Marker Techniques for Studying the Mechanism of Scaling of Metals Based on the Use of the O[sup 18](p,n)F[sup 18] Nuclear Reaction

Abstract

A general mechanistic approach to the study of scaling reactions is proposed which dispenses with the conventional inert marker. Instead, radioisotopes of the reacting components are used to establish the nature of the diffusing species in the solid reaction products formed on metal surfaces at high temperatures. For investigating the mechanism of oxidation of metals, O¹⁸ is employed as a tracer and the stable isotope is subsequently activated by proton bombardment via the nuclear reaction . The basis for the technique is outlined and its application to the scaling of iron and of zirconium is described. The experiments with iron establish conclusively that, at temperatures up to 1050°C, iron is the only diffusing component in the dense, adherent wüstite layer which forms adjacent to the metal. Direct evidence is also presented for the vapor phase transport of oxygen by a dissociative mechanism during the growth of porous, non‐adherent wüstite scales on iron in atmospheres. It is shown that oxygen is mobile in thick scales formed on Zr in a water vapor atmosphere at 1200° C; however, the unusual form of the O¹⁸ distribution in the scale indicates that open channels must have existed in the outer portion of the scale during oxidation which allowed direct penetration of oxygen to levels deep within the layer. The advantages and limitations of the present technique are discussed with particular reference to conventional marker methods. Major sources of difficulty in the interpretation of inert marker experiments are identified and a set of experimental requirements is given which, if satisfied, should permit reliable transport information to be obtained using conventional marker techniques.