Corrosion Cause and Control

Internal Pipe Corrosion Internal pipe corrosion usually occurs in unlined metal distribution system piping and building plumbing in contact with soft water of low hardness, pH, and alkalinity containing carbon dioxide and oxygen. In serious cases, water heaters are damaged, the flow of water is reduced, the water is red or rusty where unprotected iron pipe is used, and the inside surface of pipe and fittings is dissolved, with consequent release of trace amounts of possibly harmful chemicals and weakening or pitting of pipe. Dissolved iron may be redeposited as tubercules with a reduction of pipe diameter and water flow. Biochemical changes take place in pipe where iron bacteria such as crenothrix and leptothrix use iron in their growth. High water velocities, carbon dioxide, dissolved solids, and high water temperatures [(140°-150oF) (60°-66°C)] all accelerate corrosion. Free chlorine residual less than 2 mg/l in water at pH 7 to 8 results in minimal corrosion. However, significant metal leaching (copper, cadmium, zinc, and lead) can occur in home water systems served with private wells when the water has high pH and hardness.107

Although much remains to be learned concerning the mechanism of corrosion, a simple explanation as related to iron may aid in its understanding. Water in contact with iron permits the formation of soluble ferrous oxide and hydrogen gas. Gaseous hydrogen is attracted to the pipe and forms a protective film if allowed to remain. But gaseous hydrogen combines with oxygen usually present in "aggressive" water, thereby removing the protective hydrogen film and exposing the metal to corrosion. High water velocities also remove the hydrogen film. In addition, ferrous oxide combines with the water and part of the oxygen usually present to form ferric hydroxide when the carbonate concentration is low, which redeposits in other sections of pipe or is carried through with the water. When the carbonate concentration is high, ferrous carbonate is formed. Another role is played by carbon dioxide. It has the effect of lowering the pH of the water since more hydrogen ions are formed, which is favorable to corrosion.

Pipe Materials and Corrosion Lined steel and ductile iron pipe, asbestos-cement, wood-stave, plastic, vitrified clay, and concrete pressure pipe are corrosion resistant.* Plastic pipe may be polybutylene (PB), polyethylene (PE), or polyvinyl chloride (PVC). The PVC pipe comes in diameters of 0.5 to 30 inches, 10 to 20 foot lengths, and 100 to 235 psi working pressures. It is very resistant to corrosion. Fiberglass-reinforced plastic pipe is available in diameters up to 144 inches and lengths up to 60 feet. Polyethylene pipe comes in 18 to 120 inches in diameter and 20 foot lengths. Fiber-epoxy pipe comes in 20-foot lengths and 2 to 12 inches in diameter and is easily installed. It combines light weight with high tensile and compressive strength. The pipe withstands pressures of 300 psi, electrolytic attack, as well as embrittlement associated with

* See the AWWA/ANSI C104/A 21.4-85 Standard for Cement-Mortar Lining for Ductile-Iron Pipe and Fittings for water. Thermoplastic pipe should have the National Sanitation Foundation seal of approval. The AWWA standards C900-89 and C905 apply to 4- to 12-in. PVC pipe.

cold temperatures and aging. Ductile iron pipe comes in diameters of 4 to 54 inches and for pressures of 250 to 350 psi. Concrete pressure pipe withstands pressures of 400 psi and is available in diameters from 16 to 60 inches. With soft waters, calcium carbonate tends to be removed from new concrete, cement-lined, and asbestos-cement pipe for the first few years. Salt used in deicing can seep through the ground and greatly weaken reinforced concrete pipe and corrode the steel. Wood-stave, vitrified clay, and concrete pipes have limited applications. Iron and steel pipes are usually lined or coated with cement, tar, paint, epoxy, or enamel, which resist corrosion provided the coating is unbroken. Occasionally, coatings spall off or are imperfect, and isolated corrosion takes place. It should be remembered that even though the distribution system is corrosion resistant, corrosive water should be treated to protect household plumbing systems.

