CORROSION AND TUBERCULATION IN IRON PIPES: -

SURFACE OF THE CAST IRON PIPE: -

The freshly cast, underground, interior surface of cast-iron pipe has been found to have a high resistance to corrosion by water. When the surface was scraped off of the interior of the cast iron, the black phase (between the metal surface and the gray phase) was found to be ferromagnetic magnetite (Fe3O4) with a cubical structure. The gray phase on the internal, exposed surface (adjacent primarily to the magnetite) showed two or more components under polarized light long, needle-like pyroxenes of monoclinic structure, such as CaSiO3, or FeSiO3 ; and polyhedron garnets of cubical structure, such as Ca3Al2(SiO4)2 or Fe3Al2(SiO4 ) 2 . This silicate "skin" is apparently responsible for the excellent resistance to corrosion by water. It is, however, quite brittle and therefore only of academic interest.

Corrosion and Tuberculation: -

Aesthetic effects that are a result of corrosion of iron are characterized by “pitting” and are a consequence of the deposition of ferric hydroxide and other products and the solution of iron; this is known as tuberculation reduces the hydraulic capacity of the pipe. Corrosion of iron can cause customer complaints of reddish or red-dish-brown staining of plumbing fixtures and laundry. Corrosion of copper lines can cause customer complaints of bluish or blue-green stains on plumbing fixtures. Sulfide corrosion of copper and iron lines can cause a blackish color in the water. The by-products of microbial activity (especially iron bacteria) can cause foul tastes and odors in the water. The economic effects of corrosion may include the need for water main replacement, especially when tuberculation reduces the flow capacity of the main. Tuberculation increases pipe roughness, causing an increase in pumping costs and reducing distribution system pressure. Tuberculation and corrosion can cause leaks in distribution mains and household plumbing. Corrosion of household plumping may require extensive treatment, public education, and other actions under the Lead and Copper Rule. Other effects of corrosion include short service life of household plumbing caused by pitting. The buildup of mineral deposits in the hot water system may eventually restrict hot water flow. Also the structural integrity of steel water

storage tanks may deteriorate, causing structural failures. Steel ladders in clear wells or water storage tanks may corrode, introducing iron into the finished water. Steel parts in flocculation tanks, sedimentation basins, clarifiers, and filters may also corrode.

Types of CorrosionThree types of corrosion occur in water mains: galvanic, tuberculation, and/or pitting:

1. Galvanic: - When two dissimilar metals are in contact and are exposed to a conductive environment, a potential exists between them and current flows. This type of corrosion is the result of an electrochemical reaction when the flow of electric current is an essential part of the reaction.

2. Tuberculation: - This refers to the formation of localized corrosion products scattered over the surface in the form of knob-like mounds. These mounds increase the roughness of the inside of the pipe, increasing resistance to water flow and decreasing the C-factor of the pipe.

3. Pitting: - Localized corrosion is generally classier as pitting when the diameter of the cavity at the metal surface is the same or less than the depth.

Chemical Addition: Corrosion Control Parameters

1. If the product of the calcium hardness times the alkalinity of the water is less than 100, treatment may be required. Both lime and CO2 maybe required for proper treatment of the water.

2. If the calcium hardness and alkalinity levels are between 100 and 500, either lime or Na2CO3 will be satisfactory. The decision regarding which chemical to use depends on the cost of the equipment and chemicals.

3. If the product of the calcium hardness times the alkalinity is greater than 500, either lime or caustic (Noah) may be used. Soda ash will be ruled out because of the expense.

4. The chemicals chosen for treatment of public drinking water supplies modify the water characteristics, making the water less corrosive to the pipe. Modifications of water quality can increase the pH of the water, reducing the hydrogen ions available for galvanic corrosion and the

solubility of copper, zinc, iron, lead, and calcium. Modification of water quality also increases the possibility of forming carbonate protective films.5. Calcium carbonate stability is the most effective means of controlling corrosion. Lime, caustic soda, or soda ash is added until the pH and the alkalinity indicates the water is saturated with calcium carbonate. Saturation does not always assure non corrosiveness. Utilities should also exercise caution when applying sodium com-pounds, since high sodium content in water can be a health concern for some customers.

6. By increasing the alkalinity of the water, the bicarbonate and carbonate available to form protective carbonate film increase.

7. By decreasing the DO of the water, the rate of galvanic corrosion is reduced, along with the possibility of iron tuberculation.

8. Use of inorganic phosphates:

A. Zinc phosphates — it is strongly recommended that this phosphate be used. It causes algae blooms on open reservoirs.

B. Sodium silicate — Individual customers, such as apartments, houses, and office buildings use this method of treatment.

C. Sodium polyphosphates (tetra sodium pyrophosphate or sodium hexa Meta phosphate) —these chemicals control scale formation in supersaturated waters and are known as sequestering agents.

D. Silicates (SiO2) Silicates form a film. An initial dosage of 12 to 16 mg/L for about30 d will adequately coat the pipes. Then 1.0mg/L concentration should be maintained.