Will flux corrode copper pipe?

[Scott Vroom]

A plumber told me today that flux left on the outside of copper pipes will corrode the pipe over time. Is this true?

[kreemoweet]

It's absolutely true. Some fluxes are very strong, and will cause visible blue/green corrosion on copper within hours. Others may take days or weeks. When you
see copper pipe that is all gnarley with green/blue corrosion on it, especially around the joints, it usually means some lazy plumber neglected to clean off the
excess flux, as he is required to do by at least some plumbing codes, and certainly by the dictates of Good Practice.

[dlarrivee]

You can always tell hack plumbing by the green corrosion left behind.

[jadnashua]

Water soluable fluxes aren't necessarily as bad as the older acid based paste fluxes, but wiping the joint clean after soldering is still a good practice, regardless.

[hj]

I seldom "wipe" the joints, because if the soldering was done correctly the flux was evaporated by the heat. Now, however, if the plumber cools the joint by applying flux, called "expensive cooling", that flux will remain on the joint. Byt, I have seldom seen any copper really "corroded" by it.

[nestork]

I don't know about water based fluxes, but conventional soldering fluxes contain a chemical called "zinc chloride".

At soldering temperatures zinc chloride becomes acidic and it dissolves copper oxide (the brown stuff that forms on copper) much more aggressively than it dissolves copper metal. But at room temperatures, zinc chloride is so mild that you often see soldering flux being sold in metal containers. The concern is that the acidity from the zinc chloride might do some harm over a long enough period of time.

I'd tell you to clean it off too, but I'd also add that it's more important to clean it off of hot water piping than cold water piping, especially copper pipes used in hot water heating systems where the temperatures can be much higher.

Conventional soldering fluxes are made from petroleum jelly (Vaseline) so you can use mineral spirits (aka: paint thinner) to clean with.