Document L16q5DKwoedgxoeGEZO3e8Rz

910 CHAPTER 42 1951' Guide film on the metal surface or to neutralization of the COi by the alkali in the silicate solution. It has been postulated that ammonia,*5 cyclohexylamine,*8 ethylene diamine, and morpholine*7 will retard corrosion of condensate fines. Tests with benzylamine have also been reported.*8 Where copper and its alloys are involved, the use of alkaline inhibitors is believed inadvisable. The use of small amounts of sodium hexametaphosphate has been suggested too, but tests*9 indicate that this salt accelerates rather, than decelerates, the rate of attack of steel by condensate containing CO>;and oxygen. Whether chemical treatment of steam or condensate is feasible, must be determined not only upon the basis of the acuteness of corrosion troubles, but also upon the uses to which the steam or condensate is put. Use of Resistant Metals. For economic reasons, the metals known to resist corrosion can seldom be used exclusively for condensate lines in any RED BRASS 10 20 24 26 O 4 0 12 16 20 24 28 32 36 40 . AVERAGE PENETRATION IN INCHES PER TEAR X IOOO Fio. 6. Comparative Corrosion Resistivity op 10 Materials Exposed to Condensate . sizable enterprise. Nevertheless, there may be instances where the use df'a limited amount of the more costly, but resistant, materials can be justified. The data in Fig. 6 are the result^ of tests30 designed to reflect the corrosion resistance of the more commonly used metals to attack by condensate containing oxygen and COj. In contemplating the use of a resistant metal, as a section of a conden sate fine, it should be remembered that, if other conditions are right, corrosive attack will merely be transferred down stream in the system. Galvanic corrosion resulting from the contact of dissimilar metals in a condensate-fine seldom occurs. No paint or similar protective, coating has thus far proven satisfactory. Tests of cement .fined and vitreous lined pipe have shown the finings to be readily dissolved by hot con densates. ' ATMOSPHERIC CORROSION Most of the problems originated by-atmospheric corrosion Occur in connection with the fire-side of boilers and furnaces (including their flues .Corrosion and Water Formed Deposits^ Causes and Prevention 911 and stacks), sewer vents, air ducts, coal and ash handling equipment. Usually such equipment is fabricated from common types of ferrous metals. Generally little or no atmospheric corrosion occurs at temperatures higher than the boiling point of water, because at such temperatures little or no condensate is formed. If it does form at the higher temperatures, only negligible amounts of carbon dioxide and oxygen present in the atmosphere, will dissolve in the hot liquid, but sulphur gases may dissolve and cause rapid attack. Oxygen, sulphur dioxide, sulphur trioxide, and carbon dioxide are the deleterious gases most frequently accountable for corrosion in moist atmospheres. Coal Storage and Handling Equipment Virtually all coals contain-sulphur in the form of pyrite, and some moisture. In storage, the pyrite is likely to be decomposed by oxidation. Moisture dissolves the products of decomposition forming sulphurous and sulphuric acid. The acid solutions vigorously attack the supporting metal. Rubber finings have been developed for coal chutes and bins that effectively resist corrosion and the abrasive action of the coal,. but they are expensive.31 Concrete finings for steel bunkers have also been ef fectively employed.3* The use of high chromium steels is not always a sure cure, especially with coals treated with dust allaying agents high in chlorides. Flues, Stacks, and Fire-side of Boilers The surfaces of flues and boilers contacting the products of combustion, seldom experience corrosive attack when the equipment is in operation. Breechings, smoke hoods and canopies in contact with flue gas may, how ever, be subject to attack during the warming-up period of an appliance, or when the rate of operation is so low that the temperature of the flue gas is below the dew-point. It is common practice to use cast-iron or acid resistant vitreous enameled steel in flue gas' connections to appliances, to prolong the life of these parts. The shut-down period, when condensation of moisture occurs on the metal surfaces, is usually the time when most damage is done.33 In those sections'.of the stacks where flue gas tempera ture drops below the dew-point, corrosion is inevitable dining operation. It is clear that where long shut-down periods are anticipated, a practical method for' mitigating corrosion is to clean the surface thoroughly and to provide adequate clean, dry. air circulation to prevent condensation (See also Care of Idle Heating Boilers, Chapter 15).. Protective coatings with organic binders are destroyed rather rapidly above 400 F because of the decomposition of the organic materials.. The surfaces of metals, whose temperature does not exceed 400 F, may be pro tected by periodically applying paints such as those specified in the. fol lowing paragraphs entitled Air Ducts. . Air Ducts The most practical method for protecting air duct surfaces made of steel from atmospheric corrosion, is to apply protective paints. One of the most effective protective coatings is red lead paint. Three coats of paint should be applied, of which the first two coats should be rust mhibitive paint such as red lead paint, with the second coat tinted to a fight brown color with carbon black, and the finishing coat may