Galvanic series, metals

Galvanic Corrosion. Galvanic corrosion occurs when two dissimilar metals are in contact in a solution. The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. The driving force for galvanic corrosion is the electric potential difference that develops between two metals. This difference increases as the distance between the metals in the galvanic series increases. 

Table 4 shows a galvanic series for some commercial metals and alloys. When two metals from the series are in contact in solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. 

Galvanic Corrosion Galvanic corrosion is the corrosion rate above normal that is associated with the flow of current to a less active metal (cathode) in contact with a more active metal (anode) in the same environment. Tables 28-1 7 and 28-li show the galvanic series of various metals. It should be used with caution, since exceptions to... 

The use of dissimilar metals in contact with each other should generally be minimized, particularly if they are widely separated in their nominal positions in the galvanic series (see Table 28-1 ). If they are to be used together, consideration should be given to insulating them from each other or making the anodic material area as large as possible. 

Generai description. Galvanic corrosion refers to the preferential corrosion of the more reactive member of a two-metal pair when the metals are in electrical contact in the presence of a conductive fluid (see Chap. 16, Galvanic Corrosion ). The corrosion potential difference, the magnitude of which depends on the metal-pair combination and the nature of the fluid, drives a corrosion reaction that simultaneously causes the less-noble pair member to corrode and the more-noble pair member to become even more noble. The galvanic series for various metals in sea water is shown in Chap. 16, Table 16.1. Galvanic potentials may vary with temperature, time, flow velocity, and composition of the fluid. 

A galvanic series has been constructed that lists numerous industrial metals according to their galvanic potential in sea water. Table 16.1 is such a listing for metals often found in cooling water systems. 

TABLE 16.1 Galvanic Series of Selected Metals in Sea Water ... 

The galvanic series in Table 16.1 is generally useful for predicting the tendency toward galvanic corrosion between coupled metals. The arrangement of this series is, however, based on data generated under controlled laboratory conditions on clean, bare metals. 

The possible effects of fluid velocity on galvanic corrosion are sometimes overlooked. Fluid velocity can affect the apparent potential of metals in a given environment. Depending on the environment, a metal under the influence of relatively rapid flow may assume either a more noble or a more active character than that indicated by the galvanic series. Occasionally, this shift in potential may result in galvanic corrosion that would not occur under stagnant or low-flow conditions. 

Remember that the galvanic series was constructed from laboratory data using sea water as the exposure fluid. When there is a question about galvanic corrosion tendencies in actual industrial environments involving fluids substantially different from sea water, appropriate testing of candidate metals in these fluids may be warranted. 

If a metal or alloy combination is to be selected, choose combination of metals as close together in the galvanic series as possible. 

The metal that corrodes in any couple is that which has the most negative corrosion potential in the galvanic series in that environment. As guidance. Table 53.1 is of general but not universal applicability. Thus in the case of copper... 

The metal with the more negative corrosion potential in the environmental conditions prevailing (note that the standard electrode potentials are seldom applicable and the galvanic series can be misleading)... 

Fig 21 -1 Practical galvanic series Of metals and alloys indicating. . approximate potentials... 

Table 21.8. / Galvanic series of some commercial metals and alloys in...

Fig. 21.1 Practical galvanic series of metals and alloys showing potentials on the hydrogen scale. (Note that the potentials shown are typical values that will vary according to the nature of the solution.) (after Butler, G. and Ison, H. C. K., Corrosion and its Prevention in Water, Leonard Hill, London (1966))... 

Galvanic Series a list of metals and alloys based on their relative potentials in a given specified environment, usually sea water. 

TABLE 25-1 Practical Galvanic Series of Metals and Alloys... 

If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic metal will be increased, as the metal lower in the electrochemical series will readily act as a cathode. The galvanic series in sea water for some of the more commonly used metals is shown in Table 7.4. Some metals under certain conditions form a natural protective film for example, stainless steel in oxidising environments. This state is denoted by passive in the series shown in Table 7.4 active indicates the absence of the protective film. Minor... 

Following this system of classification, the metals known in antiquity can be listed in what is usually referred to as the galvanic series of metals a... 

Nickel hydroxides, 17 111 Nickel—iron alloys, 17 101 Nickel—iron—aluminum catalyst, 17 121 Nickel—iron cells, 3 491—493 Nickel—iron—chromium alloy 825 in galvanic series, 7 805t Nickel—iron—chromium alloys, 17 102—103 Nickel—iron plating, 9 821 Nickel itch, 12 691, 701 Nickel—matrix composites, 17 104 Nickel metal, forms of, 17 95—99 Nickel metal hydride cells, 3 431, 471, 509-512... 

Under either of these circumstances, the more active metal (less noble) serves as the anode and the less active metal (more noble) as the cathode. Metal loss from a small anode can be extremely high if connected to a cathode with a larger area. The closer together metals are in the galvanic series, the less the corrosion rate. The further apart, the greater the corrosion rate. See APPENDIX 4 for a listing of the galvanic series of metals and alloys. 

Because corrosion is electrochemical, we can use our knowledge of redox reactions to combat it. The simplest way to prevent corrosion is to protect the surface of the metal from exposure to air and water by painting. A method that achieves greater protection is to galvanize the metal, which involves coating it with an unbroken film of zinc (Fig. 12.16). Zinc lies below iron in the electrochemical series, so if a scratch exposes the metal beneath, the more strongly reducing zinc releases electrons to the iron. As a result, the zinc, not the iron, is oxidized. The zinc itself survives exposure on the unbroken surface because, like aluminum, it is passivated by a protective oxide. 

Many other parameters tend to influence the corrosion of metals immersed in sea water. When two metals of different potentials are galvanically coupled, the acceleration of the attack on the less noble metal of the two is observed frequently. A small area of an anodic metal coupled to a large area of a second metal that is cathodic can be particularly dangerous. A useful guide to help predict unfavorable combinations is the galvanic series of metals in sea water (0). The reverse situation—namely, a small cathode coupled to an anode that is large in area—often proves satisfactory in service. 

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