The Point Defect Model for the Passive State

A review is presented of the point defect model (PDM) for the growth and breakdown of passive films on metal and alloy surfaces in contact with aqueous solutions. The model provides a reasonable account of the steady-state properties of cation-conducting and anion-conducting barrier layers on nickel and tungsten, respectively, in phosphate buffer solu-tions; of the impedance characteristics of passive films on nickel; of the breakdown of passive films on a wide range of metals and alloys; o £ the distributions in the breakdown parameters (breakdown voltage and induction time); of the role of alloying elements in enhancing the resistance of alloys to passivity breakdown; of transpassive dissolution and electro-polishing; of erosion-corrosion; and of photoinhibition of pit nucleation. Additionally, the PDM has allowed us t0 formu-late a set of principles for designing new alloys and has led to the development of a deterministic method for predicting localized corrosion damage functions. The passivity of metals and alloys is the single most im-portant phenomenon responsible for our metals-based civ-ilization. We all accept metals technology in our everyday lives, but few of us realize that most structural metals are viable in an engineering sense only because of the existence