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240 CHAPTER 15 1965 Guide And Data Boole For new subsurface structures, it is-therefore good practice to include corrosion'control measures-in the`design. Such cor rosion control measures must be selected and adapted to meet the specific conditions in which the structure will exist- A' cor rosion survey of some form is usually necessary before corro sion control measures can be designed for underground struc tures.* Protective Coatings-^-Electrically Insulating ; Protective coatings are applied to underground structures to isolate them from the soil environment and to insulate them from electrical effects. Such coating materials must possess long term qualities of high electrical resistance, inertness to the environment, low water absorption, high- resistance to deformation by soil pressures and.the temperatures.at.which they are operated, and good adhesion. Most coatings require reinforcing, shielding, or both, to resist soil stresses. The per formance of.afl coating systems, even those.of the best ma^ terial specifications, are.dependent upon-'the-metal surface preparation, application procedure and conditions, backfilling and the physical, chemical, biological, and electrical stresses that occur in service. The coatings most commonly, used on underground struc tures include the following: 1. Hot-applied coal tar enamel. ':r' 2. Hot-applied petroleum base coating,-- such as asphalt enamels and waxes. 3. Polyethylene or polyvinyl (tapes and extruded polyethyl ene). 4. Coal tar epoxy resin. 5. Cold-applied bituminous'and asphalt emulsions,' cutback solvents, greases, etc. ............... Types of Soils Hie corrosivity of a soil is affected by. its porosity (aera tion), electrical resistivity, dissolved salts.(depolarizers and inhibitors), moisture,' and. acidity or alkalinity. Although corrosion rates and characteristics cannot be exactly related to the individual factor, the attack on a metallic structure buried in nonuniform soil will generally be greatest on those surfaces in contact with the least porous (air-free), least resistive, most.saline,.most moist, or the most acidic soil. The National Bureau of.Standards hag compiled the extensive data*7 on a series of field tests on various metals and coatings in. many .typical soils throughout, the United States. These studies, begun in 1910 and continued until 1955, are the most important source of information on soil,corrosion available. . Bacterial High rates of corrosion in some air-free environments have been found to be associated with the presence of sulphate re ducing bacteria (sporovibrio desulfuricans). The role of these bacteria in the corrosion process is being-vigorously studied.** Tests on bacterial growth have shown some coating materials are susceptible to biological attack. Bacterial activity- should not be considered a special and new form, of, corrosion. The conventional use .of well applied coatings,, and cathodic .protection will control corrosion on underground metallic structures in the presence of these bacteria. Coatings which disbond, allowing water'and cor rodent to reach.the metal surface, will shield the exposed metal from the effects of applied cathodic protection. The. performance of the cold-applied coatings is generally found to be the least effective. Each of the coatings is formulated by its manufacturers for specific application and service conditions. The Technical Practices Committees of the NACE have prepared statements on minimum requirements for coaltar coatings,** asphalt-type protective coatings* and prefabricated plastic films.** Cathodic Protection . : ,i-> Although cathodic protection can bcapt>Iied to bare struc tures underground, it is most widely,used to provide corrosion control for exposed metal at the costing flaws.(holidays) which; inevitably occur on the costing during and after installation.- The rate of metal penetration at coating flaws is often greater than that on bare surfaces. , The cathodic protection of underground pipelines and tanks can be accomplished by either galvanic anodes (magnesium or sine) or impressed current systems. Impresed current may. be applied from anodes distributed,alongside,the. structure,, from remote anodes, or from deep well anodes.. , The method used depends upon the economics and the site conditions .affecting the cathodic protection design require^, meats. . , .. , Full cathodic protection is achieved when the current ap-' plied to the structure is an amount sufficient to prevent cur rent flow into the soil from any and'all 'points on the surface of the structure; 1. '* - Specifically, this protective current must be applied to the' structure in an amount sufficient to maintain its external sur face, at every point, negative by at least 0.85 volts to a copper- saturated copper sulfate half ceil in the immediate proxim ity thereof.