Mssr.Jim: The corrosion that causes the pin holes of which you speak are caused by internal influences not external.
The walls of the Iron (steel) pipe is subject to electrolysis in much the same manner as Copper pipe
is. As the corrosion cell disolves the Iron by electrolytic decomposition , the wall section of the pipe becomes thinner and thinner in that area untill a pin hole breaks through the pipe wall and the leak starts.
There are simple methods available to control this type of pin hole producing corrosion in all types of metallic piping as well as those familiar in copper and galvanized steel pipe failures.