obtained thus far [35] demonstrate that this method is an effective tool for... 

Levels of volatility that would lead to unacceptable rates of vapor transport-driven sintering, attrition of catalytically-active materials, or corrosion of catalytic materials or support oxides by transport from contaminants or substrate materials can be estimated given equilibrium vapor pressures and a few assumptions about evaporation rates and mass transport. In particular, the rate of condensation of a vapor species on its source solid phase at high temperatures is almost certainly non-activated and may show little configurational restriction. Therefore, using the principle of microscopic reversibility, we can take the rate constant for condensation to be approximately equal to the collision frequency. 

The term high-temperature requires definition. In contrast to aqueous corrosion, the temperatures considered in this book will always be high enough that water, when present in the systems, will be present as the vapour rather than the liquid. Moreover, when exposed to oxidizing conditions at temperatures between 100 and 500 °C, most metals and alloys form thin corrosion products that grow very slowly and require transmission electron microscopy for detailed characterizahon. While some principles discussed in this book may be applicable to thin films, high temperature is considered to be 500 °C and above. 

In addition to high temperature, the refractory concrete to be produced must also be resistant to possible chemical corrosion. Just as in applications at ordinary temperatures, porosity—apart from the chemical nature of the solid phases—is the main factor that controls the chemical resistance of refractory concrete. The same general principles as outlined for the production of a good non-refractory, low-temperature concrete for civil engineering applications must also be maintained in the production of refractory concrete. 

Focus here is on service tests that evaluate materials in applications that involve both high-temperature and corrosive conditions. B lsic chemical, thermodynamic, and physical principles are discussed. Specimen preparation, environment synthesis and evaluation are briefly reviewed and, where appropriate, reference is made to more detailed information and studies available in the technical literature. 

Kinetics, chemical, thermodynamic, and physical principles will all be operating in high-temperature service test environments, requiring each investigator to have an adequate huniliarily of basic mechanisms and corrosion phenomena. A brief introduction to these aspects of service testing is presented here. 

---