Thanks aerodog for being the voice of reason.
This discussion was beginning to sound less than professional. Once in awhile, a question may seem outright basic, or border on stupidity based on the knowledge and experience of the person reading it. But don't forget, no one engineer is all-knowing, and everyone is subject to different experiences. This forum should be a place that an engineer, or engineer-in-training, can ask a basic question, if need be. It should also be a place where a very involved technical question can be asked, as the more opinions gathered, the better the possible solution. I have found many helpful hints, or at least some information that has pointed me in the right direction in these fora. Many engineers are working as the only engineer in a construction firm, or a manufacturing facility, and may have no peer with which to consult. I don't believe a professional should ever put down a fellow professional for asking a question. It never hurts to get a second opinion. My only warning to all (and I believe most of you already are aware of this) is that because of the variety of experience levels of the many persons in these fora, the information provided should always be verified with further document research, etc. You alone, as the professional-of-record, are liable for your final work product.
Now, back to the subject at hand . .
I believe, as aerodog does, that the jury is still out. However, there are several items to consider . . .
Large pipes and tubes, such as single-pole sign posts can be "seal" welded using E60 electrodes after the structural welding is complete. Even with this, the large interior volume is susceptible to moisture and subsequent condensation. Usually, the deterioration of these poles begins at the weld between the pole and the base plate, where the condensation usually collects.
Smaller pipes and tubes are best protected by galvanizing inside and out, followed by capping, etc. Many assemblies have small penetrations (bolt holes, etc.) where water can enter, particularly when rain is followed by a drop in temperatures, and the reduced pressure inside will suck the moisture inside. Other assemblies, that may be fully sealed, still seem to allow moisture penetration, as the welding and sealing may not be perfect. It doesn't take very much moisture to begin corrosion. If moisture can penetrate, so can the air that is needed for oxidation. Low points in the assembly are collection points for the condensation, and this is usually where the corrosion begins. Ambient temperature changes may cause the water to evaporate and condense within the assembly, exposing the steel to further oxidation potential. I have seen many welded steel tube amusement rides, railings, etc. that have corroded from the inside with no apparent means of moisture penetration.
I believe it is virtually impossible to completely seal steel pipes and tubes against corrosion from within. There is no easy answer, and in some cases, it may be more suitable to provide small weep holes at low collection points, such that any condensate can weep out.
In short, you should study each pipe and tube assembly as an individual case, and ascertain which is the most effective means of protecting against corrosion from the inside.
