nor materal should Poor Anode Rolling direction End grain Good Exposed Poor Copper piping 二 Cladding Fig. 5 Design details that can affect galvanic corrosion. (a) Fasteners should be more noble than the components being fastened; undercuts should be avoided, and insulating washers and spaces should be used to completely isolate the fastener .(b)Weld filler metals should be more noble than base metals. Transition joints can be used when a galvanic couple is anticipated at the design stage, and weld beads should be properly oriented to minimize galvanic effects. (c) local damage can result from cuts across heavily worked areas. End grains should not be left exposed.(d) Galvanic corrosion is possible if a coated component is cut. When necessary, the cathodic component of a couple should be coated.(e)lon transfer through a fluid can result in galvanic attack of less noble metals In the example shown at left, copper ions from the copper heater coil could deposit on the aluminum stirrer. A nonmetallic stirrer would be better at right the distance from a metal container to a heater coil should be increased to minimize ion transfer (f wood treated with copper preservatives can be corrosive to certain nails. Aluminum cladding can also be at risk.(g) Contact of two metals through a fluid trap can be avoided by using a drain, collection tray, or a deflector Where dissimilar materials are to be joined, it is advisable to use a more noble metal in a joint(Fig 5a and b) Unfavorable area ratios should be avoided. Metal combinations should be used in which the more active metal or alloy surface is relatively large. Rivets, bolts, and other fasteners should be of a more noble metal than the material to be fastened Dissimilar-metal crevices, such as at threaded connections, are to be avoided, if possible. Cr should be sealed, preferably by welding or brazing, although putties are sometimes used effectively. Re de sections of the more active member should be used at joints, or the corrosion allowance of this should be increased. or both. Effective insulation can be useful if it does not lead to crevice corrosion Some difficulties arise in the use of adhesives which may not be sealants
Fig. 5 Design details that can affect galvanic corrosion. (a) Fasteners should be more noble than the components being fastened; undercuts should be avoided, and insulating washers and spaces should be used to completely isolate the fastener. (b) Weld filler metals should be more noble than base metals. Transition joints can be used when a galvanic couple is anticipated at the design stage, and weld beads should be properly oriented to minimize galvanic effects. (c) Local damage can result from cuts across heavily worked areas. End grains should not be left exposed. (d) Galvanic corrosion is possible if a coated component is cut. When necessary, the cathodic component of a couple should be coated. (e) Ion transfer through a fluid can result in galvanic attack of less noble metals. In the example shown at left, copper ions from the copper heater coil could deposit on the aluminum stirrer. A nonmetallic stirrer would be better. At right, the distance from a metal container to a heater coil should be increased to minimize ion transfer. (f) Wood treated with copper preservatives can be corrosive to certain nails. Aluminum cladding can also be at risk. (g) Contact of two metals through a fluid trap can be avoided by using a drain, collection tray, or a deflector. Where dissimilar materials are to be joined, it is advisable to use a more noble metal in a joint (Fig. 5a and b). Unfavorable area ratios should be avoided. Metal combinations should be used in which the more active metal or alloy surface is relatively large. Rivets, bolts, and other fasteners should be of a more noble metal than the material to be fastened. Dissimilar-metal crevices, such as at threaded connections, are to be avoided, if possible. Crevices should be sealed, preferably by welding or brazing, although putties are sometimes used effectively. Replaceable sections of the more active member should be used at joints, or the corrosion allowance of this section should be increased, or both. Effective insulation can be useful if it does not lead to crevice corrosion. Some difficulties arise in the use of adhesives, which may not be sealants
