A steel-body vessel, suitable for storing ultra-high purity gases, has a relatively inert metal lining which covers the interior surface of the steel vessel body, preventing the stored gas from making any contact and reacting with the steel vessel body. The relatively inert metal lining extends into the vessel end pieces which are interiorly threaded, in the steel, proximate the relatively inert metal lining. The surface of the inert metal lining is conditioned to be smooth, preferably to 15 nullRa or better. The vessel end pieces are sealed with end closure devices, which are secured by a securing portion carrying exterior threads to matingly engage the interior threads on the vessel end pieces. The end closure devices contain a sealing portion constructed of the relatively inert metal, which retains an annular sealing gland to seal the vessel in a removable, non-contaminating manner at service pressures when the end closure device is seated within the vessel end piece by means of the securing portion.