* NACE and National Bureau of Standards References Most of the significant papers presented at meetings of the National. Association of Corrosion Engineers, which are pertinent to underground corrosion and protection are pub lished in its journals, Corrosion and Materials Protection. Spe cific recommendations (minimum .requirements) for under ground pipeline corrosion control are developed by Technical Practice Committee T-2. A statement prepared by'this Committee on minimum re quirements for corrosion control of underground pipelines*? reads as follows: 1. Underground .steel;piping .transporting petroleum, gas or - related products, where. tnmayerring populated areas or mating' in such,manner as-to create anasard to public safety, Shouldbe coated'with a protective material of per manent high electrical resistivity. Such coatings/ if not so . compounded or re-inforced as to resist soil stresses, shall be shielded in such manner as to receive negligible damage from such stresses. . 2. Where environment indicates coating, as set out in No. 1, - the protective'coating should be supplemented byapplica- - - tion of adequate cathodic protection of such coated pipe. 3. Where question, may arise as to the applicability of these requirements due to existing and long established methods, the problem should be passed to'the local electrolysis com< mittee for study and recommendation. It is recommended that such committees be composed ,of qualified engineers who are cognizant of the overall-problem and are free and capable of making objective derisions. The most' comprehensive, long-term'study of underground corrosion has been conducted by the National Bureau of Standards. The studies of soil corrosivity, materials, coatings,' cathodic protection, stray currents, etc., conducted over a period of 45 years have been published.*7 CorrosioiCand-Deposits ' c 241: Insulatiwi failures ....:: The shut-down period, when condensation of moisture oc curs on metal surfaces, is usually.the time'when the .greatest ` Jkiost tVapal insulation does not provide corrosion protect damage is done. When long shutdown periods are anticipated; tion in soils or water.Catastrophic corrosion occurs'on hot a practical method for minimising fire-side corrosion .is :to mpe surface when, intermittently, contacts! by,water/* This clam the surface thoroughly and to provide adequate dean, corrosion process cannoi be controlled by cathodic protection: dry air circulation, in order to prevent' condensation. (See insulation,- surii as' fiber glass,* magnesia-asbestos, etc., also Care of Idle Heating Boilers, Chapter 52.) ,` must be kept dry in underground runs through conduits'. Notable examples of corroaon have been experienced within :- . REFRIGERANT SIDE CORROSION a few1 months,after construction.on magnesia-asbestos insu- lated snow melting pipe manifolds extending below the slab into''soil: ` /' ` Natural asphalt materials,used for underground pipe, heat Corrosion problems which develop,on the refrigerant.side of equipment, and methods for their control, are discussed in Chapters48,-49, and'50. ' insulation ,exhibit some. corroaori .protective effect. How ever, they-are relatively permeable to water,-and repeated toting and'cooling 'initiates attack under the insulation: Similarly, common attack has been experienced at' locations rf1fluctuating'water table. . ;--- - . . Radiant heat and snow melting pipe embedded in slabs constructed with venniculite-filled concrete, brick, orconcrete block supports, and open expansion joints, have been subject to severe corrosion. Severe corrosion of radiant beating pipe embedded in sand or porous concrete under terraszo flooring has also been experienced. It is good construction practice, in * ,5 REFERENCES ^ I Manual on Industrial Water and Industrial Waste Water . (Special Technical Publication No.- 148-D, 2nd ed., American Society for Testing Materials, Philadelphia, 1959). * U. R. Evans: The Common and Oxidation of Metals (Edward Arnold Ltd., London, 1930). * P. Hamer, J. Jackson, and E. F. Thurston: Industrial Water Treatment Practice (Butterworth A Co., Ltd., London. 