obtained by using polarization curves, critical potentials, and the mixed potential of the galvanic couple. In chloride-bearing environments, galvanically induced localized corrosion of many stainless steels occurs in couples with copper or nickel and their alloys and with other more noble materials. However, couples of stainless and copper alloys are often used with impunity in freshwater cooling systems. Iron and steel tend to protect stainless steel in aqueous environments when galvanically coupled. The behavior of stainless steels makes them easy to polarize; thus, galvanic effects on other metals or alloys tend to be minimized However, galvanic corrosion of steel can be induced by stainless steel, particularly in aqueous environments and with adverse area ratios Lead, Tin, and Zinc. These three materials occupy similar positions in the galvanic series, although zinc is the most active. The oxide films formed on these materials can shift their potentials to more noble values. Thus, in some environments they may occupy more noble positions than one might otherwise expect. For example, the tin coating in tin cans is anodic to steel under anaerobic conditions in the sealed container, but becomes cathodic when the can is opened and exposed to air. Zinc is an active metal. It is susceptible to galvanic corrosion and is widely used for galvanic anodes, in cathodic protection as a sacrificial coating(for example, galvanizing or electroplating), and as a pigment in certain types of coatings Copper and its alloys occupy an intermediate position in the galvanic series. They are not readily polarized in chloride-bearing aqueous solutions; therefore, they cause severe accelerated corrosion of more active metals, such as aluminum and its alloys and the ferrous metals. Somewhat similar to the nickel alloys, they lie between the active and passive positions for stainless steels(Fig. 3)and therefore induce localized corrosion of the active alloys Nickel and its alloys are not readily polarized and will therefore cause accelerated corrosion of more active materials, such as aluminum and ferrous alloys. In chloride-bearing solutions, nickel is somewhat more noble than copper, and the cupronickel lie somewhere in between. Nickel and its alloys are similar to copper alloys in their effects on stainless steels. In some environments, the cast structure of a nickel weld may be anodic to the wrought parent metals The combination of their passive surface with their inherent resistance places nickel-chromium alloys such as Inconel alloy 600 and Hastelloy alloy C-276 in more noble positions in the traditional galvanic series. In chloride-bearing solutions, Inconel alloy 600 is reported to occupy two positions because of existence of active and passive states in a manner similar to the stainless steels(Fig 3). It is highlighted in black to indicate this These alloys are readily polarized, and galvanic effects on other less noble metals and alloys therefore tend to be minimized Cobalt-base alloys, most of which are chromium bearing, are resistant to galvanic corrosion because of their oble position in the galvanic series. However, in environments in which their passive film is not stable, they occupy a more active position and can be adversely affected by more noble materials. The fact that they polarize readily tends to reduce their galvanic effects on less noble materials Reactive metals(titanium, zirconium, and tantalum)are extremely noble because of their passive films. In general, these alloys are not susceptible to galvanic corrosion, and their ease of polarization tends to minimiz adverse galvanic effects on other metals or alloys. Because of the ease with which they pick up hydrogen in the atomic state, they may themselves become embrittled in galvanic couples. Tantalum repair patches in glass- lined vessels have been destroyed by contact with cooling coils or agitators made of less noble alloys. Tantalum is susceptible to attack by alkalis, such as may form in the vicinity of a cathode in neutral solutions Noble Metals. The term noble metal is applied to silver, gold, and platinum group metals. This designation in itself describes their position in the galvanic series and their corresponding resistance to galvanic corrosion However, they do not polarize readily and can therefore have a marked effect in galvanic couples with other metals or alloys. This effect is observed with gold and silver coatings on copper, nickel, aluminum, and their References cited in this section Water mmended Practice for Cathodic Protection of Aluminum Pipe Buried in Soil or Immersed in Mater. Protec., Vol 2(No 10), 1963 p 106 8. F W. Hewes, Investigation of Maximum and Minimum Criteria for the Cathodic Protection of Aluminum in Soil, Oil Week, Vol 16(No 24-28), Aug-Sept 1965 Thefileisdownloadedfromwww.bzfxw.com
