Atmospheric corrosion continued marine

Pitting can also occur under atmospheric conditions. The corrosion starts at the break and continues to undercut the coating, forming a rather heavy tubercle of hard rust or scale with the pit underneath the original metal. The corrosion products help to isolate the aggressive medium inside the pit. These are common in marine environments as well as various industries where strong corrosive conditions exist.26 Also, pits with their mouth open (uncovered) exist and are responsible for loss of thickness and can act as stress raisers. 

In 1993 epoxy- lysulphide based coatings were used to coat two 250 MW thermal power projects at Dhanu, about 100 km north of Bombay. The site is on the coast, less than 1 km from the sea and as such the nature of the local atmosphere is highly corrosive, with continuous sea breezes, and the soil is saturated with marine salt. The coating has shown satisfactory performance (Figure 10.7) (Morton International, undated c). This project was constructed under the control of Bombay Suburban Electricity Supply Co. Ltd (BSES). 

In a normal atmospheric exposure, a Iresh lead surface will slowly be oxidized into a thin, protective lead oxide, which halts further oxidation of the metal. The rate of formation of lead oxide is determined by the absorption of oxygen and water vapor into the lead. Such factors as industrial and marine pollution, humidity, temperature, and rainfall profoundly affect the aggressiveness of the atmosphere, and most metals suffer accordingly. However, the protective films formed of lead are so effective that corrosion is insignificant in most natural atmospheres. The extent of this protection is demonstrated by the survival of lead roofing and auxiliary products after hundreds of years of atmospheric exposure that may continue for a much longer time if these films are not damaged [2]. 

Waterfront structures are exposed to a variety of marine environments. The resistance of materials to each of these environments may vary considerably, as weU as appHcabil-ity of various forms of corrosion control in mitigating the anticipated corrosion. The waterfront environment can be divided into five exposure zones sediment, immersion, intertidal, splash/spray, and atmospheric. In most cases, a single type of material will be used for the sediment, immersion, and intertidal zones. In some cases another material may be used for the splash and spray and atmospheric zones of the structure. An example of this would be the use of a reinforced concrete deck over steel pilings. Due to differences in corrosion activity between these zones, the corrosion performance of many materials is substantially different when exposed to two or more of these zones. Figure 1, taken from Ref 4, shows the result of a classical experiment where the corrosion of a continuous strip of... 

Rain is not always corrosive. It may even have a beneficial effect by washing away atmospheric pollutants that have settled on exposed surfaces. This effect is particularly noticeable in marine environments. On the other hand, if the rain collects in pockets or crevices, it may accelerate corrosion by supplying continued wetness in such areas as shown in Fig. 9.6. 

Grade 2 also has good impact properties at low temperatiires and excellent resistance to erosion and to corrosion by seawater and marine atmospheres. Grade 2 can be used in continuous service up to 425 °C (800 °F) and in intermittent service up to 540 °C (1000 °F). 

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