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Liquid Process Piping Episode 4

Tham khảo tài liệu 'liquid process piping episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EM 1110-1-4008 5 May 99 Chapter 4 Metallic Piping Systems 4-1. General The metallic materials that are commonly used in liquid process piping systems can be categorized as ferrous ductile iron carbon steel stainless steel and alloys with iron as the principal component and non-ferrous alloys of nickel aluminum copper and lead. Metallic piping systems other than those addressed in this chapter are available e.g. zirconium 416 SS . Such materials may be used if cost and technical criteria are met. Applicable design principles from this manual are applied to use these materials. 4-2. Corrosion When metallic components are used corrosion of some type s will occur. US ACE policy requires that all underground ferrous piping be cathodically protected. Chapter 12 TM 5-811-7 and MIL-HdBK-1004 10 contain guidance pertaining to cathodic protection of underground pipelines. Conditions which promote corrosion are - contact between dissimilar metals which may become immersed in a conductive medium - exposure of piping to corrosive soils or water - high temperatures - low-velocity stagnant-type flow conditions - abrasive effects that may cause the surfaces of metals to be eroded - application of tensile stresses within a corrosive environment - highly acidic solutions combined with holes near metal-to-metal surfaces or near sealing surfaces and - any metals close to sources of atomic hydrogen. a. Theory of Corrosion Corrosion occurs by an electrochemical process. The phenomenon is similar to that which takes place when a carbon-zinc dry cell generates a direct current. Basically an anode negative electrode a cathode positive electrode electrolyte corrosive environment and a metallic circuit connecting the anode and the cathode are required for corrosion to occur. Dissolution of metal occurs at the anode where the corrosion current enters the electrolyte and flows to the cathode. The general reaction which occurs at the anode is the dissolution of metal as ions M M n e where M .