Conditions that cause serious corrosion problems include airborne contamination, dust, pollution, acid rain, and proximity to a seacoast. Designing to avoid retention of such contaminants is critical. Thus, design specifications for construction projects must include normal and extremes of environment conditions Design change may also include the isolation of dissimilar metals. The article"Forms of Corrosion"in this Volume includes a discussion of galvanic corrosion and protection. Isolation may include the use of coatings and nonmetallic insulators Corrosion is sometimes caused or aggravated by applied stress. For instance, SCC of stainless steel will occur when the steel is exposed to chlorides in the presence of tensile stresses. The magnitude of stresses obviously affected by the fabrication techniques and the component design. Changes in the design of the components in areas exposed to chlorides that serve to reduce the stress concentrations may be enough to alleviate the possibility of SCC. Areas of stress concentration such as sharp radii or joints are vulnerable. Surface conditions such as rough surfaces, dents, or scratches may also provide preferential sites for corrosion to occur. Designing to minimize these conditions will reduce corrosion Galvanic protection can be either cathodic protection or anodic protection. Cathodic protection reduces the corrosion rate by shifting the corrosion potential of the electrode toward a less oxidizing potential through use of an external electromotive force. Anodic protection is defined as a technique to reduce the corrosion rate of a metal by polarizing it into its passive region, where dissolution rates are low Cathodic protection systems require an anode, a cathode, a continuous electric circuit between the cathode and the anode, and the presence of an electrolyte. The two types of cathodic protection are sacrificial-anode protection and impressed-current protection Sacrificial-anode protection is the simplest method. A material that is more anodic than the metal to be protected becomes sacrificial. A typical cathodic protection design is shown in Fig. 8(Ref 6) Insulated wire JM Mn MM wwv Buried coated Thermit weld steel pipelin Prepackaged magnesium anode in a porous cloth bag with bentonite clay Fig8 Cathodic protection of a buried pipeline using a buried magnesium anode Source Ref 6 Magnesium, zinc, and aluminum alloys are common sacrificial anodes. Magnesium anodes are most comn used for buried soil applications. Zinc is most often used for freshwater and saltwater marine applicat Aluminum alloys are most often used for offshore structures. Prevention of passivation of aluminum is key for effective protection. Alloying elements of tin, antimony, and mercury are used for this purpose Impressed-current protection requires buried anodes and an electrical connection between the protected structure and the current source. a power rectifier supplies direct current to the buried electrodes(the structure and the anode) The direct-current source reverses the natural polarity and allows the materials to act as anodes Thefileisdownloadedfromwww.bzfxw.com
Conditions that cause serious corrosion problems include airborne contamination, dust, pollution, acid rain, and proximity to a seacoast. Designing to avoid retention of such contaminants is critical. Thus, design specifications for construction projects must include normal and extremes of environment conditions. Design change may also include the isolation of dissimilar metals. The article “Forms of Corrosion” in this Volume includes a discussion of galvanic corrosion and protection. Isolation may include the use of coatings and nonmetallic insulators. Corrosion is sometimes caused or aggravated by applied stress. For instance, SCC of stainless steel will occur when the steel is exposed to chlorides in the presence of tensile stresses. The magnitude of stresses obviously is affected by the fabrication techniques and the component design. Changes in the design of the components in areas exposed to chlorides that serve to reduce the stress concentrations may be enough to alleviate the possibility of SCC. Areas of stress concentration such as sharp radii or joints are vulnerable. Surface conditions, such as rough surfaces, dents, or scratches may also provide preferential sites for corrosion to occur. Designing to minimize these conditions will reduce corrosion. Galvanic protection can be either cathodic protection or anodic protection. Cathodic protection reduces the corrosion rate by shifting the corrosion potential of the electrode toward a less oxidizing potential through use of an external electromotive force. Anodic protection is defined as a technique to reduce the corrosion rate of a metal by polarizing it into its passive region, where dissolution rates are low. Cathodic protection systems require an anode, a cathode, a continuous electric circuit between the cathode and the anode, and the presence of an electrolyte. The two types of cathodic protection are sacrificial-anode protection and impressed-current protection. Sacrificial-anode protection is the simplest method. A material that is more anodic than the metal to be protected becomes sacrificial. A typical cathodic protection design is shown in Fig. 8 (Ref 6). Fig. 8 Cathodic protection of a buried pipeline using a buried magnesium