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Corrosion Basics: Steam Generation in Power Plants

The greatest use of high-temperature water and steam is in electrical power generation. Historically, fossil fuels (i.e., wood, coal, gas, and oil) were used almost exclusively to heat water and make steam until the introduction of nuclear power steam generators in the second part of the 20th century. The two types of power plants have much in common, but are sufficiently different to be discussed separately. Corrosion considerations for fossil fuel steam plants and nuclear fuel steam plants are briefly summarized below.

Fossil Fuel Steam Plants

Conventional steam generator plants or more modern cogeneration steam plants consist of many sections operating at different temperatures and pressures. These include:

• Feedwater heaters with water inside and steam outside the tubes.

• A boiler (water inside the tubes, hot combustion product gases outside), where water is heated to high temperatures under pressure and is sometimes flashed to steam.

• A steam drum, wherein steam is formed from water and water is separated from the steam. (Note: the steam drum is omitted in certain once-through systems.)

• A superheater, where the steam is further heated to even higher temperatures.

• A turbine, where the steam expands against the vanes of a wheel to drive the turbine that generates electricity.

• A condenser, where the low-pressure steam is condensed to water and returned to the feedwater heaters.

In such plants, there are special corrosion problems in each of these sections. There are also special problems associated with exposure to hot combustion gases.

The materials used in high-temperature steam and water systems include steels, stainless steels (SS), and nickel-based alloys. Copper-based alloys are employed in the intermediate and low-temperature ranges. Aluminum alloys generally are not used because of their poor resistance above ~200 °C.

Nuclear Fuel Steam Plants

There are several types of nuclear fuel steam plants. In some designs, the heat from the nuclear reaction is removed by nonaqueous media, which then exchange heat with a water system to generate steam. Molten sodium, organic liquids, and helium gases have been used in this way. In other designs, an aqueous media (e.g., heavy water or pressurized light water) removes the heat of reaction, but the steam is generated in a heat exchanger. There also are designs (e.g., boiling water reactors) in which the reactor cooling water is used to generate steam directly.

Nuclear and conventional plants have many components in common: the boiler, steam drum, turbine, condenser, and feedwater heaters. In the nuclear plant, however, most of these components, with the possible exception of the condenser, are made of austenitic SS or some other nickel-chromium-iron alloy to minimize corrosion products in the system. Low corrosion is desired, because if corrosion products become radioactive by transport through the reactor core, they constitute a radiation hazard to personnel when deposited again in areas remote from the core. In addition, when a water system surrounds the core, whether to generate steam directly or indirectly, specific problems arise that are peculiar to irradiated water.

This article is adapted by MP Technical Editor Norm Moriber, Mears Group, from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 146-148; 155-156.

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