Course: BFM 203 Water Treatment and Control

Lecture

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Corrosion

Corrosion is the gradual decomposition or description of a materials by chemical action, often due to an electrochemical reaction. Caused by stray current electrolysis, galvanic corrosion (dissimilar metals), or differential-concentration cells, corrosion starts at the surface of a material and moves inward. All parts of the treatment and distribution system (storage tanks, pipes, valves, service connections, etc.) must be protected from the effects of corrosion. What is the impact of corrosion in a water system? Excessive corrosion may necessitate replacement of the water main due to tuberculation. Tuberculation is the formation of clumps or mounds of corrosive materials. As indicated by the photo to the left (courtesy of the Pennsylvania Department of Environmental Protection), corrosion may reduce the carrying capacity of the main, thus causing increased pumping costs, decreased pressure, and (because corrosion can eventually eat completely through a pipe) leaks in a distribution system. In addition, pipe materials may deteriorate and enter the water. Toxic metals are a serious health hazard; iron and manganese cause color, taste, and odor complaints. Customer complains may be the first indication of a larger issue. Recording and trending all water quality complaints can identify specific symptoms that could be caused by corrosion.

Electrochemical Corrosion

Metallic corrosion in a distribution system is always the result of electrochemical corrosion. Electrochemical corrosion "takes place when two different metals come into contact with a conductive liquid -- usually impure water or soil moisture -- resulting in a flow of direct current electricity. The current always flows away from the anodic metal (anode), and the anode is corroded. The current flows through the electrolyte to the cathodic metal (cathode), but the cathode is not corroded. The potential that causes the current to flow is always due to some kind of difference between the anode and the cathode, such as a difference in the two metals, concentration of the conductive liquid, a difference in temperatures, a difference in the amount of oxygen present, or some other difference in conditions." (Source: http://www.sescocp.com/tutorial.php) In order for corrosion to occur, two different metals must be present. One acts as an anode and one as a cathode. An electrolyte (water) must also be present. This electrolyte provides the path on which the current flows. There must also be a direct electrical contact between the two metals to complete the electrical circuit. Changing the characteristics of the electrolyte may be the easiest way to reduce the degree of electrochemical corrosion in a distribution system.

A number of physical, chemical, and biological factors may contribute to corrosion, as shown in the following table.Once iron and sulfate reducing bacteria begin growing, it is very difficult to eliminate completely. Prevention of the conditions that encourage growth is very important. Stagnant water heaters set at low temperatures, such as those in model and show homes, form a perfect environment for bacterial growth.

Factors Influencing Corrosion
Physical Factors | Chemical Factors | Biological Factors
System construction: Type, relative size, and coating types. | Alkalinity: low alkalinity may result in very acidic water. | Iron bacteria: Present in a filamentous form, resulting in slimy, reddish or brownish masses. May result in red water, musty smells, and excessive corrosion under slime mass.
System pressure: Higher pressure = greater effect | pH: Low pH is corrosive/high pH is scale-forming. | Sulfate-reducing bacteria: Causes rotten egg smell (hydrogen sulfide). Forms sulfuric acid which is very corrosive.
Soil moisture: External corrosion on buried pipes | Dissolved oxygen: Considered the most corrosive component of water chemistry |
Stray electric current: Grounding to service lines or other power lines near mains. | Dissolved solids: Present as ions in water, increasing its conductivity |
Temperature: Higher temperature = greater effect | Hardness: Dissolved form of some of the scale-forming components in water |
Flow velocity: Very high or low flow = greater effect | Chloride and sulfate: Inhibit the formation of protective scales by keeping hardness ions in solution |
 | Phosphate and silicate: Form protective film in water systems. |
 | Trace metals: Copper, lead, iron, and zinc. |

As a chemical factor, the oxygen concentration cell is the most corrosive. This type of corrosion occurs in dead-end mains. Corrosion is caused when ferric hydroxide is formed from ferrous ions and dissolved oxygen. Ferric hydroxide causes tubercules to form, leaving pits underneath. Oxygen depletion follows, and anaerobic bacteria grows. This results in foul odors. Cul-de-sacs and stagnant sections in a distribution system with multiple looped connections must be flushed regularly to alleviate the conditions that create oxygen concentration cells.