anode. Source: Ref 6 Magnesium, zinc, and aluminum alloys are common sacrificial anodes. Magnesium anodes are most commonly used for buried soil applications. Zinc is most often used for freshwater and saltwater marine applications. Aluminum alloys are most often used for offshore structures. Prevention of passivation of aluminum is key for effective protection. Alloying elements of tin, antimony, and mercury are used for this purpose. Impressed-current protection requires buried anodes and an electrical connection between the protected structure and the current source. A power rectifier supplies direct current to the buried electrodes (the structure and the anode). The direct-current source reverses the natural polarity and allows the materials to act as anodes. The file is downloaded from www.bzfxw.com
A typical impressed-current system of a buried pipeline is shown in Fig. 9(Ref 6). Impressed-current anodes must be corrosion resistant and durable in the environment of use. There are several materials that are used for impressed-current anodes: high-silicon cast irons, graphite, polymeric-coated wires, precious metals, lead alloys, and ceramics. Each material has specific applications. Reference 6 provides detailed information regarding the applications of the impressed-current anodes. Impressed-current cathodic protection must have an external power source. This power source is most often rectified alternating-current power, but batteries, solar cells, and other alternative power supplies can be used in remote locations Z》多 Buried coated Thermit weld Buried grap Fig9 Impressed-current cathodic protection of a buried pipeline using graphite anodes. Source: Ref 6 Anodic protection is based on the phenomenon of passivity. A limited number of metals can achieve passivity Anodic protection requires that the potential of the metal be controlled. Shifting the potential of the metal to the passive range can reduce the corrosion rate of an active-passive metal. The anodic polarization of the metal results from the formation of the passive layer, which is relatively insoluble in the chemical environment. The most common uses for anodic protection include storage tankS, process reactors, heat exchangers, and transportation vessels. Anodic protection has been successfully employed in sulfuric acid environments to extend tank life Use of Inhibitors. Inhibitors are defined as a chemical substance or combination of substances that, when present in the environment, prevent or reduce corrosion without significant reaction with the components of the environment. Chemical treatment or inhibitors can be used for corrosion control in aqueous environments Inhibitors are classified as anodic inhibitors. cathodic inhibitors. and mixed inhibitors. Anodic inhibitors suppress the rate of metal ions being transferred into the aqueous environments. Cathodic inhibitors impede the oxygen-reduction reaction. Mixed inhibitors hinder both reactions Anodic Inhibitors. There are two types of anodic inhibitors, oxidizing and nonoxidizing Chromates and nitrites act in the absence of oxygen. Chromates are effective in protecting ferrous and nonferrous alloys. An oxidation
A typical impressed-current system of a buried pipeline is shown in Fig. 9 (Ref 6). Impressed-current anodes must be corrosion resistant and durable in the environment of use. There are several materials that are used for impressed-current anodes: high-silicon cast irons, graphite, polymeric-coated wires, precious metals, lead alloys, and ceramics. Each material has specific applications. Reference 6 provides detailed information regarding the applications of the impressed-current anodes. Impressed-current cathodic protection must have an external power source. This power source is most often rectified alternating-current power, but batteries, solar cells, and other alternative power supplies can be used in remote locations. Fig. 9 Impressed-current cathodic protection of a buried pipeline using graphite anodes. Source: Ref 6 Anodic protection is based on the phenomenon of passivity. A limited number of metals can achieve passivity. Anodic protection requires that the potential of the metal be controlled. Shifting the potential of the metal to the passive range can reduce the corrosion rate of an active-passive metal. The anodic polarization of the metal results from the formation of the passive layer, which is relatively insoluble in the chemical environment. The most common uses for anodic protection include storage tanks, process reactors, heat exchangers, and transportation vessels. Anodic protection has been successfully employed in sulfuric acid environments to extend tank life. Use of Inhibitors. Inhibitors are defined as a chemical substance or combination of substances that, when present in the environment, prevent or reduce corrosion without significant reaction with the components of the environment. Chemical treatment or inhibitors can be used for corrosion control in aqueous environments. Inhibitors are classified as anodic inhibitors, cathodic inhibitors, and mixed inhibitors. Anodic inhibitors suppress the rate of metal ions being transferred into the aqueous environments. Cathodic inhibitors impede the oxygen-reduction reaction. Mixed inhibitors hinder both reactions. Anodic Inhibitors. There are two types of anodic inhibitors, oxidizing and nonoxidizing. Chromates and nitrites act in the absence of oxygen. Chromates are effective in protecting ferrous and nonferrous alloys. An oxidation
