ON SOME FUNDAMENTAL FACTORS IN THE EFFECT OF ALLOYING ELEMENTS ON PASSIVATION OF ALLOYS

Passsivity promoters and dissolution moderators are identified among m etallic elements and classified by considering two fundamental propert ies, which are the metal-oxygen bond strength and the metal-metal bond strength. Passivity promoters are elements which combine a high metal -oxygen bond strength (high heat of adsorption of oxygen or OH (Delta H-ads(ox)) With a low metal-metal bond energy (epsilon(M-M)), because the transition from adsorbed O or OH overlayer to 3D oxide requires th e breaking of metal-metal bonds. In contrast, dissolution moderators a re expected to be elements with a high metal-metal bond energy. Groups of metals appear clearly in a diagram of Delta H-ads (oxygen) and eps ilon(M-M).