OEM coolant tubes are generally made of cold roll steel and finished with a powder coating. As soon as this powder coating gets chipped, moisture infiltrates the raw steel causing it to begin to rust. Once this process starts it is not long until that rust continues to spread under the powder coating of the area surrounding the initial point of entry causing the tube to eventually start leaking.
Our aluminized coating used on our standard replacement tubes allows a much stronger barrier to prevent rusting as the coating is actually etched right into the metal, not simply painted on. The Stainless Steel models are not coating and are guaranteed to never rust as long as it is in service.