bEr ■cad Low-carbon steel, cast iro □ Low-alloy steel H Austenitic nickel cast iron Naval brass, yellow brass,red brass a Tin I co 50Sn-50Pb solder D Admiralty brass, aluminum brass Manganese bronze D silicon bronze Tin bronzes Stainless steel (AISI types 410, 416) ■ Nickel silver 8OCU-20Ni Stainless Steel (AISI type 430) Nickel-aluminum bronze Silver brazing alloys Nickel 200 ■ Silver Stainless steel(AISI types 302, 304, 321, 347) Monel 400, Monel K-500 Stainless steel (AISI types 316, 317) Alloy 20 stainless steels, cast and wrought B Hastelloy B D Hastelloy C Graphite 08 0 Potential(E, V versus SCE Thefileisdownloadedfromwww.bzfxw.com
The file is downloaded from www.bzfxw.com
Fig 3 Galvanic series of metals and alloys in seawater Alloys are listed in order of the potential they exhibit in flowing seawater; those indicated by the black rectangle were tested in low-velocity or poorly aerated water and at shielded areas may become active and exhibit a potential near-05 V Adapted from Ref 2 Although the measurement of potentials has limitations as previously noted, galvanic series based on seawater or other standard electrolytes are worthwhile for initial materials selection for multiple metal/alloy systems in a given environment. A properly prepared galvanic series is easy to use and can answer simple galvanic- corrosion questions regarding the positions and relative separation of metals and alloys in a galvanic series However, the comparison of galvanic potentials is only a ranking of general susceptibility; it does not address all the conditions and factors that influence galvanic corrosion. Limitations on the use of galvanic series include No information is available on the rate of corrosion Active-passive metals may display two, widely differing potentials Small changes in electrolyte can change the potentials significantly Potentials may be time dependent Creating a galvanic series is a matter of measuring the corrosion potential of various materials of interest in the electrolyte of interest against a reference electrode half-cell, such as saturated calomel, as described by the procedure in ASTMG 82(Ref 3). To prepare a valid galvanic series for a given materials and environment of interest, all the factors affecting the potential must be addressed. This includes material composition, heat treatment, surface preparation(mill scale, coatings surface finish, etc. ), environmental composition(trace contaminants, dissolved gases, etc. ) temperature, flow rate, solution concentration, and degree of agitation or aeration. In addition, corrosion product films and other changes in surface composition can also occur in some environments, and so exposure time is important, too With certain exceptions, the galvanic series in seawater is broadly applicable in other natural waters and in uncontaminated atmospheres. Bodily fluids are similar to saltwater, so as a first approximation the galvanic series is useful for implants However, metals behave differently in different environments. The relative positions of metals and alloys in the galvanic series can vary significantly from one environment to another. The position of alloys in the galv series for seawater is not necessarily valid in nonsaline solutions. For example, aluminum is anodic to zinc in an aqueous 1 M sodium chromate(Na2 CrO4) solution and cathodic to iron in an aqueous 1 M sodium sulfate 2SO4) The emf series is a table that lists in order the standard electrode potentials of specified electrochemical reactions. The potentials are measured against a standard reference electrode when the metal is immersed in a solution of its own ions at unit activity (Table 1). Similar to the galvanic series, it is a list of metals and alloys arranged according to their relative potentials in a given environment. Generally, the relative positions of metals and alloys in both emf and galvanic series are the same. An exception is the position of cadmium with respect to iron and its alloys. In the emf series, cadmium is cathodic to iron, but in the galvanic series(at least in seawater), cadmium is anodic to iron. Thus, if only the emf series were used to predict the behavior of a ferrous metal system, cadmium would not be chosen as a sacrificial protective coating, yet this is the principal use for cadmium plating on steel Table i standard emf series of metals Metal-metal ion equilibrium Electrode potential versus unit activity) SHE at25°C(75°F),v Noble or cathodic au-Au 498 Pt Pd-Pd 0.987 0.799
Fig. 3 Galvanic series of metals and alloys in seawater. Alloys are listed in order of the potential they exhibit in flowing seawater; those indicated by the black rectangle were tested in low-velocity or poorly aerated water and at shielded areas may become active and exhibit a potential near -0.5 V. Adapted from Ref 2 Although the measurement of potentials has limitations as previously noted, galvanic series based on seawater or other standard electrolytes are worthwhile for initial materials selection for multiple metal/alloy systems in a given environment. A properly prepared galvanic series is easy to use and can answer simple galvaniccorrosion questions regarding the positions and relative separation of metals and alloys in a galvanic series. However, the comparison of galvanic potentials