Area Relationships in Galvanic Corrosion Available to Purchase

The effect of variations of the area of two metals in a galvanic couple is discussed for three common cases. In Case 1, it is assumed that the only significant process on the more active metal (A) at the galvanic potential (ϕ_g) is metal oxidation (dissolution), while the only significant process on the more noble metal (C) is reduction of the oxidizer (H⁺, H₂O, O₂) and Tafel behavior is observed. Various possibilities to present the galvanic current as a function of electrode areas, which might have lead to some confusion in the literature, are discussed. In Case 2, it is assumed that metal (A), the anode in the galvanic couple, is polarized only slightly from its corrosion potential. It is shown that in this case the galvanic current density $(igA)$ is not equal to the dissolution rate $(idA)$ of the anode. A correlation between these two values is given. In Case 3, it is assumed that the cathodic process on both metals is entirely diffusion controlled. In this case, the dissolution rate of the anode is shown to be proportional to the area ratio (A^C/A^A) of anode and cathode for higher values of A^C/A^A. This is the so-called catchment principle.