—Never attempt to take readings on operating machinery. We have to understand that even with the best tools, it is the person behind the tool who makes things work. ABFs can take measurements accurately and new parts to be installed can be on hand. If the one who finally assembles the pump does not know how to torque a casing or pipe flange, he or she can destroy all the hard work and money that have been put into the job. For maintenance and repair on all equipment, use the appropriate technical manuals. CORROSION CONTROL LEARNING OBJECTIVES: Describe the types of corrosion the ABF will confront. Identify their signs and explain the corrective action. The most effective repair protection is prevention. A thorough maintenance program continuously carried out prevents most equipment failure. With higher strength and closer tolerances being demanded of met- als, equipment would rapidly become inoperable with- out regular anticorrosion maintenance. Corrosion endangers the equipment by reducing the strength and changing the mechanical characteristics of the metals used in its construction. Materials are de- signed to carry certain loads and withstand given stresses as well as to provide an extra margin of strength for safety. Corrosion can weaken the structure, thereby reducing or eliminating this safety factor. Corrosion may take place over the entire surface of a metal from chemical reaction with the surrounding environment. It may be electrochemical in nature be- tween two metallic materials or two points on the sur- face of the same alloy, differing in chemical activity. The presence of moisture is essential in both types of attacks. The most familiar example of corrosion is rust found on iron or steel. All metals are affected to some extent by the atmos- phere. Water and water vapor containing salt combine with oxygen in the atmosphere and produce the main source of corrosion. There are many forms of corrosion; the form of corrosion depends upon the metal involved, atmospheric conditions, and the corrosion-producing agents present. For this discussion, we may consider corrosion as three general types-surface, galvanic, and intergranular corrosion. SURFACE CORROSION The effect of the atmosphere produces a corrosion that appears on the surface of a metal as a general roughening, etching, or pitting. Iron rust is the most common example of surface corrosion. Although aluminum, magnesium, and other nonfer- rous metals do not rust, these metals are subject to surface corrosion. Surface corrosion on unpainted alu- minum alloy is evident as white or gray powdery depos- its on the metal surface. The condition is first indicated by the powdery residue deposited on the area of contact; later pitting and searing appear on the aluminum sur- face, and finally complete deterioration of the alumi- num. Corrosion on painted aluminum-alloy surfaces cannot be recognized by either the roughened surface or by the powdery deposit. Instead, the paint or plating appears to lift off the surface, indicated by a blistered appearance and/or discoloration that results from the pressure of the underlying accumulation of the corro- sion products. Surface corrosion on magnesium alloys can be rec- ognized by powdered or roughened surfaces. Magne- sium corrosion products are white and quite large compared to the size of the base metal being corroded. The deposits have a tendency to raise slightly, and the corrosion spreads rapidly. When white, puffy areas are discovered on magnesium, prompt attention is required to prevent the corrosion from penetrating entirely through the structure. This can occur in a very short time. It has been generally established that surface cor- rosion is caused by moisture in the air. Since this type of corrosion is visible, it can be detected in its early stages by close visual inspection. Surface corrosion can be prevented or retarded by protecting the metal surface with a plating or paint and by keeping the plating or paint in good condition. GALVANIC CORROSION Galvanic (or electrolytic) corrosion occurs when two different metals are connected and exposed to an electrolyte such as water, especially salt water. When aluminum pieces are attached with steel bolts or screws, galvanic corrosion may occur between the aluminum and steel in the presence of moisture. An electrical potential is setup, current flows between the two metals, and an effect similar to that which occurs in batteries is produced. Galvanic corrosion can usually be recog- nized by the presence of a buildup of corrosion products at the joint between two metals. Preventive measures include painting and plating. 2-32