Volume 13 - Corrosion Part 11 potx

However, corrosion engineers have determined that the best measure of the potential corrosivity of a given soil is its electrical resistance measured in ohms per cubic centimeter. The electrical resistivity of the soils is determined by the nature and concentration of the ions formed by the chemical salts dissolved in the soil moisture. These ions may also determine the progress of corrosion. If the primary product of corrosion is relatively insoluble and deposits as a film on the metal surface, further corrosion may be reduced or completely stifled. Numerous inspections of existing structures have established that in sandy and loamy soils with resistivity of 4500 · cm and higher the engineer need not be concerned with the corrosion on the soil side of galvanized steel pipe (Table 13). Table 13 Relationship of soil corrosion to electrical resistivity Soil Class Corrosion resistance Electrical resistivity, ·cm Sandy Excellent 6000-10,000 Loams Good 4500-6000 Clays Fair 2000-4500 Peat/muck Bad 0-2000 When the engineer wishes to confirm his judgment of service life requirements or when the corrosion and abrasion aspects of the proposed site may be in question, reference to the California Division of Highways method for determining service life is recommended. This comprehensive method uses measurable environmental corrosion criteria and is applicable to all parts of the country. The California method correlates pH with the electrical resistivity of soils to determine the number of years necessary to perforate a galvanized steel sheet. For example, an engineer considering a typical site environment with a resistivity of 10,000 · cm and pH of 7 can refer to Fig. 4 and determine that it would take 40 years to perforate 16-gage ( 1.5-mm, or 0.06-in.) galvanized steel pipe. Fig. 4 Method of estimating service life as developed by the California Division of Highways. This correlates pH with the electrical resistivity of soils to determine year s to perforate a steel sheet. Local durability records are used for confirmation or control. Multiply years to perforation by factor for increase in metal gage. This figure is based on 16-gage galvanized steel pipe with a coating thickness of 1.6 mm (0.064 in.). Source: Ref 12 The engineer can determine the years to perforation for other gages of pipe by using the appropriate multiplying factor. For example, years to perforation for 8-gage ( 4.2-mm, or 0.16-in.) pipe would be 40 times 2.8, or 112 years. Moreover, perforations resulting from the pitting action of galvanic cells will not affect the useful service of a storm drainage structure, as compared to a pressure conduit from which a valuable or dangerous product would be lost. With proper maintenance, the life of corrugated steel pipe can be extended indefinitely. Except in the most corrosive soils, the maximum depth of pitting in steel specimens exposed for approximately 12 years was more than 11 times that in zinc specimens, although the ratio for the rates of corrosion was only approximately one- half that figure (Ref 13). This resistance to pitting, combined with the fact that rusting does not appear to start until nearly all of the zinc and zinc-alloy layers have corroded away, reduces the risk of premature failure in galvanized piping. It cannot be overemphasized that, although these results serve as a useful guide to the performance of zinc coatings in particular soils, local experience should always be sought. The Corrosion of Zinc in Chemical Environments As indicated previously, the use of zinc for corrosion-resistant applications accounts for about one-half of the total consumption of the metal. However, the available information reveals that in most applications the resistance of zinc to the atmosphere and in various waters is the primary corrosion criterion. In one sense, these may be considered chemical environments, because the chemical reactions between zinc and the constituents of pure or polluted atmospheres and waters determine the life of the zinc exposed to air or water. Zinc is usually not considered to be a useful metal in the acidic or strongly alkaline chemical environments encountered, for example, in the chemical-processing industries (see the article "Corrosion in the Chemical Processing Industry" in this Volume). The corrosion of zinc increases in aqueous chemical solutions on either side of the 6 to 12 pH range. This should not be considered a fixed rule, because many other factors, such as agitation, aeration, temperature, polarization, and, in some cases, the presence of inhibitors, may have considerable influence on the corrosion. There is considerable interest in the use of zinc in milder chemical environments. For example, zinc in used in contact with many organic chemical and chemical specialties, such as detergents, insecticides, and agricultural chemicals. In most cases, zinc comes in contact with such chemicals during the handling, packaging, and storage of