Boiler tubing oxidation, surface

Oxygen is a particularly reactive element. Actually, oxygen is a very potent gas and reacts aggressively with exposed metal surfaces to form oxides. Of all the corrosive substances encountered in a process plant, few exceed oxygen in reactivity with steel pipes. If a considerable amount of oxygen is left in boiler feedwater, interior corrosion of the boiler tubes will be rapid. The bulk of the oxygen, or dissolved air, is stripped out of the boiler feedwater in a deaerator. 

The initial surface composition of boiler tubing, prior to its installation will have an important impact on the amount and type of activated corrosion products in an aqueous reactor coolant. Consequently, the type of thermal pre-treatment the tubing undergoes, for example, for mechanical stress release,will affect the surface oxide film, and ultimately, the corrosion behavior. This particular work has been directed toward characterization of surface oxide films which form on Inconel 600 (nominal composition 77% Ni, 16% Cr, 7% Fe, — a tradename of Inco Metals Ltd., Toronto Canada) and Incoloy 800 (nominal composition 31% Ni, 19% Cr, 48% Fe 2% other, — a tradename of Inco Metals Ltd., Toronto, Canada) heated to temperatures of 500-600°C for periods of up to 1 minute in flowing argon. These are conditions equivalent to those experi enced by CANDU(CANadian Deuterium Uranium)ractor boiler hairpins during in situ stress relief. 

When severe oil contamination requires a boiler tube to be taken offline, the oil must be removed from the boiler s heat-transfer surfaces. This can be accomplished by alkaline boilout. If this is insufficient, an oxidizer must be applied. 

Figure 5 shows a comparison of van der Waals and the gravitational forces on small ash particles as these approach a collecting surface. Plots A and B indicate that the sub-micron sized particles are readily held on a surface by van der Waals forces. The capture of small particles of ash on boiler tube is further enhanced by surface irregularities of oxidized metal (19). Also, it has been suggested that electrostatic attraction forces enhance the transport and retention of sub-micron sized particles on steel probes inserted in the flue gas of coal fired boilers (7,20). A layer of... 

Ash deposits on boiler tubes can be keyed to the surface of metal oxide by mechanical and chemical bonds. Mechanical bonding is enhanced by extending surface at the interface as shown in Figure 6a. Boiler tubes are not polished and thus have an extended surface that is further increased by oxidation and chemical reactions between the oxide layer and ash deposits. It is therefore evident that a comparatively rough surface of boiler tubes constitute an anchorage for keying ash deposits to the heat exchange elements. 

Under certain corrosive conditions, many metals form covering layers. If these are sufficiently dense, they act as protective films against the corrosive removal of the material. An example of this is the protective layer of iron oxide formed in unalloyed or low-alloy boiler tubes. Corrosion with erosion is understood as the combined action of mechanical surface removal and corrosion. With some soft and loose layers, the shear forces obtained with pure flowing liqnids at medium flow velocities are sufficient to damage the protective layer without the involvement of abrasive solid particles. Where drop impingement or cavitation is involved, the mechanical removal of material is understandable. 

Oxidation of sulfur dioxide to sulfur trioxide occurs mostly in flames where (transient) atomic oxygen species are thought to be prevalent by interactions of hydrogen atoms with oxygen and by interactions of carbon monoxide with oxygen and therefore may not occur in the stoichiometric manner shown earlier. The process can, however, be catalyzed by the ferric oxides that form on boiler tube surfaces and show excellent catalytic activity for sulfur dioxide oxidation at approximately 600°C (1110 F), that is, at temperatures that occur in the superheater section of a boiler. 

With a few exceptions, coatings and linings are not used on the water and steam sides. In an EPRI project, about 50 turbine blade coatings have been evaluated, but none of these are being routinely applied. To reduce steam side oxidation in reheaters and superheaters, chromizing and chromating have been developed but these treatments are also not routinely applied. There is little use of composite materials with the exception of condenser tube sheets, which could be made of explosively clad stainless steel or titanium on carbon steel, and of the surfaces in the primary cycles of nuclear units where carbon or low alloy steels are protected by weld-deposited stainless steels. In pulp mill black liquor recovery boilers, stainless steel clad boiler tubes are often used. 

Excessive corrosion of the condensate system can lead not only to costly equipment failure and increased maintenance costs, but can also cause deposition of metal oxide corrosion products on boiler heat transfer surfaces if the condensate is recovered as feedwater. Metal oxide deposition on boiler heat transfer surfaces will result in lower fuel to steam efficiency and higher fuel costs. The deposition may also lead to tube failure due to long-term overheating. 

