acid mine drainage or in brackish marshes. Well- mixed  concrete,  usually  a  mix  of  one  part  portland cement  to  two  parts  sand,  may  be  applied  to pipelines.  The  thickness  of  the  coating  applied may be up to 2 inches. If the concrete is properly mixed  and  tamped  around  the  pipe,  it  may  last 40  years.  However,  concrete  has  a  tendency  to absorb moisture and crack, which in many ways limits its use. In fact, in places where the coating cracks, electrolysis immediately starts to corrode the  metal.  This  corrosion  can  be  partially prevented by painting the pipe with a bituminous primer  before  coating  it. Metallic Coatings Metallic  coatings  such  as  galvanizing  (zinc coating) are very effective in protecting metallic structures  or  pipes  against  atmospheric  corrosion. This type of coating is ideal for cold-water lines and   metals   exposed   to   normal   atmospheric temperatures. However, metals such as iron cor- rode rapidly when used in high-temperature equip- ment  because  at  a  critical  temperature  of approximately 140°F iron becomes anodic to zinc. This results in the iron’s becoming the sacrificial anode  that  corrodes  readily. Plastic Wrapping Plastic tapes for wrapping come in rolls. They may  be  procured  in  various  widths.  The  tape  is wrapped around the pipes before they are laid in the  trench.  The  wrappings  are  applied  by  a simple  device  that  is  clamped  on  the  pipe  and turned by the UT. Pipe joints are wrapped after the pipes are laid in the trench. GALVANIC  CATHODIC PROTECTION Galvanic  cathodic  protection  is  a  method used to protect metal structures from the action of  corrosion.  As  explained  before,  galvanic  cell corrosion  is  the  major  contributing  factor  to  the deterioration  of  metal  by  electrochemical  reaction. The area of a structure that corrodes is the anode or positive side of the cell. Corrosion occurs when the positive electric current leaves the metal and enters  the  electrolyte.  Galvanic  cathodic  protec- tion is designed to stop this positive current flow. 7-25 When the current is stopped, the corrosive action stops and the anodes disappear. This type of pro- tection  depends  upon  the  neutralization  of  the  cor- roding  current  and  the  polarization  of  the  cathode metal areas. METHODS  OF  GALVANIC CATHODIC   PROTECTION Galvanic  cathodic  protection  is  a  means  of reducing  or  preventing  the  corrosion  of  a  metal surface   by   the   use   of   sacrificial   anodes   or impressed  currents.  When  sacrificial  anodes  are used, it is known as the galvanic anode method. If impressed currents are used, it is known as the impressed  current  method.  These  two  methods can  be  used  separately  or  with  each  other, depending  upon  the  corrosive  characteristics  of the  electrolyte  surrounding  the  structure. Galvanic Anode Method The  galvanic  anode  method  of  cathodic  pro- tection uses an electrode referred to as a sacrificial anode  that  corrodes  to  protect  a  structure.  This sacrificial  anode  is  electrically  connected  to  and placed in the same electrolytic area of the struc- ture.  The  anode  used  to  protect  iron  or  steel structures should be made of magnesium or zinc so it will produce a sufficient potential difference to cause the structure to become a cathode. The action  of  this  type  of  galvanic  protection  causes the  electric  current  to  flow  from  the  sacrificial anode through the electrolyte to the structure to be  protected.  The  electrical  connection  between the two metals completes the circuit and allows the current to return to the corroding metal. The sacrificial   anode   becomes   the   anode   of   the established  dissimilar  metal  galvanic  cell,  and the  structure  to  be  protected  becomes  the cathode.  The  current  from  the  sacrificial  anode is  intense  enough  to  oppose  or  prevent  the positive  current  from  leaving  the  anodes  in  the structure to be protected. These structure anodes are   then   suppressed,   and   the   metal   in   the structure  becomes  a  cathode.  The  prevention  of these  positive  currents  from  the  anodic  areas in  the  structure  reduces  the  corrosion  rate  to almost  zero. Galvanic cathodic protection is used in areas where the corrosion rate is low and electric power is  not  readily  available.  A  typical  example  of