Galvanic coupling noble metal

Locations. Galvanic corrosion of any type is most severe in immediate proximity to the junction of the coupled metals. Galvanic corrosion of weld metals is frequently microstructurally localized. The less-noble weld material will corrode away, leaving behind the skeletal remnants of the more-noble metal (Figs. 15.1 and 15.2). 

Galvanic corrosion is location specific in the sense that it occurs at a bimetallic couple (Fig. 16.2). It is metal specific in the sense that, typically, corrosion affects the metal that has less resistance in the environment to which the couple is exposed. Hence, in principle, we would anticipate galvanic corrosion of relatively reactive metals wherever they are in physical contact with relatively noble metals in a sufficiently aggressive, common environment. Experience has shown, however, that all such couples do not necessarily result in unsatisfactory service. This is because of the interplay of various critical factors that influence galvanic corrosion. These critical factors are discussed in the next section. 

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

When possible, avoid coupling materials having widely dissimilar galvanic potentials. If this cannot he avoided, make use of favorable area ratios by giving the active metal a large exposed area relative to the noble metal. For example, copper or copper-based alloy tubes may be joined to a steel tube sheet. Because of the favorable area ratio in this case, a relatively inexpensive steel tube sheet may be intentionally substituted for a bronze or a brass tube sheet if thickness specifications allow for a small amount of galvanic corrosion of the steel. 

An interesting effect is sometimes observed when cupronickels are galvanically coupled to less noble materials. The corrosion rate of the active metal is increased and the corrosion rate of the cupronickel is diminished, as expected. The diminished corrosion rate of the cupronickel can, however, diminish its fouling resistance since reduced production of copper ions lowers toxicity to copper-ion-sensitive organisms. 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

A form of corrosion resulting from the presence of two dissimilar metals such as steel and copper in an electrolyte such as water forming a galvanic couple, whereby the less noble anodic metal (in this case steel) corrodes. 

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. 

Coupling with noble metal causes galvanic corrosion... 

There are many permutations of galvanic corrosion that can occur when two or more dissimilar metals or alloys are coupled in a cooling water electrolyte. The less noble metal actively corrodes, the extent of which is determined by a number of factors. 

In the absence of copper ions in the slurry or copper metal on the wafer, / , is the only means by which titanium is removed. As copper ions are added to the slurry, either as a by-product of copper polishing or by the addition of a copper salt such as Cu(N03)2, titanium removal also occurs via the galvanic couple with Cu ions, / 2- Increasing the concentration of Cu, increases Ecu2f/cu and moves the copper reduction curve in Figure 4.44b in the noble (positive) direction. Consequently, the corrosion current density for the Cu -Ti exchange reaction increases, increasing R2... 

Fig. 4.21 Schematic representation of polarization curves for the analysis of galvanic coupling when one metal is significantly more noble. Tafel polarization is represented.

Fig. n Influence of area ratio on galvanic corrosion for an active metal/noble metal couple. 

Galvanized steel is a common example of galvanic coupling where steel (Fe) with a standard electrode potential of—0.440 Vvs. SHE is cathodicaUy protected by a coating of zinc with a more active standard electrode potential of—0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects the steel from corrosion because of its sacrificial properties. Because zinc is less noble than steel, it acts as the anode. The sacrificial anode is continuously consumed by anodic dissolution and protects the more positive metal from corrosion. In practice, sacrificial anodes are... 

The anode and cathode corrosion currents, fcorr.A and fcorr,B. respectively, are estimated at the intersection of the cathode and anode polarization of uncoupled metals A and B. Conventional electrochemical cells as well as the polarization systems described in Chapter 5 are used to measure electrochemical kinetic parameters in galvanic couples. Galvanic corrosion rates are determined from galvanic currents at the anode. The rates are controlled by electrochemical kinetic parameters like hydrogen evolution exchange current density on the noble and active metal, exchange current density of the corroding metal, Tafel slopes, relative electroactive area, electrolyte composition, and temperature. 

Mixed potential theory is used to estimate the galvanic current and the galvanic potential in an active-passive metal that passivates at potentials less noble than the reversible hydrogen potential. A galvanic couple between titanium and platinum of equal area of 1 cm is exposed to 1 M HCl. The electrochemical parameters for the active-passive alloy are eeq xi = —163 V vs. SHE anodic Tafel, b Ti = 0.1 exchange current density, ixi= 10 A/cm passivation potential, pp= —0.73 V passivation current, 7pass= 10 A/cm transpassive potential, = 0.4 V vs. SHE and activity of dissolved species [Ti ] = 1 M. The exchange current densities, i°, on platinum and titanium... 

The series of standard reversible potentials of the various metals (Section 3.6) are now and then used to explain and estimate the risk of galvanic corrosion. This can be very misleading, because 1) these potentials express thermodynamic properties, which do not tell us anything about the reaction rate (e.g. passivation tendencies are not taken into account), and 2) if the potential difference between the two metals in a galvanic couple is large, the more noble metal does not take part in the corrosion process with its own ions. Thus, under this condition, the reversible potential of the... 

Equation (7.1) expresses also that the total corrosion rate of the less noble metal ( oorrA) cousists of a Contribution of galvanic corrosion (/g iv) and a contribution of self-corrosion due to the cathodic reaction on the same metal (/catA)- The corrosion current densities of a galvanic couple are finally derived by dividing the corrosion... 

If there is a crevice on a component made of a material liable to crevice corrosion, and this component is connected to a more noble material with free surfaces, crevice corrosion may be intensified strongly. Such a case is a couple of an aluminium component (with a crevice) and a steel plate in water containing some chloride. The corrosion form can be called galvanic crevice corrosion. The crevice corrosion rate will be particularly high if the more noble metal acts as an efficient cathode in the given environment. The explanation is the same as for ordinary galvanic corrosion. 

If the cathodic process c and the anodic a take place on different metals C and A, respectively, the corrosion processes may evolve if the two metals are connected both electi onically and ionically. In this case, one speaks of galvanic coupling between C and A. It is said that C is cathodic or more noble compared to A, which is said to be anodic or less noble. In a galvanic coupling between C and A, if the electronic or ionic connection are eliminated, corrosion cannot occur. 

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