the commercial products. The corrosion resistance of zinc to the chemicals is usually the primary consideration, but in some cases, the effect of zinc corrosion on a consumer product or chemical is of greater concern than the actual corrosion rate of zinc. For example, zinc in contact with certain organic chemicals may in rare cases cause polymerization or catalyze some other undesirable change that would alter the original composition of properties of the product. In other cases, some change in the appearance or texture of a consumer product may be caused by the relatively slight corrosion of zinc. Thus, zinc would be considered incompatible, although it does not corrode excessively. There are many situations in the chemical industry in which zinc serves a useful purpose. Zinc-coated tanks and cylinder are widely used in oil refineries and other plants for storing oil and petroleum products, chlorine, CO 2 , and other industrial gases (see the articles "Corrosion in Petroleum Production Operations" and "Corrosion in Petroleum Refining and Petrochemical Operations" in this Volume). Refrigerating plants and cooling equipment, as well as degreasing plants, are almost universally protected by zinc coatings. Zinc-coated steel is also used on structural steelwork around chemical plants, where it is exposed to high humidity and a variety of chemical fumes. Galvanized steel is extensively used for roofing and siding on pulp and paper processing buildings (see the article "Corrosion in the Pulp and Paper Industry" in this Volume). Other uses of zinc-coated steel in the chemical industry include applications in floating roof type storage tanks for volatile liquids as well as galvanized wire cloth and mesh belts for the movement of chemicals through various production stages. Galvanized steel containers are used to store strategic chemicals in outdoor locations. From these representative examples it is apparent that zinc and zinc-coated steel are definitely useful in the chemical industry. Corrosion in Dissolved Salts, Acids, and Bases Zinc is not used in contact with acid and strong alkaline solutions, because it corrodes rapidly in such media. The section "Corrosion of Zinc in Water" in this article indicates the safe range in which it may be used. Very dilute concentrations of acids accelerate corrosion rates beyond the limits of usefulness. Alkaline solutions of moderate strength are much less corrosive than corresponding concentrations of acid, but are still corrosive enough to impair the usefulness of zinc. Zinc-coated steel is used handling refrigeration brines that may contain calcium chloride (CaCl 2 ). In this case, the corrosion rate is kept under control by adding sufficient alkali to bring the pH into the mildly alkaline range and by the addition of inhibitors, such as sodium chromate (Na 2 CrO 4 ). Certain salts, such as the dichromates, borates, and silicates, act as inhibitors to the aqueous corrosion of zinc. Nonaqueous Corrosion Organic Compounds. Many organic liquids that are nearly neutral in pH and substantially free from water do not attack zinc. Therefore, zinc and zinc-coated products are commonly used with gasoline, glycerine, and inhibited trichlorethylene. The presence of free water may cause local corrosion because of the lack of access to oxygen. When water is present, zinc may function as a catalyst in the decomposition of such solutions as trichlorethylene, with acid attack as the result. Some organic compounds that contain acidic impurities, such as low-grade glycerine, attack zinc. Although neutral soaps do not attack zinc, there may be some formation of zinc soaps in dilute soap solutions. Gases. Zinc may be safely used in contact with most common gases at normal temperatures if water is absent. Moisture content stimulates attack. Dry chlorine does not affect zinc. Hydrogen sulfide (H 2 S) is also harmless because insoluble zinc sulfide (ZnS) is formed. On the other hand, SO 2 and chlorides have a corrosive action because water-soluble and hygroscopic salts are formed. Indoor Exposure. Zinc corrodes very little in ordinary indoor atmospheres of moderate relative humidity. In general, a tarnish film begins to form at spots where dust particles are present on the surface; the film then develops slowly. This attack may be a function of the percentage of relative humidity at which the particles absorb moisture from the air. However, moisture has little effect on the tarnish formation up to 70% relative humidity. The degree of corrosion is related to the relative humidity at and above this point because the zinc corrosion products absorb enough moisture or stimulate the attack to a perceptible rate. Rapid corrosion can occur where the temperature decreases and where visible moisture that condenses on the metal dries slowly. This is related to the ease with which such thin moisture films maintain a high oxygen content because of the small volume of water and large water/air interface area. Considerably