Polycyclic aromatic carbons, some of which are known to be carcinogenic, are picked up from bituminous lining of the water distribution system, not from oil-derived tarry linings. On general principles, bituminous linings are being discontinued in England by the Department of the Environment.108 The WHO recommends that polynuclear aromatic hydrocarbon (PAH) levels in drinking water not exceed 10 to 50 ^g/1, the levels found in unpolluted groundwater. Since some PAHs are carcinogenic in laboratory animals and may be carcinogenic in humans, the WHO also recommends that the use of coal-tar based and similar materials for pipe linings and coatings on water storage tanks should be discontinued. This recommendation was made with the knowledge that food contributes almost 99 percent of the total exposure to PAHs and that drinking water contributes probably less than 1 percent. Tetrachloroethylene can leach from vinyl-toluene-lined asbestos-cement pipe at dead-end or low-flow sections. The health risk is considered negligible.109 Petroleum distillates, such as gasoline, can pass through PB and PE pipe and impart taste and odor to drinking water, but PVC pipe is penetrated to a lesser extent by gasoline. The PE, PB, PVC, asbestos-cement, and plastic joining materials may permit permeation by lower molecular weight organic solvents or petroleum products. The manufacturer should be consulted as to whether pipe may pass through contaminated soil.110 See also "Drinking Water Additives," in Chapter 1.

Corrosion Control The control of corrosion involves the removal of dissolved gases, treatment of the water to make it noncorrosive, building up of a protective coating inside pipe, use of resistant pipe materials or coating, cathodic protection, the insulation of dissimilar metals, prevention of electric grounding on water pipe, and control of growths in the mains. Therefore, if the conditions that are responsible for corrosion are recognized and eliminated or controlled, the severity of the problem will be greatly minimized. The particular cause(s) of corrosion should be determined by proper chemical analyses of the water as well as field inspections and physical tests. The applicable control measures should then be employed.

The gases frequently found in water and that encourage corrosion are oxygen and carbon dioxide. Where practical, as in the treatment of boiler water or hot water for a building, the oxygen and carbon dioxide can be removed by heating or by subjecting the water, in droplets, to a partial vacuum. Some of the oxygen is restored if the water is stored in an open reservoir or storage tank.

Dissolved oxygen can also be removed by passing the water through a tank containing iron chips or filings. Iron is dissolved under such conditions, but it can be removed by filtration. The small amount of oxygen remaining can be treated and removed with sodium sulfite. Ferrous sulfate is also used to remove dissolved oxygen.

All carbon dioxide except 3 to 5 mg/l can be removed by aeration, but aeration also increases the dissolved-oxygen concentration, which in itself is detrimental. Sprays, cascades, coke trays, diffused air, and zeolite are used to remove most of the carbon dioxide. A filter rate of 25 gpm/ft2 in coke trays 6 inches thick may reduce the carbon dioxide concentration from 100 to 10 mg/l and increase pH from about 6.0 to 7.0.m The carbon dioxide remaining, however, is sufficient to cause serious corrosion in water having an alkalinity caused by calcium carbonate of less than about 100mg/l. It can be removed where necessary by adding sodium carbonate (soda ash), lime, or sodium hydroxide (caustic soda). With soft waters having an alkalinity greater than 30 mg/l, it is easier to add soda ash or caustic soda in a small water system to eliminate the carbon dioxide and increase the pH and alkalinity of the water. The same effect can be accomplished by filtering the water through broken limestone or marble chips. Well water that has a high concentration of carbon dioxide but no dissolved oxygen can be made noncorrosive by adding an alkali such as sodium carbonate, with pH adjusted to 8.1 to 8.4. Soft waters that also have a low carbon dioxide content (3 to 5 mg/l) and alkalinity (20 mg/l) may need a mixture of lime and soda ash to provide both calcium and carbonate for the deposition of a calcium carbonate film.*

Sodium and calcium hexametaphosphate, tetrasodium pyrophosphate, zinc phosphates, sodium silicate (water glass), lime, caustic soda, and soda ash are used to build up an artificial coating inside of pipe. Health department approval of chemical use and pilot plant studies are usually required. The sodium concentration in drinking water is increased when sodium salts are added.

Sodium hexametaphosphate dissolves readily and can be added alone or in conjunction with sodium hypochlorite by means of a solution feeder. Concentrated solutions of metaphosphate are corrosive. A dosage of 5 to 10 mg/l is normally used for 4 to 8 weeks until the entire distribution system is coated, after which the dosage is maintained at 1 to 2 mg/l with pH maintained at 7.2 to 7.4. The initial dosage may cause precipitated iron to go into solution with resultant temporary complaints, but flushing of the distribution system will minimize this problem. Calcium metaphosphate is a similar material, except that it dissolves slowly and can be used to advantage where this property is desirable.

* One grain per gallon (17.1 mg/l) of lime, caustic soda, and soda ash remove, respectively, 9.65, 9.55, and 7.20mg/l free CO2; the alkalinity of the treated water is increased by 23.1, 21.4, and 16.0 mg/l, respectively. One milligram per liter of chlorine decreases alkalinity (as CaCO3) 0.7 to 1.4 mg/l and 1 mg/l alum decreases natural alkalinity 0.5 mg/l.

