Correlation between microstructure, phosphine evolution and spontaneous crumbling in iron-silicon alloys

Commercial ferrosilicon alloys of the composition Fe-75wt%Si (FeSi75) are known to evolve the toxic gas PH₃ when they are exposed to water vapor. FeSi75 alloys also have a tendency to spontaneously disintegrate or crumble if specific impurities are present. PH₃ evolution and spontaneous disintegration of FeSi75 alloys have been the cause of several accidents in the last 100 years, many of which were fatal or resulted in a significant loss of property. The purpose of the present study was to determine the mechanisms that control PH₃ evolution and spontaneous disintegration from FeSi75 alloys and devise methods to reduce or eliminate these processes.; The mechanisms that control PH₃ evolution and spontaneous disintegration are directly related to specific impurity phases in the alloys. Reactive phosphides of aluminum, calcium and magnesium form in commercial FeSi75 alloys at the end of solidification and these phosphides react with water vapor to produce PH₃ gas. The water vapor reaches the phosphides by penetrating the FeSi75 alloy through pre-existing microcracks. The reactive phosphide phase can also lead to spontaneous disintegration by expanding as it reacts with water vapor. The expansion of the phosphides can generate stresses that are capable of nucleating and propagating cracks in the material. When the volume fraction of phosphides is large enough, this cracking can cause the spontaneous disintegration of the alloy.; The relationship between the evolution of PH₃ and the concentration of reactive phosphide forming elements (specifically aluminum, calcium and magnesium) was quantified. For a fixed phosphorus concentration, less PH ₃ evolves when aluminum, calcium or magnesium are added individually compared to when they are added in combination. A maximum of 75% of the total phosphorus content in the alloy will evolve as PH3 when all three elements are present in the alloy. The remaining “inert” phosphorus is present in solid solution in the silicon phase and will not form reactive phosphides.; Only phosphorus in the form of aluminum, calcium and/or magnesium phosphides contributes to the evolution of PH₃. Thus, the evolution of PH ₃ and spontaneous disintegration can both be reduced by suppressing the formation of these reactive phosphide phases in FeSi75 alloys. These phases can be suppressed by increasing the amount of phosphorus in solid solution in the silicon phase or by preferentially forming a chemically inert phosphide phase. The amount of phosphorus in the silicon phase can be increased by either increasing the volume fraction of the silicon phase in the alloy or by increasing the cooling rate during solidification. A chemically inert phosphide phase can be formed by adding the appropriate alloying elements to the FeSi75 alloy. In this study, the chemically inert compound MgSiP₂ was formed preferentially by adding magnesium in quantities >2.5wt%. Reductions in PH ₃ evolution of an order of magnitude were achieved with this method when sufficient amounts of magnesium were added. The required amount of magnesium to achieve these reductions was found to be dependent upon the concentration of the other phosphide forming elements present in the alloy. It is important to note that if insufficient amounts of magnesium are added to the alloy, the evolution of PH₃ will actually increase due to the formation of a reactive magnesium phosphide phase.