I was given a very special Krups coffee machine that grinds and makes the coffee, for a big birthday. Can I use citric acid rather than extremely expensive Krups descaling powder?
Technically you can use whatever you want to descale a machine.
Keep in mind the manufacturer recommends using certain things for a reason. It could be the material inside the machine are sensitive to certain thing, or how much of the descaling solution is left behind.
You should always follow the manufacturer's instructions. Breville for instance recommends using white vinegar (acetic acid) and water at a 50/50 mix for descaling.
If you do not want follow the manufacturers instruction, I would use white vinegar at a 50/50 mix. This method has been used for ages to descale coffee machines.