is only a ranking of general susceptibility; it does not address all the conditions and factors that influence galvanic corrosion. Limitations on the use of galvanic series include: · No information is available on the rate of corrosion · Active-passive metals may display two, widely differing potentials · Small changes in electrolyte can change the potentials significantly · Potentials may be time dependent Creating a galvanic series is a matter of measuring the corrosion potential of various materials of interest in the electrolyte of interest against a reference electrode half-cell, such as saturated calomel, as described by the procedure in ASTM G 82 (Ref 3). To prepare a valid galvanic series for a given materials and environment of interest, all the factors affecting the potential must be addressed. This includes material composition, heat treatment, surface preparation (mill scale, coatings surface finish, etc.), environmental composition (trace contaminants, dissolved gases, etc.), temperature, flow rate, solution concentration, and degree of agitation or aeration. In addition, corrosion product films and other changes in surface composition can also occur in some environments, and so exposure time is important, too. With certain exceptions, the galvanic series in seawater is broadly applicable in other natural waters and in uncontaminated atmospheres. Bodily fluids are similar to saltwater, so as a first approximation the galvanic series is useful for implants. However, metals behave differently in different environments. The relative positions of metals and alloys in the galvanic series can vary significantly from one environment to another. The position of alloys in the galvanic series for seawater is not necessarily valid in nonsaline solutions. For example, aluminum is anodic to zinc in an aqueous 1 M sodium chromate (Na2CrO4) solution and cathodic to iron in an aqueous 1 M sodium sulfate (Na2SO4) solution (Ref 4). The emf series is a table that lists in order the standard electrode potentials of specified electrochemical reactions. The potentials are measured against a standard reference electrode when the metal is immersed in a solution of its own ions at unit activity (Table 1). Similar to the galvanic series, it is a list of metals and alloys arranged according to their relative potentials in a given environment. Generally, the relative positions of metals and alloys in both emf and galvanic series are the same. An exception is the position of cadmium with respect to iron and its alloys. In the emf series, cadmium is cathodic to iron, but in the galvanic series (at least in seawater), cadmium is anodic to iron. Thus, if only the emf series were used to predict the behavior of a ferrous metal system, cadmium would not be chosen as a sacrificial protective coating, yet this is the principal use for cadmium plating on steel. Table 1 Standard emf series of metals Metal-metal ion equilibrium (unit activity) Electrode potential versus SHE at 25°C (75 °F), V Noble or cathodic Au-Au3+ 1.498 Pt-Pt2+ 1.2 Pd-Pd2+ 0.987 Ag-Ag+ 0.799
Hg-Hgt 0.788 Cu-Cu 0.337 H 0.000 Pb-Pb -0.136 0.250 0.277 Cd-Cd- 0.403 0.440 Cr-C 0.744 Zn-znt 0763 Ti-Ti -1210 1.630 A1-Alf 1.662 Mg-Mg 2.363 -2.714 2925 Active or anodic olarization Electron flow occurs between metals or alloys in a galvanic couple. This current flow between the more active and more noble members causes shifts in potential, because the potentials of the metals or alloys tend to approach each other over time. This shift, where the open-circuit electrode potential changes as a result of the passage of current, is due to polarization of the electrodes. For example, during electrolysis, the potential of an anode becomes more noble, and the potential of the cathode becomes more active than their respective open- circuit(or reversible) potentials. Often polarization occurs from the formation of a film on the electrode surface The magnitude of the shift depends on the environment, as does the initial potential. If the more noble metal or alloy is more easily polarized, its potential is shifted more toward the more active metal or alloy potential. The shift in potential of the more active metal or alloy in the direction of the cathode is therefore minimized so that accelerated galvanic corrosion is not as great as would otherwise be expected. On the other hand, when the more noble metal or alloy is not readily polarized, the potential of the more active metal shifts further toward the cathode( that is, in the direction of anodic polarization) such that appreciable accelerated galvanic corrosion occurs Polarization measurements on the members of a galvanic couple can provide precise information concerning their behavior. The polarization curves and the mixed potential for the galvanically coupled metals in a particular environment can be used to determine the magnitude of the galvanic-corrosion effects as well as the type of corrosion An important application in the use of polarization measurements in galvanic corrosion is the prediction of localized corrosion. Polarization techniques and critical potentials are used to measure the susceptibility to pitting and crevice corrosion of metals and alloys coupled in chloride solutions. In addition, this technique is valuable in