Document MJrmqzjw4nEx3r1yBZg649zEM

228 CHAPTER 15 1965 .Guide And Data Book b sometimes used as a preliminary step in other cleaning methods..-,) _2- AliaH doming is generally preferred over solvent donning Hie solution can t applied by spray or fringe depending,on the size of the article. Alkaline cleaners are water soluble and are _ generally used at high temperature, although cold types are available^ 3. Evudeifiable advent deeming is used to remove oils and greases. Emulsiliable solvents offer the advantage of flushing the aoQ away during the rinsing operation, leaving only a very thin film of solvent on the surface, which can later be removed if neeeosiy. The emulsifiable solvents are either used in dip tanka or are sprayed or brushed on the surface and allowed to stand for_ a few minutes. The loosened and dissolved matter is then rinsed with water. 4. Steam deeming involves abrading surfaces with high velocity, unpact of steam. Inis type of cleaning is very effective, but must often be augmented by wire brushing or sandblasting. 5. Add doming generally consists of treating the metal with an add containing oil solvents such as alcohols, ethers, ketones, etc., to aid in removing oil and oil-type products. Detergents _ wetting agents are incorporated to assist in wetting the nrfn^ and, ramoving soils. Add cleaners of thin type effectively remove grease, oil, and other surface contaminants, but acids alone are not effective on greases. This method differs from alkali or emul sion cleaning in that it removes light rust and minutely etches the surface of the metal, thus improving the adhesion properties_ of the coatings by providing a good mechanical base. The use of a; phosphoric acid cleaner has the added advantage of reacting with' the steel to produce a thin film of insoluble iron phosphate, which enhances paint adhesion. If mill scale is too heavy, phosphoric acid solutions may require too much time, in which case, sulfuric add pickling, may be em ployed where practical. Shop Applied Coatings. Shop applied coatings offer the best overall opportunity for the best results. This is true' because better conditions usually exist in the shop.. . .. v- cor.r,Table;3 Typical Paint Systems Sfttetat Adrtmhjg** Duodvoatogt Alkyd 1. Good adhesion: to 1. The principal disad most substrates. 2. Skill required for ap vantage is that they have Only minimn^ plication is minimal. 3. Reasonably high sol ids allow for maxi mum mil thifkrqa resistance to add or alkali contamination. in a minimum numft ' ber of coats.' High solids facilitate cover- tern so that proper `' protection is achieved. In general, alkyd for- , muJations will takecontinuous operat ing temperatures of around 200 F. Vinyls . 1. Excellent gloss. and 1. Relatively tow oper color retention. : i 2. The beat known air-- dry, thin1 film, add ating temperature limit in ranges of 130140 F. resistant coating. 2. In relation to other 3. Excellent moisture coatingB, relatively resistance, both im low solids with the mersion and splash.- high in the range of 30 percent by volume. 3. Need for better sur- face.'preparation. than other types of coatings because of poor-ability to wet. Hie use of a piece of equipment will largely determine- ' the type of surface preparation and coating to be used. If the -. equipment will be used under very severe conditions, either, by nature of the surrounding environment or the environ ment of its own manufacture, shop posting is recommended"* if at all possible. It is at this stage that heavy members will probably.be sandblasted and lighter members chemically treated. There - -Converted or 1. Excellent alkali re 1. Chalks readily, al Catalyzed sistance though notadegrading Epoxy 2. J High film build with type ` of chalk,.- as minimum-number of would be experienced coats. in oil films. 3. Chemical conversion 2. Always a two compo ' - assuringabettercure. nent system, necessi 4. Excellent abrasion re- tating a relatively -. sistance. : ' ' short pot life: are of course, many types of coatings available. In a broad sense, however, there are three basic types of environment, and three generic types of coating can be considered. should be scheduled after installation to detect mw.Wnii.nl Normal mild atmospheric corrosion ean be effectively pre damage. Damaged areas should be noted and repaired. .. vented by alkyd resin systems. These systems can undergo lees than perfect cleaning and still give reasonably good serv Cathodic Protection ice. A vinyl resin system along with the best surface prepara- ; tion is recommended when exposure to add conditions is anticipated. If the equipment is to be used under severe alkali conditions, the best type coating will be a catalyzed epoxy. _ Some of the advantages and disadvantages for each of the The cathodic protection principle is widely used to control corrosion in hot and cold water storage tanka, heat exchange water boxes, water and chemical processing equipment such'as filters, reactors, and clarifiers, and-the external surfaces of submerged ^and- underground tanks, piping, and piling Cathodic protection may be used with iron, aluminum, lead,' broad classifications are given in Table 3. A guide for wtaiingg based on Federal Specification materials by end use is given- in Table 4. -^ Field Applied Coatings. Field applied coatings can, in gen stainless qteels, etc.,On new steel'structures, optimum corro-sion^control design is often achieved by applying cathodic .protection in combination' with coatings, environment condi tioning, or both. eral, be of the same types as shop applied coatings. The limiting factor will be accessibility.' Once equipment is ,in place, it becomes very difficult to use the type of surfaoe preparation that-will do the best job due to obstructions and!' accessibility. Maintenance of Protective Coatings. Defects in a nhating are virtually inescapable.