Because of high operating temperatures, harsh environmental conditions, and highly abrasive materials, corrosion is a major cost to the power generating industry.
The majority of electric power worldwide is generated by fossil-fuel and nuclear supply systems. Currently, in the United States, coal-fired plants contribute approximately 46% of the nation’s electric power, according to recent government estimates, while nuclear and natural gas-fired plants each contribute about 21%. Combined, natural gas and coal account for the largest share of total world electricity generation, at more than 60% of global supply.
Conventional steam generator plants, or 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 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.
Meanwhile, there are several types of nuclear fuel steam plants. In some designs, 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 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 nuclear plants, 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. 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 from irradiated water.
The power industry depends on technological expertise as well as a wide variety of applications, including coatings for corrosion resistance. For 25 years, the NACE International Coating Inspector Program has set the standard for inspections in the protective coatings industry, and it is the world’s most recognized coating inspector certification program.
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