Howdy Doc,
It's not often that the moderators look to me for an answer, unless the question at hand refers to tasteless humor or walking on thin ice (ironic how closely related those 2 specialties are).
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I believe Bruce refers to my background in steel. I regret, though, that after this post, my notable specialties will remain the original 2. I don't come with a definitive answer to your query.
Yes, I sell steel, but the technical metallurgy and chemistry background required to thoroughly analyze and quantitatively compare the 2 metal products you specify are not within my grasp. I will offer what I do know.
Carbon steel rusts (epiphany!) There are grades of carbon steel that are melted and produced with a small percentage of chrome, nickel, and copper.
Corten , a US Steel product, is this type steel. It is used on building exteriors, bridges, and other exposed structures. No other protective coating is applied. Numerous buildings and structures in Chicago use corten steel on their exterior, without coating. The Daley Center is one of those (rusty patina; building on left
)
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Carbon based steel pipe of the size we speak of will be rolled and welded seam. I don't know if pipe like that would be available made from corten, or if the welded seam might ultimately be the weak link...? What I would suggest, though, is to seriously consider the more corrosion resistant products like aluminum or plastics...even galvanized. Like Otto noted above
I have never replaced aluminum pipe, but have two steel pipes being replaced this year
Another consideration for steel is the cost. This year has been tumultuous in the steel industry. Since January of this year, prices for carbon based steel products has escalated some 50 - 70%; incredible! I am unfamiliar with the non-ferrous markets (ie; aluminum), but current pricing of steel pipe may have already provided some of your answer as to which product to use.
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Lusk is quick to remind all budding pondmeisters with the hankering to build a pond that the decisions they make today will affect at least 4 generations beyond. I keep this in mind for all my projects at the pond, right down to using construction methods on my docks that will allow easy replacement of worn and rotted structural members that will likely never even fail in my lifetime (I hope). Building the big hole in the ground got the same treatment. Always build it to last; don't pull up short.