Surface composition boiler tubing

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. 

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. 

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. 

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. 

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. 

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