Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
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M AT E R I A L S S E L E C T I O N & D E S I G N FIguRE 1 ing system is cool, usually during a shutdown or stagnant fow in a pipe. If there is suffcient moisture, or if a direct path to the pipe surface exists because of poor installation or physical damage, the water will eventually come into contact with the pipe surface. (a) Pitting and crevice corrosion and (b) tee-repaired using FRP wraps. As the pipe is heated and restored to service, the water nearest the pipe is converted to steam and deposits any soluble FIguRE 2 compounds that may have been transported in the liquid water. With successive wetting and drying cycles, the concentration of deposited chemicals can become high enough to be a signifcant factor in the local chemical reactions that routinely take place. Effect of Chlorides One of the main substances that causes corrosion is chloride. Refneries and tank farms are frequently located in close proximity to large ports and seagoing facilities. Unfortunately, the same locations that are most economically attractive for processing plants are also the most chemically hostile. The coastal zone, with its associated salt spray and high humidity, provides a ready and Modulus of composite repairs. unlimited source of moisture and chlorides, which cause corrosion. also signifcantly more brittle. While undergoing the reaction, the physical volTypes of Corrosion under Insulation ume of the metallic structure increases, There are several means of attack that causing the familiar swelling, bulging, or must be considered to successfully protect blistering often associated with red rust. against CUI. Galvanic corrosion, pitting This change in geometry is accompanied and crevice corrosion, chloride stress cor- by increased brittleness and non-magrosion cracking (SCC), and alkaline cor- netic adherence. The oxide readily disrosion are among the most common. places itself from the parent metal, and Each of these may affect large areas or exposes new, fresh surfaces that are again cause localized pitting depending on the susceptible to attack. Carbon steels, iron metallurgy and chemistry of the environ- alloys possessing between 0.05% and 1% ment. carbon, are relatively inexpensive and General corrosion refers to broad area offer excellent strength, but their general attack. When allowed to dry, the reaction corrosion resistance tends to be low. continues, leading to a non-binding heGalvanic corrosion is perhaps the most matite that is much harder than iron, but understood. The electrochemical potenNACE International, Vol. 52, No. 5 tial of various metals in a conductive environment leads to corrosion. Over time, this reaction leads to loss of metal. In CUI, the presence of even two slightly dissimilar metals can create a galvanic cell when the insulation becomes wet. The moisture in the insulation acts as the conductive environment, while one of the metals becomes the anode and the other becomes the cathode. The plant owner may not even realize that a galvanic cell has been created if periodic inspections are not performed. Crevice corrosion occurs in small, sheltered areas such as those in gasketed joints, bolts, washer contact zones, mud deposits, and other small, crack-like May 2013 MATERIALS PERFORMANCE 75