ÿþ<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>CORROSION PROTECTION AND PREVENTION UNDERGROUND</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <style> A { color: #003399; } A:HOVER { color: #FF0000; } A:VISITED { color: #800080; } .FourthSeparator { color: #999999; } .HeaderTitle { font-family: Tahoma; font-size: 12.5pt; color: #FFFFFF; font-weight: bold; } .HeaderLine { color: #6A7BA7; } .HeaderTable { background: #1D3679; } .Footer { background: #1D3679; font-family: Tahoma; font-size: 8.5pt; color: #C9CADE; } .Footer A { color: #FFFFFF; } .Fields { font-family: Tahoma; font-size: 8.5pt; color: #FFFFFF; } .Values { font-weight: bold; } .NoResults { font-family: Tahoma; font-size: 8.5pt; color: #757575; font-size: bold; } .Result { font-family: Verdana; font-size: 7.5pt; } .ResultTitle { font-size: 10pt; } .ResultInfo { color: #2B3366; } div{color:#000} div{font-family:arial,sans-serif} .g{margin:1em 0} .r{font-weight:normal;margin:0;display:inline} .r{font-size:1em} a:link{color:#00c} td{color:#000} td{font-family:arial,sans-serif} .j{width:34em} .a{color:green} H1 { FONT-WEIGHT: bold; FONT-SIZE: 14pt; COLOR: #7a7a7a; FONT-FAMILY: Arial, Helvetica, sans-serif } .style2 { font-family: Verdana; font-size: 7.5pt; width: 92px; } .style3 { width: 92px; } .style4 { text-align: justify; } .style6 { font-family: Verdana; font-size: 7.5pt; text-align: justify; } .style8 { text-align: center; } .style9 { font-family: Verdana; font-size: 7.5pt; width: 92px; height: 22px; } .style10 { font-family: Verdana; font-size: 7.5pt; height: 22px; } .style11 { font-weight: bold; } .style12 { text-decoration: none; } .style13 { font-family: Tahoma; font-size: 8.5pt; color: #FFFFFF; width: 991px; } .style14 { font-size: 10pt; } .style15 { width: 348px; height: 294px; } .style16 { font-size: 10pt; text-align: center; } </style> <meta name="keywords" content="buried corrosion, corrosion underground, corrotion soil, soil electrolyte"> <meta name="description" content="CORROSION PROTECTION AND PREVENTION UNDERGROUND. "> </head> <body leftmargin="0" topmargin="0" marginwidth="0" marginheight="0"> <table width="100%" border="0" cellpadding="10" cellspacing="0" class="HeaderTable"> <tr> <td class="HeaderTitle" bgcolor="#FFFFFF"> <p align="left"><font color="#000000">Corrosionist&nbsp; <font size="2">The Website of Corrosion <span lang="it">Protection </span>and Corrosion Control</font></font></td> </tr> </table> <table width="100%" border="0" cellpadding="0" cellspacing="0" class="HeaderTable"> <tr> <td width="100%" height="1" bgcolor="#000080"><hr class="HeaderLine" size="1" noshade></td> </tr> </table> <table width="100%" border="0" cellpadding="10" cellspacing="0" class="HeaderTable"> <tr> <td class="style13" bgcolor="#FFFFFF"> <b><font color="#000000">You are here &gt;&gt;&gt; </font> <a href="index.htm"> <font color="#000000"><span lang="it">Corrosion Protection H</span>ome</font></a><font color="#000000"> - <font size="2"><u> <a href="http://www.corrosionist.com/corrosion_prevention.htm">Corrosion <span lang="it">Prevention</span></a></u> - <o:p><span class="style11" lang="it">CORROSION PROTECTION AND PREVENTION </span>UNDERGROUND. </o:p> </font></font></b></td> <td class="Fields" bgcolor="#FFFFFF"> &nbsp;</td> </tr> </table> <table width="100%" border="0" cellpadding="0" cellspacing="10"> <tr> <td class="style9" style="text-align: center"> </td> <td class="style10" style="text-align: center" width="97%"> <script type="text/javascript"><!-- google_ad_client = "pub-6464343554360654"; /* 728x90, creato 26/09/08 */ google_ad_slot = "4501101590"; google_ad_width = 728; google_ad_height = 90; //--> </script> <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script></td> <td class="style10" style="text-align: center" width="97%"> </td> </tr> <tr> <td class="style2"> &nbsp;</td> <td class="style6" width="97%"> <font color="#FFFFFF"><script type="text/javascript"><!-- google_ad_client = "pub-6464343554360654"; //728x15, creato 09/11/07 google_ad_slot = "0960947182"; google_ad_width = 728; google_ad_height = 15; //--></script> <p> <o:p><b> <script type="text/javascript"><!-- google_ad_client = "pub-6464343554360654"; /* 728x90, creato 27/09/08 */ google_ad_slot = "2783268396"; google_ad_width = 728; google_ad_height = 90; //--> </script> <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script></b> </o:p> <o:p>SACRIFICIAL ANODE CATHODIC PROTECTION SYSTEMS. In sacrificial anode systems the high energy electrons required for cathodic protection are supplied by the corrosion of an active metal. This is illustrated in Figure 10-2. 