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 (http://phosadd.com/foods/foodselection.htm)

And another one:
Phosphorus In Foods Guide (http://www.righthealth.com/topic/Phosphorus_In_Foods/overview/adam20?fdid=Adamv2_002424)

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