External Corrosion Prevention

The generic coating systems commonly used for cross country pipelines are as follows  

Asphalt mastic is a heavy duty system that is shop-applied in thicknesses up to 1 in. (25.4 mm). Although asphalt mastics perform well, they are very costly. Their use is usually limited to river crossing and swamp areas. Coal-tar enamel has more than 50 y of successful performance underground, but it is subject to cracking when the temperature is below 20° F (-7°C), and there are environmental constraints. The EPA has shut down applicators in the middle of production because of air pollution. These problems rule out asphalt mastic and coal-tar enamel as feasible materials for most pipelines. In addition, coal-tar enamels are often ruled out because of long-term deterioration. Asphalt enamel is generally considered inferior to coal-tar enamel therefore, its use is limited. Polyvinyl chloride tape is becoming obsolete because of the superior performance of polyethylene tape. 

Polyethylene tape, polyethylene extrusions, and fusion-bonded epoxy are the remaining viable choices. All have been applied successfully, and all have had problems. To obtain a satisfactory coating, careful attention must be given to the following  

Polyethylene tape. There are more than 30 y of experience with polyethylene tape. It has been applied over the ditch, at the railhead, and in the shop. Surface preparation, an indispensable element in proper coating adhesion, is very difficult in an over-the-ditch operation. The generally poor performance of over-the-ditch tape applications has caused most pipeline companies to abandon them. Occasionally, however, one still hears of a pipeline company that has used its own highly trained crews and has obtained a good over-the-ditch application. 

Cathodic disbondment is associated with over-the-ditch applications of tape and, to a lesser extent, with shop- or railhead-applied tape. Moisture penetration into the overlap is also a potential problem. For this reason, and to minimize damage during handling, a double wrap is preferred. The potential problems with polyethylene tape have been avoided in many cases, and there is a long history of successful applications. 

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Provide purging for electronics to prevent external corrosion... 

The bath is normally at a temperature in the range 620-710°C, depending on whether the coating material is an aluminium-silicon alloy (for use in high-temperature conditions) or pure aluminium (for corrosion prevention). It is heated by inductors, by resistance heaters or by an external flame. The pot will usually be refractory lined unless cast-iron pots are needed to ensure adequate heat transfer from an external flame. As molten aluminium is extremely aggressive towards ferrous metals, replacement of cast-iron pots is fairly frequent. Refractory-lined pots obviously do not have this drawback, although the bath hardware, in particular the sinker roll and support mechanism, will still be attacked and need replacement at intervals. 

Besides the risk of external damage, another implementation problem for C02 pipelines is corrosion prevention. Corrosion can be caused by the presence of water in the C02, which generates the corrosive acid H2C03. Therefore, it is important to transport C02 in a dry condition, which is achieved by the dehydration after the initial compression. 

When insulation is used to fireproof steel that will be at or below ambient conditions, caution should be taken to prevent external corrosion of the steel caused by condensation when the steel is below the atmospheric dew point. 

Condensate preheater (economizer) temperature control is unique to HRSGs and is not used on conventional fired boilers. Its purpose is to (1) prevent external condensation on the tubes of the preheater, which could cause external corrosion and (2) to obtain the correct inlet temperature to the deaerator. 

Corrosion protection refers to a situation in which all the inherent factors to prevent corrosion have been optimized and external intervention is deemed necessary to minimize the corrosion that is beyond the scope of corrosion-preventative factors such as metallurgical, design and life prediction analysis. Corrosion protection can be achieved by (i) addition of inhibitors and (ii) use of protective coatings. 

The most economical way to prevent external corrosion of underground piping is to coat the pipe and use cathodic protection. ... 

The painting areas of a ship are shown in Figure 11.1. The main requirement of marine coating systems is corrosion prevention. Detailed requirements vary with the particular internal or external area (e.g., chemical resistance in cargo tanks resistance to seawater in ballast spaces heat resistance in engine rooms impact and abrasion resistance on boottoppings, external hulls, and decks cosmetic qualities on superstructure and topsides). 

Recognize corrosion problems in materials used at the site and make monitoring a normal part of the operation. Sour oil and gas operations are often conducted under high pressure and corrosive conditions. Therefore, in addition to temperature and pressure considerations, system designs for the wellhead, downhole equipment, and pipeUnes must have features to minimize the effects of corrosion and prevent an accidental release of H2S. Corrosion-inhibiting fluids can be used to prevent internal corrosion and cathodic protection can be used to prevent external corrosion. Also, during extended periods of shut-in and injection into pipelines, inhibitor applications may be beneficial. 

One company s pipeline team, comprised of a first-level supervisor and other personnel responsible for operating and maintaining the pipeline system, established an objective of developing a more effective approach to serious-incident prevention based on the eight-element process model. The team s assessment of risks included evaluating causes of past U.S. pipeline accidents. Department of Transportation (DOT) data (Fig. 18.5) identify third-party damage as the most frequent cause of pipeline incidents, followed by defective equipment or repair, external corrosion, internal corrosion, and operator error. The team s thorough evaluation of potential risks has helped ensure that the appropriate tasks and safe... 

Corrosive chemicals and external exposure can cause tank failure. Materials of construction should be chosen so that they are compatible with the chemicals and exposure involved. Welding reduces the corrosion resistance of many alloys, leading to localized attack at the heat-affected zones. This may be prevented by the use of the proper alloys and weld materials, in some cases combined with annealing heat treatment. 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

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