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888 . CHAPTER 42 1950. Guide film on-the metal surface, or. to neutralization of the COj by the alkali in the silicate solution. . It has been postulated that ammonia,22 cyclohexylamine,26 ethylene diamine, and morpholine27 will retard corrosion of condensate lines. Tests with benzylamine have also been reported.21. 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 COj 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 8C33CMER STEO. nb 6ata aI: 90 DAY TEST eo bat' test 30 DAT TEST III COO.EO SPECIMEN* Q UNCOILED SPECIMENS I UNMEASURABLE VALUES IMIIII 4 6 12 16 20 24 28 0 4 8 12 10 20 34 2S 0 4 6 12 IS 20 24 2B 3a 36 40 AVERAGE PENETRATION ' IN INCHES PER TEAR X 1000 Fig. 6. Comparative Corrosion Resistivity, op 10 Materials Exposed to Condensate sizeable 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 tests20 designed to reflect the corrosion resistance of the. more commonly used metals to attack by condensate containing oxygen and C02. 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 vitreouslined 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', V Corrosion and Water Formed Deposits; Causes and Prevention 889 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 boding 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 hotliquid, 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 thesupporting metal. : Rubber linings have been developed for coal chutes arid bins that effectively resist corrosion and-the abrasive action of the coal, but they are expensive.11 Concrete linings for steel bunkers have also been ef fectively employed.22 ,: ' 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 copomon practice to .use cast-iron or acid resistant vitreous enameled steel in flue gas cphririctidhs' tiyappliarices,' to prolong the life of these parts. The shut-down period,: wheircoridehsatiofl of moisture occurs on the metal surfaces, is usually the`time when most damage is done.21 In those sections of the stacks where flue gas teinperature drops below the dew-point, corrosion is inevitable during operation. . It. is clear that where long shut-down periods are anticipated, a practical iriethod for mitigating corrosion is to clem the Surface thoroughly and to provide adequate clean, dry air circulation to prevent cbnderisation. .(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, iriay be pro tected by periodically applying paints such as those specified in the fol lowing paragraphs entitled Air Ducts. . ; > . . '." Air Ducts . '' : 1; . 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. . i i. 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