If you are worried about rust, you can put Boeshield or something like it on them. That is what I use on my cast iron tables. I wouldn't think this would cause them to slip.
I have had the pony pipe clamps for a few years and they have not rusted, but my shop is always heated at 50 to 55 degrees. I have had some rust on my table saw.
As the others have said, put some wax paper over the pipes and you wont get the black transfer to the wood. This especially happens with oak.
Rich (the Yooper)
"To the world, you may be one person, but to one person, you may be the world."
"Common sense is not so common."