Bimetallic corrosion potentials

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. 

Fig. 1.62 Potential/current curves for a metal polarised (a) cathodically and (b) anodically. The horizontal intercepts xy, x y, x"y" with AB and CB represent the local cell currents respectively, and yz, y z, y z" the externally applied currents (cathodic and anodic). In bimetallic corrosion yz, y z, etc. will be the galvanic current /gjjv, flowing from to (see...

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. 

It is evident from previous considerations (see Section 1.4) that the corrosion potential provides no information on the corrosion rate, and it is also evident that in the case of a corroding metal in which the anodic and cathodic sites are inseparable (c.f. bimetallic corrosion) it is not possible to determine by means of an ammeter. The conventional method of determining corrosion rates by mass-loss determinations is tedious and over the years attention has been directed to the possibility of using instantaneous electrochemical methods. Thus based on the Pearson derivation Schwerdtfeger, era/. have examined the logarithmic polarisation curves for potential breaks that can be used to evaluate the corrosion rate however, the method has not found general acceptance. 

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 greater the difference of potential between the two metals, the greater is the magnitude of bimetallic corrosion. Figure 8.4 shows a valve from a condensate pipe. The cast iron valve was incorporated in AISI 304 stainless steel condensate pipe of a copper heat exchanger. The difference of potential between copper, steel and cast iron caused bimetallic corrosion. 

Deminerahzed water also may be used sometimes as makeup. Characteristics of makeup water are important with respect to corrosion in high-temperature hot-water systems. If the circulating water pH is properly adjusted, much of the corrosion potential can be minimized. In all-steel systems, the pH can be adjusted to 11.0 to minimize corrosion. However, in bimetallic systems, pH values should not be allowed to reach this level because of possible alkaline reaction with brass, bronze, copper, and/or aluminum. 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

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