layer of a chromium oxide( Cr2O3)and a ferrous oxide(fe_O3)is formed. A critical concentration of chromates must be present to protect against aggressive ions, such as chlorides and sulfates Similarly, nitrites also require critical concentration levels to maintain optimal corrosion protection. Nitrites are commonly used in closed recirculating systems. The pH levels are usually maintained above 7.5 to avoid extensive pitting of carbon steel materials. Borax-nitrite has been used for open recirculating cooling water systems in conjunction with the use of biocides Molybdates are typically combined with other inhibitors for optimal treatment. An oxidizing agent such as oxygen is required to form a protective film. Molybdates are generally most effective in the range of 5.5 to 8.5 pH. Phosphates also require the presence of oxygen in order to form a protective oxide layer. Phosphates are generally used in alkaline environments of pH greater than 8 Cathodic inhibitors suppress the corrosion rate by restricting the availability of oxygen or by altering sites favorable for cathodic hydrogen evolution. While it is generally true that cathodic inhibitors are not as effective as anodic inhibitors, they are considered safer. Additionally, they do not cause localized attack, as anodi Precipitating inhibitors produce insoluble films on the cathode in high pH conditions. By this means, /f nhibitors do when used in insufficient amounts. There are several different kinds of cathodic inhibite cathode is isolated from the environment. Zinc ions may be used as a means to precipitate zinc hydroxide at the cathode. The protective film is enhanced by the combination of other inhibitors of a multicomponent treatment program Polyphosphate-inhibitor treatments are widely used in part due to the economics of the program. The pH of the environment greatly affects the protective nature of the phosphate film. The ph should be maintained in a near neutral range, between 6.5 and 7.5, if both steel and copper alloys are part of the system Phosphonates are slightly different than polyphosphates. Phosphonates form direct phosphorus-carbon bonds. while polyphosphates form a phosphorous-oxygen bond. The phosphates are sensitive to water quality and temperature There are numerous methods developed that use multicomponent systems. The combinations have been developed to suit a specific environment or varying environment conditions. Copper inhibitors are widely used These inhibitors are used to prevent excessive copper deposition on steel components occurring from the presence of dissolved copper in the circulating waters. These inhibitors control the corrosion of the copper materials Oil and Gas Refinery Inhibitors. A number of inhibitors are designed specifically to meet the needs of the oil and gas and refinery industries. An extensive discussion regarding the use of inhibitors, the application techniques, and the monitoring of the effectiveness of inhibitor use can be found in the article"Corrosion Inhibitors for Oil and Gas Production"in Corrosion, Volume 13 of the ASM Handbook(Ref 7) Factors affecting the inhibition systems include the corrosivity of the environment, the type of environment (oxidizing or nonoxidizing ), the temperature, the pH, the metallurgy of the system components, the design of the system, and the economics of the inhibition program. Government regulations for use of the chemicals employed, regulations of the particular industry for which the inhibitor is used, potential human handling and consumption concerns, and side effects on processing fluids are all significant considerations Velocity may also play a major role in the effectiveness of an inhibitor. The higher the velocity of the fluid environment, the greater the amount of inhibitor reaching the surface. This increased velocity, in essence, the same effect as increasing the inhibitor concentration. In addition, the fast-moving solution prevents buildup of debris and deposits on the surface. This allows for better wetting of the inhibitor on the surface Use of Nonmetallic Coatings and Linings. The most widely used methods of corrosion control today are the application of nonmetallic coatings and linings. This includes organic coatings and liners, porcelain enamel and chemical-setting ceramic linings. There are constant improvements in this area of protection. More surface area of metals is protected by this method than all other methods combined ( ref 8) Surface preparation is very important for coating life and effectiveness. The Steel Structures Painting Council SSPC)and NACE International have published surface preparation and cleaning standards. Standards published by the Swedish Standards Institute have been widely used in Europe and Asia. To ensure that the coating has been applied properly, a third-party inspection may be appropriate in some circumstances. NACE International provides training and certification of such inspectors This section focuses primarily on liquid applied coatings used in atmospheric and immersion service. The application of coatings is most successful when the base metal corrosion does not exceed 1.3 mm/yr(0.050 in /yr). The resin or organic binding of the coating material is the most influential factor in determining the Thefileisdownloadedfromwww.bzfxw.com
