Interfacial reactions between a lead borosilicate glass containing CuO and an Al/Si alloy — evidence for galvanic cells

A PbO-rich borosilicate glass with the additions of CuO and Al2O3 has been developed as a thick-film, dielectric coating for a high Si content Al/Si alloy produced by the Osprey process (Al–70Si wt.%). Wetting behaviour and interfacial reactions were investigated using sessile drop experiments at temperatures between 520 and 583°C under air and argon atmosphere. The wetting behaviour of the glass was found to be a function of atmosphere and wetting angle decreased with increasing temperature and time. Coatings applied in air in the range 520–565°C showed <1% porosity, whereas those applied at 583°C in argon exhibited relatively high porosity (∼8%) and both coating and un-wetted substrate surface supported large Al–Si spheres containing ∼21 wt.% Si. Coatings heated in air at 520 and 540°C show good bonding with no interfacial reaction products. However, those samples heated in air at 565°C consistently show evidence of excessive interfacial reactions which are the components of a galvanic process, leading to the oxidation and dissolution of the Al regions (anodic) and the deposition of copper at the glass/Si interface (cathodic). From observations of the rate of copper deposition, the current density arising from the electron flow through the alloy has been calculated to be ∼0.91 A m−2. The galvanic process is suppressed at the periphery of the coating where oxygen diffused from the atmosphere enables interface oxidation. The onset of the galvanic process between 540 and 565°C occurs when dissolution rate exceeds oxidation rate, exposing the fresh Al anode to the glass melt. Under inert atmosphere (at 583°C), air oxidation is not possible and galvanic cell redox reactions generate an excessive copper interlayer as the system attempts to sustain the oxide layer at the Al anode.