obtained by using polarization curves, critical potentials, and the mixed potential of the galvanic couple. In chloride-bearing environments, galvanically induced localized corrosion of many stainless steels occurs in couples with copper or nickel and their alloys and with other more noble materials. However, couples of stainless and copper alloys are often used with impunity in freshwater cooling systems. Iron and steel tend to protect stainless steel in aqueous environments when galvanically coupled. The passive behavior of stainless steels makes them easy to polarize; thus, galvanic effects on other metals or alloys tend to be minimized. However, galvanic corrosion of steel can be induced by stainless steel, particularly in aqueous environments and with adverse area ratios. Lead, Tin, and Zinc. These three materials occupy similar positions in the galvanic series, although zinc is the most active. The oxide films formed on these materials can shift their potentials to more noble values. Thus, in some environments they may occupy more noble positions than one might otherwise expect. For example, the tin coating in tin cans is anodic to steel under anaerobic conditions in the sealed container, but becomes cathodic when the can is opened and exposed to air. Zinc is an active metal. It is susceptible to galvanic corrosion and is widely used for galvanic anodes, in cathodic protection as a sacrificial coating (for example, galvanizing or electroplating), and as a pigment in certain types of coatings. Copper and its alloys occupy an intermediate position in the galvanic series. They are not readily polarized in chloride-bearing aqueous solutions; therefore, they cause severe accelerated corrosion of more active metals, such as aluminum and its alloys and the ferrous metals. Somewhat similar to the nickel alloys, they lie between the active and passive positions for stainless steels (Fig. 3) and therefore induce localized corrosion of the active alloys. Nickel and its alloys are not readily polarized and will therefore cause accelerated corrosion of more active materials, such as aluminum and ferrous alloys. In chloride-bearing solutions, nickel is somewhat more noble than copper, and the cupronickels lie somewhere in between. Nickel and its alloys are similar to copper alloys in their effects on stainless steels. In some environments, the cast structure of a nickel weld may be anodic to the wrought parent metals. The combination of their passive surface with their inherent resistance places nickel-chromium alloys such as Inconel alloy 600 and Hastelloy alloy C-276 in more noble positions in the traditional galvanic series. In chloride-bearing solutions, Inconel alloy 600 is reported to occupy two positions because of existence of active and passive states in a manner similar to the stainless steels (Fig. 3). It is highlighted in black to indicate this. These alloys are readily polarized, and galvanic effects on other less noble metals and alloys therefore tend to be minimized. Cobalt-base alloys, most of which are chromium bearing, are resistant to galvanic corrosion because of their noble position in the galvanic series. However, in environments in which their passive film is not stable, they occupy a more active position and can be adversely affected by more noble materials. The fact that they polarize readily tends to reduce their galvanic effects on less noble materials. Reactive metals (titanium, zirconium, and tantalum) are extremely noble because of their passive films. In general, these alloys are not susceptible to galvanic corrosion, and their ease of polarization tends to minimize adverse galvanic effects on other metals or alloys. Because of the ease with which they pick up hydrogen in the atomic state, they may themselves become embrittled in galvanic couples. Tantalum repair patches in glasslined vessels have been destroyed by contact with cooling coils or agitators made of less noble alloys. Tantalum is susceptible to attack by alkalis, such as may form in the vicinity of a cathode in neutral solutions. Noble Metals. The term noble metal is applied to silver, gold, and platinum group metals. This designation in itself describes their position in the galvanic series and their corresponding resistance to galvanic corrosion. However, they do not polarize readily and can therefore have a marked effect in galvanic couples with other metals or alloys. This effect is observed with gold and silver coatings on copper, nickel, aluminum, and their alloys. References cited in this section 7. Recommended Practice for Cathodic Protection of Aluminum Pipe Buried in Soil or Immersed in Water, Mater. Protec., Vol 2 (No. 10), 1963 p 106 8. F.W. Hewes, Investigation of Maximum and Minimum Criteria for the Cathodic Protection of Aluminum in Soil, Oil Week, Vol 16 (No. 24–28), Aug–Sept 1965 The file is downloaded from www.bzfxw.com
Forms of Corrosion Uniform corrosion vised and adapted by randy k. Kent, MDE Engineers, Inc Uniform corrosion, or general corrosion, is a corrosion process exhibiting uniform thinning that proceeds without appreciable localized attack. It is the most common form of corrosion and may appear initially as a single penetration, but with thorough examination of the cross section it becomes apparent that the base material has uniformly thinned Uniform chemical attack of metals is the simplest form of corrosion, occurring in the atmosphere, in solutions, and in soil, frequently under normal service conditions. Excessive attack can occur when the environment has changed from that initially expected Weathering steels, magnesium alloys, zinc alloys, and copper alloys are examples of materials that typically exhibit general corrosion. Passive materials, such as stainless steels aluminum alloys, or nickel-chromium alloys are generally subject to