-These defects can be caused either by; coating flaws in the film during application, or by mw.Wm.Mil damage after coating. In either event, the damage must be repaired in order to eliminate. premature failure at these points. Depending upon the severity of the service, an inspection The cathodic protection principle is unique in that protec- tivsKeffects.pan be directed from .distantly. positoraed anode current sources onto existing submerged or buried structures., Corrosion control can generally be accomplished by cathodic protection without taking the' facilities out of service, exposing the.surfaces,for coating,.or specially, treating the surrounding environment. ! \ . , yCathodic protection,-by definition, is the reduction or pre vention of coriosioh by making the metal the cathode in a con ducting medium By means of directelectric curient which is" either impressed or galvanic.,, . !j(',.' , ,!.., Since this process superimposes-the effect; of. an applied Corrosion: and Dcposifs . 229 Table}4 . - - Guide for Coatings Based on Federal Specification Materials by End Use .Expoiww Pry Interior use Prinwn TT-P-636-B Synthetic Alkyd Primer fallen--dkCe ond fioiA TT-E-489-B Gloss Synthetic Enamel ' or -TT-E-508 Semi Gloss Synthetic Enamel or TT-P-51D Flat Alkyd Enamel Exterior Exposure - ' (normal weather conditions). TT-P-615-B Bade Lead Silico Chromate Alkyd Primer ....... or TT-P-636-B Synthetic Alkyd Primer Interkm-Exterior Heavy Moisture`or Add Conditions if. C. Primer MIL-P-15328-A Vinyl Wash Primer . Primer MIL-P-15929-A Vrnyl Red Lead Interior-Exterior Alkali7 Conditions -. MIL-P-23377 Primer Epoxy Polyamide TT-E-48d-B Gloss Synthetic Enamel Intermediate MIL-P-15929-A Vinyl Red Lead '' Intermediate MIL-C-22750-A Coating Epoxy Polyamide Finish MIL-C-2275-A Coating Epoxy Polyamide . current' onto* an aviating' electrochemical corrosion- system, its design'must be adapted to meet the varying needs of the specific' corrosion problem.u The electrochemical corrosion mechanisms to'which cathodic protection can be applied fall into two broad classifications: 1. Corrosion of a metal surfaoe by stray direct currents which flow in drcaits grounded at more than one point or which flow in subsurface' structures - purposely made- part of a direct current circuit. -i - - i ^,2. Corrosion of a metal surface in an electrolyte by gauamc currents originating between discrete areas of oxidation and re duction reactionsTGalvanic currents are the effect rather than the ea^sebf corrosion. *" '' While riiggiTnilar metal.couples, such as copper-iron, result in the corrosion of the more anodic metal, the corrosion cells formed by nonuniformities on a single metal surface or in the adjoining environment also conform to the requirements of the galvanic corrosion system. Complete corrosion control by cathodic protection is. ap proached when the net current'flow at any point on the metal surface either measures zero or is flowing from the corroding madia into the metal. It is not generally feasible to measure current flow'-directly at all points'bn a metal surface. A re quirement for full cathodic protection is to polarize the cathode- areas to the open circuit' potential of the anodes." The criterion for cathodic protection of-iron is met when all points on the metal surface are polarized to a potential of. --0.85 volts, of more negative, measured against a copper sulfate- reference electrode positioned at the metal surface. The protective current requirements generally vary* with factors influencing corrosion rates. Increasing oxygen concen tration,'temperature, and' velocity increases protective cur rent requirements. Resistive coatings, precipitated calcareous salts, adherent zinc or aluminum floes, silt, and electrophoreti- cally deposited particles all tend-to reduce the-protective current requirements. Adequate protective current, flow' onto a surface from sacri- 'firial or impressed current sources is equally effective in cor rosion control. Sacrificial Anodes. Coupling a more active metal to a struc-, tore will result in galvanic current flow through the corroding electrolyte, thereby providing a protection' effect- on the cathode surface. In providing the, galvanic protective current Bow, the more ictive metal- is eiectrocheroically consumed (sacrificed) and must be replaced! No outside power is re quired for protection. j:'-' -J , The properties of the more commonly used sacrificial'gal vanic anode materials are tabulated below. - Material' 0>/(eap) (rrt Aetna! tb/(amp) Crr) Potential Ccat/ib1 (Co-CnSOJ (Approx) Magnesium (AZ-63A)"' Zinc (MIL-A-18001) Aluminum (B-605) Aluminum (ERP-HP7) ' 8.5 23 6.5 6.5 17 25 12 8- -- l;55v -1.10 v* -1.05 v* -- 1.20 v* SO.35 SO.20 SO.36 ' SO.50 * Seawater. The Hfti'rifif.iftl anode consumption is greater than the theo retical electrochemical equivalent of its protective current output. This is attributable to the self-corrosion current flow superimposed upon,the protective current output.... The design of a sacrificial anode system is limited by the available driving voltage between the sacrificial, anode.and the structure to - be protected.-The resistance of the circuit between the anodes and the structure mainly determines the; protective current flow. A high resistivity electrolyte re quires a larger number of anodes than a more conductive electrolyte to obtain the same amount of protective current flow. - -r' - - fWrifirial anodes are cast in various forms and weights in order to obtain optimum design and service life. Specisl back- filla around the anodes are used in soils to maintain moisture at the anode surface.and increase efficiency. The tine on galvanized iron.provides a cathodic protection effect on. the underlying ra^tnl until.it is consumed in the pro-, tective process. . : -, ' : Zinc ftbd aluminum anodes can usually be used in combina- tion with well coated, surfaces.without accelerating coating damage. - ....... Impressed Current- An external voltageeource can be used to impress protective current flow from anodes through the conducting medium onto .the corroding surface. Alternating current power/ converted to direct current by an adjustable output gploninrn or .silicon type rectifier, is most .commonly used. 1: Sacrificial .type anodes, such as-scrap iron or. aluminum, are sometimes -used with impressed current. Nonsacrificial anndea (those not,,consumed by the electrochemical process), such as graphite, high silicon-cast iron alloy,.platinum or platinum plated or clad on titanium or. tantalum, and silverr s'