Corrosion high-temperature

The term high temperature is relative. In practical terms, it usually means a temperature about 35% of the absolute melting range of a given metal or alloy (or up to 60% for some nickel- and cobalt-based alloys). For the conventional austenitic grades of stainless steel, such as type 304, this would be any temperature above 1050°F (575°C). 

In general, the straight chromium and austenitic varieties of stainless steel have an upper limit of about 1600°F (870°C), except the more highly alloyed grades ( 20% Cr) that will tolerate slightly higher temperatures— about 2000°F (1100°C) in continuous service. 

Changes can occur in the nature of the surface film of stainless steel when exposed to high temperatures. For example, at mildly elevated temperatures in an oxidizing gas, a protective oxide film is formed. In an environment containing sulfur-bearing gases, the film will be in the form of sulfides that may also be protective. 

In more aggressive environments, with temperatures above 1600°F (871°C), the surface film may breakdown with a sudden increase in scaling. Depending on alloy content and environment, the film may be self-healing for a period of time followed by another breakdown. 

Under extreme conditions of high temperature and corrosion, the surface film may not be protective at all. Based on this, service tests are recommended. 

High-temperature corrosion problems are experienced mainly by boilers firing residual fuel oils. The corrosion is due primarily to the presence of vanadium, sodium, and sulfur compounds in the fuel oil (vanadium can be as high as 500 ppm, Na 300 ppm, and sulfur 40,000 ppm). During combustion the presence of these compounds react and give rise to complex low-melting-point materials that deposit on heat-transfer surfaces and supporting structures see reactions (17.1)—(17.3) (Niles and Sanders, 1962)  

High-temperature corrosion can be combated by the introduction of MgO into the boiler where it reacts with vanadium pentoxide to form magnesium ortho vanadate see reaction (17.8)  

Magnesium orthovanadate has a melting point of around 1190°C, which is considerably higher than that of sodium-vanadium compounds. This means that magnesium orthovanadate has a lesser tendency to deposit and, when it does, it is loose and powdery and easily removed using the soot blowers (Salooja, 1972). 

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. 

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-temjjerature 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. 

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Most common process temperatures are in the range 450 to 850°C or higher (Fig. 15.1). Materials of construction must withstand excessive metal loss by scale formation from oxidation and from penetration by internal oxidation products that could reduce the remaining cross-sectional area to a level that cannot sustain the load-bearing requirements. The component will then yield and may swell or distort. In some cases the internal fluid pressures can be sufficient to burst the component releasing hot, possibly toxic or flammable fluids. 

There are several types of corrosion reactions and the specific type dictates the kind of protection needed against it  

A few high-temperature corrosive properties of ceramics are mentioned here as examples. The book by Samsonov and Vinitskii has many details on high-tempera-ture reactions with various ambients. Table 6.6 compares the corrosion resistance of several refractories in combustion gases. 

Silicon carbide (SiC) oxidizes slowly in air at a temperature of 1400°C. After 200 h the weight increase is 9.2 mg/cm. It is also highly resistant to metal melts and vapors, acids, and alkalis. It is often used in pump parts, heat exchangers, and abrasives, and is a useful high-temperature semiconductor. 

Boron carbide (B4C) is also surprisingly oxidation-resistant. Its weight decrease after 100 h in air at 1200°C is llmg/cm. It is an abrasive used in sandblasting nozzles, chemical vessels, ignitrons (semiconductor sparkers), thermoelectric energy converters, and nonlinear electronics. It is considered one of the most suitable 

The perovskite lanthanum chromite (LaCr03) is one of the exceptional ceramic compounds that are chemically very resistant to both oxidizing and reducing ambients. Moreover, being an electronic conductor, it is eminently suitable as a bipolar connector in solid oxide fuel cells. It is evident that the thermal expansion coefficients of the different components in a fuel cell (electrolyte, electrodes, bipolar connector) must be closely matched. Doping the chromite with strontium or magnesium ions is necessary to increase its electronic conductivity as well as its sinter activity. 

1 Standard free energy of formation versus temperature 

In characterizing HTC, X-Ray Diffraction is a very useful and reliable technique for indexing patterns of phases leading to phase identification in the corrosion product known as surface scale. In fact, this high temperature phenomenon is different from the mst layer formation (in the presence of moisture) since it virtually occurs in dry gaseous environments or molten salts, hi certain cases, HTC may be an internal oxidation process when oxygen diffusion is faster than the surface oxidation rate. 

Most experimental data available in the literature is based on weight gain per unit surface area. However, weight gain and thickness reduction or penetration are adequate parameters for assessing HTC. For instance, measurements of metal thickness reduction and oxide layer thickness increments are very important because the former is related to structure strength. 

One of the harmful deposits has been sodium sulfate. In the case of heat engines, ingestion of sodium takes place through intake air or fuel. Fuel contains sulfur and, on burning, it forms sulfur oxides. The reaction can be represented by Equation 5.14. 

