On second review of the photos, I don't think that the corrosion is due to electrolysis. The dielectric nipples should be adequate to prevent that and there is no sign of issues with the hot connection. As for the T&P connection, if the copper drain pipe does not come into contact with anything then there is no closed circuit for electrolysis to take place.
It looks more like corrosion due to a small leak over a long period of time. A leak that would not have always been noticeable, but would slowly get worse over time. You can see where the water has been dripping and there is signs of calcium buildup.
Like I mentioned in a previous comment, I would inspect the sacrificial anode rod and replace it if needed. As for the cold connection, you could:
- cut the copper line and unscrew the copper section from the nipple
- brush the nipple with a steel wire brush to remove the buildup
- reinstall the copper section using teflon tape and a joint compound on the nipple
- reconnect the copper line that you cut with a shark bite coupling or
a solder coupling if you know how
You could do the same with the T&P connection, just cut the vertical down pipe so that you can remove the copper pipe from the T&P and then remove the T&P, clean and reinstall.
You mentioned that the tank was installed in 2006, so it would be nearing the end of its life expectancy (most residential warranties are for 5 to 6 years) so I would keep an eye on the tank for any leaks.
Though it may seem odd, it is possible the plumber knows what he's talking about. If this is a Polybutylene pipe, it could indeed be brittle and fail if it's jostled during the installation of the filter.
It's impossible to tell from the picture what type of pipe that is, but based on the plumbers concern I'd guess it is Polybutylene.
Polybutylene
Polybutylene was used in pipes between about 1980-1994/5, and was commonly used as supply line in the 80's. It was found (in some cases) to react with oxidants in the water supply, which caused the pipe to become brittle and fail. To my knowledge, this type of piping is no longer used. Though if the house was built; or there was supply line work done, during the 80's, it's likely this is Polybutylene pipe.
High Density Polyethylene
Don't confuse Polybutylene pipe with Polyethylene pipe. High-density polyethylene is commonly used as water supply line today, and is stable and safe to use.
Cross-linked polyethylene
Cross-linked polyethylene (PEX) is not typically used as an underground supply line, as it can sometimes react with minerals in the soil. Where PEX is used underground, it's typically encased in sand (or other non-reactive materials) to prevent negative reactions. Because of this it's often more expensive to bury PEX, so other materials are used instead.
How can I tell what it is?
The only way to be sure what type of pipe this is, is to look for markings on the pipe itself. Polybutylene will be marked PB####, Polyethylene will be marked PE####, Cross-linked polyethylene will be marked PEX-x (where x = A, B, or C).
Best Answer
Based on your picture:
It is just possible that the union there is dielectric, in which case the copper above it and the ground wire above it are the main issues, but it's unlikely. A dielectric union is correctly used where the pipe material changes from copper to iron.
You could certainly start with a dielectric union and moving the ground wire (or having it moved by an electrician that knows what he or she is doing, unlike the one that put it there.)