Document VM1GkJn06YGMwOzeZBbBbDBN

:f: SF ` i* ! i\\ ; |j? i ri' i !* I I! ]i1 996 CHAPTER 43 1956 Guide 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 The age old assumption that closed systems are closed is no longer valid.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) mechanical 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 they 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. Treating Chemicals Whenever chemicals are used to control scale, corrosion, algae, and slime', competent advice from a water chemist is desirable. Often factors considered irrelevant to the layman, will be of much importance in secur- Table 7. Quantities op Sodium Dichromate to be Added to Maintain Initial Concentration Specific Gravity of Brine to be Strengthened Lb Sodium Bichromate per 100 Lb CaCl* Added 1.16 1.18 1.20 1.22 1.24 0.695 0.621 0.556 0.502 0.455 Lb of Sodium Dichromate per 100 db NaCI Added 1.12 1.14 1.16 1.175 1.79 1.47 1.32 1.18 ing effective treatment. Very often troubles are created through im proper use of chemicals and are more serious than if they were not used-23 Refrigerating Systems Corrosion in refrigerating systems is confined to surfaces in contact with brines or those in contact with the refrigerant. Brines. Refrigerating brines usually are comprised of sodium chloride, calcium 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 1.5 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 sili- Corrosion and Water Formed Deposits, Causes and Prevention 997 cate and sodium phosphate are also used at times, but tests indicate they are not as effective as is sodium dichromate. It has been suggested24 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 brinies; that when salt or calcium chloride is added to "strengthen" brine, sodium dichromate also be added in the amounts shown in Table 7. Refrigerants. The common refrigerants, except those of the hydro carbon 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 on the metal surface. With the halogenated hydrocarbons, complete elimination 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.sulphur dioxide is used, more than 50 ppm (0.005 percent) of water will cause appreciable corrosion of virtually all the common materials. Minimizing Condensate Corrosiveness There are four expedients that may be utilized to minimize corrosion in steam condensate systems: (1) treatment of the boiler feedwater So as to eliminate deleterious gases entrained with the steam, (2) design of the condensing equipment to minimize dissolution in the condensate of the deleterious gases entrained with the steam, (3) chemical treatment of the condensate, (4) use of resistant metals. Boiler Feedwater Treatment. Elimination of oxygen from boiler feedwater and, therefore, from the steam developed, can be accomplished either mechanically or chemically. In some steam generating stations, both expedients are employed. Tests25 have indicated that in small low-pressure heating boilers, where the boiler input contains less than about 50 ppm of carbonate hardness, the CO2 in the steam can be controlled by adding calcium hydroxide to the boiler. In Fig. 6 are shown the equilibria conditions proposed for boilers operating at pressures up to about 5 psi gage. This expedient may not be used in higher pressure boilers, because of the possibilities of scale and sludge formations. In the latter, the only method used to date for treating the feedwater consists of removing the' alkaline earth salts, i.e., softening, and subsequent acidulation followed by deaeration at tempera tures near the atmospheric boiling point of water.26 Design of Condensing Equipment. In the design of water heaters and comparable types of condensing equipment,27 it is possible to shift the accumulation of non-condensable gases to a location away from the con densate level and, subsequently, vent these gases to the atmosphere. Venting an amount of steam equal to about one-half percent of the total steam entering the condenser is the optimum vent rate. Venting is of little practical value when the CO2 content of the in coming steam is below about 5 ppm. When the steam contains more than 5 ppm, venting provides a means of producing a condensate containing a minimum of about 3 ppm. However, even as little as 3 ppm of dissolved COi can produce active corrosion if large amounts of condensate are flowing. Chemical Treatment of Condensate. Condensates containing compara tively large amounts of oil, are practically non-corrosive, due to the pro tective film provided by the oil. When oil is intentionally added to con-