Document Qgy9q2q24Lz14YM6jbQvkqo6R

900 CHAPTER 51 1948 Guide 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, howr 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 done33. 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 surface thoroughly and to provide adequate clean, dry air circulation to prevent condensation (See also Care of Idle Heating Boilers, p. 357, Chapter 18). 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 following 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 be red lead paint tinted to a black or brown color, black paint made according to Federal Specification TT-P-61; red iron oxide paint con forming to Federal Specification TT-P-31, or white or light tinted paint made according to Federal Specification TT-P-40. Another paint which has had some use for priming iron and steel is zinc chromate paint. Under some conditions, a chlorinated rubber base paint made according to Federal Specification TT-P-91 may be' used for the finishing coat, particularly where the presence of highly corrosive gases or contact with strong alkaline water would injure the standard paints. Rubber base paints should be used only for the finishing coat, over regular priming and second coats. BURIED PIPE LINES Lines that are cold and in intimate contact with the earth are corroded from the same causes as in mineral waters, but pitting is usually more intense due to variations in concentration of salts and oxygen in solution and presence of solid materials, such as cinder in contact with metal pipe. Galvanic currents induced by contact of certain dissolved constituents in the soil often act over a large area, and accelerate corrosion where they leave the pipe line. Corrosion and Water Formed Deposits Causes and Prevention 901 - . Certain bacteria that thrive in the absence of oxygen have the power to f obtain hydrogen and dissociate sulphates in the soil with a resultant pro duction of hydrogen sulfide which attacks the iron to form iron sulphide. Other bacteria have preference for cathodic hydrogen and by removing it keep natural corrosion going on at a rapid rate. ' Stray electric currents from electric power generating, stations some times find their way into buried steel structures and do damage in pro portion to the current density where the current leaves the metal to enter the ground. Pipe Materials Under many conditions where steel would be corroded, the use of corrosion-resistant metals other than steel may be desirable even if greater in first cost. Stainless steel, copper, red brass, arid bronze will resist corrosiori and may, at times, be used to advantage. Sheet lead linings are excellent to prevent corrosion, and lead pipe will resist corrosion both on the exterior and interior when used for buried pipe lines as well as for pipe for general purposes. Galvanized iron pipe will resist corrosion - for various periods of time depending on the soil and how long the galvanized coating lasts. The zinc used for the galvanized coating is on the electro chemical protective side of the iron and the zinc is corroded and changed to ziric compounds before the iron is attacked. This accounts for the pro-' tection afforded by galvanized iron. Evert if some protection is obtained, eventually the galvanized coatings are destroyed by chemical action and the corrosion of the steel begins. Protective Coating Protective coatings for buried pipe lines are in a class by themselves because of the unusual service conditions and because it is not possible to maintain them by recoating when necessary. Buried steel pipe lines have . been protected against corrosion with considerable success by the use of very thick bituminous coatings applied in molten condition. The best results are obtained by applying the bituminous coatings over a standard priming coat such as red lead or a bituminous paint, arid for long service it has been found that after the bituminous coatings are applied, a wrap ping of asbestos fabric saturated with bitumens will prevent movement and displacement of the bituminous coatings and. add greatly to the length of time satisfactory protection will be maintained. Cathodic Protection Protection' is obtained by rendering the structure cathodic to the sur rounding water or soil by means of. a controlled difference of potential. This method, which has proved satisfactory and economical on a number of gas and oil pipe lines underground, has also beeri applied with some success to the protection of the inside of water storage tanks and. other structures that are in contact continuously with water. Protective coatings that insulate a large portion of the metal surface will reduce very materially the total arnount of protective current that it is necessary to impress on bare anodic areas to arrest corrosion. For each structure, it is necessary to determine or estimate the mini mum current density required and design the anode or anodes so that the necessary protection can be obtained most economically. In water having -relatively high electrical conductivity such as in sea water, this is com paratively easy compared with fresh water. In the latter, the composition