Document OR1gNqz4Jg6MaXQ0X6MbRLGj

, 740 CHAPTER 55 1959 Gvidi through systems, where large quantities of water are used, intermittent treatment a few times each day will usually result in satisfactory slime removal and chemical economies. Neither the phenols nor copper sulfate' may be used for the removal of slime already formed. For this pur pose, chlorine gas or suitable chlorine liberating compounds may be used. After being cleaned, the other chemicals may be used to prevent the reestablishment of slime in the system. The removal of green algae from a cooling tower should never be used as an indication that the true slime forming organisms on heat exchanger surfaces have been removed. The more resistant slime formers, which so ma terially reduce heat transfer efficiency, will often be un affected by treatment which completely eliminates algae. Copper sulfate must be used with care because it can cause serious corrosion of steel in a system. It is also in effective in alkaline water because the copper is precipitated from the water. Gosed Orce-Through Systems In equipment where light is excluded, slime formations are due to fungi. Usually, they predominate on the heat exchange surfaces. Bacteria form thick, soft slime. Yeast and molds form tough rubbery slimes. Chlorine and hypo chlorite solutions, fed intermittently, are usual preventatives. UNDERWATER CORROSION When deleterious substances are present in water, the corrosivity of the solution is increased in proportion to the amount of deleterious substances present, the tem perature, and usually the rate of flow of the solution over the metal surfaces. There are other relevant factors, but their influence in general is subordinate to those mentioned. Dissolved oxygen, acid gases, and chloride salts are the corrosion accelerators most frequently encountered. Neutral and. slightly alkaline waters saturated with air, corrode iron at a rate about triple that for the same water free of air. Hot water containing oxygen will corrode iron at a rate three to four times that for the same water when cold. Corrosion of iron decreases as the pH of water solutions increases, and practically ceases at a pH of 11. Filmforming agents such as chromium, nickel, and silicon, can be added in the manufacturing of metals to provide increased corrosion resistance. In some processes, inhib itors, such as chromates, can be added to the water to minimize corrosion. Soft water, as for example the effluent from zeolite softeners, is likely to be several times more corrosive to iron than hard waters. In small installations, the use of copper or brass pipe usually is a practical expedient. Ce ment-Lined pipe and tanka suitably resist attack. Where the water contains slime-forming organisms, espe cially those bacteria that thrive on iron, chlorination of the water is imperative to inhibit tuberculation and subsequent pitting. Bitumastic paints, applied at regular intervals upon well cleaned surfaces, will measurably prolong the life of equip ment handling cold waters. It is generally agreed that the rate at which oxygen reaches the surface determines the rate of corrosion of ferrous metals. Underwater, the oxygen diffusion to a surface is often restricted by films and scale that form on the surface, and consequently the rate of corrosion differs from that of surfaces exposed to the atmosphere or to alternately wet and dry conditions. All ferrous metals will not corrode at the same rate. A thorough knowledge of the conditions to be encountered and experience gained from tests of the materials in service are required when attempting to predict performance or life in a given service. Copper, aluminum, and other non-ferrous metals and their alloys have been found effective in resisting many types of underwater corrosion. Corrosion of ferrous metals with which these metals are in contact, may be accelerated in water of good conductivity. The environmental conditions are responsible for length of service of any metal. No single material is suitable for all types of service. Once-Through Systems Where corrosion can be expected in once-through sys tems, it may be minimized through the use of one of the following methods: 1. Forming a protective film of calcium carbonate on the metal surfaces. 2. Providing mechanical or ehemic&l deaeration, or both, of the water. 3. Using organic or inorganic corrosion inhibitors, or both, in low concentrations. Formation of a thin protective calcium carbonate film is accomplished by adjustment of the water until the Langelier Index has a value greater than +0.5. This is rather difficult in systems which have a wide variation in temperature. Since the Langelier Index is dependent on temperature, when a water is adjusted to form a protective film on those surfaces of higher temperature, no film will form on colder surfaces. Therefore, this method of corrosion control is more applicable to large municipal distribution systems. Where heat transfer is involved, the method is usually unpractical except -under unusual circumstances. When this method is used, pH is iruM-p-ft-oH by using an inexpensive alkali such as lime, caustic soda, or soda ash. Lime is usually used for waters of low calcium content. Caustic soda or soda ash are used for waters of high calcium content. Mechanical" and chemical deaeration are not often used in once-through systems because of relatively high operating costs. Mechanical deaeration alsn requires the use of costly equipment. Chemical deaeration is usually accomplished by raising pH with caustic soda and by continuously feed ing catalyzed sodium sulfite' in proportion to water flow. The use of catalyzed sulfite is not permitted in water to be used for potable purposes. . There are several corrosion inhibitors such as chromate, polyphosphates, and silicates which are effective in oncethrough systems." Polyphosphate at a concentration of 2 to 5 ppm has been found useful in controlling tuberculation of iron pipe and in reducing overall corrosion. Usually, more effective treatment is to use chromate polyphosphate at concentrations of less than 60 ppm. Sodium silicate is often used in relatively soft waters by increasing the silica content about 8 ppm It is used pri marily to reduce corrosion in potable water supply systems. Open Recirculating Systems Corrosion in open recirculating systems, such as air washers and cooling towers, is usually controlled by use of Corrosion and Water-Formed Deposits, Causes and Prevention 741 corrosion inhibitors such as chromates, polyphosphates, a combination of chromate and polyphosphate, and nitrites."