Investigation into the behaviour of aluminium and steel under melt/freeze cyclic conditions

In the current study aluminium has been cycled around its melting temperature (660 °C) in stainless and carbon steel crucibles. The interaction between the crucibles and aluminium have been studied using scanning electron microscopy (SEM) and auger electron spectroscopy (AES), while the phase change behaviour of the aluminium has also been studied. It could be seen that after 10 cycles a black carbonaceous layer forms on the surface of the crucibles preventing aluminium and steel interaction. After 60 cycles this layer is still present on the stainless steel samples but has been removed, from the carbon steel surface, most likely from thermal cycling. This layer has resulted in much fewer instances of aluminium penetration into the stainless steel over the carbon steel. Similar results are seen for the 100 cycle samples. In instances where aluminium has been in contact with the steel, Fe₂Al₅ and FeAl₃ have been present. It is suggested that the presence of these products is the likely cause of the change in aluminium phase change performance. Overall, it was found that under the conditions present in the study that stainless steel suffered from far less aluminium intrusion than the carbon steel samples. It was hypothesised that the carbon layer found on the surface of the samples largely prevented any aluminium interaction, preventing the loss of stainless steel at the interface. In contrast to the mild steel samples, the carbon layer was found to adhere to the stainless steel much more effectively, preventing aluminium and steel interaction. The potential for this carbon layer to act as a barrier to corrosion between stainless steel and aluminium warrants further investigation.