predicting galvanic corrosion among three or more coupled metals or alloys Polarization measurements can be made by generating stepped potential or potentiodynamic polarization curves or by obtaining potentiostatic information on polarization behavior. The objective is to obtain a good indication of the amount of current required to hold each material at a given potential. Because all materials in the galvanic system must be at the same potential in systems with low solution resistivity, such as seawater, and because the sum of all currents flowing between the materials must equal 0 by Kirchoft's Law, the coupled potential of all materials and the galvanic currents flowing can be uniquely determined for the system. The corrosion rate can then be related to galvanic current by Faraday's Law. Faraday's Law establishes the proportionality n current flow and the amount of material dissolved or deposited in electrolysis Additional information is provided in the article"Kinetics of Aqueous Corrosion, in Corrosion, Vol 13 of the ASM Handbook Thefileisdownloadedfromwww.bzfxw.com
Hg-Hg2+ 0.788 Cu-Cu2+ 0.337 H2-H+ 0.000 Pb-Pb2+ -0.126 Sn-Sn2+ -0.136 Ni-Ni2+ -0.250 Co-Co2+ -0.277 Cd-Cd2+ -0.403 Fe-Fe2+ -0.440 Cr-Cr2+ -0.744 Zn-Zn2+ -0.763 Ti-Ti3+ -1.210 Ti-Ti2+ -1.630 Al-Al2+ -1.662 Mg-Mg2+ -2.363 Na-Na+ -2.714 K-K+ -2.925 Active or anodic Polarization Electron flow occurs between metals or alloys in a galvanic couple. This current flow between the more active and more noble members causes shifts in potential, because the potentials of the metals or alloys tend to approach each other over time. This shift, where the open-circuit electrode potential changes as a result of the passage of current, is due to polarization of the electrodes. For example, during electrolysis, the potential of an anode becomes more noble, and the potential of the cathode becomes more active than their respective opencircuit (or reversible) potentials. Often polarization occurs from the formation of a film on the electrode surface. The magnitude of the shift depends on the environment, as does the initial potential. If the more noble metal or alloy is more easily polarized, its potential is shifted more toward the more active metal or alloy potential. The shift in potential of the more active metal or alloy in the direction of the cathode is therefore minimized so that accelerated galvanic corrosion is not as great as would otherwise be expected. On the other hand, when the more noble metal or alloy is not readily polarized, the potential of the more active metal shifts further toward the cathode (that is, in the direction of anodic polarization) such that appreciable accelerated galvanic corrosion occurs. Polarization measurements on the members of a galvanic couple can provide precise information concerning their behavior. The polarization curves and the mixed potential for the galvanically coupled metals in a particular environment can be used to determine the magnitude of the galvanic-corrosion effects as well as the type of corrosion. An important application in the use of polarization measurements in galvanic corrosion is the prediction of localized corrosion. Polarization techniques and critical potentials are used to measure the susceptibility to pitting and crevice corrosion of metals and alloys coupled in chloride solutions. In addition, this technique is valuable in predicting galvanic corrosion among three or more coupled metals or alloys. Polarization measurements can be made by generating stepped potential or potentiodynamic polarization curves or by obtaining potentiostatic information on polarization behavior. The objective is to obtain a good indication of the amount of current required to hold each material at a given potential. Because all materials in the galvanic system must be at the same potential in systems with low solution resistivity, such as seawater, and because the sum of all currents flowing between the materials must equal 0 by Kirchoff's Law, the coupled potential of all materials and the galvanic currents flowing can be uniquely determined for the system. The corrosion rate can then be related to galvanic current by Faraday's Law. Faraday's Law establishes the proportionality between current flow and the amount of material dissolved or deposited in electrolysis. Additional information is provided in the article “Kinetics of Aqueous Corrosion,” in Corrosion, Vol 13 of the ASM Handbook. The file is downloaded from www.bzfxw.com
Potentiodynamic polarization curves are generated by connecting the specimen of interest to a scanning potentiostat. This device applies whatever current is necessary between the specimen and a counterelectrode to maintain that specimen at a given potential versus a reference electrode half-cell placed near the specimen. The current required is plotted as a function of potential over a range that begins at the corrosion potential and Ich material of interest in the system. Additional information on the method for generating these ated for proceeds in the direction(anodic or cathodic)required by that material. Such curves would be generated for available in ASTM G 5(Ref 5). The scan rate for potential must be chosen such that sufficient time is allowed for completion of electrical charging at the