Hydrogen evolution during thermal spray Firstly, let me state that I'm no chemist. My specialty is in thermally applied coatings such as electric-arc metalizing. 

During the course of my career I've always been presented with vague warnings to use "proper" ventilation to prevent hydrogen buildup within a contained area when spraying Al alloys.

Could someone please explain the process that creates hydrogen gas when arcing between two Al electrodes? 

Also, what would the "proper" ventilation be? Would dry dust collection/ventilation suffice or would wet collection methods be required? Would this same reaction occur when using other alloys commonly used in thermal deposition processes?
The culprit in the undesired production of hydrogen gas from electric-arc metalizing is water vapor, as it is the only significant source of hydrogen atoms in the atmosphere. The concentration of water vapor is then of course a major factor in how much hydrogen gas can be produced. In very humid conditions the air can contain as much as a few percent water vapor. The chemical reaction for the conversion of water vapor to hydrogen gas under the conditions of this process is as follows:  

$$\ce{H2O &lt;=&gt; H2 + 1/2O2}$$  

A few percent water vapor may not seem like enough to produce explosive levels of hydrogen gas, but hydrogen gas becomes explosive in air at levels as low as $\pu{4\%}$.  

A recirculating dust filtration system, dry or wet, is not an appropriate system for removing hydrogen gas. The only reliable way to prevent the possibility of hydrogen buildup in a closed area is with an appropriate fume hood (or similar approved device) that vents the gases out of the building.  

I can't give an exhaustive list of the metals/alloys that could result in hydrogen production via this process, although base metals like copper, lead, nickel and zinc have the potential to do so.