Ф 03-03 Kasymova Zh. Zh

28.2. Reading Passage
Corrosion is deterioration of intrinsic properties in a material due to reactions with its environment. It is the oxidation of metals reacting with water or oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemical corrosion. This is commonly known as rust. This type of damage usually affects metallic materials, and typically produces oxide(s) and/or salt(s) of the original metal. Corrosion also includes the dissolution of ceramic materials and can refer to discoloration and weakening of polymers by the sun's ultraviolet light.
Corrosion of steel pipelines is a natural process and steel pipes, when buried, invariably suffer from external corrosion unless adequately protected. Pipelines are usually coated to protect the external surfaces of the steel pipe against corrosion. There are essentially two methods: impressed current method and sacrificial anode method. For onshore pipelines it is common to use an impressed current method. In this system, a DC (direct current) is supplied to the pipeline and is made to flow between the pipe and an anode ground bed via the soil. The current is adjusted to generate higher driving potentials in the pipe than those existing naturally in the corrosion cell. This neutralizes or reverses the effects of corrosion. This system requires transformer stations and is usually monitored by output voltage readings. A sacrificial anode method, on the other hand, relies upon the installation of anodes on or near the pipeline. The pipeline becomes the cathode of the system and the anodes, which corrode are sacrificed to arrest corrosion of the pipeline. When water is present on the transported fluid, corrosion of the internal pipe surfaces can also occur.
Water may be present, either alone, or in combination with CO2, H2O, O2 or other salts. The severity of corrosion depends upon the operating temperature, pressure, conductivity, soil condition, pH and fluid velocity and composition. Corrosion control measures include water removal and drying, chemical injection and corrosion allowance on wall thickness. Wall thickness – the most important element in pipeline mechanical design is the determination of pipeline wall thickness. Wall thickness is a function of the pipeline maximum allowable operating pressure and the yield strength of the steel pipe used. Operating pressure and wall thickness determine the number and locations of pump or compressor stations along the pipeline. If a higher pipeline operating pressure is chosen, the power at each station can be greater, and the stations can be farther apart.