Scales high-temperature corrosion

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 %. 

Corrosion Resistance Possibly of greater importance than physical and mechanical properties is the ability of an alloy s chemical composition to resist the corrosive action of various hot environments. The forms of high-temperature corrosion which have received the greatest attention are oxidation and scaling. 

In selecting metals and alloys as materials of construction, one must have knowledge of how materials fail, for example is, how they corrode, become brittle with low-temperature operation, or degrade as a result of operating at high temperatures. Corrosion, embrittlement, and other degradation mechanisms such as creep will be described in terms of their threshold values. Transient or upset operating conditions are common causes of failure. Examples include start-ups and shutdowns, loss of coolant, the formation of dew point water, and hot spots due to the formation of scale deposits on heat transfer surfaces. Identification and documentation of all anticipated upset and transient conditions are required. 

Since for protection against high-temperature corrosion, slow-growing, dense, well-adherent oxide scales are of greatest importance, later on the research concentrated on the growth and adherence of such scales, that is, mainly Cr203 and... 

The integrity of the floor liner in the case of thermal expansion of its total area was confirmed at the Monju design stage however, an examination of its integrity in the case of localized temperature rises during medium-scale and small-scale leaks will now also be performed. Further work is requited on damage due to high temperature corrosion by combustion residues. 

The corrosion of AISI 316L in return water proceeds in short intervals. The steel has some tendency to passivate, but the reaction product layer spalls off every time (Fig. 6.2). The shape of the curve resembles that of the semilinear scale growth seen in high temperature corrosion, i.e. short successive... 

Parabolic Reaction Kinetics Parabolic reaction kinetics describe high-temperature corrosion where the rate determining step is diffusion. This is often associated with protective scales and with internal penetration by such corrosive species as oxygen, carbon, or sulfur in certain materials. 

High-Temperature Corrosion on carburized steels that forms a porous scale of several iron oxide phases. 

Chapter 10 deals with high temperature corrosion, in which the thermodynamics and kinetics of metal oxidation are included. The Pilling Bedworth Ratio and Wagner s parabolic rate constant theories are defined as related to formation of metal oxide scales, which are classified as protective or nonprotec-tive. 

Small-scale experiments are being conducted to develop unit operations for these processes. Containment vessel materials are being developed for use in the process steps, and testing is being conducted to determine their performance in the high temperature, corrosive environment. Experiments are also being done to develop methods for recycle of the... 

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... 

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