Preventive Measures. In particular cases, it is possible to reduce or eliminate galvanic-corrosion effects between widely dissimilar metals or alloys in a particular environment by altering one or more of the three key factors necessary for galvanic corrosion. Electrochemical techniques to mitigate galvanic corrosion include electrical isolation, use of transition materials, and cathodic protection. Electrical Isolation. The joint between dissimilar metals can be isolated to break the electrical continuity. Use of nonmetallic inserts, washers, fittings, and coatings at the joint between the materials will provide sufficient electrical resistance to eliminate galvanic corrosion. The design of the insulation or maintenance must ensure that the insulator is not bridged by accumulated debris Transition Materials. In order to eliminate a dissimilar-metal junction, a transition piece can be introduced. The transition piece consists of the same metals or alloys as in the galvanic couple bonded together in a laminar structure. The transition piece is inserted between the members of the couple such that the similar metals mate with one another. The dissimilar-metal junction then occurs at the bond interface, excluding the electrolyte Cathodic Protection. Sacrificial metals, such as magnesium or zinc, may be introduced into the galvanic assembly. The most active member will corrode while providing cathodic protection to the other members in the galvanic assembly (for example, zinc anodes in cast iron waterboxes of copper alloy water-cooled heat exchangers ). Cathodic protection is a common and economical method of corrosion protection that is often used for the protection of underground or underwater steel structures. The use of cathodic protection for long-term corrosion prevention for structural steels, underground pipelines, oil and gasoline tanks, offshore drilling rigs, well-head structures, steel piling, piers, bulkheads, offshore pipelines, gathering systems, drilling barges, and other underground and underwater structures is a fairly standard procedure. Magnesium, zinc, and aluminum galvanic(sacrificial)anodes are used in a wide range of cathodic protection applications The electric potential of an object can be changed to protect it. a direct-current potential can be applied on it through the use of a rectifier, battery, or solar cell. Conversely, stray currents can be a source of increased alvanic degradation Altering the Electrolyte. The use of corrosion inhibitors is effective in some cases. Elimination of cathodic depolarizers( deaeration of water by thermomechanical means plus oxygen scavengers such as sodium sulfite or hydrazine)is very effective in some aqueous systems Metallic Coatings. Two types of metallic coatings are used in engineering design: noble metal coatings and sacrificial metal coatings. Noble metal coatings are used as barrier coatings over a more reactive metal Galvanic corrosion of the substrate can occur at pores, damage sites, and edges in the noble metal coating Sacrificial metal coatings provide cathodic protection of the more-noble base metal, as in the case of galvanized steel or alclad aluminum Forms of corrosion Evaluation of galvanic corrosion In terms of general evaluation, the most common method of predicting galvanic corrosion is by immersion testing of the galvanic couple in the environment of interest. Although time consuming, this is the most desirable method of investigating galvanic corrosion In design and materials selecti ning tests are conducted to eliminate as many candidate materials as possible. These screening tests consist of one or more of the following electrochemical techniques Potential measurements Polarization measurements Potential measurements are made to construct a galvanic series of metals and alloys, as described in the next section. As a first approximation, the galvanic series is a useful tool. However, it has several limitations. Metal alloys that form passive films will exhibit varying potentials with time and are therefore difficult to position e series with certainty. Also, the galvanic series does not provide information on the polarization Thefileisdownloadedfromwww.bzfxw.com
Preventive Measures. In particular cases, it is possible to reduce or eliminate galvanic-corrosion effects between widely dissimilar metals or alloys in a particular environment by altering one or more of the three key factors necessary for galvanic corrosion. Electrochemical techniques to mitigate galvanic corrosion include electrical isolation, use of transition materials, and cathodic protection. Electrical Isolation. The joint between dissimilar metals can be isolated to break the electrical continuity. Use of nonmetallic inserts, washers, fittings, and coatings at the joint between the materials will provide sufficient electrical resistance to eliminate galvanic corrosion. The design of the insulation or maintenance must ensure that the insulator is not bridged by accumulated debris. Transition Materials. In order to eliminate a dissimilar-metal junction, a transition