Badly Rusted Cupro-Nickel Coins. Can They Be Cleaned ?

Cupro-Nickel does not rust.
Vinegar should remove the tarnish, but may damage the coins.
Were going to need to see some examples.

nice job on that 50p clean up Rob.

Vinegar = impure acetic acid, so it shouldn't surprise that those two treatments produced the same negative result.

I would suggest ammonia. It might turn the coin a funky orange colour, but the corrosion ought to disappear. Lye (sodium hydroxide) solution ought to have a similar effect. The advantage of strongly alkaline solutions is that, unlike acids, they won't attack uncorroded metal.

That's... not typical cupronickel corrosion.

Just to check: the coins don't stick to a magnet, do they?

Then they're not cupronickel - they're steel replicas. And the brown stuff is actual rust.

Anything that takes rust away ought to do the trick - but it'll need to be quite corrosive or abrasive. Rust on iron or steel coins is much worse than corrosion on more noble metals, it tends to totally destroy the design underneath it.

This reminds me of the time when someone visited our coin club meeting with some " Morgan dollars" they'd picked up in Vietnam. I happened to have my coin-testing neodymium supermagnet at the meeting, and offered to test his coins with my magnet to see if they were silver.

CLANG.

Darn things stuck to the magnet so fast they nearly took my fingers off.