-" Mechanical or chemical deaeration is not practicable in open systems because of the high rate of aeration. It is not practica ble to use high pH values of 11 or more in systems containing wood. High pH will cause serious delignification of wood. High pH values prevent control of scale. Corrosion con trol is usually carried out in the pH range of 6.5-8. Chromates are by far the most effective corrosion inhibi tors. It is extremely important, however, to maintain an adequate concentration of 300-500 ppm, which is effective for most systems. If for economic reasons substantially lower concentrations are used, serious pitting corrosion may occur. Polyphosphates are most effective in reducing tubercula tion. It is not usually possible to reduce overall corrosion to anywhere near the degree possible with chromate. Where economy of treatment is of primary importance, a substantial reduction of pitting and overall corrosion can be obtained by using as little as 60 ppm of a mixture of poly phosphate and chromate. Close control over pH is a re quirement for good corrosion control by this process. Sodium nitrite has not had widespread use as a corrosion inhibitor in open recirculating systems. It has been reported that difficulty may be encountered in maintaining effective concentrations. Considerable'field experience is needed to' further qualify this inhibitor. Brines. Refrigerating brines usually are comprised of so dium chloride, ealoiiim chloride, or calcium and magnesium chlorides. The corrosivity of dilute brines is higher than their more concentrated solutions. The corrosivity of sodium brines, other conditions being fixed, is about 15 times greater than brines of the alkaline earth metals. Brines are excellent electrolytes. Contact of dissimilar metals of wide potential differences, when in contact with brines, results in rapid corrosion by galvanic action. The leakage of air, acid refrigerants, or both, accelerates the corrosivity of brines. Ammonia precipitates calcium and magnesium salts, thus clogging the system at restricted points. The addition of caustic soda and sodium dichromate to brine solutions to inhibit corrosion of iron, is a more or less general practice. Sodium silicate and sodium phosphate are also used at times, but tests indicate they are not as effective as is sodium dichromate. It has been suggested" that 125 lb of sodium bichromate per 1000 cubic feet of calcium chloride brine, and 200 lb per 1000 cubic feet of sodium chloride brine, be added to inhibit brines; that when salt or calcium chloride is added to "strengthen" brine, sodium dichromate also be added in the amounts shown in Table 6. - Table 6 .... Quantifies of Sodium Dichromate to be Added to Maintain Initial Concentration Closed Recirculating Systems The term Closed Recirculating System is in reality a mis nomer. Except for relatively mud! systems, most closed sys tems are open because they most usually require make-up water. Recently, tests conducted on 84 closed- systems indi cated that more than 50 percent of the systems had one or more water changes per month because of leakage. Weekly water changes were found in more than 10 percent of the systems." Continuous make-up, of course, replenishes oxygen-in the system, thus promoting corrosion. It is imperative, therefore, that corrosion control be provided for most closed systems. The age old assumption that closed systems are closed is no longer valid. Corrosion control is usually accomplished by (1) mechani cal or chemical deaeration or (2) use of corrosion inhibitors such as chromates and nitrites: The use of polyphosphates is not generally recommended for closed systems because thgy- will revert to ineffective orthophosphates unless there is a Large replacement of water containing polyphosphate. Higher concentrations of chromate are usually maintained in closed systems as compared -with open systems. This is primarily due to the fact that, since water losses are usually small, the cost of maintaining excess chromate as a safety factor is small. Speofie Gravity of Brin* to bo Strengthened 1.16 1.18 1.20 1.22 1.24 1.12 1.14 1.16 1.175 -lb Sodium Dichromate per 100 lb Cad* Added 0.695 0.621 0.556 0.502 0.455 15 of Sodium Dtdirotaate per 100 lb Nod Added 1.79 1.47 1.32 1.18 1 Refrigerants." The common refrigerants, except those of the hydrocarbon type, will attack the common metals and alloys if moisture is present. Even a very small amount of water may cause severe corrosion with certain refrigerants. The amount required need only be sufficient to produce a water film oh the metal-surface. - With the halogen&ted hydrocarbons, complete elimination Treating Chemicals Whenever chemicals are used to control scale, corrosion, algae, and sfime, competent advice from a water chemist is desirable. Often factors considered irrelevant to the layman, will be of much importance in securing effective treatment. Very often troubles are created through improper use of chemicals and are more serious than if- they were not used." of water is much to be.desired. Where ammonia is used, copper and its alloys, - aluminum and zinc, are- attacked especially at elevated temperatures. When sulfur' dioxide is used, more than 50 ppm .(0.005 percent) of water will cause appreciable corrosion of virtually all the common ma terials. Minimizing Steam Condensate Corrosion Refrigerating Systems Corrosion in refrigerating systems is confined to surfaces in contact with brines or those in contact with refrigerant. . - There are four expedients that may be utilized to minimize corrosion in steam condensate systems: (1) treatment of the boiler feed water go-as to eliminate deleterious gases en trained with the steam, (2) design of the condensing equip-