AQUEOUS CORROSION STUDY OF ALPHA-TI3AL SIC COMPOSITES/

Aqueous corrosion behavior of (alpha(2)-Ti-24% Al-11% Nb (at%)/ silico n carbide (SiC) fiber composite was studied by direct current IDC) pol arization and alternating current (AC) impedance techniques in pH 6, 0 .5 N sodium sulfate (Na2SO4) and sodium chloride (NaCl) solutions. The composite was manufactured by the foil fiber-foil process using comme rcial (SCS-6) SiC fibers with an external C coating. Experiments were conducted on several types of electrodes: the composite, a SiC electro de composed of SCS-6 fibers only, and a bulk matrix of similar composi tion. Composite electrodes were prepared with two orientations, a cros s section perpendicular to the fiber axis and a cross section parallel to the fibers such that no fibers were exposed. Though pitting potent ials (E-pit) for the composite were somewhat lower than the matrix mat erial, they were still quite high (approximate to 1.0 V-SCE). Passive current density in NaCl for the composite was 2 orders of magnitude hi gher than that of the matrix, which was attributed to the presence of crevices at the fiber/matrix interface. This was consistent with the o bservation that more extensive surface preparation increased damage to the fiber/ matrix interfaces, resulting in higher passive currents, a decrease in impedance at low frequency, and the introduction of a sec ond time constant in the phase angle plots. An estimate of the galvani c current obtained from mixed-potential theory suggested that galvanic corrosion resulting from fiber/matrix coupling was not expected to be significant. For conditions that led to pitting (e.g., high potential s and the presence of chloride), severe pitting of the matrix between fibers seemed to initiate in the Al-rich alpha(2) phase.