layer of a chromium oxide (Cr2O3) and a ferrous oxide (Fe2O3) is formed. A critical concentration of chromates must be present to protect against aggressive ions, such as chlorides and sulfates. Similarly, nitrites also require critical concentration levels to maintain optimal corrosion protection. Nitrites are commonly used in closed recirculating systems. The pH levels are usually maintained above 7.5 to avoid extensive pitting of carbon steel materials. Borax-nitrite has been used for open recirculating cooling water systems in conjunction with the use of biocides. Molybdates are typically combined with other inhibitors for optimal treatment. An oxidizing agent such as oxygen is required to form a protective film. Molybdates are generally most effective in the range of 5.5 to 8.5 pH. Phosphates also require the presence of oxygen in order to form a protective oxide layer. Phosphates are generally used in alkaline environments of pH greater than 8. Cathodic inhibitors suppress the corrosion rate by restricting the availability of oxygen or by altering sites favorable for cathodic hydrogen evolution. While it is generally true that cathodic inhibitors are not as effective as anodic inhibitors, they are considered safer. Additionally, they do not cause localized attack, as anodic inhibitors do when used in insufficient amounts. There are several different kinds of cathodic inhibitors. Precipitating inhibitors produce insoluble films on the cathode in high pH conditions. By this means, the cathode is isolated from the environment. Zinc ions may be used as a means to precipitate zinc hydroxide at the cathode. The protective film is enhanced by the combination of other inhibitors of a multicomponent treatment program. Polyphosphate-inhibitor treatments are widely used in part due to the economics of the program. The pH of the environment greatly affects the protective nature of the phosphate film. The pH should be maintained in a nearneutral range, between 6.5 and 7.5, if both steel and copper alloys are part of the system. Phosphonates are slightly different than polyphosphates. Phosphonates form direct phosphorus-carbon bonds, while polyphosphates form a phosphorous-oxygen bond. The phosphates are sensitive to water quality and temperature. There are numerous methods developed that use multicomponent systems. The combinations have been developed to suit a specific environment or varying environment conditions. Copper inhibitors are widely used. These inhibitors are used to prevent excessive copper deposition on steel components occurring from the presence of dissolved copper in the circulating waters. These inhibitors control the corrosion of the copper materials. Oil and Gas Refinery Inhibitors. A number of inhibitors are designed specifically to meet the needs of the oil and gas and refinery industries. An extensive discussion regarding the use of inhibitors, the application techniques, and the monitoring of the effectiveness of inhibitor use can be found in the article “Corrosion Inhibitors for Oil and Gas Production” in Corrosion, Volume 13 of the ASM Handbook (Ref 7). Factors affecting the inhibition systems include the corrosivity of the environment, the type of environment (oxidizing or nonoxidizing), the temperature, the pH, the metallurgy of the system components, the design of the system, and the economics of the inhibition program. Government regulations for use of the chemicals employed, regulations of the particular industry for which the inhibitor is used, potential human handling and consumption concerns, and side effects on processing fluids are all significant considerations. Velocity may also play a major role in the effectiveness of an inhibitor. The higher the velocity of the fluid environment, the greater the amount of inhibitor reaching the surface. This increased velocity, in essence, has the same effect as increasing the inhibitor concentration. In addition, the fast-moving solution prevents the buildup of debris and deposits on the surface. This allows for better wetting of the inhibitor on the surface. Use of Nonmetallic Coatings and Linings. The most widely used methods of corrosion control today are the application of nonmetallic coatings and linings. This includes organic coatings and liners, porcelain enamel, and chemical-setting ceramic linings. There are constant improvements in this area of protection. More surface area of metals is protected by this method than all other methods combined (Ref 8). Surface preparation is very important for coating life and effectiveness. The Steel Structures Painting Council (SSPC) and NACE International have published surface preparation and cleaning standards. Standards published by the Swedish Standards Institute have been widely used in Europe and Asia. To ensure that the coating has been applied properly, a third-party inspection may be appropriate in some circumstances. NACE International provides training and certification of such inspectors. This section focuses primarily on liquid applied coatings used in atmospheric and immersion service. The application of coatings is most successful when the base metal corrosion does not exceed 1.3 mm/yr (0.050 in./yr). The resin or organic binding of the coating material is the most influential factor in determining the The file is downloaded from www.bzfxw.com
