Electrochemical properties of metallurgical-grade silicon in hydrochloric acid

The electrochemical properties of metallurgical-grade silicon (MGS) and its native oxide (passive layer) were investigated using standard potentiostatic, potentiodynamic and impedance methods. The corrosion rate was found to reach up to ∼37 μm year−1 in undiluted hydrochloric acid. The passive oxide layer was found to thicken with time as well as with potential. Increasing the potential from −0.5 to 1.0 V vs. SHE increases the thickness of the passive layer from approximately 2 to 6 nm. The interfacial impedance data for MGS in contact with the electrolyte was modeled. The model takes into account the capacitive behaviour of the space charge region and passive film. Due to the non-ideal behaviour of the passive film, a constant phase element (CPE) was incorporated in the model. Extraction of Mott–Schottky plot data from the EIS tests showed that the passive layer on the surface of the MGS is a p-type semiconductor. Scanning electron microscopy was used to determine that the impurities in the silicon (mostly iron, aluminum and calcium) were located almost exclusively at grain boundaries.