Abstract
An apparent inverse dependence of oxidation rate on temperature was discovered between approximately 270°C and 295°C using 60% tin-40% lead alloy as a result of a preliminary investigation carried out to determine the effect of temperature on the oxidation and drossing rates of stirred tin-lead alloy melts. This effect was found to be dependent on time and the composition of the alloy, and coincided with a considerable rise in drossing rate. Furthermore the oxidation rate of stirred 60% tin-40% lead alloy showed a very unusual increasing rate with time. Therefore a systematic study was carried out to determine the factors which control the oxidation and drossing rates of liquid tin-lead alloys, pure tin and pure lead at temperatures between approximately 200°C and 400°C. The results of the study are reported in two parts. In the first part the effects of time, temperature, the concentration of tin in the alloy, stirring rate and the effects of additions of up to 0.2% of copper, cadmium, zinc, aluminium, antimony silver and phosphorus on the weight gain and the weight of dross formed using stirred tin-lead alloys are reported. It was found that except for a phosphorus addition of 0.01% which reduces the weight gain of stirred 60% tin-40% lead alloy by at least 50%, the additions generally show only minor effects on the weight gain dependence of the alloy on temperature. The oxidation kinetics of stirred 60% tin-40% lead alloy have been interpreted using the Arrhenius equation. Drosses were examined and analysed using scanning electron microscopy, metallography, X-ray diffraction, X-ray photoelectron spectroscopy and atonic absorption spectrophotometry. In the second part of the study the effects of temperature and air pressure on the oxidation kinetics of static melts of 60% tin-40% lead alloy, pure tin and solid lead were determined using an electromicrobalance. Generally parabolic kinetics were observed indicating that the oxidation processes involved are diffusion controlled. It was also found that the oxidation gate of static melts of 60% tin-40% lead alloy at 270°C exceeds that at 295°C using an air pressure of 250 Pa although this effect is eliminated by reducing the air pressure to 50 Pa. Furthermore the oxidation rates of liquid 60% tin-40% lead alloy, pure tin and solid lead were in general higher using the lower air pressure of 50 Pa. The results obtained in this part of the study have also been inteipreted using the Arrhenius equation. Furthermore the oxide layers were examined and analysed using scanning electron microscopy and X-ray diffraction techniques. It appears that changes in the morphology of the oxide layer on liquid 60% tin-40% lead alloy may account for the observed changes in the oxidation rate of the alloy with, temperature and air pressure. A comparison of the oxidation rates of static and stirred melts of 60% tin-40% lead alloy show clearly that the oxidation rate of the stirred alloy is controlled by the additional surface area exposed as a result of drossing. Furthermore the nature of the oxide layer appears to have a significant effect on the drossing rate of the alloy. It was also shown by calculation that the observed time dependence of the fall in weight gain of stirred 60% tin-40% lead alloy with temperature could be explained in terms of the total surface area of the alloy exposed to the atmosphere.