piece can be introduced. The transition piece consists of the same metals or alloys as in the galvanic couple bonded together in a laminar structure. The transition piece is inserted between the members of the couple such that the similar metals mate with one another. The dissimilar-metal junction then occurs at the bond interface, excluding the electrolyte. Cathodic Protection. Sacrificial metals, such as magnesium or zinc, may be introduced into the galvanic assembly. The most active member will corrode while providing cathodic protection to the other members in the galvanic assembly (for example, zinc anodes in cast iron waterboxes of copper alloy water-cooled heat exchangers). Cathodic protection is a common and economical method of corrosion protection that is often used for the protection of underground or underwater steel structures. The use of cathodic protection for long-term corrosion prevention for structural steels, underground pipelines, oil and gasoline tanks, offshore drilling rigs, well-head structures, steel piling, piers, bulkheads, offshore pipelines, gathering systems, drilling barges, and other underground and underwater structures is a fairly standard procedure. Magnesium, zinc, and aluminum galvanic (sacrificial) anodes are used in a wide range of cathodic protection applications. The electric potential of an object can be changed to protect it. A direct-current potential can be applied on it through the use of a rectifier, battery, or solar cell. Conversely, stray currents can be a source of increased galvanic degradation. Altering the Electrolyte. The use of corrosion inhibitors is effective in some cases. Elimination of cathodic depolarizers (deaeration of water by thermomechanical means plus oxygen scavengers such as sodium sulfite or hydrazine) is very effective in some aqueous systems. Metallic Coatings. Two types of metallic coatings are used in engineering design: noble metal coatings and sacrificial metal coatings. Noble metal coatings are used as barrier coatings over a more reactive metal. Galvanic corrosion of the substrate can occur at pores, damage sites, and edges in the noble metal coating. Sacrificial metal coatings provide cathodic protection of the more-noble base metal, as in the case of galvanized steel or Alclad aluminum. Forms of Corrosion Evaluation of Galvanic Corrosion In terms of general evaluation, the most common method of predicting galvanic corrosion is by immersion testing of the galvanic couple in the environment of interest. Although time consuming, this is the most desirable method of investigating galvanic corrosion. In design and materials selection, screening tests are conducted to eliminate as many candidate materials as possible. These screening tests consist of one or more of the following electrochemical techniques: · Potential measurements · Polarization measurements · Current measurements Potential measurements are made to construct a galvanic series of metals and alloys, as described in the next section. As a first approximation, the galvanic series is a useful tool. However, it has several limitations. Metals and alloys that form passive films will exhibit varying potentials with time and are therefore difficult to position in the series with certainty. Also, the galvanic series does not provide information on the polarization The file is downloaded from www.bzfxw.com
characteristics of the materials and so is not helpful in predicting the probable magnitude of galvanic effects Polarization measurements are discussed in the section "polarization"' in this article Measurement of galvanic currents between coupled metals or alloys is based on the use of a zero-resistance milliammeter Zero-resistance electrical continuity between the members of the galvanic couple is maintained electronically, while the resulting current is measured with the ammeter. Use of this technique should take into account certain limitations. First, when localized corrosion such as pitting or crevice corrosion is possible in the galvanic couple, long induction periods may be required before these effects are observed Test periods must be of sufficient duration to take this effect into account also the measured galvanic current is not al ways a true measure of the actual corrosion current, because it is the algebraic sum of the currents due to anodic and cathodic reactions. When cathodic currents are appreciable at the mixed potential of the galvanic couple, the measured galvanic current will be significantly lower than the true current. Therefore, large differences between the true corrosion rate calculated by weight loss and that obtained by galvanic current measurements have been observed Evaluating