Water – What could cause corrosion in a water heater

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.

For starters, I'd suggest you shut down the heater, drain it completely, and replace both electrodes.

The danger with a water heater this old is that in draining it and dislodging sediment within you could expose small leaks.

At 16 years old, I hate to say it, but you'll probably find it more efficient just to replace it.