XI. PROTECTIVE SYSTEMS FOR MATERIALS 

 1 . Corrosion . 



a. G eneral . To understand the reason for using coatings or applying 

 cathodic protection, a brief review of corrosion fundamentals is necessary. 

 These principles apply to any metal structure. Corrosion is defined as 

 the deterioration of a material, usually a metal, or of its properties 

 because of a reaction with its environment. Three conditions are necessary 

 for metallic corrosion to occur: 



(1) There must be an electrical potential difference between 

 two metallic electrodes, anode and cathode. This can exist because 

 of metallic composition differences, metallic surface condition 

 differences, or because of differences in the environment contacting 

 the electrodes; 



(2) the contacting environment (electrolyte) must be 

 electrically conductive with positively and negatively charged 

 ions present; and 



(3) there must be a metallic connection between the electrodes. 



b. Corrosion Process . Corrosion is a natural process involving 

 electrochemical reactions with a resulting flow of direct current from 

 anodic areas of the substructure (corroding areas) to cathodic areas of 

 the substructure, through the surrounding and contacting electrolyte (soil 

 or water environment) . A simplified diagram of the corrosion process on 

 iron or steel in water is shown in Figure 90. The circuit is completed 

 through the metallic connection between anode and cathode. They both may 

 be part of the same structure. This current flow is called galvanic 

 current and usually is in microampere or milliampere quantities. 



With steel substructures, corrosion (loss of metal) takes place only at 

 anodic areas as the result of current flow into the electrolyte from 

 anodic areas. In the case of iron or steel, metal loss amounts to about 

 90 newtons (20 pounds) per ampere-year of current flowing from the metal 

 into the contacting electrolyte. Loss of metal is directly proportional 

 to the amount of current. One milliampere of current leaving the substructure 

 from one point into the electrolyte will cause penetration of 9.5 millimeters - 

 (3/8 inch) steelplate in less than 1 year. See Table 40 for corrosion 

 rates of other metals, including anode materials. With iron or steel 

 substructure the electrochemical reaction prevents corrosion of areas 

 where current flows from the electrolyte into cathodic areas of the 

 substructure. Anodes, cathodes, and corrosion current as related to 

 steel in water are shown in Figure 91. 



c. Corrosivity of the Environment . In air the corrosivity of the 

 environment on a structure will depend on the temperature (ranges and mean 

 averages), relative humidity, wind conditions, proximity to the water, 

 rainfall, and chemical fumes (from cargo or nearby plants). Corrosivity 

 generally increases with increases in temperature, relative humidity, wind 

 velocity (particularly off water); with closeness to the water; with 

 increased rainfall; and with higher concentrations of chemical fumes. 



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