Galvanic Corrosion as a Synergistic Corrosion Mechanism. Uniform corrosion, pitting, and crevice corrosion can all be exacerbated by galvanic conditions. In addition, any of the tests used for the more conventional forms of corrosion, such as uniform attack, pitting, or stress corrosion, can be used, with modifications, to determine galvanic-corrosion effects. The modifications can be as simple as connecting a second metal to the system or as complex as necessary to evaluate the appropriate parameters. a change in the method of data interpretation is often all that is needed to convert conventional test methods into galvanic- corrosion tests Example 1: Galvanic Corrosion Leading to Fatigue Failure of a Helicopter Tail Rotor(Ref 6). A tail rotor blade spar shank was constructed of 2014-T652 aluminum alloy. The spar was hollow with the cavity filled with a lead wool ballast material and sealed with a thermoset material. One blade separated in flight, but fortunately a successful emergency landing was made. The outboard section that had separated was recovered Examination. There was a large flat fracture with beach mark features typical of fatigue. The fatigue crack had initiated on the inner surface of the spar at the cavity( Fig. 6a and b). Fluid was found within the cavity, and the thermoset seal cap was broken. Microscopic examination revealed pitting corrosion on the inner surface of the spar cavit (b) Fig 6 Fatigue cracking of a helicopter tail rotor blade.(a) Scanning electron micrograph of the blade showing lead wool ballast in contact with the 2014-t652 aluminum spar bore cavity wall at the failure origin-13x.(b) Greater magnification(63x)in this same area shows the multiple pits and associated corrosion products at the failure origin. The beach marks are seen emanating from the pits, typical of fatigue failure mode Source: Ref 6 Conclusion. The seal had failed allowing moisture into the cavity. This served as an electrolyte for galvanic corrosion between the lead and aluminum components. Corrosion pits formed at contact points of the lead wool
characteristics of the materials and so is not helpful in predicting the probable magnitude of galvanic effects. Polarization measurements are discussed in the section “Polarization” in this article. Measurement of galvanic currents between coupled metals or alloys is based on the use of a zero-resistance milliammeter. Zero-resistance electrical continuity between the members of the galvanic couple is maintained electronically, while the resulting current is measured with the ammeter. Use of this technique should take into account certain limitations. First, when localized corrosion such as pitting or crevice corrosion is possible in the galvanic couple, long induction periods may be required before these effects are observed. Test periods must be of sufficient duration to take this effect into account. Also the measured galvanic current is not always a true measure of the actual corrosion current, because it is the algebraic sum of the currents due to anodic and cathodic reactions. When cathodic currents are appreciable at the mixed potential of the galvanic couple, the measured galvanic current will be significantly lower than the true current. Therefore, large differences between the true corrosion rate calculated by weight loss and that obtained by galvanic current measurements have been observed. Evaluating Galvanic Corrosion as a Synergistic Corrosion Mechanism. Uniform corrosion, pitting, and crevice corrosion can all be exacerbated by galvanic conditions. In addition, any of the tests used for the more conventional forms of corrosion, such as uniform attack, pitting, or stress corrosion, can be used, with modifications, to determine galvanic-corrosion effects. The modifications can be as simple as connecting a second metal to the system or as complex as necessary to evaluate the appropriate parameters. A change in the method of data interpretation is often all that is needed to convert conventional test methods into galvaniccorrosion tests. Example 1: Galvanic Corrosion Leading to Fatigue Failure of a Helicopter Tail Rotor (Ref 6). A tail rotor blade spar shank was constructed of 2014-T652 aluminum alloy. The spar was hollow with the cavity filled with a lead wool ballast material and sealed with a thermoset material. One blade separated in flight, but fortunately a successful emergency landing was made. The outboard section that had separated was recovered. Examination. There was a large flat fracture with beach mark features typical of fatigue. The fatigue crack had initiated on the inner surface of the spar at the cavity (Fig. 6a and b). Fluid was found within the cavity, and the