resistance and properties. Pigments, solvents, and additives will also influence the application properties and protective capability of the applied film Auto-Oxidative Cross-Linked Resins. All of the coatings in this class contain drying oils, which consist mainly of polyunsaturated fatty acids and undergo film formation by oxidation drying. Cross-linked resins include alkyd resins, modified alkyd resins, and epoxy resins The alkyd resins are the most common coating systems used to combat corrosion. These are not known for their exceptional chemical, alkaline, or moisture resistance, however, the ease of application, low cost, and ability to penetrate the surface, even surfaces that are poorly prepared, make alkyds one of the best choices for steel otection not directly in contact with harsh chemical environments. Some of the most common applications for kyds are water tanks, highway bridges, structural steel, and machinery housings lkyd modifications include additives that improve specific properties of the material. There are a numbe modifications that are available for different applications. Reference 8 identifies several different variations and their properties poxy esters are usually prepared by reacting a drying oil with the epoxy resin at elevated temperature Compared to the alkyd resins, epoxy resins have better adhesion, moisture, and chemical resistance and a slight Increase in cost Thermoplastic resins are characterized by softening at elevated temperatures. The most useful thermoplastic resins are the vinyls, chlorinated rubbers, thermoplastic acrylics, and bituminous resins(coal tar and asphalt). In general, these resins have good resistance to weathering, acid or alkali exposure, and moisture. The coal tars chlorinated rubbers, and the vinyls are excellent choices for superior moisture resistance. Vinyl-solution patings are most often used in applications requiring toughness and water resistance. Submersion applicatio such as locks and dams may use vinyl-solution coatings. Vinyl paints in conjunction with zinc-rich primers are used for bridge applications Acrylic coatings exhibit excellent chemical resistance to weathering environments and moisture; however, they are not considered suitable for immersion service or strong chemical environments. Typical properties include excellent color retention, gloss, and ultraviolet stability. Thermoplastic acrylics are often used in the automotive industry in finishing and refinishing. One of the most familiar uses of acrylics is for water-base indoor and outdoor paints. The major benefit of acrylic-based paint is the water cleanup Bituminous coatings are most commonly used for roof coatings, highway or pavement sealers, underground coatings, and waterproofing compounds. Asphaltic materials are less affected by ultraviolet light and have better atmospheric weathering resistance than coal tar coatings, so the asphaltic coatings are usually used for above-grade applications. However, coal tar coatings are more resistant to moisture, acids, and alkalies than are the asphaltic Cross-linked thermosetting coatings refer to chemically cured coatings that harden by a chemical reaction either with a copolymer or with moisture. Examples of chemically cross-linked thermosetting coatings include po high-temperature curing silicones, phenolic linings, and certain urethanes Chemically cured coatings that react with water include moisture-cured polyurethanes and most of the inorganic zinc-rich coatings. Chemically cross-linked coatings and linings are considered highly corrosion resistant to acids, alkalies, and moisture. They can be formulated to be resistant to abrasion, ultraviolet radiation, and thermal degradation. These properties are optimized when sufficient curing occurs. The curing time is dependent on weather conditions, but may take 7 days. Detailed discussion of specific cross- linked thermosetting coatings is provided in Ref 8 Zinc-rich coatings have the unique ability to provide galvanic protection to ferrous substrates due to the metallic zinc dust pigment. Metal-to-metal contact of the coating and substrate creates a galvanic couple. Zinc is anodic to the steel and thus provides protection to the steel substrate. There are two categories, organic zinc- rich coatings and inorganic zinc-rich coatings, based on the binders used. The great advantage of this coatin system is the elimination of pitting and subfilm corrosion. Zinc-rich coatings are widely used in industrial and marine environments. These coating are often used as primers or first coats, with a topcoat then applied to complete the protection system Porcelain Enamels. Porcelain coatings or glass coatings are applied primarily to steel, cast iron, and aluminum They are matured at 425C(800F). The porcelain coatings offer barrier protection to the substrate, which must be free of defects. Common applications of porcelain enamels include major appliances, water heater tanks, chemical-processing vessels, heat exchangers, storage tanks, piping, and pump components. The choice of porcelain enamels is primarily for chemical resistance, corrosion protection, weather resistance, mechanical
