Corrosion Control for Water System Piping and Storage TanksWater Law Resource
September 24, 2012 — 985 views
Whether coming from distant reservoirs or local municipal wells, potable water supplies must pass through miles of pipes, valves, pumps, storage tanks and treatment facilities before entering homes. Corrosion in these systems results in billions of dollars of unnecessary costs resulting from water leaks, blockages and repairs. These costs will increase as water supply infrastructures age.
This corrosion is the result of oxidation of the metal in water carrying pipes or water tanks. This oxidation is an electrolytic reaction that involves the movement of electrons (current) from an anode to a cathode formed within or between pipes, tanks or fixtures. Current flow and corrosion is maintained by the movement of electrolytes in the water to form a “corrosive cell.”
The anode is the site of oxidation where the metal releases electrons and reacts with water or oxygen to form oxides as rust. The cathode releases the electrons to form hydrogen from water. This combination of anode and cathode is termed an electrolytic corrosive cell. This usually slow process is accelerated by water acidity. Corrosion may also result from oxygen corrosion cells and galvanic corrosion cells.
Oxygen cells result from any differences in oxygen concentration along a pipe or tank wall. To balance oxygen concentration, a corrosive anode forms where oxygen concentration is low. In response, oxygen is consumed at the cathode. These cells commonly form in stagnant water at system dead-ends or tanks. Anodes also form where dirt or bacteria attach to the pipe or tank wall and block oxygen access.
Galvanic cells form when different metals come in contact in a distribution system. Metals vary in their tendency to oxidize - their galvanic potential. The metal more easily oxidized will form an anode where corrosion occurs while the other metal serves as the cathode. Common piping or tank metals like iron, steel or copper differ in galvanic potential. Contact between them will form a corrosive cell.
Corrosion prevention requires both proper design of water distribution systems and the control of the characteristics of the water flowing within the system. Corrosive or unstable water results from high acidity. Chemical or physical methods are used to prevent corrosive water effects. Addition of lime, soda ash, or caustic soda to the water can raise the pH and “stabilize” the water.
Alternatively, corrosion inhibitors such as inorganic phosphates or silicates added to the water form a protective pipe coating. In a physical or design approach, pipes and water tanks may be lined with cement mortar, bituminous compounds, epoxy paints or polyethylene.
Minimization of the aeration of groundwater, the design of systems to prevent stagnant water, the filtration of dirt and the killing of bacteria will decrease oxygen cell corrosion. To prevent galvanic corrosion, designs should include non-conductive couplings between corrosive metal pairs.