Chemically homogeneous alloys, localized

We wish to determine under isothermal and isobaric conditions the concentration of defects as a function of the solid solution composition (e.g NB in alloy (A, B)). Consider a vacancy, the formation Gibbs energy of which is now a function of NB. In ideal (A, B) solutions, we may safely assume that the local composition in the vicinity of the vacancy does not differ much from Ns and /VA in the undisturbed bulk. Therefore, we may write the vacancy formation Gibbs energy Gy(NE) (see Eqn. (2.50)) as a series expansion G%(NE) = Gv(0) + A Gv Ab+ higher order terms, so that AGv = Gv(Nb = l)-Gv(AfB = 0). It is still true (as was shown in Section 2.3) that the vacancy chemical potential /Uy in the homogeneous equilibrium alloy is zero. Thus, we have (see Eqn. (2.57))... 

Therefore it is not the average chemical composition of the steel that is responsible for its corrosion resistance, but rather the locally present chemical composition. All metallurgic processes that result in poorer homogeneity reduce corrosion resistance. This includes in particular the separation of carbides or intermetallic phases, withdra ving alloying elements from the matrix that can then not contribute to the local pitting resistance equivalenf . 

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