resistance and properties. Pigments, solvents, and additives will also influence the application properties and protective capability of the applied film. Auto-Oxidative Cross-Linked Resins. All of the coatings in this class contain drying oils, which consist mainly of polyunsaturated fatty acids and undergo film formation by oxidation drying. Cross-linked resins include alkyd resins, modified alkyd resins, and epoxy resins. The alkyd resins are the most common coating systems used to combat corrosion. These are not known for their exceptional chemical, alkaline, or moisture resistance; however, the ease of application, low cost, and ability to penetrate the surface, even surfaces that are poorly prepared, make alkyds one of the best choices for steel protection not directly in contact with harsh chemical environments. Some of the most common applications for alkyds are water tanks, highway bridges, structural steel, and machinery housings. Alkyd modifications include additives that improve specific properties of the material. There are a number of modifications that are available for different applications. Reference 8 identifies several different alkyd variations and their properties. Epoxy esters are usually prepared by reacting a drying oil with the epoxy resin at elevated temperature. Compared to the alkyd resins, epoxy resins have better adhesion, moisture, and chemical resistance and a slight increase in cost. Thermoplastic resins are characterized by softening at elevated temperatures. The most useful thermoplastic resins are the vinyls, chlorinated rubbers, thermoplastic acrylics, and bituminous resins (coal tar and asphalt). In general, these resins have good resistance to weathering, acid or alkali exposure, and moisture. The coal tars, chlorinated rubbers, and the vinyls are excellent choices for superior moisture resistance. Vinyl-solution coatings are most often used in applications requiring toughness and water resistance. Submersion applications such as locks and dams may use vinyl-solution coatings. Vinyl paints in conjunction with zinc-rich primers are used for bridge applications. Acrylic coatings exhibit excellent chemical resistance to weathering environments and moisture; however, they are not considered suitable for immersion service or strong chemical environments. Typical properties include excellent color retention, gloss, and ultraviolet stability. Thermoplastic acrylics are often used in the automotive industry in finishing and refinishing. One of the most familiar uses of acrylics is for water-base indoor and outdoor paints. The major benefit of acrylic-based paint is the water cleanup. Bituminous coatings are most commonly used for roof coatings, highway or pavement sealers, underground coatings, and waterproofing compounds. Asphaltic materials are less affected by ultraviolet light and have better atmospheric weathering resistance than coal tar coatings, so the asphaltic coatings are usually used for above-grade applications. However, coal tar coatings are more resistant to moisture, acids, and alkalies than are the asphaltics. Cross-linked thermosetting coatings refer to chemically cured coatings that harden by a chemical reaction either with a copolymer or with moisture. Examples of chemically cross-linked thermosetting coatings include epoxies, unsaturated polyesters, high-temperature curing silicones, phenolic linings, and certain urethanes. Chemically cured coatings that react with water include moisture-cured polyurethanes and most of the inorganic zinc-rich coatings. Chemically cross-linked coatings and linings are considered highly corrosion resistant to acids, alkalies, and moisture. They can be formulated to be resistant to abrasion, ultraviolet radiation, and thermal degradation. These properties are optimized when sufficient curing occurs. The curing time is dependent on weather conditions, but may take 7 days. Detailed discussion of specific cross-linked thermosetting coatings is provided in Ref 8. Zinc-rich coatings have the unique ability to provide galvanic protection to ferrous substrates due to the metallic zinc dust pigment. Metal-to-metal contact of the coating and substrate creates a galvanic couple. Zinc is anodic to the steel and thus provides protection to the steel substrate. There are two categories, organic zincrich coatings and inorganic zinc-rich coatings, based on the binders used. The great advantage of this coating system is the elimination of pitting and subfilm corrosion. Zinc-rich coatings are widely used in industrial and marine environments. These coating are often used as primers or first coats, with a topcoat then applied to complete the protection system. Porcelain Enamels. Porcelain coatings or glass coatings are applied primarily to steel, cast iron, and aluminum. They are matured at 425 °C (800 °F). The porcelain coatings offer barrier protection to the substrate, which must be free of defects. Common applications of porcelain enamels include major appliances, water heater tanks, chemical-processing vessels, heat exchangers, storage tanks, piping, and pump components. The choice of porcelain enamels is primarily for chemical resistance, corrosion protection, weather resistance, mechanical