10-5 Figure 10-2 Sacrificial Anode Cathodic Protection System Sacrificial anode systems depend on the differences in corrosion potential that are established by the corrosion reactions that occur on different metals or alloys. For example, the natural corrosion potential of iron is about -0.550 volts in seawater. The natural corrosion potential of zinc in seawater is about -1.2 volts. Thus if the two metals are electrically connected, the corrosion of the zinc becomes a source of negative charge which prevents corrosion of the iron. Sacrificial anode cathodic protection systems are illustrated in Figure 10-3. The materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminum alloys that have been specifically developed for use as sacrificial anodes. In applications where the anodes are buried, a special backfill material surrounds the anode in order to insure that the anode will produce the desired output. Sacrificial anodes are normally supplied with either lead wires or cast-m straps to facilitate their connection to the structure being protected. The lead wires may be attached to the structure by welding or mechanical connections. These should have a low resistance and should be insulated to prevent increased resistance or damage due to corrosion. When anodes with cast-in straps are used, the straps can either be welded directly to the structure or the straps can be used as locations for attachment using mechanical 10-6 Figure 10-3 Cathodic Protection by the Galvanic Anode Method 10-7 fasteners. A low resistance mechanically adequate attachment is required for good protection and resistance to mechanical damage. In the process of providing electrons for the cathodic protection of a less active metal the more active metal corrodes. The more active metal (anode) is sacrificed to protect the less active metal (cathode). The amount of corrosion depends on the metal being used as an anode but is directly proportional to the amount of current supplied. The anodes in sacrificial anode cathodic protection systems must be periodically inspected and replaced when consumed.</o:p></p> <p> <span lang="EN-GB" style="font-size: 10pt; font-family: 'Verdana','sans-serif'; mso-ansi-language: EN-GB"> <span style="color: #000000"> <o:p>Source : &quot;Corrosion Control&quot; NAVFAC MO-307 September 1992 supplied by a rectifier. </o:p> </p> <p class="style14"> <o:p>As shown in Figure low energy electrons that are picked up at a non-reactive anode bed are given additional energy by the action of a rectifier to be more energetic than the electrons that would be produced in the corrosion reaction. </o:p> </p> <p class="style16"> <o:p> </o:p> <img alt="impressed current cathodic protection" class="style15" longdesc="cathodic protection impressed current" src="impressed%20current%20cathodic%20protection.jpg" width="752" height="570" /></p> <p class="style14"> <o:p>Impressed Current System</o:p></p> <p class="style14"> <o:p>The energy for the  electron energy pump action of the rectifier is provided by ordinary alternating current. The effect of these electrons at the structure being protected is the same as that derived from the sacrificial anode type of cathodic protection system. However, the anode material serves only as a source of electrons and need not be consumed in providing protective current.</o:p></p> <p class="style14"> <o:p>The materials used for impressed current cathodic protection can the pass a current into the environment without being consumed at a high rate. Graphite and high silicon cast iron are the most commonly used impressed current cathodic protection anode materials, however, other materials (such as magnetite, platinum, and newly developed ceramic materials) have been successfully used.</o:p></p> <p class="style14"> <o:p>For buried anodes, a backfill of carbonaceous material is used to surround the anode to decrease the electrical resistance of the anode, to provide a uniform, low resistivity environment surrounding the anode and to allow gasses produced at the anode surface to vent. In practice, materials such as graphite are used for impressed current cathodic protection system anodes that are slowly consumed. </o:p> </p> <p class="style14"> <o:p>Anodes in impressed current systems must be inspected and replaced if consumed or otherwise damaged. As is the case for any electrical equipment, rectifiers used for impressed current cathodic protection systems require preventative maintenance to insure proper operation. </o:p> </p> <font color="#FFFFFF"> <p> <span lang="EN-GB" style="font-size: 10pt; font-family: 'Verdana','sans-serif'; mso-ansi-language: EN-GB"> <span style="color: #000000"> <o:p>Source : &quot;Corrosion Control&quot; NAVFAC MO-307 September 1992</o:p> </span></span> </p> <p class="style8"> <script type="text/javascript"><!-- google_ad_client = "pub-6464343554360654"; /* 728x90, creato 26/09/08 */ google_ad_slot = "4501101590"; google_ad_width = 728; google_ad_height = 90; //--> </script> <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script> </p> <p class="style8"> <span lang="it"> <font size="2"><a href="http://www.corrosionist.com/corrosion_prevention.htm"> Corrosion Prevention<a href="http://www.corrosionist.com/corrosion_prevention.htm" class="style12">| <a href="http://www.corrosionist.com/corrosion_control_methods.htm">Corrosion Control</a> | </font> <a href="http://www.corrosionist.com/Frequently_Asked_Questions_Corrosion_Rust.htm"> <font size="2">Corrosion Rust FAQ</font></a> | <a href="http://www.corrosionist.com/how_does_rust_form.htm"><font size="2"> Metal Corrosion</font></a> | <a href="http://www.corrosionist.com/copper_pipe_corrosion.htm"><font size="2"> Copper Pipe Corrosion</font></a></span></p> <p> &nbsp;</p> </font> </td> <td class="Result" width="97%"> <span lang="it">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span>&nbsp;<span lang="it">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span></td> </tr> <tr> <td class="style3">&nbsp;</td> <td class="style4"><hr class="FourthSeparator" size="1" noshade> </td> <td>&nbsp;</td> </tr> </table> </body> </html>