Inexpensive and simple pot-type feeders that are particularly suitable for small water supplies are available. Sodium pyrophosphate is similar to sodium hexam-etaphosphate. All these compounds are reported to coat the interior of the pipe with a film that protects the metal, prevents lime scale and red water trouble, and resists the corrosive action of water. However, heating of water above 140° to 150°F (60°-66°C) will nullify any beneficial effect. The phosphate in these compounds may stimulate biological growths in mains. In any case, the corrosion control method used should be monitored to determine its effectiveness.

Sodium silicate in solution is not corrosive to metals and can easily be added to a water supply with any type of chemical feeder to form calcium silicate, provided the water contains calcium. Doses vary between 25 and 240 lb/million gal, 70 lb/million gal being about average. The recommendations of the manufacturer should be followed in determining the treatment to be used for a particular water.

Adjustment of the pH and alkalinity of a water so that a thin coating is maintained on the inside of piping will prevent its corrosion. Any carbon dioxide in the water must be removed before this can be done, as previously explained. Lime* is added to water to increase the alkalinity and pH so as to come within the limits shown in Figure 2.7. The approximate dosage may be determined by the marble test, but the Langelier saturation index, Ryznar index, and Enslow stability indicator are more accurate methods. Under these conditions, calcium carbonate is precipitated from the water and deposited on the pipe to form a protective coating, provided a velocity of 1.5 to 3.0 fps is maintained to prevent heavy precipitation near the point of treatment and none at the ends of the distribution

Caco3 Solubility
FIGURE 2.7 Solubility of CaCO3 at 71°F (Baylis curve). (Source: C. R. Cox, Water Supply Control, Bulletin No. 22, New York State Department of Health, Albany, 1952, p. 185.)

*At a pH above 8.3, calcium carbonate is soluble to 13-15 mg/l.

system. The addition of 0.5 to 1.5mg/l metaphosphate will help obtain a more uniform calcite coating throughout the distribution system. The addition of lime must be carefully controlled so as not to exceed a pH of 8.0 to 8.5 to maintain chlorination disinfection effectiveness. Calcium carbonate is less soluble in hot water than in cold water. It should be remembered that the disinfecting capacity of chlorine (HOCl) decreases as the pH increases; hence, the free available chlorine concentration maintained in the water should be increased with the higher pH. See Table 2.1. Also note that soft corrosive water with a high pH will increase corrosion of copper and zinc; old, yellow brass plumbing can be dezincified and galvanizing can be removed from iron pipe.112

The Langelier saturation index (the difference between the measured pH and the calculated pH) can be used to determine the point of calcium carbonate stability for corrosion control with waters having an alkalinity greater than 35 to 50 mg/l. A positive Langelier index is indicative primarily of calcium carbonate (scale) deposition; a negative index number is indicative of increasing water corrosivity with -2.0 considered high. Slightly positive is the goal. The point of calcium carbonate stability is also indicated by the Ryznar index. A Ryznar index number of less than about 6.0 is indicative primarily of the start of calcium carbonate (scale) deposition; an index number greater than 6.0 to 7.0 is indicative of increasing water corrosivity. Other measures are the Enslow stability indicator and the aggressiveness index. The Caldwell-Lawrence diagram113 is useful for solving water-conditioning problems, but raw water concentration of calcium, magnesium, total alkalinity, pH, and TDS values must be known. See Standard Methods114 for procedures. Do not rely on only one method.

The AWWA recognizes the coupon test to measure the effects of physical factors and substances in water on small sections of stainless steel and galvanized iron inserted in a water line for 90 days. Measurement of the weight loss due to corrosion or weight gained due to scale formation can thus be determined under the actual use conditions. The gain on stainless steel should not exceed 0.05mg/cm2; the loss from the galvanized iron should not exceed 5.0mg/cm2. Temperature, pH, velocity, dissolved oxygen, and water quality affect corrosion rates. Coupons should preferably remain in the pipe for 1 year or longer. The test does not show the inside condition of the pipe.115

The danger of lead or zinc poisoning and off-flavors due to copper plumbing can be greatly reduced when corrosive water is conducted through these pipes by simply running the water to waste in the morning. This will flush out most of the metal that has had an opportunity to go into solution while standing during the night. Maintenance of a proper balance between pH, calcium carbonate level, and alkalinity as calcium carbonate is necessary to reduce and control lead corrosion by soft aggressive water. Formation and then maintenance of a carbonate film are necessary. See Figure 2.7. In a soft, corrosive water, sodium hydroxide can be used for pH adjustment and sodium bicarbonate for carbonate addition. Lead pipe should not be used to conduct drinking water. Low lead solders and use of plastic pipe and glass-lined water heaters will minimize the problems associated with corrosion in the home. See "Lead Removal," this chapter.