or electrical properties, appearance, color, cleanability, and thermal shock capability. There are two types of porcelain enamels, ground-coat or cover-coat enamels Ground-coat enamels contain oxides that promote the dherence of the coating to the substrate. Cover-coat enamels are applied to improve appearance and properties of the coating Chemical-Setting Ceramic Linings. Inorganic chemical-setting ceramic linings are widely used as a protective lining for construction materials. Their characteristics include resistance to high temperatures and aggressive corrosion media. Applications include floors, vessels, tankS, scrubbers, air pollution control equipment, and chimneys in a variety of industries. The linings have resistance to strong acids and solvents, temperatures up to 870C(1600F), good compressive and flexural and abrasion resistance. Monolithic linings are applied by cast or gunite(shotcreting) methods over old and new steel, concrete, and brick and mortar masonry. Inorganic monolithic linings have a certain amount of permeability, and over time acid can penetrate the lining and eventually reach the substrate. Dual-lining systems can combat this problem Metallic coatings are another method of corrosion protection for materials. Typical application methods include electroplating, hot dipping, cladding, thermal spray coatings, electroless process, and vapor-deposition process Metals with greater corrosion resistance than the base material are applied Electroplated Coatings. Electroplating is deposition of a metal or alloy in an adherent form on an object serving as a cathode. Electrodeposition is used for cosmetic purposes, to improve conductivity and other surface properties, and for corrosion protection. Specific applications for electroplating include automobile bumpers, trim on appliances, electronic circuits, construction and architectural hardware, plumbing fittings, fasteners, gun bores, and food packaging. The selection of the coating is primarily based on the galvanic series. Coatings are difficult to apply without any imperfections. Thus, the most desirable situation is to choose a coating material that is anodic to the substrate and one that has a very low corrosion rate. Typical materials used as electrodeposited coatings include nickel, nickel-phosphorus, cadmium, zinc, chromium, tin, lead, copper, and Several factors that influence the performance of the coatings are the design of the part, the stresses in the substrate, the surface profile, and susceptibility to hydrogen absorption. Advantages of electroplating including the variety of coatings available, versatility of applications, variety of application methods, galvanic protection, color options for aluminum, precise thickness control, ductility, and formability after coating, There are some disadvantages to the electroplated coatings, which include color limitations, substrate incompatibilities, desig limitations, and the difficulty to plate large objects lot-dip coatings are applied by immersing a metal in a molten bath of coating metal. Typical hot-dip coated materials include zinc-coated steel (galvanized steel), aluminum-coated steel (aluminized), and aluminum-zinc alloy coated steel that are produced by batch or continuous methods. The advantages of these coatings include the ability to coat recessed areas with minimum coating thickness, resistance to mechanical damage, and good resistance to corrosion. There are, however, limitations regarding hot-dip coatings. The coating must melt at a reasonably low temperature, and the steel base metal must not undergo property alterations during the coating process. Galvanized steel has been used for more than 100 years and remains in high demand today Aluminum-coated steel requires a more difficult process, but is used in major applications including chain links, roofing panels, and automotive exhaust components. Aluminum-zinc alloy coated steel provides a combination of galvanic protection and low corrosion rate. Applications include metal roofing, automotive components, appliances, and corrugated pip Vapor-deposited coatings have been used to modify the surface properties of the materials. These coatings are most often used to improve the wear properties of a material, but they have also been used as corrosion- resistant coatings. The two major categories of these coatings are physical vapor deposition(PVD)and chemical vapor deposition(CVD). Sputtering, the process of transporting material from a source to a substrate by bombardment of a target with gas ions, is the most widely used method of application for PVD. This method is optimal for thin metal films where good adhesion and uniformity are necessary. However, the limited thickness and high cost are disadvantages of this method. Chemical vapor deposition is a process used in the semiconductor and cutting tool industry for thick, dense, and high-quality films Thermal Spray Coatings. Thermal spraying is a process in which fine molten metallic or nonmetallic material sprayed onto a substrate to form a coating. Advantages of the thermal spray process are the ability to apply thick coatings, excellent paintability of the surface, no heat distortion of the substrate, no curing time required and the capability of applying coatings in the field(Ref 9). Long-term corrosion protection of iron and steel is chieved by thermal spray coatings. The bond between the sprayed coating and the substrate is generally Thefileisdownloadedfromwww.bzfxw.com