accelerated corrosion can then take place with the formation of a film that is too thick. Chromate protective films are used to a considerable extent to prevent attack where accidental or limited contact with water is expected. Atmospheres inside industrial buildings can be corrosive, particularly where heated moisture and gases, such as SO 2 , condense near a cool room. Contact With Food Products. Zinc should not be used in contact with acidic foodstuffs unless they can be expected to remain dry. Otherwise, the zinc must be adequately protected by copper-nickel-chromium plating or another satisfactory impervious coating. The slight acidity present in many foodstuffs can attack the zinc, and this may give the food a metallic taste. For the same reason, the zinc die casting used in any equipment to hold or dispense drinks should also be plated or otherwise protected. Consumption of food contaminated with zinc may cause nausea but is not dangerous, and this rarely arises because of the taste. A summary of the compatibility of untreated zinc with various media is presented in Table 14. Table 14 Compatibility of untreated zinc with various media Medium Media descriptor Compatibility Aerosol propellants . . . Excellent Weak, cold, quiescent Fair Acid solutions Strong Not recommended Anhydrous Good Water mixtures Not recommended Alcohols Beverages Not recommended Up to pH 12.5 Fair Alkaline solutions Strong Not recommended Carbon tetrachloride . . . Excellent Cleaning solvents Chlorofluorocarbon Excellent Detergents Inhibited Good Diesel oil Sulfur free Excellent Fuel oil Sulfur free Excellent Gas (a) Towns, natural, propane, butane Excellent Glycerine . . . Excellent Printing Excellent Inks Aqueous writing Not recommended Dry Excellent Insecticides In solution Not recommended Mineral, acid free Excellent Lubricants Organic Not recommended Paraffin . . . Excellent Perchlorethylene . . . Excellent Petroleum (a) . . . Excellent Refrigerants Chlorofluorocarbon Excellent Soaps . . . Good Trichloroethylene . . . Excellent (a) Chromate passivation treatment recommended because of the possible presence of moisture traces. Stress-Corrosion Cracking and Corrosion Fatigue Stress-Corrosion Cracking (SCC). The effects of corrosion and stress on the performance of a material are often treated as separate concerns. However, in conjunction, the two can cause the phenomenon of SCC, which can destroy a metallic component faster than either stress or corrosion separately. In essence, SCC is a process in which cracks in the metal grow under the combined effects of tensile stress and a corrosive environment. When the cracks become sufficiently large, the component fractures. Often, but not always, the stress-corrosion resistance of an alloy is very dependent on its microstructure. Information on the mechanisms of SCC is provided in the article "Environmentally Induced Cracking" in this Volume. Testing and interpretation of SCC data can be found in the article "Evaluation of Stress-Corrosion Cracking" in this Volume. Corrosion Fatigue. When metal is subjected to vibration or cyclic stress, fatigue strength is more important than ultimate tensile strength. Under corrosive conditions, even the advantages of alloy additions and heat treatments that considerably increase the tensile strength and fatigue limit of steel have only a minor effect on corrosion fatigue behavior. Corrosion fatigue is the weakening that occurs in a metal after it has been stressed repeatedly in the presence of corrosive agents. Generally, the corrosion fatigue characteristics of low-alloy steels are little or no better than those of ordinary low- carbon steel unless the metal has a protective coating. The plastic deformation that occurs in the metal at the bottom of a crack during repeated stressing raises its energy and thus increases its susceptibility to chemical attack. The strengthened material that would otherwise prevent the cracks from spreading is therefore destroyed by corrosion in preference to the surrounding metal, and the crack continues to grow. This process continues by further plastic deformation of metal, which is in turn destroyed. In addition to this basic mechanism of corrosion fatigue, certain subsidiary effects also play a part. For example, the strains occurring in the surface of the stressed metal tend to disrupt such protective films as would otherwise be formed by the corrosion products, with the damage to the protective film often taking the form of small cracks or crevices. The small areas of metal exposed by these cracks are anodic to the surrounding film and corrode at an accelerated rate with the formation of pits; this sets the mechanism of corrosion fatigue in motion. The total amount of corrosion involved in corrosion fatigue is often extremely small. It is important to prevent corrosion from the start because once it has begun cyclic stressing may lead to early failure even though further attack is prevented. These facts clearly point