I'm replying to you, but this also is for all the others who have kindly commented on my initial post.
First, I will keep the steel tubes, indeed closed sections, for some 10-12 months before using them for the new roof. Not longer. That would be doing the work in January next year. The reason I'm now delaying the work is that I'm leaving Thailand in a few days, and I absolutely do not want to have any work done on the house in my absence, even by a team of guys we have used a few times before and who do good work - certainly compared with a few others who disappointed me.
I'm back for a good length of time between August and October. The job could be done them but I thought I would rather wait until January because of the risk of heavy rains.
Right now, the tubes are neatly lined up, on cement blocks to keep them off the ground. I could space them up a bit more from each other to facilitate air ventilation. The idea of a tarpaulin cover but, again, with some ventilation space around, is easy to implement.
I do not have enough space in the car port for the tubes. But, btw, I have had left over tubes from a previous roofing job (our second house just next door). It's exactly the same sort of steel as our new stock - they are white (or silver?), definitely not black - maybe that's the galvanised type (not sure...). The point is that the car port is not totally closed, it lets a lot of rain come in, and those old steel left overs (2 or 3 years) have not shown any sign of corrosion whatsoever. So I thought that leaving our new stock with no special treatment for 1 year would not be such an issue.