1961). * E. Nordell: Water Treatment for Industrial and Other Uses (Reiahold Publishing Co., New York, 1961, 2nd ed.). * S. T. Powell: Water Conditioning for Industry (McGraw- such hating systems, to avoid severe corrosion problems by mting a minimum thickness of 1 in. of cement-rich concrete to completely surround the piping, a water-proof membrane under the slab, and steel saddle pipe supports. The piping should be coated at expansion joints and pipe should not come in contact with the reinforcing steel. Lightweight or insulat ing concrete produces local cell activity at the nonunifonnities in contact with the piping. The piping should be insulated from all other metallic structures. Magnesium anodes buried in the adjoining Boil may be attached to the piping to provide protective effect Although cathodic protection has been applied to radiant heating piping, it is not always possible to direct adequate amounts of protective current to corroding surfaces. The best corrosion control measures are those integrated into the original design and construction of the heating system. FIRE-SIDE AND HIGH TEMPERATURE CORROSION The surfaces of flues and boilers which come into contact with combustion products are seldom corroded while the equipment is in operation. Breechings, smoke hoods, and canopies in contact with flue gas may, however, be subject to attack during warmup periods or when the rate of operation is so low that the temperature of the flue gas is below its dew point. In those sections of stacks in which flue gas temperatures drop below the dew point, corrosion is inevitable during opera tion. It is common practice to use case iron or acid-resistant vitre ous enamelled steel in flue gas connections to appliances in order to prolong the life of these parts. Protective coatings with organic binders are destroyed rather rapidly above 400 F, because of the decomposition of the organic materials. Metal surfaces with temperatures that do not exceed 400 F may be protected by periodic applications of paints. Protection of Hill Book Co., New York, 1954). * H. H. Uhlig: The Corrosion Handbook (John Wiley A Sons, Inc., New York, 1943). ' tl. R. Evans: An Introduction to Metallic Common (Edward Arnold, LtdL, London, 1963, 2nd ed.). * H. H. Uhlig: Corrosion and Corrosion Control (John Wiley A Sons, Inc., New York, 1963). * W. H. J. Vernon: A laboratory study of the atmospheric corrosion of metals (Faraday Society Transactions, VoL 31, 1935, p. 1668). "H. R- Copsou: Effects of velocity on corrosion by water (Industrial A Engineering Chemistry, Vol. 44, 1952, p. 1745). II Corrosion and Its Prevention (Air Conditioning and Refriger ation Institute, Arlington, Va., 1958). u L. P. Sudrabin: A review of cathodic protection theory and practice (Materials Protection, VoL 2, No. 5, 1963, p. 8). u R. H. Brown and R. B. Mears: Cathodic protection (Trans actions of Electrochemical Society, VoL 81, 1942, p. 455). u L. P. Sudrabin: Designing automatic controls for cathodic protection (Materials Protection, Vol. 2, No. 2, 1963, p. 64). LP. Sudrabin et al: Some effects of cathodicprotection on conventional paints (Common, Vol. 8, 1952, p. 109). u Bets Handbook of Industrial Water Conditioning (Bets Labo ratories, Philadelphia, 1962, 6th cd.). 1T W. D. Collins: Typical water analyses for classification with reference to industrial use (American Society for Testing Ma terials Proceedings, Vol. 44, 1944, p. 1057). " S. Sussman and I. L. Portnoy: Water composition changes in air conditioning equipment (Journal cd the American Water Works Association, VoL 51, 1959, p. 953). 17 S. Sussman and J. B. Fullman: Common in closed circu lating water systems (Heating and Ventilating, October 1953, P- 77). ** R. EU&ssen, R. T. Skrinde, and W. B. Davis: Experimental performance of "miracle" water conditioners (Journal of the American Water Works Association, VoL SO, 1953, p. 1372). * B. Q. Welder and E. P. Partridge: Practical performance of water conditioning gadgets (Industrial and Engineering Chem istry, Vol 46, 1954, p. 954). ** D. Pye: Chemical fixation of oxygen (Journal of the Ameri can Water Works Association, VoL 39, 1947, p. 1121). ** W. F. Inngeiier: The analytical control of anticorrosion water treatment (Journal of the American Water Works Associa 'stacks and other surfaces which reach higher temperatures may be achieved by the use of other coatings, such as the silicones, that are resistant at the service temperatures. tion, VoL 28, 1936, p. 1500). ** J. W. Ryxnar: A new index for determining amount of cal cium carbonate scale formed by a water (Journal of the American Water Works Association, VoL 36, 1944, p. 472). Fire-side corrosion be minimimd by modification of the * W. L. Denman: Maximum re-use of cnnling water based on firing cycle so as to minimiw* the number oftimes that the flue gypsum content and solubility (Industrial and Engineering i gas temperature drops below its dew point, by specifying low Chemistry, Vol. 53, 1961, P- 817). * J. Green and J- A. Holmes: Calculation of the pH of satura sulfur content fujels, and, in certain types of ingtaHatinne, by tion of tricalcium phosphate (Journal of the American Water the use of fuel oil additives which neutralise the acidic con Works Association, Vol. 39, 1947, p. 1090). densates on the fire-dde surfaces. n S. Sussman: Non-chemical factors affecting inhibitor selec- > i