High-temperature corrosion materials behavior

The high-temperature corrosion behavior of these engineering alloys by aU possible kinds of attack has been described in a book by Lai (1), which gives extensive information on the mechanisms and kinetics of corrosion and the necessary background for the material s selection in the industry. 

The corrosion rate constant kp is actually the most important parameter for describing the resistance of a material against high-temperature corrosion. If kp is low, the overall high-temperature corrosion rate is low and the metal consumption occurs at a low rate. In this case the surface scale forms a good diffusion barrier and prevents access of the corrosive species from the reaction environment to the metal. If the value of kp is high, this means that the consumption rates of the metal are high and a situation of nonprotective behavior exists. 

Since both HI and H3PO4 are reducing in nature, it is possible to use materials that are common to both in the iodine separation reaction (see table 4.3). The material of construction used to fabricate the I2 separation reactor must be able to resist the combination of HI and H3PO4. Preliminary test results show that the corrosion behavior of the various materials tested in the HI,-H3P04 acid mixture is similar to that in HI, at high temperatures, with Ta and Nb alloys and SiC-based materials the most promising construction materials candidates. 

Hot gas corrosion behavior of various silicon-based non-oxide ceramic materials (e.g., silicon carbide, silicon nitride, etc.) can vary widely depending on the stoichiometry, structure and sintering aids. Silicon carbide materials exhibit excellent corrosion resistance towards sulfur-containing atmospheres even at a high temperature around 1,400X [Fdrthmann and Naoumidis, 1990]. 

Priority tasks have been the selection of high temperature, high strength alloys and testing of candidate commercial alloys. The material characteristics investigated included creep behavior, fatigue properties, structural stability, and corrosion resistance [32]. 

There are numerous electrochemical approaches to quantitatively assessing the corrosion resistance of bare and painted metallic materials immersed in conductive electrolytes, and they can be very sensitive and relatively easy and rapid to perform (Figure 3.8), either in the laboratory or in the field for corrosion rate monitoring. However, in some environments such as those with high-temperature gases or high-resistivity electrolytes that do not follow classic electrolyte behavior, standard electrochemical approaches either are extremely difficult or fail completely. An important research opportunity is to develop sensitive, quantitative, and accurate methods for evaluating corrosion resistance in these environments. 

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