Flux corrosion inside copper pipe

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Orange County California
Hi everyone, I have been reading this forum for months, but this is my first post. I am new to soldering, and I use the OATEY H20#5 lead free flux, I have tried their regular #5, and the #95 tinning flux, as well as their water soluble tinning flux. I really didn’t like the water soluble tinning flux, but the rest all seem ok. My question relates to the corrosion inside the pipe from the areas where the heated flux came into contact with. I don’t know if I am doing something wrong and that is why this is happening, I am very conservative with the amount of flux I use, but basically anywhere the hot flux touches, turns an orange color that looks like rust on steel. The regular paste flux that is not water soluble does not due this, but I have also noticed issues with those as well, where they leave a bunch of spots that look like they will eventually cause pitting. I already know that copper does not rust and it develops a protective patina, but I have also heard it develops a protective film inside the pipe to protect against bacteria and pathogens, and I am worried this could effect the development of that protective film. Is there a water soluble flux that does not do this? I really like the idea of the flux just washing away without any extra help. But more importantly than that I want the least amount of damage to the inside of the copper pipes. This is the main reason I am thinking about purchasing a press tool


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