to the necessity of sacrificial, electrochemical protection as a means of preventing corrosion fatigue. Steel components can be protected throughout their life by a sacrificial zinc coating to give them complete protection. If a coating with no sacrificial properties is used, a surface fault may well lead to fatal pitting. Steel and Corrosion Fatigue. Detailed investigations into the corrosion fatigue resistance of steels in moist air and saline solutions have give some surprising results. The ultimate tensile strength and indeed the pure fatigue limits of steels are increased considerably by alloying or special heat treatments, but these have only a minor effect on corrosion fatigue behavior. Thus, under practical conditions the behavior of a special steel may be little better than that of ordinary carbon steel unless it has a protective zinc coating. These are significant considerations in determining what preventive measures to adopt in practice, and that explain why sacrificial or electrochemical protection is of major importance. Because corrosion fatigue cracks may be difficult to detect until they have reached dangerous proportions, a practical safeguard is to protect steel components throughout their service lives by a sacrificial coating of zinc, which gives them complete cathodic protection. When zinc coatings are used, small to medium-sized imperfections in the coating are of relatively little importance, and corrosion is prevented even at the growing points of the microscopically small crevices that cause corrosion fatigue. Any type of coating that covers the surface completely and retards the onset of corrosion may help, but if it is not anodic to steel, it cannot exert cathodic protection at coating defects. In many general uses, the fact that nonanodic coatings cannot prevent corrosion at small defects may be unimportant. When fatigue failure is not a consideration, a very small amount of corrosion has no significant effect. Wherever corrosion fatigue can occur, however, the situation is very different. Coatings that cannot protect sacrificially may, as a result of discontinues, cause serious pitting, and trouble may be accentuated by concentrating the corrosion in small areas, thus stimulating the growth of fatigue cracks. Additional information on corrosion fatigue can be found in the articles "Mechanically Assisted Degradation" and "Evaluation of Corrosion Fatigue" in this Volume. Corrosion Protection With Zinc Anodes Historical Development of Cathodic Protection. Historically, zinc was one of the very first metal to be used as a galvanic anode; early in the 19th century, Sir Humphrey Davy secured pieces of zinc to the copper sheathing on wood hulls of British Navy vessels to prevent severe underwater corrosion of the copper. The latter was used as a barrier shield to stop penetration and destruction of the wood hulls by marine borers and to prevent attachment of barnacles to hulls. Sir Humphrey thus became the first practitioner of cathodic protection and is credited with developing the concept of the electrochemical series of the elements. He concluded from actual sea trials aboard ship that corrosion of the copper sheathing could be arrested by using zinc protectors, but the fouling problems by marine organisms were not entirely resolved. Further experiments demonstrated that by varying the area of the protector metal in relation to the copper the latter would corrode at a low, tolerable rate that was insufficient to cause perforation but adequate to exhibit antifouling characteristics. The electrolytic protection of metals from corrosion in a bulk electrolyte, as demonstrated by Sir Humphrey, has developed into a universally accepted method of corrosion control known as cathodic protection. Cathodic Protection Systems. In practice, cathodic protection is applied to potentially corrodible metals underground or underwater by galvanic anodes (self-generated current) or power-impressed systems. Galvanic anode systems do not require an external source of current, because the protective current is self-generated when the galvanic anode is electrically connected to the structure to be protected in a bulk electrolyte. The most commonly used power-impressed systems employ anodes with low anodic corrosion rates that are electrically connected to a rectifier that converts alternating current (ac) to direct current (dc). The negative terminal of the rectifier is grounded to the structure to be cathodically protected, and the positive terminal is connected to the relatively inert anodes. When the system is activated, current flows from the rectifier to the anodes, through the electrolyte to the structure (cathode) to be protected, and back to the rectifier through the return path. The anodes commonly used in impressed- current systems include, but are not limited to, high-silicon cast iron, graphite, platinized titanium, and lead-silver alloys (seawater only). The cathode, or structure to be protected, does