Is it harmfull for human if phophate added in drinking water to control the corrosion?

Well, anything that is past a certain limit is harmful. Phosphates are needed by the body for the proper formations of bones, teeth, etc, but they are required in fairly low amounts.

Here's an interesting link:
Foods high in phosphate

And another one:
Phosphorus In Foods Guide

According to the second link I posted, it seems that 2.0 mg/L is safe.