Analysis on micro-regional characters of the resistance wires surface in electrical immersion heater by SEM/EDX

We present a novel method for identifying the conditions of electrical resistance wires before fire basing on micro-regional characters. The morphology and elemental compositions of resistance wires surface under normally used, misused and external heating conditions were respectively studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The results show that the normally used wire has a smooth surface and stable metal composition. While the resistance wire under misused condition for the certain time could be characterized as a coarse surface covered by compacted and silica-embedded oxide layer, and the proportion of elemental compositions of different metals is also changed with higher content of Al and lower content of Fe and Cr. The external heating condition has little influence on inside wires. The mechanism of changes formed under misused condition was explained. We demonstrate that this approach is practical and functional to aid fire investigators in determining the cause of related fires.