not recognize the source of protective current, because it is possible to polarize most structures by using galvanic anodes or impressed-current systems. Other power sources used in impressed-current systems include, but are not limited to, the following: engine generator sets, wind-powered generators, thermoelectric generators, gas turbines, solar cells and fuel cells. Although the sources of power for supplying protective current in cathodic protection systems are numerous, zinc is the one that is most frequently used; therefore, the rest of this section will discuss the details relating to the function of zinc as a galvanic anode. Applications of Zinc Anodes. The use of zinc in commercial cathodic protection systems involves environments as diverse as seawater, brackish water, freshwater, and a wide variety of soils. Because the environment plays a significant role in determining the success or failure of zinc galvanic anode systems, seawater and soils will be treated separately. The application of cathodic protection to reduce or prevent corrosion damage occurring on the steels hulls of marine craft, such as ships, launches, barges, floating docks, buoys, and pontoons, makes it possible to increase substantially the interval between dry dockings and to reduce the amount of maintenance work to be accomplished during each dry docking period. The material and labor costs of installing a zinc anode cathodic protection system are usually small compared to such maintenance work as chipping, painting, and replacing hull plates. The economic benefits that accrue from marine cathodic protection depend to a large extent on the duty cycle of the craft under consideration. Maximum savings are obtained when cathodic protection is applied to vessels that are dry-docked only for reasons of anticorrosion maintenance. Included in this category would be most barges, dredges, buoys, pontoons, and stored ships. It is not unusual under these conditions for a cathodic protection system, designed for a 3-year life, to pay for itself within 1 year. Even in the case of merchant ships, which must be dry-docked annually for removal of marine growths and reapplication of antifouling paint, cathodic protection, if properly applied, can prove economical because it reduces or eliminates corrosion damage to exposed steel propeller shafts and to areas on the hull where the paint has become damaged. Zinc is an ideal metal for cathodic protection in seawater because it does not subject the adjacent painted surfaces of the hull to high potentials, which are injurious to many commonly used paints. Nevertheless, good marine paints, application practice, and reasonable maintenance are of course necessary. Zinc has a high ampere-hour capability per unit volume (530 A · yr/m 3 , or 15 A · yr/ft 3 ). This means that the total volume of zinc required is not large; therefore, the effect of the installation on the speed of the ship is generally very small if the anodes are correctly installed. The installation of zinc anodes on marine craft is straightforward, particularly if it is accomplished when the vessel is in dry dock. Table 15 lists the chemical composition and impurity limits of the current United States Government (MIL-A-18001H) and ASTM (B 418) zinc anode specifications. The basic zinc anode specification containing aluminum and cadmium as alloying elements has become the worldwide standard for zinc anodes used in cathodic protection systems in seawater and brackish water at ambient temperatures. Committee B-2 of ASTM cautioned that the threshold level for intergranular corrosion of type I composition zinc-aluminum-cadmium alloy is particularly severe above a temperature of about 50 °C (120 °F). For additional information on anode selection, see the articles "Marine Corrosion," "Corrosion of Magnesium and Magnesium Alloys," "Corrosion of Aluminum and Aluminum Alloys," and "Cathodic Protection" in this Volume. Table 15 Zinc anode composition specifications for seawater use Element MIL-A-18001H composition, % ASTM B 418 Type 1 composition, % Aluminum 0.10-0.50 0.10-0.4 Cadmium 0.025-0.15 0.03-0.10 Iron (a) 0.005 0.005 Lead (a) 0.006 . . . Copper (a) 0.005 . . . Silicon (a) 0.125 . . . Zinc rem rem (a) Maximum composition Prevention of Intergranular Corrosion. The zinc industry has long been aware of the susceptibility of zinc alloys containing aluminum to intergranular corrosion when exposed to elevated temperatures. Committee B-2 of ASTM on nonferrous metals and alloys recognized this potential problem when zinc alloy anodes were exposed to service conditions involving elevated temperatures and recommends the use of unalloyed low-iron zinc anodes, ASTM Type II (up-dated zinc anode specification, ASTM B 418), shown in Table 16 to avoid intergranular corrosion at elevated temperatures. Table 16 Zinc anode composition specification for elevated-temperature exposure Element ASTM B 418 Type II composition, % (a) Aluminum 0.005 Cadmium 0.003 Iron 0.0014 