Hydrogen evolution during thermal spray
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.