Copper pipe replacements in 1 pipe steam heat system.

I have recently heard that I may be in for some trouble down the line with corrosion and or failing because of pressure?

Old Dunkirk steam boiler had some pipes replaced. The old cast iron (?) pipes are connected to new copper pipes.

Was this the wrong thing to do?

If the copper was conected directly to the cast iron, (most likely, it’s actually galvanized steel) then yes, you’re in for joint failures due to galvanic corrosion. If the pipes were connected with dielectric unions, you should be OK.

Here’s a pic of an installed dielectric union - note the red (sometimes it’s orange or black) plastic ring - this electrically isolates the two sections of pipe so the joint doesn’t corrode.

You might also post your question on There are a lot of steam buffs there.

Without knowing specifics of your system, I’ll offer general comments.
The original piping is probably Sch.40 black steel (not cast iron). Corrosion isn’t an issue (normally) in a sealed system due to low oxygen levels, so galvanised piping isn’t usual.
The copper piping could be a problem depending on operating pressure and wall thickness.
Galvanic reaction will certainly occur but is likely the least of your worries- even joints in active domestic hot water systems last many years, usually longer than storage tanks, which succumb to the reaction once the sacrificial anodes are depleted.

I’m not too familiar with one pipe systems, but in a two pipe steam system the steam supply is usually schedule 40 pipe and the condensate is usually schedule 80 pipe, due to the condensate causing more wear on the pipe (due to corrosion, I think).

Not sure from the OP - what pipes were replaced - tubes in the boiler or the distribution pipes?