or electrical properties, appearance, color, cleanability, and thermal shock capability. There are two types of porcelain enamels, ground-coat or cover-coat enamels. Ground-coat enamels contain oxides that promote the adherence of the coating to the substrate. Cover-coat enamels are applied to improve appearance and properties of the coating. Chemical-Setting Ceramic Linings. Inorganic chemical-setting ceramic linings are widely used as a protective lining for construction materials. Their characteristics include resistance to high temperatures and aggressive corrosion media. Applications include floors, vessels, tanks, scrubbers, air pollution control equipment, and chimneys in a variety of industries. The linings have resistance to strong acids and solvents, temperatures up to 870 °C (1600 °F), good compressive and flexural and abrasion resistance. Monolithic linings are applied by cast or gunite (shotcreting) methods over old and new steel, concrete, and brick and mortar masonry. Inorganic monolithic linings have a certain amount of permeability, and over time acid can penetrate the lining and eventually reach the substrate. Dual-lining systems can combat this problem. Metallic coatings are another method of corrosion protection for materials. Typical application methods include electroplating, hot dipping, cladding, thermal spray coatings, electroless process, and vapor-deposition process. Metals with greater corrosion resistance than the base material are applied. Electroplated Coatings. Electroplating is deposition of a metal or alloy in an adherent form on an object serving as a cathode. Electrodeposition is used for cosmetic purposes, to improve conductivity and other surface properties, and for corrosion protection. Specific applications for electroplating include automobile bumpers, trim on appliances, electronic circuits, construction and architectural hardware, plumbing fittings, fasteners, gun bores, and food packaging. The selection of the coating is primarily based on the galvanic series. Coatings are difficult to apply without any imperfections. Thus, the most desirable situation is to choose a coating material that is anodic to the substrate and one that has a very low corrosion rate. Typical materials used as electrodeposited coatings include nickel, nickel-phosphorus, cadmium, zinc, chromium, tin, lead, copper, and gold. Several factors that influence the performance of the coatings are the design of the part, the stresses in the substrate, the surface profile, and susceptibility to hydrogen absorption. Advantages of electroplating including the variety of coatings available, versatility of applications, variety of application methods, galvanic protection, color options for aluminum, precise thickness control, ductility, and formability after coating, There are some disadvantages to the electroplated coatings, which include color limitations, substrate incompatibilities, design limitations, and the difficulty to plate large objects. Hot-dip coatings are applied by immersing a metal in a molten bath of coating metal. Typical hot-dip coated materials include zinc-coated steel (galvanized steel), aluminum-coated steel (aluminized), and aluminum-zinc alloy coated steel that are produced by batch or continuous methods. The advantages of these coatings include the ability to coat recessed areas with minimum coating thickness, resistance to mechanical damage, and good resistance to corrosion. There are, however, limitations regarding hot-dip coatings. The coating must melt at a reasonably low temperature, and the steel base metal must not undergo property alterations during the coating process. Galvanized steel has been used for more than 100 years and remains in high demand today. Aluminum-coated steel requires a more difficult process, but is used in major applications including chain links, roofing panels, and automotive exhaust components. Aluminum-zinc alloy coated steel provides a combination of galvanic protection and low corrosion rate. Applications include metal roofing, automotive components, appliances, and corrugated pipe. Vapor-deposited coatings have been used to modify the surface properties of the materials. These coatings are most often used to improve the wear properties of a material, but they have also been used as corrosionresistant coatings. The two major categories of these coatings are physical vapor deposition (PVD) and chemical vapor deposition (CVD). Sputtering, the process of transporting material from a source to a substrate by bombardment of a target with gas ions, is the most widely used method of application for PVD. This method is optimal for thin metal films where good adhesion and uniformity are necessary. However, the limited thickness and high cost are disadvantages of this method. Chemical vapor deposition is a process used in the semiconductor and cutting tool industry for thick, dense, and high-quality films. Thermal Spray Coatings. Thermal spraying is a process in which fine molten metallic or nonmetallic material is sprayed onto a substrate to form a coating. Advantages of the thermal spray process are the ability to apply thick coatings, excellent paintability of the surface, no heat distortion of the substrate, no curing time required, and the capability of applying coatings in the field (Ref 9). Long-term corrosion protection of iron and steel is achieved by thermal spray coatings. The bond between the sprayed coating and the substrate is generally The file is downloaded from www.bzfxw.com