Biochemical actions such as the decomposition of organic matter in the absence of oxygen in the dead end of mains, the reduction of sulfates, the biochemical action within tubercles, and the growth of crenothrix and leptothrix, all of which encourage corrosion in mains, can be controlled by the maintenance of at least 0.3 mg/l free residual chlorine in the distribution system.

External Pipe Corrosion External corrosion of underground pipe may be caused by stray direct electric currents; buried defective electric, telephone, and TV cables and grounding connections to water mains; grounding of household systems, appliances, and equipment; direct current welding equipment; acidic soils; abrasions and breaks in external coating; anaerobic bacteria; and dissimilar metals in contact. Stray currents from electric trolleys and subways also contribute to the problem. Soil around pipe serves as the electrolyte and the pipe serves as the conductor.

Corrosion caused by electrolysis or stray direct electric currents can be prevented by making a survey of the piping and removing grounded electrical connections and defective electric cables. Moist soils will permit electric currents to travel long distances. A section of nonconducting pipe in dry soil may confine the current. In the vicinity of power plants, this problem is very serious and requires the assistance of the power company involved.

Where dissimilar metals are to be joined, a plastic, hard rubber, or porcelain fitting can be used to separate them. It must be long enough to prevent the electric charge from jumping the gap. A polyethylene tube or encasement around cast-iron and ductile iron pipe and mastic, coal-tar enamel, epoxy, or similar coating protected with a wrapping will protect pipe from corrosive soil.

Corrosion of water storage tanks and metal pipelines can be controlled by providing "cathodic protection," in which a direct current is imposed to make the metal (cathode) more electronegative than an installed anode. But it is necessary to repaint the metal above the water line in a water storage tank with an approved coating. Consult with the provider of cathodic protection equipment. A number of galvanic anodes, which are higher in the galvanic electromotive series,* such as magnesium or zinc, may be used adjacent to pipelines. The higher metal in the electromotive series will be the anode and will corrode; the lower metal (the pipe) is the cathode. The current flows from the anode to the cathode. The moist soil serves as the electrolyte. Eventually, the anode will have to be replaced.

Well Clogging and Cleaning A common problem with wells in anaerobic zones is the reduction in production capacity, usually due to clogging of the formation or incrustation of the well screen openings. This may be due to mineral scale precipitation formed around the screen and on the screen; to bacteria that oxidize iron such as crenothrix, leptothrix, and gallionella; and plugging when silt, fine sand, and clay build-up in the formation or gravel pack around the well

*Galvanic series from most active to least active: sodium, magnesium, zinc, aluminum, steel, iron, lead, tin, brass, copper, nickel, silver, gold, platinum.

screen. Anaerobic waters may contain sulfides or iron and possibly manganese. If sulfates predominate, the water will contain sulfides. Iron bacteria are found where dissolved oxygen and dissolved iron are present. The source may also be surface water contamination.

As much information as possible should be obtained concerning the well-water characteristics to suggest a possible cause of reduced well capacity before any unclogging work is done. Chemical analysis of a representative water sample and a marble test or calculation of the Langelier saturation index or Ryznar index can show if calcium carbonate could precipitate out on the well screen or if iron or manganese and incrustations are present. Comparison with analyses made when the well was new may provide useful information. High bacterial plate counts may indicate organic growths in or on the well screen. Microscopic examination can show if iron bacteria or other objectionable growths are present.116

Treatment methods include the use of acids to dissolve mineral scale and bacterial iron precipitate, but care is necessary to minimize corrosion of the well, screen, and pump. Chlorination (sodium hypochlorite) to disinfect a well will also remove and retard growth of the iron bacteria. A 1-mg/1 copper sulfate solution or a quaternary ammonium compound might also be effective. Sodium polyphosphates have been found effective in unplugging wells caused by clay and silt particles.117 Repeat treatment is usually needed, including well surging to purge the well screen and adjacent aquifer.