thermoset seal cap was broken. Microscopic examination revealed pitting corrosion on the inner surface of the spar cavity. Fig. 6 Fatigue cracking of a helicopter tail rotor blade. (a) Scanning electron micrograph of the blade showing lead wool ballast in contact with the 2014-T652 aluminum spar bore cavity wall at the failure origin ~13×. (b) Greater magnification (~63×) in this same area shows the multiple pits and associated corrosion products at the failure origin. The beach marks are seen emanating from the pits, typical of fatigue failure mode. Source: Ref 6 Conclusion. The seal had failed allowing moisture into the cavity. This served as an electrolyte for galvanic corrosion between the lead and aluminum components. Corrosion pits formed at contact points of the lead wool
and the aluminum spar wall. These pits served as the point of origin for the fatigue crack leading to failure of Corrective Actions. To prevent the root cause of the failure, the causes of galvanic corrosion needed to be eliminated. The integrity of the seal needed to be ensured. a barrier coating between the lead and aluminum wall could be added. Alternate means of encapsulating the lead wool were also suggested The corrective action taken was not reported Reference cited in this section 6. R.H. McSwain, Fatigue Fracture of a Helicopter Tail Rotor Blade Due to Field-Induced Corrosion, Handbook of Case Histories in Failure Analysis, Vol 2, ASM International, 1993, p 30-32 Forms of Corrosion Examples of Factors Contributing to Galvanic Corrosion on transier results in the deposition of active and noncompatible deposits on a metal surface. For example, an aluminum stirrer plate used in water was extensively pitted because the water bath was heated by a copper heater coil(Fig. 5e). The pits resulted from deposition of copper ions from the heater element Nonmetallic Conductors. Less frequently recognized is the influence of nonmetallic conductors as cathodes in galvanic couples. Carbon brick in vessels is strongly cathodic to the common structural metals and alloys Imperilled polymers or metal-matrix composites can act as noble metals in a galvanic couple us graphite, especially in heat-exchanger applications, is cathodic to the less noble metals and alloys Metal Ion Deposition Ions of a more noble metal may be reduced on the surface of a more active metal-for example, copper on aluminum or steel, silver on copper. This process is also known as cementation, especially with regard to aluminum alloys. The resulting metallic deposit provides cathodic sites for further galvanic corrosion of the more active metal Forms of Corrosion Performance of Alloy groupings Magnesium occupies an extremely active position in most galvanic series and is therefore highly susceptible to galvanic corrosion. Metals that combine active potentials with higher hydrogen overvoltages, such as aluminum, zinc, cadmium, and tin, are much less damaging, although not fully compatible with magnesium Aluminum alloys containing small percentages of copper(7000 and 2000 series and 380 die-casting alloy)may cause serious galvanic corrosion of magnesium in saline environments. Very pure aluminum is quite compatible, acting as a polarizable cathode; however, when iron content exceeds 200 ppm, cathodic activity becomes significant(apparently because of the depolarizing effect of the intermetallic compound FeAl3), and galvanic attack of magnesium increases rapidly with increasing iron content. The effect of iron is diminished by the presence of magnesium in the alloy (fig. 7) Thefileisdownloadedfromwww.bzfxw.com
and the aluminum spar wall. These pits served as the point of origin for the fatigue crack leading to failure of the assembly. Corrective Actions. To prevent the root cause of the failure, the causes of galvanic corrosion needed to be eliminated. The integrity of the seal needed to be ensured. A barrier coating between the lead and aluminum wall could be added. Alternate means of encapsulating the lead wool were also suggested. The corrective action taken was not reported. Reference cited in this section 6. R.H. McSwain, Fatigue Fracture of a Helicopter Tail Rotor Blade Due to Field-Induced Corrosion, Handbook of Case Histories in Failure Analysis, Vol 2, ASM International, 1993, p 30–32 Forms of Corrosion Examples of Factors Contributing to Galvanic Corrosion Ion transfer results in the deposition of active and noncompatible deposits on a metal surface. For example, an aluminum stirrer plate used in water was extensively pitted because the water bath was heated by a copper heater coil (Fig. 5e). The pits resulted from deposition of copper ions from the heater element. Nonmetallic Conductors. Less frequently