The UK’s HSE has published a booklet to help manage risks when performing Hot Work (Welding, Cutting, Brazing) on “used containers”. In the USA, OSHA has some very explicit language in their welding standards…
1910.252(a)(3)(i) Used containers.
No welding, cutting, or other hot work shall be performed on used drums, barrels, tanks or other containers until they have been cleaned so thoroughly as to make absolutely certain that there are no flammable materials present or any substances such as greases, tars, acids, or other materials which when subjected to heat, might produce flammable or toxic vapors. Any pipe lines or connections to the drum or vessel shall be disconnected or blanked.
This booklet, published 5/23, covers: