Thanks Frosty to go back to the core of my question, which was which material to use and eventual suppliers.
What you describe here makes sense and is helping to solve some questions, some I was aware of and some, that are beyond my boundaries.
Indeed it will endure severe weather condition such as lots of rain, sea salt breeze and wind of the north sea and it's going to have to be relatively child safe and vandal resistant. I also have a consideration for the surroundings, which is a public square hundreds of yard away, behind rows of building, from the sea shore...
Yes, I would be more interested in technical issue, such as steel quality or the finish process, as you mentioned, such as patination.
I called a British blacksmith this morning, recommended by the BBA, who just told me to get some low carbon steel, construction grade, the rest is too tough to work with.
What I want to do with the rods as a starting point is to, in a way, is to weave them, in a quite simple movement, like a rope...to me it make sense, I don't want to reproduce a tree as it is with the bark on the trunk, it has to be quite suggestive or abstract...