Bimetallic corrosion distribution

Fig. 1.72 Schematic representation of calculated bimetallic corrosion distribution (a) connected header, (b) electrically-insulated heater...

For further discussion of cathode to anode area ratio effects see References " and also refer to the section entitled Distribution of Bimetallic Corrosion in Real Systems, p. 1.238. 

Fig. 1.69 Effect of resistivity of solution on the distribution of corrosion on the more negative metal of a bimetallic couple, (a) Solution of very low resistivity and (b) solution of very high resistivity. Note that when the resisitivity is high the effective areas of the cathodic and anodic metals are confined to the interface between the two metals...

Hot in hot hard waters, the scale that forms at water temperatures above about 55°C has a coarse-grained structure and adheres less well to the zinc surface. Corrosion of the zinc will still occur locally, as a result of discontinuities in the scale or local electrochemical action. Above about 60°C, zinc usually becomes cathodic to steel, and in bimetallic contacts the steel will then corrode first. Pouibaix (1970) has performed success-fill laboratory simulations of the corrosion of galvanized water distribution pipes. 

Figure 1. Evans diagram for estimating the galvanic current density. The bimetallic electrode consists of a carbon fibre embedded in a 6061 aluminium alloy (metal matrix composites). Two galvanic regimes can be reached as function of the aeration of the corrosive media. This analysis does not take into account the spatial distribution of the carbon fibers in the metallic matrix (see Fig. 12). Reprinted with permission from/nrernahono/Mafen-alsReviews, 39 (1994) 245.Copyright 1994 Maney Publishing. ...

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