localized corrosion. Under specific conditions, however, each material may vary from its normal mode of corrosion Uniform corrosion commonly occurs on metal surfaces having homogeneous chemical composition and microstructure. Access to the metal by the attacking environment is generally unrestricted and uniform. Any gradient or changes in the attacking environment due to the corrosion reaction or stagnancy can change the degradation mode away from uniform corrosion. At the microlevel, uniform corrosion is found to be an electrochemical reaction between adjacent closely spaced microanodic and microcathodic areas Consequently uniform corrosion might be considered to be localized electrolytic attack occurring consistently and evenly over the surface of the metal This is well illustrated in binary alloys that contain phases of differing corrosion potentials. The grain in one phase becomes anodic to grain in the second phase in the microstructure, thus producing an electrolytic cell when the proper electrolyte is present Another illustration is ductile cast iron, where there are three compositionally different phases within the material: ferrite, pearlite, and graphite. The corrosion process will initiate as uniform corrosion with the formation of an electrolytic microcell between the graphite cathode and ferrite or pearlite anode(ref 9, 10) The process can turn into a pitting mechanism if the microcells develop enough to produce acidic gradients within the micropits where the graphite nodules lie or are dislodged The uniform corrosion occurring by the microelectrolytic or galvanic cell in the ductile cast iron is shown in Fig. 8. Observe the initial stages of micropits being formed by these microcells
Forms of Corrosion Uniform Corrosion Revised and adapted by Randy K. Kent, MDE Engineers, Inc. Uniform corrosion, or general corrosion, is a corrosion process exhibiting uniform thinning that proceeds without appreciable localized attack. It is the most common form of corrosion and may appear initially as a single penetration, but with thorough examination of the cross section it becomes apparent that the base material has uniformly thinned. Uniform chemical attack of metals is the simplest form of corrosion, occurring in the atmosphere, in solutions, and in soil, frequently under normal service conditions. Excessive attack can occur when the environment has changed from that initially expected. Weathering steels, magnesium alloys, zinc alloys, and copper alloys are examples of materials that typically exhibit general corrosion. Passive materials, such as stainless steels, aluminum alloys, or nickel-chromium alloys are generally subject to localized corrosion. Under specific conditions, however, each material may vary from its normal mode of corrosion. Uniform corrosion commonly occurs on metal surfaces having homogeneous chemical composition and microstructure. Access to the metal by the attacking environment is generally unrestricted and uniform. Any gradient or changes in the attacking environment due to the corrosion reaction or stagnancy can change the degradation mode away from uniform corrosion. At the microlevel, uniform corrosion is found to be an electrochemical reaction between adjacent closely spaced microanodic and microcathodic areas. Consequently, uniform corrosion might be considered to be localized electrolytic attack occurring consistently and evenly over the surface of the metal. This is well illustrated in binary alloys that contain phases of differing corrosion potentials. The grain in one phase becomes anodic to grain in the second phase in the microstructure, thus producing an electrolytic cell when the proper electrolyte is present. Another illustration is ductile cast iron, where there are three compositionally different phases within the material: ferrite, pearlite, and graphite. The corrosion process will initiate as uniform corrosion with the formation of an electrolytic microcell between the graphite cathode and ferrite or pearlite anode (Ref 9, 10). The process can turn into a pitting mechanism if the microcells develop enough to produce acidic gradients within the micropits where the graphite nodules lie or are dislodged. The uniform corrosion occurring by the microelectrolytic or galvanic cell in the ductile cast iron is shown in Fig. 8. Observe the initial stages of micropits being formed by these microcells
Fig.8 Scanning electron micrograph of ductile cast iron graphite nodules and ferritic phase after corrosion tests. Note the loss of material at the interface of the nodule 2000x. Source: Ref 11. 12 References cited in this section 9. R. Kent, Anodic Polarization Measurements of Alloyed DI, " NACE Int Conf, National Association of Corrosion Engineers, 1987 0.R. Kent, "Potential Anodic Polarization Measurements and Mechanical Property Analysis of Ferritic Nodular Iron Alloyed with Nickel, University of Washington, 1986 11. F. Mansfeld and V. Bertocci, Electrochemical Corrosion Testing, STP 727, American Society for Testing and Materials. 1981 12. M. Henthorne, Corrosion Causes and Control, Carpenter Technology Corp., 1972, p. 30 Forms of Corrosion Surface Conditions All metals are affected by uniform corrosion in some environments; the rusting of steel and the tarnishing of silver are typical examples of uniform corrosion. In some metals, such as steel, uniform corrosion produces a Thefileisdownloadedfromwww.bzfxw.com
