High-temperature corrosion types

High temperature corrosion type of furnace radiant tubes... 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

High-temperature corrosion is induced by accelerated reaction rates inherent in any temperature reaction. One phenomenon that occurs frequently in heavy oil-firing boilers is layers of different types of corrosion on one metal surface. 

A further type of chemical process, which is analogous to high-temperature Corrosion, is the reaction of metals with organic sulphur compounds, which follow the equation... 

The majority of corrosion reactions fall into three categories (1) aqueous corrosion (2) nonaqueous corrosion and (3) high-temperature corrosion. The corrosion media consist of four types ... 

The basic types of thermodynamic diagrams useful in interpreting the results of high-temperature corrosion experiments have been discussed. While no attempt at a complete discussion of this subject has been made, it is hoped this chapter will facilitate a clearer understanding of the high-temperature corrosion phenomena to be described in the remainder of this book. 

Hot corrosion refers to corrosion between a metal-oxide and a molten salt deposit. It occurs at the solid-gas interface. Molten salts are extremely corrosive and their presence increases the rate of corrosion by two orders of magnitude when compared to high-temperature corrosion at similar temperatures and conditions [27—29]. They act as solvents, preventing the formation of a stable oxide, or they chemically react with the oxide layers. By transporting through, the salts may damage the protective oxide layers. Two different types of hot corrosion exist, namely. Type I and II. 

This chapter deals with a specific type of corrosion that occurs in dry and rather high-temperature environments. Hot ash corrosion accounts for about 60% of the failure occurring in coal-fired power generation stations in addition to the erosion by ash (Prakash et al. 2001). Fundamentally, high-temperature corrosion is caused by the action of molten species produced in situ on the construction materials of the heating system. 

IRRAS can be extremely useful for studying in situ the corrosion and anticorrosion mechanisms [295]. For example, in order to understand high-temperature corrosion processes on AISI type 304 stainless steel, Guillamet et al. [284] measured the spectra by IRRAS of a steel plate exposed for 1 min to air at high temperatures. Comparison with the vlo bands of a series of oxides indicated that the main product is a-Fc203 (not Fc304, as suggested earlier for corrosion of... 

Reactions depicting high-temperature corrosion can be most commonly classified as one of three types linear, parabolic, or logarithmic [/]. 

Table 2-1. Summary of the applications, processes, temperatures, and types of high temperature corrosion observed in industry .

The types of high temperature corrosion mentioned so far will be discussed in more detail in later sections. Before doing so, however, fire next section will deal with the aspect of high temperature corrosion protection and the fundamentals of oxide scale growth. The reason for this is that the key... 

Alloying to Resist Specific Types of High-Temperature Corrosion. .. 212... 

Table 5-4 summarizes the types of high-temperature corrosion which occur in various industrial processes (Lai et al., 1985). As can be inferred from this table, many high-temperature processes involve atmospheres which contain oxidants in addition to O2, H2O, or CO2 the most prevalent of these are sulfur, carbon, nitrogen, and chlorine. The corrosive effects of these additional oxidants depend on the oxygen potential in the atmosphere, which is measured by the equilibrium Pq. In relatively high- multi-oxidant atmospheres, such as those resulting from the combustion of hydrocarbon fuels with excess air, the corrosive effects tend to be minimal or even beneficial. An example of a beneficial effect is that of traces of sulfur in the combustion environment of a reheat furnace which acts to slow the oxidation rate of steel as a result of a surface-poisoning effect (Lee, 1997). 

The variables to be considered here include both the type of ceramic and the environment to which it is exposed. Non-oxide ceramics include borides, nitrides, and carbides. Most high temperature corrosion environments contain oxygen and hence the emphasis of this chapter will be on oxidation processes of the type (Shaw et al., 1987 Jacobson and Opila, 1999) ... 

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 Tanunaim and, independently, by Pilling and Bedworth. 

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