Document x5OoyB09RR9gZG4ezk63d87O1

226 CHAPTER 15 1965 Guide And Data Book be protective. Calcium carbonate scale in hard waters repre sents such an example. Or, temperature ma" affect the nature of the corrosion product, which may be relatively stable and protective in certain temperature ranges, and unstable and nonprotective in other temperature ranges. Zinc in distilled water represents such a case, the corrosion prodact being nonprotective in the range of about 140 to 190 F, but reason ably protective at other temperatures. Thus, the effect of temperature on a particular system is difficult to predict without specific knowledge of the characteristics of the metals involved and the constituents of the media. Pressure As is the case with temperature, it is difficult to predict the effects of pressure on corrosion. Where dissolved such as oxygen and carbon dioxide may affect corrosion rate, pres sure on the system may increase their solubility.and thus increase corrosion. Similarly a vacuum on the system will reduce dissolved gas solubility and thus reduce corrosion. In a heated system pressure may raise the boiling point of the media, and thereby affect corrosion rate through the effects of temperature. It is seldom possible or practical to control corrosion of a metallic system simply by means of pressure control Velocity The effects of velocity of flow of the media in a system will depend upon the characteristics of the particular metal or alloy. In media where oxygen increases corrosion rate, for example iron or steel in water, velocity of flow can increase corrosion rate by making more oxygen available to the metal surface for reaction. Under the same circumstances, alloys dependent upon thin oxide films for corrosion resistance may have such films enhanced due to velocity, and remain quite resistant up to very high velocities. Such is frequently-the case with BtainUma steels. For metal systems in which corrosion products retard corrosion by acting as a physical barrier, velocity of media flow may sweep away these products and, therefore, permit corro sion to proceed at its initial rate. In specific media, and for a specific metal, there may be a critical velocity below which the corrosion product film is adherent and protective. Turbulent media flow may cause uneven attack, involving localised erosion as well as corrosion. Such attack is refereed to as erosion-corrosion, and may occur in piping systems at sharp bends if the designed flow velocity is rather high! Very high velocity may lead to localized cavitation, a damage mechanism involving bubble formation and enllnp*. which literally tears out particles of metal Such attack may occur in pump impellers and mir^r propeller blades. For more information on the effects of velocity, referNto Copson.* PREVBTTIVE AND PROTECTIVE MEASURES Materials Selection In theory, it is possible to construct almost any .piece of equipment for a heating or air-conditioning system.with materials which will not corrode to any significant degree under service conditions. However, this is rarely pcesible be cause of economic and physical limitations. For example, silver is a corrosion-resistant metal with excellent heat trans^ fer properties, but it is much too expensive for common use in condenser tubes. Many plastics resist the chomifal action of flue gases hut they are not-used -for fireside service because they melt or burn at flue gss temperatures.............. When selecting materials for the construction of a given Piece of equipment, it is necessary to consider the corrosion resistance of each proposed metal or other material to the service environment, the nature of the corrosion products which may be formed, and their effects upon the functioning of the equipment, the suitability of the materials to handling by standard fabrication methods, and the effects of design mid fabrication limitations upon tendencies toward local corrosion (as discussed in the preceding section). The overall economic balance during the projected life of the equipment should also be considered. For example, it may be less expend sive in the long run to pay more for a corrosion resistant material and avoid the need for regular painting or other corrosion control measures, than to use a less expensive ma terial and be required to cany out a regular corrosion control maintenance program throughout the life of the equipment. Other considerations include selection of materials used during fabrication and the methods of fabrication to be em ployed. Acid fluxes, for example, if not property removed from a piece of equipment, can aggravate localized corrosion after the product goes into service. Both metallic and nonmetallic materials of construction are utilized in heating, refrigerating, and