Abstract: A weld protecting device by gas diffusion including several elements mounted one after the other and hinged to each other, and including a front element, intermediate elements mounted one after the other and behind the front element. A back element is mounted behind the last intermediate element, each element including a housing open downwards and towards each other adjacent element, forming a gas diffusion chamber, wherein two adjacent elements are hinged to each other and they are free to pivot relative to each other, regardless of the operating conditions of the protecting device. A facility for welding two opposite edges of two pieces, a weld protecting device, and a system for guiding the protecting device are disclosed.