Fig. 8 Scanning electron micrograph of ductile cast iron graphite nodules and ferritic phase after corrosion tests. Note the loss of material at the interface of the nodule. 2000×. Source: Ref 11, 12 References cited in this section 9. R. Kent, “Anodic Polarization Measurements of Alloyed DI,” NACE Int. Conf., National Association of Corrosion Engineers, 1987 10. R. Kent, “Potential Anodic Polarization Measurements and Mechanical Property Analysis of Ferritic Nodular Iron Alloyed with Nickel,” University of Washington, 1986 11. F. Mansfeld and V. Bertocci, Electrochemical Corrosion Testing, STP 727, American Society for Testing and Materials, 1981 12. M. Henthorne, Corrosion Causes and Control, Carpenter Technology Corp., 1972, p. 30 Forms of Corrosion Surface Conditions All metals are affected by uniform corrosion in some environments; the rusting of steel and the tarnishing of silver are typical examples of uniform corrosion. In some metals, such as steel, uniform corrosion produces a The file is downloaded from www.bzfxw.com
somewhat rough surface by the oxidation/reduction reaction, in which the end product(oxide)either dissolves in the environment and is carried away or produces a loosely adherent, porous coating now greater in thickness Because of the porosity in the oxidation(rust), this metal is still considered active or able to continue degrading Coating specialists have been able to formulate a phosphoric-acid-based compound that reacts with the rust to produce an oxide that makes the surface passive and seals the surface from water and other solutions In contrast to the active surfaces formed typically on steels, some metals form dense insulated tightly adherent passive films from uniform-corrosion processes with the metal surface remaining somewhat smooth. Examples of these include the tarnishing of silver in air, lead in sulfate-containing environments, and stainless steel in ing Forms of Corrosion Classification of uniform Corrosion In general terms, uniform corrosion can be classified further according to the specific conditions of environmental or electrochemical attack. For example, uniform thinning can be attributed to various conditions Atmospheric corrosion Aqueous corrosion Galvanic corrosion Stray-current corrosion(which is similar to galvanic corrosion, but does not rely on electrochemically induced driving forces to cause rapid attack) Biological corrosion(which is a microbial-assisted form of attack that can manifest itself as uniform corrosion by forming weak or cathodic oxides, or it can also produce a localized form of attack) Molten salt corrosion and liquid-metal corrosion(which have become more of a concern as the demand for higher-temperature heat-transfer fluids increases) High-temperature(gaseous)corrosion Corrosive attack for these conditions is not necessarily restricted to just uniform thinning. Other forms of corrosive attack( such as stress-corrosion cracking, dealloying, or pitting) may also be operative Following are some examples of uniform-corrosion-related failures and its prevention and evaluation Appropriate methods of controlling uniform corrosion in some situations are noted. Selection of a metal that has a suitable resistance to the environment in which the specific part is used and the application of paints and other types of protective coatings are two common methods used to control uniform corrosion. Modification of the environment by changing its composition, concentration, pH, and temperature or by adding an inhibitor is also effective Example 2: Uniform Corrosion of Carbon Steel Boiler Feedwater Tubes. The carbon steel tubes shown in Fig 9 have been corroded to paper thinness, revealing a lacelike pattern of total metal loss. These steel tubes are from a boiler feedwater heater feeding a deaerator. As part of the boiler-water treatment program, it was decided to inject a chelate to control scale formation in the boiler tubes. Unfortunately, the chelate was added ahead of the preheater, where the boiler water still contained oxygen(O2). As the chelate removed iron oxide, the O2 formed more iron oxide, and this uniform dissolution of steel reduced the tubing to the totally corroded condition shown. Moving the chelate addition to a point after the deaerator stopped the corrosion and allowed reuse of
somewhat rough surface by the oxidation/reduction reaction, in which the end product (oxide) either dissolves in the environment and is carried away or produces a loosely adherent, porous coating now greater in thickness. Because of the porosity in the oxidation (rust), this metal is still considered active or able to continue degrading. Coating specialists have been able to formulate a phosphoric-acid-based compound that reacts with the rust to produce an oxide that makes the surface passive and seals the surface from water and other solutions. In contrast to the active surfaces formed typically on steels, some metals form dense insulated tightly adherent passive films from uniform-corrosion processes with the metal surface remaining somewhat smooth. Examples of these include the tarnishing of silver in air, lead in sulfate-containing environments, and stainless steel in humid air creating passive films. Forms of Corrosion Classification of