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<br />Corrosion can attack the inside or outside of the culvert barrel. The <br />chemicals in drainage water can attack the material on the interior of the <br />culvert. Culverts subject to continuous flows or standing water with <br />aggressive chemicals are more likely to be damaged than those with <br />intermittent flows. The exterior of culverts can be attacked by chemicals in <br />the ground water which can originate in the soil, be introduced through <br />contaminates in the backfill soil, or be transported by subsurface flow. <br /> <br />Corrosion affects all metals and alloys, although the rates can vary widely <br />depending both upon the chemical and physical properties of the metal and upon <br />the environmental condition to which it is exposed. When a metal corrodes a <br />very low voltage electrical current is established between two parts of a <br />metal surface that have different voltage potential. The difference in <br />voltage potential may be caused by slight variations in the material, changes <br />in surface condition, or the presence of foreign materials. The current <br />removes metallic ions from one location and deposits them at another location, <br />causing corrosion, as shown in exhibits 27 and 28. The chemicals present in <br />the water greatly influence its effectiveness as an electrolyte. <br /> <br /> <br />'f-, <br /> <br />, <br />/ <br /> <br /> <br />. <br /> <br />, <br /> <br />. <br />, <br />f Jo , <br /> <br /> <br />~ ..~'...- <br /> <br />'&-'~~~" .~~ <br />...-. -. .s:. <br />~ ~'~.- <br />.. :.~. t~ <br />..... I' ~...~.j'..~ <br /> <br />Exhibit 27. Severe corrosion of corrugated steel <br />culvert due to aggressive water. <br /> <br />38 <br /> <br />