Ail of the kinetic tests were conducted by using the same batch of tubing obtained from an operational northeastern U.S. drum boiler. The tubes were machined to a constant OD (3.34 cm) and length (4.74 cm) with a total surface area of 102.8 cm. The interior surfaces were coated with about 1 g of oxide and 50 mg of Cu. The composition of the scale as determined by x-ray diffraction and the chemical composition of the boiler tube are shown in Table 2. In each of the dissolution tests, two AISI 1010 carbon steel coupons in a PTFE mount were added to give a total wetted surface area of 190 cm. The chelating agents tested were obtained from commercial sources and were used without further purification. The chelants and their iron formation constants are described in Table 3. 

Nonchloride species are probably rapidly converted to oxides (Na20, K2O) on leaving the flame front. The volatile alkalies may condense on the surfaces of fly-ash particles carried by the flue gas or on cooler boiler surfaces. Wibberly and Wall [j ] performed drop-tube experiments in which silica particles were exposed to synthetic combustion gases containing sodium at temperatures of 1200 to 1600°C. Sodium silicate layers ranging in thickness from 0.03 to 0.3 pm were observed on the particle surfaces, and sintered deposits formed rapidly on stainless steel probes inserted into the lower part of the furnace. Such alkali-silicate layers are molten at the temperatures of interest. The thickness of the sodium silicate layers was decreased by a factor of three when the sodium was introduced in the form of NaCl, rather than in chlorine-free forms. 

Weak Adhesive Bond of Deposits and Austenitic Steels. The adhesive bond between the austenitic steel surface and ash deposit is relatively weak as a result of the composition and thermal incompatibility of the steel oxide and the silicate material. The temperature fluctuations on changeable boiler load conditions can cause sufficiently high thermal stresses for deposit to skid off the austenitic steel tubes. 

Depending upon the source of the cmde oil, the refining process, and the fuel grade, varying amounts of sulfur may be present in different types of fuel oils. Combustion of the sulfur containing fuel oils produces sulfur oxides, which pollute the atmosphere and cause corrosion problems in boiler equipment. They may form sodium and vanadium complexes, and such deposits on external surfaces of superheater tubes, economizers, and air heaters cause equipment corrosion and loss of thermal efficiency. 

With other very exothermic reactions, such as air oxidation of aromatic hydrocarbons, the number of beds would have to be uneconomically large to limit the temperature increase per bed, so that the multitubular reactor is definitely preferred Cooling the reactor with. he incoming reactant would be insufficient, however, and require too much heat exchanging surface. Such reactors are therefore cooled by means of circulating molten salts which in turn give off their heat to a boiler. The phthalic anhydride synthesis reactor shown in Fig. 11.3-6 [6] may contain up to 10,000 tubes of 2.5 cm inside diameter. The tube diameter has to limited to such a small value to avoid excessive overtemperatures on the axis, a feature that is discussed later in this chapter. 

A common problem in boilers is the occurrence of calcium oxide build-up on the heating elements. This is not a corrosion problem in itself, because it is caused by a chemical reaction in the water at high temperatures. However, a scale deposit present on a metal surface may cause corrosion under the deposit. This type of underdeposit corrosion can be aggravated when corrosive species such as sulfides and/or chlorides are present in the water. While scale deposits reduce the thermal conductivity of the steel, and thereby increase energy costs, corrosion of the heating element can lead to a catastrophic tubing failure, which requires costly repairs. 

The boiler has a water-circulating tube wall (waterwall) construction. A layer of smelt freezes on the fireside of the wah due to coohng from the water in the tubes, and this frozen smelt provides a barrier between the tube and the reducing and/or oxidizing gases. Gas composition has been implicated as a major determinant of the corrosion rate on the fireside surfaces. The lower waterwaU is the most critical component. Unlike conventional boilers, a waterwall tube leak cannot be tolerated in a recovery boiler, since a smelt-water reaction has the potential for a catastrophic explosion. 

Superheater/reheater corrosion in fossil-fuel-fired boilers is caused by the deposition of alkali sulphates on to the tube surface The corrosion rates increase rapidly at temperatures above 600° C as the sulphates become molten. These molten sulphates contain free SO3 which dissolves the protective oxide to form Fe-based sulphates. The corrosivity of the molten sulphates depends strongly upon their melting points, which are themselves strongly dependent upon the ratio of Na and K in the deposits. 

The wet oxidation process uses water molecules instead of dry oxygen as the oxygen source to oxidize sUicon. As a matter of fact, water molecules contact the silicon furnace in a normal wet oxidation process. Water molecules dissociate at high temperature and form hydroxide (HO) prior to reaching the silicon surface. Hydroxide has faster diffusion mobility in silicon dioxide than pure O2, which explains why wet oxidation has a higher growth rate than dry oxidation. Wet oxidation is used to form thick oxides such as the LOCOS oxide, masking oxide and field oxide. As shown in Fig. 7, several systems have been used to deliver water vapor into the process tube. The boiler system is the simplest setup which vaporizes ultra-pure water then drives the water vapor in to the process tube via heated gas lines. However, it is dif-... 

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