recognized is the influence of nonmetallic conductors as cathodes in galvanic couples. Carbon brick in vessels is strongly cathodic to the common structural metals and alloys. Impervious graphite, especially in heat-exchanger applications, is cathodic to the less noble metals and alloys. Carbon-filled polymers or metal-matrix composites can act as noble metals in a galvanic couple. Metal Ion Deposition. Ions of a more noble metal may be reduced on the surface of a more active metal—for example, copper on aluminum or steel, silver on copper. This process is also known as cementation, especially with regard to aluminum alloys. The resulting metallic deposit provides cathodic sites for further galvanic corrosion of the more active metal. Forms of Corrosion Performance of Alloy Groupings Magnesium occupies an extremely active position in most galvanic series and is therefore highly susceptible to galvanic corrosion. Metals that combine active potentials with higher hydrogen overvoltages, such as aluminum, zinc, cadmium, and tin, are much less damaging, although not fully compatible with magnesium. Aluminum alloys containing small percentages of copper (7000 and 2000 series and 380 die-casting alloy) may cause serious galvanic corrosion of magnesium in saline environments. Very pure aluminum is quite compatible, acting as a polarizable cathode; however, when iron content exceeds 200 ppm, cathodic activity becomes significant (apparently because of the depolarizing effect of the intermetallic compound FeAl3), and galvanic attack of magnesium increases rapidly with increasing iron content. The effect of iron is diminished by the presence of magnesium in the alloy (Fig. 7). The file is downloaded from www.bzfxw.com
o<0.01%Mg in Al ·2.10% Mg in Al E 20F4 4.80% Mg in AI E 2HUncoupled AZ31B 001 Iron content of coupled aluminum, Fig. 7 Corrosion rates in 3% NaCl solution of magnesium alloy Az31B coupled with aluminum containing varying amounts of iron and magnesium. The corrosion rate of uncoupled az3lB is shown for comparison Aluminum and its alloys also occupy active positions in the galvanic series and are subject to failure by galvanic attack. In chloride-bearing solutions, aluminum alloys are susceptible to gal vanically induced localized corrosion, especially in dissimilar-metal crevices. In this type of environment, severe galvanic effects are observed when aluminum alloys are coupled with more noble metals and alloys Cementation effects are also observed in the presence of dissolved heavy-metal ions such as copper, mercury or lead. Some aluminum alloys are used for sacrificial anodes in seawater. An active, anodic alloy is used to clad aluminum, protecting it against pitting in some applications Contact of aluminum with more cathodic metals should be avoided in any environment in which aluminum by itself is subject to pitting corrosion. Where such contact is necessary, protective measures should be implemented to minimize sacrificial corrosion of the aluminum. In such an environment, aluminum is alread polarized to its pitting potential, and the additional potential imposed by contact with the more cathodic metal eatly increases the corrosion current In the absence of chlorides or with low concentrations, as in potable water, aluminum and its alloys may be les active because of greater stability of the protective oxide film. Galvanic effects are not as severe under these conditions. Corrosion of aluminum in contact with more cathodic metals is much less severe in solutions of most nonhalide salts, in which aluminum alone normally is not polarized to its pitting potential In many environments, aluminum can be used in contact with chromium or stainless steels with only slight cceleration of corrosion; chromium and stainless steels are easily polarized cathodically in mild environments so that the corrosion current is small despite the large differences in the open-circuit potentials between these metals and aluminum. Galvanic current between aluminum and another metal also can be reduced by removing oxidizing agents from the electrolyte. Thus, the corrosion rate of aluminum coupled to copper in seawater is greatly reduced wherever the seawater is deaerated The criterion for cathodic protection of aluminum in soils and waters has been published by the National Association of Corrosion Engineers(Ref 7). The suggested practice is to shift the potential at least-015 V, but not beyond the value of-1 20 V as measured against a saturated copper sulfate( Cu/CusO4 reference electrode In some soils, potentials as low as-1.4 V have been encountered without appreciable cathodic corrosion(Ref Iron and steel are fairly active materials and require protection