Water Softening Water softening is the removal of minerals causing hardness from water. For comparative purposes, one grain per gallon of hardness is equal to 17.1 mg/l. Water hardness is caused primarily by the presence of calcium bicarbonate, magnesium bicarbonate (carbonate hardness), calcium sulfate (gypsum), magnesium sulfate (epsom salts), calcium chloride, and magnesium chloride (noncarbonate hardness) in solution. In the concentrations usually present these constituents are not harmful in drinking water. The presence of hardness is demonstrated by the use of large quantities of soap in order to make a lather*; the presence of a gritty or hard curd in laundry or in a basin; the formation of hard chalk deposits on the bottom of pots and inside of piping causing a reduced water flow; and the lowered efficiency of heat transfer in boilers caused by the formation of an insulating scale. Hard water is not suitable for use in boilers, laundries, textile plants, and certain other industrial operations where a zero hardness of water is needed.

In softening water the lime or lime-soda ash process, zeolite process, and organic resin process are normally used. In the lime-soda ash method, the soluble bicarbonates and sulfates are removed by conversion to relatively insoluble forms. In the zeolite process, the calcium and magnesium are replaced with sodium,

*With a water hardness of 45 mg/l the annual per-capita soap consumption was estimated at 29.23 lb; with 70 mg/l hardness, soap consumption was 32.13 lb; with 298 mg/l hardness, soap consumption was 39.89lb; and with 555 mg/l, soap consumption was 45.78 lb. (M. L. Riehl, Hoover's Water Supply and Treatment, National Lime Association, Washington, DC, April 1957.)

forming sodium compounds in the water that do not cause hardness but add to the sodium content. With synthetic organic resins, dissolved salts can be almost completely removed. Table 2.6 gives ion exchange values. Caustic soda can also remove both carbonate and noncarbonate hardness, but it is more costly.

Lime-soda ash softening requires the use of lime to convert the soluble bicarbonates of calcium and magnesium (carbonate hardness) to insoluble calcium carbonate and magnesium hydroxide, which are precipitated. Prior aeration of excess lime is needed to remove carbon dioxide if it is present in the raw water. The soluble calcium and magnesium sulfate and chlorides (noncarbonate hardness) are converted to insoluble calcium carbonate and magnesium carbonate by the addition of soda ash and lime and precipitated. Lime softening will also remove 90 to 99 percent of the bacteria and viruses but does not remove the need for disinfection. The sodium chloride and sodium sulfate formed remain in the water. Excess lime is needed to achieve a pH of about 9.5 to precipitate calcium carbonate, and a pH of about 11 is needed to precipitate magnesium hydroxide when it is greater than about 40 mg/l. Then pH adjustment is needed to control calcium carbonate precipitation on filters and in the distribution system. Carbon dioxide gas is usually added (recarbonation) to change the calcium hydroxide to calcium carbonate to improve precipitation and to adjust the pH to 8.6 or less in the finished water. Carbon dioxide is usually produced by the burning of gas, oil, coke, or coal. A coagulant such as aluminum sulfate (filter alum), ferrous sulfate (copperas), ferric sulfate, or sodium aluminate is usually used to coagulate and settle the compounds formed, followed by filtration to remove turbidity and color. Large volumes of sludge with high water content are produced. Disposal may present a problem. Options include reclamation and land disposal. The lime-soda ash method is not suitable for softening small quantities of water because special equipment and technical control are necessary. The process is more economical for softening moderately hard water. As water hardness increases, the lime requirement increases, which makes the zeolite process more attractive. The lime-soda ash process is usually controlled to reduce hardness to about 50 to 80 mg/l.

The zeolite and synthetic resin softening methods are relatively simple ion exchange processes that require little control. Only a portion of the hard water need be passed through a zeolite softener since a water of zero hardness is produced by the zeolite filter. The softener effluent can be mixed with part of the untreated water to produce a water of about 50 to 80 mg/l hardness. The calcium and magnesium in water to be treated replace the sodium in the zeolite filter media, and the sodium passes through with the treated water. This continues until the sodium is used up, after which the zeolite is regenerated by bringing a 5 to 10 percent solution of common salt in contact with the filter media. Units are available to treat the water supply of a private home or a community. Water having a turbidity of more than 5 units will coat the zeolite grains and reduce the efficiency of a zeolite softener. Iron in the ferric form and organic substances are also detrimental. Iron or manganese or iron plus manganese should not exceed 0.3 mg/l. Pretreatment to remove turbidity, organic matter, and iron would be

TABLE 2.6 Ion Exchange Materials and Their Characteristics

Exchange

Exchange Capacity

Effluent

Regeneration

Remarks

Material

(grains/ft3)

Content

Material

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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