Uniform Corrosion In general terms, uniform corrosion can be classified further according to the specific conditions of environmental or electrochemical attack. For example, uniform thinning can be attributed to various conditions such as: · Atmospheric corrosion · Aqueous corrosion · Galvanic corrosion · Stray-current corrosion (which is similar to galvanic corrosion, but does not rely on electrochemically induced driving forces to cause rapid attack) · Biological corrosion (which is a microbial-assisted form of attack that can manifest itself as uniform corrosion by forming weak or cathodic oxides, or it can also produce a localized form of attack) · Molten salt corrosion and liquid-metal corrosion (which have become more of a concern as the demand for higher-temperature heat-transfer fluids increases) · High-temperature (gaseous) corrosion Corrosive attack for these conditions is not necessarily restricted to just uniform thinning. Other forms of corrosive attack (such as stress-corrosion cracking, dealloying, or pitting) may also be operative. Following are some examples of uniform-corrosion-related failures and its prevention and evaluation. Appropriate methods of controlling uniform corrosion in some situations are noted. Selection of a metal that has a suitable resistance to the environment in which the specific part is used and the application of paints and other types of protective coatings are two common methods used to control uniform corrosion. Modification of the environment by changing its composition, concentration, pH, and temperature or by adding an inhibitor is also effective. Example 2: Uniform Corrosion of Carbon Steel Boiler Feedwater Tubes. The carbon steel tubes shown in Fig. 9 have been corroded to paper thinness, revealing a lacelike pattern of total metal loss. These steel tubes are from a boiler feedwater heater feeding a deaerator. As part of the boiler-water treatment program, it was decided to inject a chelate to control scale formation in the boiler tubes. Unfortunately, the chelate was added ahead of the preheater, where the boiler water still contained oxygen (O2). As the chelate removed iron oxide, the O2 formed more iron oxide, and this uniform dissolution of steel reduced the tubing to the totally corroded condition shown. Moving the chelate addition to a point after the deaerator stopped the corrosion and allowed reuse of steel in the preheater
Fig9 Uniform corrosion of steel tubes in boiler feedwater containing oxygen(O2) and a chelating water-treating chemical Example 3: Uniform Corrosion of a Copper Pipe Coupling. The 25 mm(1 in. ) copper coupling shown in Fig 10 has been uniformly degraded around most of the circumference of the bell and partially on the spigot end One penetration finally occurred through the thinned area on the spigot end of this pipe. The pipe was buried in noncorrosive sandy soil, but was found to incur stray currents of 2 v direct current in relation to a Cu/ CuSO4 half cell. Eliminating, moving, or shielding the source of stray current are obvious solutions Fig. 10 A 25 mm(I in copper coupling from a potable water system that had degraded uniformly from stray currents. The spigot end has been penetrated near the edge of the bell. Courtesy of MDE Engineers, Inc. Example 4: Uniform Corrosion of Copper Piping Caused by Microbiological Attack. Microbiological attack of copper piping has been well documented and was found in a closed-loop water heater system. Figure 11 shows the microbes that were cultured from the corrosion product. They were found to be sulfur-reducing bacteria Uniform thinning occurred when the resultant oxide on the copper pipe surface eroded in low flow rates. The threshold for erosion by turbulent waters is much lower with this type of oxide and found to penetrate this same pipe, as shown in Fig. 12 Thefileisdownloadedfromwww.bzfxw.com
Fig. 9 Uniform corrosion of steel tubes in boiler feedwater containing oxygen (O2) and a chelating water-treating chemical Example 3: Uniform Corrosion of a Copper Pipe Coupling. The 25 mm (1 in.) copper coupling shown in Fig. 10 has been uniformly degraded around most of the circumference of the bell and partially on the spigot end. One penetration finally occurred through the thinned area on the spigot end of this pipe. The pipe was buried in noncorrosive sandy soil, but was found to incur stray currents of 2 V direct current in relation to a Cu/CuSO4 half cell. Eliminating, moving, or shielding the source of stray current are obvious solutions. Fig. 10 A 25 mm (1 in.) copper coupling from a potable water system that had degraded uniformly from stray currents. The spigot end has been penetrated near the edge of the bell. Courtesy of MDE Engineers, Inc. Example 4: Uniform Corrosion of Copper Piping Caused by Microbiological Attack. Microbiological attack of copper piping has been well documented and was found in a closed-loop water heater system. Figure 11 shows the microbes that were cultured from the corrosion product. They were found to be sulfur-reducing bacteria. Uniform thinning occurred when the resultant oxide on the copper pipe surface eroded in low flow rates. The threshold for erosion by turbulent waters is much lower with this type of oxide and found to penetrate this same pipe, as shown in Fig. 12. The file is downloaded from www.bzfxw.com