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The corrosion of any metal is due to an electrochemical reaction and is always accompanied by a flow of current. This current flows from the metal through the electrolyte (soil, water, or any material conductive to electricity) and back to the metal. The points from which the current leaves the pipe wall are called anodic. The points where the current returns to the pipe are called cathodic. This phenomenon is commonly known as galvanic cell, thus the name, galvanic corrosion. Current leaves the anode to travel to the cathode, and the loss of current from the anode results in a loss of metal.
Many factors contribute to galvanic corrosion on pipelines such as different types and textures of soil, moisture content, oxygen supply, and soil resistivity. These soil conditions create various pipe to soil voltages or potentials along the entire length of the pipeline. The higher potential readings are anodic to the lower readings (cathodic).
Because corrosion is caused by current leaving the metal surface, it can then be assumed that if this current is stopped, corrosion can also be halted. Three basic methods are used to subdue the current flow.
The prime purpose of coating is to prevent the current from leaving the metal pipeline. Ideally, if a pipeline could be 100% coated and completely insulated from all surrounding earth or foreign structures, no current could leave. However, coating wil never be perfect due to material defects, poor handling, and improper backfilling practices.
Therefore, it becomes necessary to provide a supplementary method to block out any current leaving the pipe wall at locations where coating is not perfect. If all sections of a pipeline could be made cathodic, and thereby receiving current, there would be no corrosion. The process of making the structure a cathode is called cathodic protection. The current received by the cathodic structure must come from some anodic source. This source can be galvanic anodes which are higher on the electromotive series of metals than steel. Metals can also be used with current being discharged from them by batteries or rectifiers. By either method a direct current flow is created.
Many companies in recent years have laid coated lines with anodes installed at various spacing along the line and expected complete cathodic protection. To their dismay, in five years or less, leaks appeared on these sections of line. The leaking problems were caused by the failure to insulate the new coated sections from old bare lines.
When a new coated section of steel line is tied or shorted to any other metal with a less negative pipe to soil potential, a corrosion cell is created with the new pipe being the anode. The shorter the section of new line, the more severe corrosion will be. It is then of the utmost importance that every section of new coated line being laid be insulated and isolated from any other structure before effective cathodic protection can be obtained.
With the proper application of coating, cathodic protection, and insulation, we can prevent corrosion and expect many trouble-free years of service from our pipeline systems.
The anodes used by Southeast Corrosion & Engineering are primarily 3 lb., 9 lb., 17 lb., and 32 lb. magnesium on coated structures. On bare structures, larger anodes are sometimes used where current requirements are greater, and the magnesium will be consumed much more rapidly. Several hundred feet of well coated line have been protected with a single 17 lb. anode.
We must keep in mind that a single magnesium anode will only produce current in milliamperes due to the limited driving voltage and surface of the anodes. For example, if a pipeline measures -.6 volt to a copper sulfate electrode and the magnesium anode measures -1.5 volt to the same electrode, we would then start with a 0.9 volt driving voltage. If the pipeline then later reads a polarized potential of -.95 volt, the driving voltage would then be 0.55 volt.
The important advantages of magnesium anodes are:
The useful life of an anode is determined by the weight of the anode and the amount of current discharged. It is generally agreed that there are approximately 500 ampere hours per pound of magnesium. If it required .05 amperes to cathodically protect a line, a 17 lb. magnesium anode discharging this amount of current would last about 19 years. Anode life, however, cannot be calculated exactly due to polarization, soil conditions, etc.
Southeast Corrosion & Engineering serves all of South Florida including Miami Dade County, Broward County, Palm Beach County, Aventura, Bal Harbour, Boca Raton, Boynton Beach, Coconut Creek, Cooper City, Coral Gables, Coral Springs, Cutler Ridge, Dania Beach, Davie, Deerfield Beach, Delray Beach, Doral, Fontainbleau, Fort Lauderdale, Golden Glades, Hallandale Beach, Hialeah, Hollywood, Homestead, Jupiter, Kendale Lakes, Kendall, Kendall West, Key Biscayne, Lake Worth, Lantana, Lauderdale Lakes, Lauderhill, Margate, Miami, Miami Beach, Miami Gardens, Miami Lakes, Miami Shores, Miami Springs, Miramar, North Lauderdale, North Miami, North Miami Beach, Oakland Park, Palm Beach Gardens, Parkland, Pembroke Pines, Perrine, Pinecrest, Plantation, Pompano Beach, Riviera Beach, South Miami Heights, Sunrise, Surfside, Tamarac, Tamiami, Wellington, West Palm Beach, Weston, Wilton Manors.