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942 CHAPTER 43 1953 Guide film-on the metal surface or to neutralization of the CO; by the alkali in theisilicate solution. It .has been postulated that ammonia,15 cyclohexylamine,26 ethylene diamine, and morpholine27 will retard corrosion of condensate lines. Tests with benzylamine have also been reported.28 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 tests29 indicate that this salt accelerates rather than decelerates, the rate of attack of steel by condensate containing C02 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 des&cmer steel REO BRASS PHOSPHORIZED COPPER PHOSPHORIZED ADMIRALTY V PHOSPHORIZED ADMIRALTY "B* ARSENICAL COPPER ARSENICAL AOMIRALTY CUPRO NICKEL 00/20 TIN COATED COPPER ALUMINUM STAINLESS STEEL V STAINLESS STEEL *B* INCONEL Fig. 6. Comparative Corrosion Resistivity op 10 Materials Exposed to Condensate sizable enterprise. Nevertheless, there may be instances where the use of a limited amount of the more costly, but resistant, materials can be justified. The data in Fig. 6 are the results 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 line, 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 line seldom occurs. No paint or similar protective coating has thus far proven satisfactory. Tests of cement lined and vitreous lined pipe have shown the linings 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 943 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 linings have been developed for coal chutes and bins to ef fectively resist corrosion and the abrasive action of the coal, but they are expensive.31 Concrete linings for steel bunkers have also been ef fectively employed.32 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 fltie 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 fife 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 during operation. It is clear that where long shut-down periods are anticipated, a practical method for mitigating corrosion is to clean the surfa.ce thoroughly and, to provide adequate clean, dry air circulation to prevent condensation. (See also Care of Idle Heating Boilers, Chapter 16). 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 inhibitive paint such as red lead paint, with the second coat tinted to a light brown color with carbon black, and the finishing coat may.