All metal pipes tend to corrode with time due to their continuous contact with water, and the presence of hard minerals in the tap water accelerates the process of corrosion. The areas most affected are the joints between the pipes, where due to high pressure, some water oozes out and when it comes in contact with air, rust is formed.
If you want to repair a part of plumbing affected with rust, you will have to loosen it first. Using some form of rust remover or a lubricant is a good way to soften corroded plumbing parts.
Close the water supply to the area where you want to do plumbing work. Shut the main water valve and open the safety valve to drain out the water from the pipes. If you cannot locate the safety valve, open all the taps in the washroom to drain water from the pipes. If you want to repair a sink or a flush, close the valves attached to these respective fixtures as well.
There will be rust visible if any of the joints are leaking, you can use WD-40 spray to remove this outside rust with ease. However, the real problem is not the outdoor corrosion, it’s the rust deposits which are within the pipes which slow down the speed of transfer of water, and lower the quality of tap water.
If you are certain there are deposits inside the walls of the pipes, you will have to take some drastic measures. You will either have to add a rust removing agent in the main water tank and allow it to circulate to each nook and corner of the piping system, or separate small areas of the piping and clean them up for rust by following the same procedure.
The first method will clean out pipes of all forms of rust, but for that you will have to find alternate source of water supply for about a day or two, and it will also waste hundreds of litres of water.
Use a wire brush to remove corrosion on the pipe joints from outside. You may pour two or three table spoons on the corroded parts to help remove the mineral deposits from the surface of the pipe. After cleaning the rust, spray a lubricant on the joints and nuts to soften the pipes.