Galvanic series in seawater

Figure 30 Partial list of galvanic series in seawater. (From H.P. Hack, Metals Handbook, Vol. 13, Corrosion, 9lh ed ASM, Metals Park, OH, p. 234, 1987.)...

The galvanic series in seawater is often used to estimate the risk of galvanic corrosion in other media, for which the series may not be available. The risk of galvanic corrosion depends as much on the corrosivity and conductivity of the medium as on the separation of the two metals in the galvanic series. Fresh waters generally have neither the corrosivity nor the conductivity to support galvanic activity. 

Figure 3.3. Galvanic series in seawater [5]. (Reprinted with permission of ASM International. All rights reserved, www.asminternational.org.)...

Figure 2 shows two bars made of zinc and copper immersed in an electrolyte (seawater). Needless to say, after some time, the zinc bar shows signs of corrosion. In this situation an electrical potential difference is established between the two metals which can be measured and is in agreement with the table of galvanic series in seawater. Here, the zinc and copper bars acts as an anode and a cathode, respectively. The simple experiment demonstrates the basic idea behind the concept of cathodic protection. The zinc bar (anode) is being sacrificed to protect the copper bar (cathode). This method of cathodic protection is known as the sacrificial anodic method. 

Fig. 4. Galvanic series in flowing seawater. Certain alloys may become more active in low velocity or poorly aerated seawater and the potentials exhibited...

Table 7.7 Galvanic series in flowing, aerated seawater [1].

Each environment requires a different galvanic series, for example, a galvanic series in static seawater cannot be used to predict galvanic corrosion in turbulently flowing seawater. 

The galvanic series in Table 17.2 indicates the relative reactivities in seawater of a number of metals and alloys. When two alloys are conpled in seawater, the one lower in the series experiences corrosion. Some of the alloys in the table are grouped in brackets. Generally the base metal is the same for these bracketed alloys, and there is little danger of corrosion if alloys within a single bracket are conpled. It is also worth noting from this series that some alloys are hsted twice (e.g., nickel and the stainless steels), in both active and passive states. 

The galvanic series of metals and alloys in seawater has been given in Figure 1.6 whenever dissimilar metals are used, it is pmdent to select a pair of metals with minimum difference in potential, in the particular environment and temperature of exposure. 

Figure 6.11 Galvanic series for some metals in seawater (Hack)5...

The galvanic series of metals and alloys in seawater is given in Table 7.20. From this series it is clear that steel and 2024 aluminum are in close proximity. From their positions it is inferred that steel is cathodic and aluminum is anodic in seawater. The corrosion potentials of iron and aluminum measured after immersion in various media for 24 h are given in Table 7.21. It is seen from these data that the corrosion potentials of iron and aluminum are very nearly the same in 0.1M sodium chloride. Some studies on the galvanic action of the steel-aluminum couple in fresh waters such as pure, river, lake and underground water and salt solutions are noted in Table 7.22. In one of the studies, the... 

Fie. 4.20 Galvanic series of various metals in flowing seawater at 2.4 to 4.0 m/sat 5 " to 30 °C (volts vs. saturated calomel reference electrode). Note Dark boxes indicate active behavior of active-passive alloys. Source Ref 12 and 1 3... 

Figure 1.7 Galvanic series of metals in seawater. (Reproduced by permission. National Association of Corrosion Engineers International (11).)...

A new galvanie series of different materials in seawater at 10°C as well as 40°C has been established, see Figure 7.6. It is shown that the eorrosion potentials of the most eorrosion-resistant materials depend considerably on the temperature. This is due to a microbiologieal (baeterial) slime layer that is formed at temperatures below about 30°C in northern waters, and below 40°C in warmer waters like that in the Mediterranean. The baeterial aetivity lifts the potential of these materials (see Sections 6.2.1 and 7.5.4, Figure 7.23). Previously published galvanic series do not show this effect, probably because the exposure periods were too short to imply the potential increase. In addition to the temperature, the flow velocity may have some effeet upon the eorrosion potential. The data in Figure 7.6 were recorded at 1 m/s. 

Figure 7.6 Galvanic series of different materials in seawater at 10 Cand40 C [7.5, 7.6].

Chandler KA. Marine and Offshore Corrosion. London Butterworths, 1985. Recommended practice. RP-series on Cafliodic Protection, NACE, Houston. Gartland PO, Drugli JM. Methods for evaluation and prevention of local and galvanic corrosion in chlorinated seawater pipeUnes. Corrosion/92. Nashville. Paper no. 408,1992. 

Cathodic protection from sacrificial anodes is based on the principle of galvanic corrosion. This means that a less noble material is connected to the structure (metal) to be protected. To select the right saeriflcial anode material, the galvanic series is important. Table 19.1 shows the galvanic series for seleeted materials in seawater. The table indicates that magnesium, zinc, and alnminum alloys are well suited as sacrificial anodes when protecting steel. 

Table 9.25. Galvanic series of metals and alloys in seawater Metal or alloy...

FIG. 2—Galvanic series of various metals In flowing seawater at 2.4-4.0 m/s for 5-15 days at 5- 0°C (redrawn from original In Ref 21 taken from ASTM G 82). 

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