air-conditioning equip ment. Ferrous metals`are widely used and include cast iron and cast steel, wrought iron, mild steel, and many alloy steels, including the so-called stainless steels. The less corrosion resistant forms of ferrous metals may be protected in many ways including galvanizing and other forms of zinc coating,cadmium plating or other types of plating, painting, and plastic dip-coating. Copper and a variety of copper-base alloys, inelndfag brasses, bronzes, and cupronickel, are widely used for con denser tubing and other applications requiring greater corro sion resistance. Aluminum and its alloys are also nwl under suitable environmental conditions. Nonmetallic materials of construction include wood and concrete (frequently used for cooling tower construction), rubber, plastics, fiberglass, and fabrics. A variety of protec tive coatings are also in use. ' Materials of construction (metallic or nonmetallic) fre quently used for components of air-conditioning and refrigera tion equipment are summarized in Table 2." Protective Coatings Surface Preparation. One of the most important factors in corrosion control is the condition of the substrate prior to coating. For this reason, surface preparation should be considered with great care. The environment in which the system components will eventually operate, will largely determine the type of coating needed. Each coating has a particular tolerance for surface contamination which will determine the degree of surface preparation necessary. Coatings which require the removal of all contaminants are not necessary in some environmental conditions. The presence of rust and mill scale often presents a problem in surface preparation. Mill scale is a source of trouble be cause it does not have the -same coefficient of expansion as steel and, as a result, it may become cracked and loosened during fabrication and shipping.or during expansion and contraction of the metal. As a result, moisture and con taminants may gain access to exposed areas Mid cause corro sion. Surface preparation is generally divided into two types, mechanical and chemical. Mechanical deeming methods are generally described as followa, and are more effective in descending order of listing. ' Confcion^and Deposits Table 2 .... Composite-Reference of Suitable Materials for Equipment Components*'* 227 qu(paie8f ruling Towers Evaporative Condensers Cooling Cods (condensation) Air Washers Room'Aif-Conditionera (condensers and evaporators) -' Air-Cooled Condensers Heading CoOs Ducts and - Support) GridyKn. ETCmfoaton Pipe* Heat Vatves Duct* Transfer Surface* G c G. c 0 p. P p s ss w -S3 w S s- CG cG cG c G' cG c 0. p :P. .P : p 0 p. p. s ss w. SS S ss s A C *G cG A c G^ c p P. p P. s ss W 8 ' 'ss ss s ss AcG G cG c cG c 0 p P, P p 0 p. P. s ss ,W. SS; S ss ss ss s A c G A Ci G A c G AcG c 0 p Pe 0 P ; P p P. p s ss W SS W ss ss s ss A c G A c G A- c G . A- c G A c G. A c 0 ;p P, 0 p. p O P P. s ss W 8 ss W S_. ss s. ss W S ss s ss A c G A c, G A c G AcGAc 0 p* P. 0 p*l P s ss W s ss S ss P. S ss S AcG O s ss W Fans - * Below ISO F. -r -- . ? Bated as date Iron Rcfennee II., h Metallic or Noo^eteflk material* oasd Ira O -- Organic Fioiibca a toOem: * ` K' i-C, Copper and & ` P ---piutia V? I Wood Fara CG P. SS G P,- s G P. ss AcG p. P. ss AcG P. S ss AcG p P, S S8 1. Hand cleaning ie one of the oldest and most commonly used methods ofcleaning, accomplished by use of a wire brush, scraper, or chipping hammer. 2. Power tool denning is done with rotary wire brushes, de- scalers, and rotary sea]era. ' 3. Flame cleaning utilizes a torch which b played on the steel to take advantage of the difference in the rate of expansion between the base metal and the mill scale. An advantage.of this type of cleaning, in some'case&'is that the steel can be painted while it is still warm, thereby ptiminatirig for all practical^piirposes, the problem of adsorbed moisture. '/ ' 4. Blast ckoTUng involves abrading' surfaces withhigh velocity impact of abrasive particles. I Chemical Cleaning. Many types of chemical surface prep arations are available, and selection depends on.the type and degree of contamination. A brief description of each follows. 1. Solvent wiping and vapor degreasing is the least effective of Id] types of chenural surface preparation, but it is still one of the most widely used. In solvent wiping, the surface is wiped witha suitable solvent such as naphtha or toluol In vapor degreasing, the item to be cleaned is exposed to the solvent in its vapor phase. The solvent condenses on the metil surface and washes off the contaminant. `This method is'more satisfactory than wiping, because it can clean irregular surfaces better, arid solvent contamination is not a factor. Solvent wiping