interface Potentiodynamic polarization is particularly effective for materials with time-independent polarization behavior. It is fast, relatively easy, and gives a reasonable, quantitative prediction of corrosion rates in many systems. However, potentiostatic techniques are preferred for time-dependent polarization. To establish larization characteristics for time-dependent polarization, a series of specimens are used, each held to one of a series of constant potentials with a potentiostat while the current required is monitored as a function of time After the current has stabilized or after a preselected time period has elapsed, the current at each potential is recorded. Testing of each specimen results in the generation of one potential/current data pair, which gives a point on the polarization curve for that material. The data are then interpolated to trace out the full curve. This technique is very accurate for time-dependent polarization, but is expensive and time cor individual specimens can be weighed before and after testing to determine corrosion rate as a function of potential, thus enabling the errors from using Faraday's Law to be easily corrected The process of predicting galvanic corrosion from polarization behavior can be illustrated by the example of a steel-copper system. Steel has the more negative corrosion potential and will therefore suffer increased corrosion upon coupling to copper, but the amount of this corrosion must be predicted from polarization curves If the polarization of each material is plotted as the absolute value of the log of current density versus potential and if the current density axis of each of these curves is multiplied by the wetted surface area of that material in the service application, then the result will be a plot of the total anodic current for steel and the total cathodi current for copper in this application as a function of potential ( Fig. 4) dic Iron anodic Ecorr (Cu) ouple coupled(Cu) coupled( Fe) Ecor(Fe) io (F Iron cathodic Log current Fig4 Prediction of coupled potential and galvanic current from polarization diagrams. i current; io, exchange current; Ecorr, corrosion potential Furthermore, when the two metals are electrically connected, the anodic current to the steel must be supplied by the copper; that is, the algebraic sum of the anodic and cathodic currents must equal 0. If the polarization curves for the two materials, normalized for surface area as above, are plotted together, this current condition is satisfied where the two curves intersect. This point of intersection allows for the prediction of the coupled potential of the materials and the galvanic current flowing between them from the intersection point. This procedure works if there is no significant electrolyte resistance between the two metals; otherwise, this resistance must be taken into account in a complex manner that is beyond the scope of this article
Potentiodynamic polarization curves are generated by connecting the specimen of interest to a scanning potentiostat. This device applies whatever current is necessary between the specimen and a counterelectrode to maintain that specimen at a given potential versus a reference electrode half-cell placed near the specimen. The current required is plotted as a function of potential over a range that begins at the corrosion potential and proceeds in the direction (anodic or cathodic) required by that material. Such curves would be generated for each material of interest in the system. Additional information on the method for generating these curves is available in ASTM G 5 (Ref 5). The scan rate for potential must be chosen such that sufficient time is allowed for completion of electrical charging at the interface. Potentiodynamic polarization is particularly effective for materials with time-independent polarization behavior. It is fast, relatively easy, and gives a reasonable, quantitative prediction of corrosion rates in many systems. However, potentiostatic techniques are preferred for time-dependent polarization. To establish polarization characteristics for time-dependent polarization, a series of specimens are used, each held to one of a series of constant potentials with a potentiostat while the current required is monitored as a function of time. After the current has stabilized or after a preselected time period has elapsed, the current at each potential is recorded. Testing of each specimen results in the generation of one potential/current data pair, which gives a point on the polarization curve for that material. The data are then interpolated to trace out the full curve. This technique is very accurate for time-dependent polarization, but is expensive and time consuming. The individual specimens can be weighed before and after testing to determine corrosion rate as a function of potential, thus enabling the errors from using Faraday's Law to be easily corrected. The process of predicting galvanic corrosion from polarization behavior can be illustrated by the example of a steel-copper system. Steel has the more negative corrosion potential and will therefore suffer increased corrosion upon coupling to copper, but the amount of this corrosion must be predicted from polarization curves. If the polarization of each material is plotted