Zinc rem (a) Maximum composition Underground Zinc Anodes. The cost of installing cathodic protection as a means of stopping the corrosion of coated steel distribution piping is usually small when compared to the cost of repairing leaks and making replacements. The perforation of the pipe wall as a result of corrosion may occur sooner on a well-coated pipe than on a bare pipe, because corrosion current is concentrated at holidays or damaged areas in the coating. The total metal loss, however, of a coated pipe will usually be negligible compared to that which would occur on a bare pipe, and the danger of developing general structural weakness is therefore greatly reduced. Fortunately, it is relatively easy and inexpensive to apply cathodic protection to a coated pipeline as compared to a bare pipeline, because the current requirements are only a small fraction of that required for the bare pipeline. The combination of a good pipe coating plus cathodic protection has proved to be both economical and successful to such an extent that the current practice is to provide both a coating and cathodic protection on virtually all new transmission pipelines. This practice is often carried out even in areas where there is no certainty that severe corrosion would occur. Because the environment around an anode is significant when determining the type and nature of anodic films or coatings, successful anode performance is related to the presence of a friendly environment in contact with the zinc. Soils containing significantly more dissolved sulfates and chlorides than carbonates, bicarbonates, nitrates, and phosphates are often compatible with zinc anodes. However, by packing a prepared backfill consisting of hydrated gypsum, bentonite clay, and sodium sulfate (Na 2 SO 4 ) around an anode, a friendly environment is created, and such a packaged anode can be used in almost all soils. The most popular consists of 75% gypsum, 20% bentonite clay, and 5% Na 2 SO 4 ; this environment provides a relatively low resistivity and thus permits high current output levels. In time, however, the Na 2 SO 4 tends to leach out of the backfill, and higher resistivity occurs. The principal zinc specification (ASTM B 418 Type II) used for underground cathodic protection systems given in Table 16 is the unalloyed high-purity zinc with iron controlled to 0.0014% maximum. The literature does not show any long- term field tests comparing the unalloyed Type II materials, with the zinc-aluminum-cadmium composition of MIL-A- 18001H, or the ASTM B 418 Type I given in Table 15. Although the latter alloy anodes prepackaged in prepared backfill have been installed, comparative results are lacking. Zinc Coating Processes Seven methods of applying a zinc coating to iron and steel are in general use: hot dip galvanizing, continuous-line galvanizing, electrogalvanizing, zinc plating, mechanical plating, zinc spraying, and painting with zinc-bearing paints. This section and the section "Painting With Zinc-Bearing Paints" in this article contain brief descriptions of each process, the nature of the coating formed, and the practical advantages and limitations of each method. There is usually at least one process that is applicable to any specific purpose. Because the processes are complementary, there are rarely more than two processes to be seriously considered as the best choice for a particular application. Additional information on zinc coating processes can be found in the articles "Hot Dip Coatings" (continuous and batch processes are described), "Electroplated Coatings," "Thermal Spray Coatings," "Corrosion of Carbon Steels," and "Corrosion in the Automotive Industry" in this Volume. Reference to Surface Engineering, Volume 5 of the ASM Handbook is also recommended. Hot Dip Galvanizing In hot dip galvanizing, the steel or iron to be zinc coated is usually completely immersed in a bath of molten zinc. It is by far the most widely used of the zinc coating processes and has been practiced commercially for almost two centuries. The modern hot dip galvanizing process is conducted in carefully controlled plants by applying the results of scientific research, and it is far removed from that of years ago, although it is still dependent on the same basic principles. The process is primarily applied to finished parts and to semifabricated materials, such as sheet, strip, wire, and tube, on the continuous automated lines of the steel producers. There is an obvious advantage in galvanizing after fabrication in that the zinc completely seals edges, rivets, and welds so that there are no uncovered parts at which rusting can begin. Continuous Galvanizing. In 1936, a revolutionary new process for continuously coating coils of sheet steel by hot dipping was introduced in the United States. This process, known as the Sendzimir process, uses a small amount of aluminum in the zinc bath and produces a coating with essentially no iron-zinc alloy and with sufficient ductility to permit deep drawing and folding without damage to the coating. Other processes for continuous zinc coating of sheet steel without alloy layer formation were later developed and joined the Sendzimir process. Today, nearly all hot dip galvanized sheet steel is produced by continuous methods. [...]