Aqueous corrosion study of α2-Ti3Al/SiC composites

• Published in: Corrosion (Houston, Tex.) Vol. 54; no. 11; pp. 877 - 886
• Main Authors: SAFFARIAN, H. M, WARREN, G. W
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.11.1998
• Subjects: Alternating current; Aluminum; Applied sciences; Coated electrodes; Corrosion; Corrosion environments; Corrosion tests; Cross-sections; Current density; Direct current; Electrode polarization; Electrodes; Exact sciences and technology; Fiber composites; Fiber-matrix interfaces; Fibers; Foils; Galvanic corrosion; Impedance; Interfaces; Metals. Metallurgy; Niobium; Potential theory; Silicon carbide; Sodium; Sodium chloride; Sodium sulfate; Surface preparation; Time constant; Titanium aluminides; Water corrosion; Fibre reinforced metal; Intermetallic compound; Niobium alloy; Aluminium alloy; Sodium chloride; Titanium base alloys; Composite material; Pitting corrosion; Silicon carbide; Sodium sulfate; Pitting potential; Current density; Passivation
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3284807

Aqueous corrosion behavior of α2-Ti-24% Al-11% Nb (at%)/silicon carbide (SiC) fiber composite was studied by direct current (DC) polarization 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 commercial (SCS-6) SiC fibers with an external C coating. Experiments were conducted on several types of electrodes: the composite, a SiC electrode composed of SCS-6 fibers only, and a bulk matrix of similar composition. Composite electrodes were prepared with two orientations, a cross section perpendicular to the fiber axis and a cross section parallel to the fibers such that no fibers were exposed. Though pitting potentials (Epit) for the composite were somewhat lower than the matrix material, they were still quite high (≈ 1.0 VSCE). Passive current density in NaCl for the composite was 2 orders of magnitude higher than that of the matrix, which was attributed to the presence of crevices at the fiber/matrix interface. This was consistent with the observation 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 second time constant in the phase angle plots. An estimate of the galvanic 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 potentials and the presence of chloride), severe pitting of the matrix between fibers seemed to initiate in the Al-rich α2 phase.