# 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:

$$\ce{H2O <=> H2 + 1/2O2}$$

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.

• Hmm. While I also like to err on the side of safety - hydrogen spreads out into a room extremely rapidly and mixes well with air and would be vented out. It would take quite a lot to make a significant (25% LEL) buildup in a normal room with normal ventilation. However, in an unventilated room - things will go boom. – Stian Yttervik Aug 14 '17 at 21:01
• @StianYttervik Yea, since the OP mentioned warnings about proper ventilation in his procedure, I figured I should address the issue anyway. In any kind of open space it shouldn't be a problem, but as you say, there is the potential for going boom in small unventilated areas ;) – airhuff Aug 14 '17 at 21:07
• Typically when I am working in a spray containment, I size my negative pressure collection systems to cycle the air 1-2 times per minute. Some of these systems can handle up to 100,000 CFM and for larger contained areas we would use multiple units to ventilate different zones. I'm not familiar with flow rates of fume hoods. How many full evacuations per minute are achieved by a basic fume hood? – DooderMcDood Aug 16 '17 at 0:48
• There are a couple different basic kinds of fume hoods: those that cycle air through some kind of filter and thus don't actually move any air out of the area at all, and those that vent the air out of the area, typically up a stack to the outside. The 100,000 CFM hood sounds like the later, which is what you would need to remove hydrogen gas. The number of evacuations per minute is a function of the flow rate and the size of the room, but if you are working inside the hood that isn't a big issue with the small amount of hydrogen you will produce. – airhuff Aug 16 '17 at 1:18