as the absolute value of the log of current density versus potential and if the current density axis of each of these curves is multiplied by the wetted surface area of that material in the service application, then the result will be a plot of the total anodic current for steel and the total cathodic current for copper in this application as a function of potential (Fig. 4). Fig. 4 Prediction of coupled potential and galvanic current from polarization diagrams. i, current; io, exchange current; Ecorr, corrosion potential Furthermore, when the two metals are electrically connected, the anodic current to the steel must be supplied by the copper; that is, the algebraic sum of the anodic and cathodic currents must equal 0. If the polarization curves for the two materials, normalized for surface area as above, are plotted together, this current condition is satisfied where the two curves intersect. This point of intersection allows for the prediction of the coupled potential of the materials and the galvanic current flowing between them from the intersection point. This procedure works if there is no significant electrolyte resistance between the two metals; otherwise, this resistance must be taken into account in a complex manner that is beyond the scope of this article
References cited in this section 3."Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance, " G 82, Wear and Erosion; Metal Corrosion, Vol 03 02, Annual Book of ASTM Standards American Society for Testing and Materials 4. E.H. Hollingsworth and H.Y. Hunsicker, Corrosion of Aluminum and Aluminum Alloys, Corrosion, Vol 13, ASM Handbook, ASM International, 1987, p 583-609 5. "Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements, G 5, Wear and Erosion; Metal Corrosion, Vol 03.02, Annual Book of ASTM Standards, American Society for Testing and Materials Forms of corrosion Combating Circumstances That Promote Galvanic Action Dissimilar Metals. The combination of dissimilar metals in engineering design is quite common--for example in heating or cooling coils in vessels, heat exchangers, or machinery. Such combinations often lead to galvanic corrosion. The obvious design considerations to minimize the adverse effects of using dissimilar metals are to Select metals with the least difference in"uncoupled"corrosion potentials Minimize the area of the more noble metal with respect to that of the active metal Insert an electrically insulating gasket between the two dissimilar metals Seal the junction from any available electroly Another possibility is coating the cathodic material for corrosion control. Where metallic coatings are used, there may be a risk of galvanic corrosion, especially along the cut edges. Rounded profiles and effective sealants or coatings are beneficial. Ineffective painting of an anode in an assembly can significantly reduce the desired service lifetime because local defects(anodes) effectively multiply the risk of localized corrosion It is often necessary to use different materials in close proximity. Sometimes, components that were designed in isolation can end up in direct contact in the plant(Fig. 5). In such instances, the ideals of a total design concept become especially apparent, but usually in hindsight Thefileisdownloadedfromwww.bzfxw.com
References cited in this section 3. “Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance,” G 82, Wear and Erosion; Metal Corrosion, Vol 03.02, Annual Book of ASTM Standards, American Society for Testing and Materials 4. E.H. Hollingsworth and H.Y. Hunsicker, Corrosion of Aluminum and Aluminum Alloys, Corrosion, Vol 13, ASM Handbook, ASM International, 1987, p 583–609 5. “Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements,” G 5, Wear and Erosion; Metal Corrosion, Vol 03.02, Annual Book of ASTM Standards, American Society for Testing and Materials Forms of Corrosion Combating Circumstances That Promote Galvanic Action Dissimilar Metals. The combination of dissimilar metals in engineering design is quite common—for example, in heating or cooling coils in vessels, heat exchangers, or machinery. Such combinations often lead to galvanic corrosion. The obvious design considerations to minimize the adverse effects of using dissimilar metals are to: · Select metals with the least difference in “uncoupled” corrosion potentials · Minimize the area of the more noble metal with respect to that of the active metal · Insert an electrically insulating gasket between the two dissimilar metals · Seal the junction from any available electrolyte Another possibility is coating the cathodic material for corrosion control. Where metallic coatings are used, there may be a risk of galvanic corrosion, especially along the cut edges. Rounded profiles and effective sealants or coatings are beneficial. Ineffective painting of an anode in an assembly can significantly reduce the desired service lifetime because local defects (anodes) effectively multiply the risk of localized corrosion. It is often necessary to use different materials in close proximity. Sometimes, components that were designed in isolation can end up in direct contact in the plant (Fig. 5). In such instances, the ideals of a total design concept become especially apparent, but usually in hindsight. The file is downloaded from www.bzfxw.com