Hot corrosion is the term used for sodium-sulfate-induced corrosion. Components made of SiC and SigN4 form a layer of Si02 on their surfaces on exposure to oxidizing atmosphere. This thin layer acts as a protection that prevents the corrosion of the substrate. Previous works have shown that, for silicon-based ceramics, the primary mode of attack is dissolution of the silica scale [54-56]. 

Sodium sulfate forms readily on heat engine parts, according to Equation 5.14. It is a highly stable molecule. Once formed, its corrosion reaction occurs above its melting point. The different reactions leading to corrosion are represented by the following equations  

These reactions indicate that, for the corrosion of the Si02, Na20 is required. Hence, any reaction that gives rise to the formation of Na20 should cause corrosion. This is possible if sodium carbonate and sodium chloride are present. The corresponding reactions are given in Equations 5.18 and 5.19. 

The corrosion processes discussed so far involve three steps—deposition, oxidation, and dissolution. The extent of corrosion in the life of a component will be decided by the kinetics of each these steps. In kinetics, one of the steps will be the slowest, and it will decide total rate of the whole process. 

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Cullis A G, Canham L T and Calcott P D J 1997 J. Appl. Phys. 82 909 Schmuki P, Erickson L E and Lockwood D J 1998 Phys. Rev. Lett. 80 4060 Kofstad P 1988 High Temperature Corrosion (London Elsevier)... 

Lai G Y 990 High-Temperature Corrosion of Engineering A//oys (Metals Park, OFI ASM International)... 

Saito Y, Oenay B and Maruyama T (eds) 1992 High Temperature Corrosion of Advanced Materials and Protective Coaf// gs (Amsterdam North-Flolland)... 

Shores D A, Rapp R A and Flou P Y (eds) 1997 Proc. Symp. on Fundamental Aspects of High Temperature Corrosion vol 96-26 (Pennington, NJ Electrochemical Society)... 

Mrowec S and Werber T 1978 Gas Corrosion of Metals (Washington, DC National Bureau of Standards) Rapp R A (ed) 1983 High Temperature Corrosion—NACE 6 (Flouston, TX NACE)... 

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. The rate of oxidation of magnesium adoys increases with time and temperature. Additions of berydium, cerium [7440-45-17, lanthanum [7439-91-0] or yttrium as adoying elements reduce the oxidation rate at elevated temperatures. Sulfur dioxide, ammonium fluoroborate [13826-83-0] as wed as sulfur hexafluoride inhibit oxidation at elevated temperatures. 

Ruthenium and osmium have hep crystal stmetures. These metals have properties similar to the refractory metals, ie, they are hard, britde, and have relatively poor oxidation resistance (see Refractories). Platinum and palladium have fee stmetures and properties akin to gold, ie, they are soft, ductile, and have excellent resistance to oxidation and high temperature corrosion. 

Tips of platinum, platinum—nickel alloy, or iridium can be resistance-welded to spark-plug electrodes for improved reHabiHty and increased lifetime. These electrodes are exposed to extremely hostile environments involving spark erosion, high temperature corrosion, thermal shock, and thermal fatigue. 

Diffusion alurninide and sihcide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 10 /yr in North America and about 50 x 10 worldwide. 

At very high and very low temperatures, material selection becomes an important design issue. At low temperatures, the material must have sufficient toughness to preclude transition of the tank material to a brittle state. At high temperatures, corrosion is accelerated, and thermal expansion and thermal stresses of the material occur. 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

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. 

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. 

Harada, Y. High Temperature Corrosion in Heavy Oil Firing Boilers, Proc. Fifth Int. Cong, on Metallic Corrosion (Houston, TX National Association of Corrosion Engineers, 1974). 

In the design of the plate heat exchanger, fouling due to coking is of no significance, since the unit cannot be used at such high temperatures. Corrosion is also irrelevant. 

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... 

Rapp, R. A. (Ed.), High Temperature Corrosion, NACE, Houston, Texas (1983)... 

The extent to which low alloy steels react to high temperature corrosive environments is the subject of this chapter. In view of the commercial importance of these steels, the published literature on this topic is extensive and is being continually enlarged. The reader is encouraged to refer to the many excellent papers and current issues of the journals, referenced at the end of the chapter, for more detailed and contemporary information on the topic. 

Mrowec et examined the resistance to high-temperature corrosion of Fe alloys with Cr contents between 0.35 and 74 at% Cr in 101 kPa S vapour. They found that the corrosion was parabolic, irrespective of the temperature or alloy composition, and noted that sulphidation takes place at a rate five orders of magnitude greater than oxidation at equivalent temperatures. At less than 2% Cr, the alloys formed Fe, j.,S growing by outward diffusion of Fe ions, with traces of FeCr2S4 near the metal core. 

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