The welder's mask is a piece of headwear in the Fallout 3 add-on The Pitt.
The mask, as its name suggests, is primarily used by slaves that use a welding torch as to keep sparks from flying into their face. The reasoning is the same with slaves that use auto axes to cut through metal, but instead of just sparks, it's sparks and rogue metal shrapnel.
It is a variant of the raider arclight helmet featuring an increased damage resistance of 5. It can be repaired with raider arclight helmets.
It can be found on a number slaves in The Pitt, and is in appearance identical to the raider arclight helmet found throughout the Capital Wasteland.