Water pipes are typically made of plastic, concrete or metal. Cast iron water lines are vulnerable to corrosion. Differences in metals, alkaline soil, water additives and electrical cables commonly factor into corrosion in cast iron water lines. Cast iron becomes a composite of cast iron and graphite as it corrodes, and the graphite will keep the shape of the pipe, often covering the extent of corrosion.
Slightly different characteristics between metals, galvanic differences, cause one metal to release electrons to another metal. Metals give up electrons to metals higher in the galvanic series (the hierarchy of which metals release electrons to other metals). Mill scale is embedded into the walls of iron pipe during its manufacturing process. When buried in the ground, the mill scale becomes negatively charged (cathodic); the pipe is positively charged (anodic), and the soil acts as an electrolyte (containing free ions that make it electrically conductive). A current leaves the pipe and passes through the mill scale to the soil. This reaction causes pitting of the metal pipe at the areas the current leaves from. The continued electrochemical action will corrode the pipe. Polished surfaces of the cast iron pipe walls can become positively charged, and corrosion will begin rapidly.
Soil that is highly alkaline (pH level greater than 7) often acts as an electrolyte, which completes an electrochemical reaction with the pipe. Currents leave the pipe and pass into the soil. The intensity of the current and the severity of the corrosion are dependent on the conductivity of the soil.
Water Treatment Additives
Chemicals are added to water during the treatment process to remove impurities. Many of these additives, such as aluminum sulfate, are acidic. These acids can inhibit corrosion protection on cast iron pipes, making them vulnerable to corrosion. Water contains metals such as calcium and magnesium. Water softener systems convert hard water, which has higher levels of calcium and magnesium, to soft water. Soft water is more vulnerable to elements that affect corrosion. For this reason, corrosion inhibitors are often used in soft water systems.
Stray Electrical Currents
Neutral cables buried underground close to cast iron water lines can contribute to the pipe’s corrosion. Direct currents pass in and out of an electrolyte (often the soil), and this can cause stray currents to move through the pipe. The corrosion doesn’t occur at the point the current enters the pope because it is usually protected, but the point where the current leaves the pipe will begin to corrode.
Soil changes caused by bacterial elements in the soil can leave slimy deposits near the metal pipe. These deposits produce corrosive substances like hydrogen sulphide, ammonia and carbon dioxide. Often colonies of bacteria that don’t produce corrosive substances will act as a protective coating to the pipe, but corrosion is still possible when these bacteria are not present and its protection can cause an uneven distribution of electrical potential, increasing the chance of corrosive action. This type of corrosion is difficult to prevent, and pipes may be given a protective coating to prevent biological corrosion.
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