You should be able to replace an old rusted nut that might have failed due to a small leak or condensation in the bathroom.

Once you have set aside the tools and time to do the job you need to consider that the nut might have been in place for years,  may not be as easy to unscrew as you might first think.

So the things that you could try to get those nuts moving  are as follows:

Firstly clean it up with a scourer or bristle wire brush to get rid of the thick rust which might be messy. When it is completely clean you need tp apply a lubricating oil and wait for it to sink in to the metal.

You then need to give it a little tap with your hammer to get your connection loose enough to undo.

This might sound odd but then you  need to tighten it up as the nut has been in place for a long time any kind of movement will be good for it. Once you have tightened it a little you then need to loosen it in the opposite direction.

Heat is good for metal as it makes metal expand so you could even get the hair dryer on it at this point or wrap a cloth around it that has been soaked in hot water.

If you try all of the above and you still it still hasn`t shifted at all you will then need to start cutting in to the nut which could be a bit more of a tricky job depending on your confidence with a hack saw. It might actually be at this point that you turn to the TRADESCOPE website and find a local trusted Plumber to help you with the task.