What can be a harmless electrolytic additive for water for creating an oxy-hydrogen flame?
Electrolysis of water has become an increasingly common means of creating torches for soldering and welding, and there are commercial devices available as well as plenty of internet do-it-yourself sites. According to this site regarding the SafeFlame consortium:   


  "[...]the European Union-funded SafeFlame consortium has developed a
  torch system that generates a flame using nothing but H2O and
  electricity.   SafeFlame utilizes an electrical current to electrolyze
  ordinary water, separating it into hydrogen and oxygen gas. Those
  gases are then mixed and ignited as they exit the torch's nozzle. By
  fine-tuning the proportions of the two gases, different types of
  flames can be produced for different applications."


A commonly used electrolyte / electrode combination is $\ce{KOH}$ or $\ce{Na2CO3}$ with stainless steel electrodes. I believe this would be pretty corrosive toward aluminum, but stainless steel for such a setup should be cheap and easy to come by.