Steels Galvanic series

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... 

Austenitic manganese steels, 75 562-563 Austenitic nickel cast iron, in galvanic series, 7 805t... 

Stainless steels Alloy 2C cast and wrought in galvanic series, 7 805t Stainless steel surgical needles, 24 206 Stain removal... 

High nitrile photocured coatings on B40 coated steel panels exhibit corrosion resistance when their open circuit potentials are either more noble or less noble than the B40 coated steel panel (Figure 3). At first, we assumed that enhanced nobility should give rise to enhanced corrosion resistance, i.e. the galvanic series. 

Table 5.10 lists the metals in order of their position in the galvanic series. The more positive or anodic metals are more active and prone to corrosion. In some installations, to prevent corrosion of a specific metal member, sacrificial anodes are installed in the ground. The sacrificial anodes are more electropositive than the metals they are protecting, so they are sacrificed to protect the structural steel. 

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... 

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. 

Galvanic series are fundamentally different from the series of standard potentials of electrode reactions (Chapter 2), which is based entirely on thermodynamic quantities. In a galvanic series, the value of the potential and the relative positions of the metals in the series can vary depending on the environment. Furthermore, individual metals may appear twice in the same galvanic series, once each for the active and the passive states. For example, according to Table 7.7, stainless steel in the active state is less noble than copper, while for the same steel in the passive slate... 

An inspection of the galvanic series will indicate that tin is more noble than steel and, consequently, the steel would corrode at the base of the pores. On the outside of a tinned container this is what happens—the tin is cathodic to the steel. However, on the inside of the container there is a reversal of polarity because of the complexing of the stannous ions by many food products. This greatly reduces the activity of the stannous ions, resulting in a change in the potential of tin in the active direction. 

Contact between dissimilar metals occurs frequently in electronic devices, components, and systems. Among the many metallization schemes found in thin film and integrated circuits are Ti/TiN/Al/TiN (see Section 9.4.1) and Ti/Pd/Au. Gold wires bonded to aluminum pads or to alumi-num/copper interfaces in devices are also commonplace. In equipment cabinets, copper ground planes may be connected to the steel frame. Dissimilar metals in electrical contact in the presence of an electrolyte solution may lead to galvanic corrosion in which the more active metal will corrode, while the more noble metal is protected from corrosion. Ranking of metals in terms of their relative activity is known as the galvanic series (Jones, 1992). 

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