against galvanic corrosion by the higher alloys They are, however, more noble than aluminum and its alloys in chloride solutions. However, in low-chloride waters,a reversal of potential can occur that causes iron or steel to become more active than aluminum. a similar reversal can occur between iron and zinc in hot waters of a specific type of chemistry Stainless Steels. Galvanic-corrosion behavior of stainless steels is difficult to predict because of the influence of passivity. In the common galvanic series, a noble position is assumed by stainless steels in the passive state, while a more active position is assumed in the active state(Fig. 3). These are noted by black blocks in the diagram. This dual position in galvanic series in chloride-bearing aqueous environments has been the cause of some serious design errors. More precise information on the galvanic behavior of stainless steels can be
Fig. 7 Corrosion rates in 3% NaCl solution of magnesium alloy AZ31B coupled with aluminum containing varying amounts of iron and magnesium. The corrosion rate of uncoupled AZ31B is shown for comparison. Aluminum and its alloys also occupy active positions in the galvanic series and are subject to failure by galvanic attack. In chloride-bearing solutions, aluminum alloys are susceptible to galvanically induced localized corrosion, especially in dissimilar-metal crevices. In this type of environment, severe galvanic effects are observed when aluminum alloys are coupled with more noble metals and alloys. Cementation effects are also observed in the presence of dissolved heavy-metal ions such as copper, mercury, or lead. Some aluminum alloys are used for sacrificial anodes in seawater. An active, anodic alloy is used to clad aluminum, protecting it against pitting in some applications. Contact of aluminum with more cathodic metals should be avoided in any environment in which aluminum by itself is subject to pitting corrosion. Where such contact is necessary, protective measures should be implemented to minimize sacrificial corrosion of the aluminum. In such an environment, aluminum is already polarized to its pitting potential, and the additional potential imposed by contact with the more cathodic metal greatly increases the corrosion current. In the absence of chlorides or with low concentrations, as in potable water, aluminum and its alloys may be less active because of greater stability of the protective oxide film. Galvanic effects are not as severe under these conditions. Corrosion of aluminum in contact with more cathodic metals is much less severe in solutions of most nonhalide salts, in which aluminum alone normally is not polarized to its pitting potential. In many environments, aluminum can be used in contact with chromium or stainless steels with only slight acceleration of corrosion; chromium and stainless steels are easily polarized cathodically in mild environments, so that the corrosion current is small despite the large differences in the open-circuit potentials between these metals and aluminum. Galvanic current between aluminum and another metal also can be reduced by removing oxidizing agents from the electrolyte. Thus, the corrosion rate of aluminum coupled to copper in seawater is greatly reduced wherever the seawater is deaerated. The criterion for cathodic protection of aluminum in soils and waters has been published by the National Association of Corrosion Engineers (Ref 7). The suggested practice is to shift the potential at least -0.15 V, but not beyond the value of -1.20 V as measured against a saturated copper sulfate (Cu/CuSO4) reference electrode. In some soils, potentials as low as -1.4 V have been encountered without appreciable cathodic corrosion (Ref 8). Iron and steel are fairly active materials and require protection against galvanic corrosion by the higher alloys. They are, however, more noble than aluminum and its alloys in chloride solutions. However, in low-chloride waters, a reversal of potential can occur that causes iron or steel to become more active than aluminum. A similar reversal can occur between iron and zinc in hot waters of a specific type of chemistry. Stainless Steels. Galvanic-corrosion behavior of stainless steels is difficult to predict because of the influence of passivity. In the common galvanic series, a noble position is assumed by stainless steels in the passive state, while a more active position is assumed in the active state (Fig. 3). These are noted by black blocks in the diagram. This dual position in galvanic series in chloride-bearing aqueous environments has been the cause of some serious design errors. More precise information on the galvanic behavior of stainless steels can be