On this subject of aluminum destruction, I found this page interesting:
http://www.newton.dep.anl.gov/askasc.../chem03682.htm
To summarize, it states that Aluminum reacts with water to release hydrogen, however, the oxidization of the aluminum creates a layer that stops the reaction continuing (which we already know). However, if the water becomes a base solution via the addition of KOH or NaOH for example, this allows the reaction to continue unhindered.
The short answer here is that we need to keep both these substances out of our engines, if they contain aluminum. We will be getting some extra hydrogen produced by a method we do not want, otherwise.
Here's a question: How can we test our HHO output for the presence of these elements so we know if our scrubbing methods are working?
Perhaps we can pump the HHO into an aluminum can and check for pitting?