Galvanic Corrosion and Localized Degradation of Aluminum-Matrix Composites Reinforced with Silicon Particulates

Abstract

Galvanic corrosion of an Al-based metal-matrix composite ͑MMC͒ reinforced with 40 wt % Si particulates was examined by considering the semiconductor/electrolyte junction properties of the Si reinforcements. Scanning capacitance microscopy and photoelectrochemical experiments indicated that the Si reinforcements were slightly p-doped. At the open-circuit condition, a wide depletion layer was expected to be present at the slightly p-doped Si surface. The blocking characteristics of the depletion layer for electrochemical reactions would thus limit the galvanic interactions between the Si reinforcements and the Al matrix. The photoelectrochemical experiments also suggested that solar irradiation may promote galvanic corrosion because of the enhanced cathodic activity of the Si reinforcements under illuminated conditions. Corrosion was found to initiate from Fe-containing intermetallics, which served as effective cathodes. A chemical decoration method indicated that, while the bulk region of the Si reinforcements was not effective for cathodic reactions, the exposed Si surfaces at the Al/Si interfaces were effective cathodic sites. It was hypothesized that ͑i͒ during the processing of the Al/Si MMC, interdiffusion caused the Si surfaces at the Al/Si interfaces to be highly p-doped with Al, and ͑ii͒ upon propagation of localized corrosion from the sites adjacent to the Fecontaining intermetallics, the highly p-doped Si surfaces were exposed to solution and served as effective cathodic sites to induce galvanic corrosion. A scanning vibrating electrode technique revealed a net cathodic current over the localized corrosion regions, which were many times larger than the intermetallic particle or the Si reinforcement particle. Accordingly, the solution near the localized corrosion regions was alkalinized, as was revealed by the scanning ion-selective electrode technique.

Electrochemical Examinations on the Corrosion Behavior of Boron Carbide Reinforced Aluminum-Matrix Composites

“…Theoretical calculations 29 indicate that Al and C readily react to form Al 4 C 3 ; whereas Al 4 C 3 rapidly hydrolyzes upon contact with water and dissolve into solution. 8 Hence, it is hypothesized that a layer of Al 4 C 3 may form at the Al=C interface during the processing of the Al=B 4 C MMC. Upon immersion of the Al=B 4 C MMC into aqueous solution, the Al 4 C 3 may hydrolyze and induce crevices at the Al=C interfaces.…”

“…7 Although the incorporation of a second phase into a matrix material can enhance the physical and mechanical properties of an MMC, such an addition could also significantly change its corrosion behavior. 3,[6][7][8][9][10][11] Galvanic corrosion is a concern in MMCs containing conductive constituents. [6][7][8][9][10][11] Boron carbide (often wrongly denoted as B 4 C, which is also used through out this text for convenience) has unique chemical, structural, and electronic properties.…”

“…3,[6][7][8][9][10][11] Galvanic corrosion is a concern in MMCs containing conductive constituents. [6][7][8][9][10][11] Boron carbide (often wrongly denoted as B 4 C, which is also used through out this text for convenience) has unique chemical, structural, and electronic properties. 12 Due to the low density and high hardness of B 4 C, B 4 C particulates have been used as reinforcements in MMCs.…”

Galvanic Corrosion and Localized Degradation of Aluminum-Matrix Composites Reinforced with Boron Carbide Particulates

“…(7) Although the incorporation of a second phase into a matrix material can enhance the physical and mechanical properties of an MMC, such an addition could also significantly change its corrosion behavior. (3,(6)(7)(8)(9)(10)(11) Galvanic corrosion is a concern in MMCs containing conductive constituents. (6)(7)(8)(9)(10)(11) Boron carbide (often wrongly denoted as B 4 C, which is also used through out this text for convenience) has unique chemical, structural, and electronic properties.…”

“…(3,(6)(7)(8)(9)(10)(11) Galvanic corrosion is a concern in MMCs containing conductive constituents. (6)(7)(8)(9)(10)(11) Boron carbide (often wrongly denoted as B 4 C, which is also used through out this text for convenience) has unique chemical, structural, and electronic properties. (12) Due to the low density and high hardness of B 4 C, B 4 C particulates have been used as reinforcements in MMCs.…”