THE ROLE OF GRAIN BOUNDARY STRUCTURE ON THE ELECTROCHEMICAL STABILITY OF PASSIVE FILMS FORMED ON HIGH PURITY POLYCRYSTALLINE NICKEL

Les techniques électrochimiques à l'aide de la microscopie électronique (SEM/ECP) sont utiliser pour examiner l'effet de la structure des joints de grain (i.e. CSL) sur la corrosion intergranulaire du nickel polycristallin de haute submergé dans l'acide sulfurique (2N). Les potentiels électrochimiques caractéristiques (Egb) de l'initiation de l'attaque intergranulaire sont obtenus dans le domaine de potentiel passif-transpassif. Ces potentiels sont fortement dépendant des paramètres structuraux (i.e., Σ, Ɗθ), avec les joints ayant une désorientation proche à cette de basse-Σ et de haute résistance à l'attaque intergranulaire. Ces résultats sont considérés en relation avec la stabilité des couches passive formées sur défauts cristallins.Electrochemical techniques were utilized in conjunction with SEM/ECP to investigate the effects of grain boundary structure (i.e. CSL) on the intergranular corrosion behaviour of high purity polycrystalline Ni (99.999%) in 2 N H2SO4. Characteristic electrochemical potentials (Egb) for the initiation of grain boundary corrosion were found to exist within the passive-transpassive potential range. These potentials were determined to be strongly structure-dependent (i.e. Σ, Ɗθ) with boundaries close to low-Σ CSL relationships displaying a high resistance to the initiation of localized attack (i.e. high Egb). These results are discussed in terms of the stability of passive films formed at crystalline defects

A TEM STUDY OF DIFFUSION-INDUCED GRAIN BOUNDARY MIGRATION IN Ni-Cu DIFFUSION COUPLES

A study was conducted on defect structures, orientation relationships, and compositional profiles at DIGM boundaries in Ni-Cu diffusion couples. TEM revealed dislocations at the initial grain boundary positions of the DIGM zones in the Ni substrate. The misorientation between the DIGM zones and the matrix (across the dislocation wall) was determined by electron diffraction and found to be less than 0.5°. Cu profiles across the DIGM zones were obtained by TEM/EDS analysis. The formation of the dislocation wall is discussed in terms of the misorientation and lattice misfit between the DIGM zones end the matrix