Abstract: A system is provided for use with structural steel members' lying in a corrosive atmospheric environment such as marine, wherein the system protects the members against corrosion. An elongated rigid envelope (20,22) that is gas tight surrounds the structural members (12,14) and extends along their lengths. A source (27) of pressured inert gas such as nitrogen is coupled through a regulator (25) to the inside of the envelope and maintains a pressure therein slightly greater than atmospheric. For wide flange members, the envelope's cross-section is preferably rectangular. Semi-globular protrusions (71,73,74,76), and (81,84,85,88) at the inside corners of the envelope and additional semi-globular protrusions (82,83,86,87) keep each side of the envelope moldings slightly spaced from the flanges of the members. The large space (26) provides a reservoir for inert gas, while providing minimum surface area of the envelope through which inert gas can be lost.