I have had 3 pinholes occur in the copper piping in my house...

My house is only 6 years old, and all the pinholes have occured within the last 4 weeks. All of them on the hotwater line coming from the hot water heater, within 15 feet of the tank.... I am on County water...

I replaced the three sections where the pinholes were, and inside the copper pipe I removed, a black film coated the interior of the pipe, and the interior of the pipe also felt like sand paper....

I dont understand why this happened or how copper 6 years old has become so bad..

1. What has caused this condition? How do I determine the cause?

3 Can i prevent it from getting worse?

2. How bad is this problem, and will I have to relpace the pipe in the whole house?

Thanks for the help... i am pretty clueless about this stuff...