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RESEARCH PRODUCT

Measurements of Positrons Lifetimes in Corroded Nickel

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It is over twenty years now that the nuclear method of conducting investigations into defects in the volume and on the surface of solids, based on positron annihilation phenomenon, has been developing on a quite intensive scale [1-3]. The present work is an attempt at adopting positron annihilation to investigations of the phenomenon of pitting corrosion in metals. Pitting occurs mainly in environments containing chlorides and affects metals and alloys covered with passivating oxide layers. Research carried out for many years by the use of various methods . has proved that pittings, which could — after some time — take sizes as large as the tenth of the millimeter order, are formed in the physically and chemically defected surface layer or the one close to the surface. Kinetics of formation of pittings has not been known thoroughly yet. It seems that in examinations of the first stages of corrosion, the method of measuring lifetimes of monoenergetic positrons implanted into investigated samples could be very effective. However, the quantitative analysis of dependence of the number of positron annihilation acts on the depth from the surface into which they penetrate shows that the quanta originating from positron annihilation from "Na source in nickel carry, in their majority, the information concerning the layers close to the surface. Because of this an attempt was made to apply the technique of measuring lifetimes of positrons emitted from radioactive 22 Na source to investigations into kinetics of pitting in nickel. An assumption was made here that the spectrum of lifetimes reflects the presence of small dimension defects on which macroscopic pittings are formed during the process of corrosion.