... alloys resisted corrosion in the tropical zones, although less so at coastal sites as compared to inland sites The tin-containing alloys were as good as, or slightly superior to, the other alloys More recent work by the same investigators included previous data plus additional information on the following cast bronzes: Cu-5Sn-5Pb-5Zn, Cu-6Sn-2Pb-3Zn-1Ni, Cu-9Sn-3Zn-1Ni, and Cu-3Sn-2Zn-6Ni (Ref 63) The... bronze (6% Sn) 1.2 High-conductivity copper 2.4 Aluminum bronze (3.5% Al) 2.1 7 0-3 0 nickel-copper 3.2 6 0-4 0 copper-zinc 18.4 7 0-3 0 copper-zinc 8.6 Source: Ref 57 A more extensive study covering 20 years and seven sites compared the behavior of a variety of alloys, including phosphor bronze (Cu-7.85Sn-0.03P), admiralty brass (Cu-29.01Zn-1.22Sn), and a nickel-tin bronze (Cu-28.6Ni-1.04Sn0.55Zn) (Ref 58)... 2): Process Thickness m mils Chemical replacement Trace-2.5 Trace-0.1 Flow-melted electrodeposition 0. 4-7 .5 0.0 2-0 .3 Electrodeposition, general 2. 5-7 5 0. 1-2 .9 Hot dipping 1. 5-2 5 0.0 6-1 Spraying 7 5-3 50 2.9 -1 3. 6 Coatings applied by any method may contain pores that will expose the base metal Porosity should be minimal for electrodeposited and hot-dipped coatings thicker than 15 m (0.6 mils) (Ref 2) However,... Bearing Alloys The most widely used babbitt bearing alloys are usually classified as tin- or lead-base and have composition ranges within the following limits: Alloy addition Composition, % Tin-base Lead-base Tin 6 5-9 1 0-2 0 Lead 0.3 5-1 8 63 (min) Antimony 4. 5-1 5 1 0-1 5 Copper 2-8 1.5 (max) The tin-base alloys are much more corrosion resistant against the action of the acids contained or formed in lubricating... both sites than the stainless, but the low-alloy steel was more severely attacked However, the bronze was free of pitting and suffered no loss in strength, which was not the case with some of the stainless steels In Ref 62, these researchers summarized the results of 16-year exposures on three tin-containing alloys (Cu-4.38Sn-0.36P, Cu-39Zn-0.84Sn, and Cu-40Zn-1Fe-0.65Sn) exposed at marine, inland semirural,... sulfates), that is adherent, protective, and has a pleasing appearance More information on the corrosion resistance of copper alloys is available in the article "Corrosion of Copper and Copper Alloys" in this Volume Atmospheric Corrosion Early studies were conducted on Cu-6.3Sn-0.08P wire and Cu-6.3Sn-0.08P-0.5Zn sheet in rural, suburban, urban, industrial, and marine environments for 1 year (Ref 56)... was Cu-5Sn-7Al Another researcher noted that such alloys could be brittle, but that the addition of 1% Fe and 1% Mn overcame this difficulty without detracting from the corrosion resistance of the alloy (Ref 66) Tin-Silver In the mid-1930s, tin-silver alloys were assessed as potential replacements for sterling silver (silver-copper alloy) in decorative applications (Ref 67) In this work, an Ag-7.5Sn... Cu-2Sn bronze was found to exhibit the lowest strength losses at all sites from a group of alloys that included brasses, aluminum bronze, and nickel-copper Outstanding corrosion resistance of a Cu-2Sn bronze exposed to sulfur-bearing gases in railway tunnels was also reported (Ref 60.) Another investigation compared the behavior of five stainless steels and a low-alloy steel with that of a Cu-4.38Sn-0.36P... Table 10 Corrosion rate of 2 5- m (1-mil) thick coatings on steel at three tropical sites after 2 years of exposure Coating Material loss, full exposure test Jungle m/yr Town mils/yr m/yr Coast mils/yr m/yr mils/yr Tin 0.18 0.007 1.02 0.04 3.02 0.12 80Sn-20Zn 0.46 0.018 1.35 0.053 2.87 0. 113 Zinc 0.53 0.021 1.45 0.057 2.90 0 .114 Weight loss, sheltered exposure test mg/dm2 Jungle Town Coast Tin 1.6 11. 3... Temperature on the Corrosion of Zinc, Ind Eng Chem., Vol 23 1931, p 90 2-9 04 9 H Grubitsch and O Illi, The Hot Water Corrosion of Zinc II, Korros Metall., Vol 16, 1940, p 197 10 H Grubitsch and H Huemer, Scanning Electron Microscope Studies of the Hot Water Corrosion of Zinc, Werkst Korros., Vol 24 (No 1), 1973, p 1-7 11 R Rosset and A Jardy, Protection of Galvanized Steel Against Cold and Hot Water Corrosion . the possible presence of moisture traces. Stress -Corrosion Cracking and Corrosion Fatigue Stress -Corrosion Cracking (SCC). The effects of corrosion and stress on the performance of a material. specifications for seawater use Element MIL-A-18001H composition, % ASTM B 418 Type 1 composition, % Aluminum 0.1 0-0 .50 0.1 0-0 .4 Cadmium 0.02 5-0 .15 0.0 3-0 .10 Iron (a) 0.005 0.005 Lead (a) . be concerned with the corrosion on the soil side of galvanized steel pipe (Table 13) . Table 13 Relationship of soil corrosion to electrical resistivity Soil Class Corrosion resistance