Bimetallic contact potential

Experiments carried out by Keir, van Rooyen and Pryor have clarified the variable behaviour of the Al-Zn couple in chloride solutions. Using high-purity aluminium and zinc electrodes of equal size coupled together in sodium chloride solution, it was found that zinc is initially anodic to aluminium but that within one day the polarity of the couple reverses and remains as such subsequently (Fig. 1.70). This reversal in polarity appears to be due to the accumulation of Zn in solution. Accordingly, with decrease in distance between the electrodes, and in solution-volume electrode-area ratio, the polarity reversal occurs much more rapidly. The accumulation of Zn in solution depresses the potential of the aluminium from an initial value of about -0-5 V to a final open-circuit value of about —1-0 V (vj. S.H.E.). The corrosion rates of both the aluminium and the zinc electrodes are greater than in the absence of bimetallic contact, but the corrosion of the aluminium is changed from the characteristic pitting, usually observed in nearly neutral chlorides, to a desirable mild uniform attack. The polarity reversal is not... 

Bimetallic contact A metallic contact between two metals differing in potential as exemplified by galvanic series. 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

Both metals are applied to copper-base alloys, stainless steels and titanium to stop bimetallic corrosion at contacts between these metals and aluminium and magnesium alloys, and their application to non-stainless steel can serve this purpose as well as protecting the steel. In spite of their different potentials, zinc and cadmium appear to be equally effective for this purpose, even for contacts with magnesium alloys Choice between the two metals will therefore be made on the other grounds previously discussed. 

Formation of a Galvanic Cell. When a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte, a galvanic cell is created. The electromotive force and current of the galvanic cell depend on the properties of the electrolyte and polarization characteristics of anodic and cathodic reactions. The term galvanic corrosion has been employed to identify the corrosion caused by the contact between two metals or conductors with different potentials. It is also called dissimilar metallic corrosion or bimetallic corrosion where metal is the conductor material. 

The SEA method can be extended to the synthesis of bimetallics and may represent a simpler, more versatile alternative to surface redox reactions. The syntheses of bimetallics are the same as described in the previous section, only that the adsorbing supported oxide, like the CO3O4 depicted in Figure 13.13, is itself reducible and, after reduction, forms a bimetallic particle in intimate contact with the second metal precursor that had adsorbed directly onto it. This process can be conducted at ambient conditions, with an intermediate calcination in air to create the first metal oxide from a deposited or adsorbed precursor. The first metal might itself be deposited by SEA in well-dispersed form by precursors such as cationic cobalt hexa-ammine on silica. Thus, there is the potential to create homogeneous bimetallic particles with very high dispersion, using simple methods with common metal precursors. 

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