Atmospheric corrosion-effect temperature

In previous designs of SCWO reactors, special materials such as Hastelloy, Inconel or Gold1 were needed to withstand such drastic operation condition of temperature and pressure to reduce corrosion effects imposed by the oxidizing atmosphere... 

High mechanical and electrical resistance, wide operation temperature range (from 260 toi - -260°C), low friction coefficient. Surpasses all known materials in chemical resistance. Resistant to atmospheric, corrosion and radiation effects. 

Atmospheric corrosion involves various forms of corrosion effects at ambient temperature in which the Earth s atmosphere is the corrosive environment. Atmospheric corrosion has been recognized for several thousand years and the atmosphere is the most abundant environment to which solid materials are exposed. Hence, its implications in our society are enormous and range from bridges, elevated highways, railway and subway systems, aircraft, automobiles, and buildings, to industrial processes, electronic components and systems, to artistic or historic objects, such as statues and monuments. In the United States, for example, the total costs for all forms of corrosion have been estimated to be around 1000 US per capita per year. A substantial part of that amount is due to consequences of atmospheric corrosion. 

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material, corrosion products and possibly other deposits, a surface layer of water (often more or less polluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, which diffuses through the surface layer of water and deposits. As shown in Section 6.2.5, the thickness of the water film may have a large effect, but it is more familiar to relate atmospheric corrosion to other parameters. The main factors usually determining the accumulated corrosion effect are time of wetness, composition of surface electrolyte, and temperature. Figure 8.1 shows the result of corrosion under conditions implying frequent condensation of moisture in a relatively clean environment (humid, warm air in contact with cold metal). 

Effect of Temperature. Temperature plays an important role in atmospheric corrosion. There is normal increase in corrosion activity which can theoretically double for each 10° increase in temperature. As the ambient temperature drops during the evening, metallic surfaces tend to remain warmer than the humid air surrounding them and do not allow condensation until some time after the dew point has been reached. As the temperature begins to rise in the surrounding air, the lagging temperature of the metal structures will tend to make them act as condensers, maintaining a film of moisture on their surfaces [60-63]. 

Atmospheric corrosion of zinc is roughly proportional to the time of wetness in a particular location, a point emphasized by Mikhailovskii et al. (1986) for areas of the former Soviet Union, provided the nature and quantity of environmental pollution do not change a high relative humidity, which can cause condensation, increases corrosion. Rain obviously increases time of wetness, but it can have an indirect beneficial effect by removing corrosive materials. In practice, time of wetness is often taken as the time when relative humidity (RH) exceeds 80% and the temperature is above 0°C. Thin layers of solutions (except acids) are more corrosive than bulk solutions (Mansfield and Tsai, 1979). The general consensus is that the significance of atmospheric humidity in the corrosion of zinc is related to the conditions that may cause condensation of moisture on the metal surface and to the frequency and duration of the moisture contact. 

Salts present on the surface due to pollution reduce the saturation vapor pressure. They also increase the ionic conductivity of the electrolyte that forms on the metal surface, thus favoring corrosion even more. Table 8.11 gives the relative humidity in equilibrium with solutions of different salts at their saturation concentration. The given values demonstrate that in presence of certain salts condensation can take place at quite low values of relative humidity. In the laboratory, one uses this effect to control the relative humidity during atmospheric corrosion experiments one exposes the sample to air in a chamber that contains a salt solution of known concentration. The saturation vapor pressure then determines the humidity of the atmosphere for any given temperature. 

Cabinet tests are the most commonly used laboratory tests in the automobile industry. These have been developed to simulate the effects of atmospheric corrosion 44. Normally, a test chamber is used and the desired environment is introduced under controlled conditions. A list of these tests is given in Table 6. The simplest of these is the humidity test whereby the temperature and relative humidity within a chamber are controlled. A condensing humidity chamber, which operates at 100 % relative humidity and 38°C, provides... 

Atmospheric corrosion depends not only on the moisture content present but on the dust content and the presence of other impurities in the air, all of which have an effect on the condensation of moisture on the metal surface and resulting corrosiveness. The temperature of the air can also be a factor. 

Carbon monoxide is a colorless, odorless, flammable toxic gas. Liquid carbon monoxide is a cryogenic liquid, which exists at a temperature of -313°F (-192°C) and atmospheric pressure. It becomes a flammable vapor upon addition of heat. If inhaled, concentrations of 0.4 percent in air prove fatal in less than 1 hour, while inhalation of high concentrations can cause sudden collapse with little or no warning. Pure carbon monoxide has a negligible corrosive effect on metals at atmospheric pressures. Impure carbon monoxide, containing water vapor, sulfur compounds, or other impurities causes stress corrosion to ferrous metals at elevated pressures. 

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