Soldering terminology

Technique for reducing or joining of metal oxides by use of the large amounts of heat released during oxidation of aluminum and other easily oxidized metals. If aluminum is used as a reducing agent, the process is called aluminothermy. In this case, the metal oxide is mixed with aluminum, manganese or an alloy of the two in powder form and ignited. The metal oxide oxidizes aluminum and manganese and is, in turn, reduced and melted into a lump by the heat of the reaction. Chromium oxide (Cr₂ O₃) is reduced by aluminum as follows :

Cr2O3+2 Al →2 Cr+Al2O3+112 Kcal

Welding and joining metal by this method is called thermite welding. For this purpose, Al or Mg powder or an Al -Mg alloy is applied and ignited, causing the parts to be joined by the oxidation heat that melts and connects the joint. Thermite welding was formerly used for the joining of iron rails.