Will UA, put me on the next avalable flight for free if it is missed due to, delays/immirgration?
Yes, if you miss your connection to PDX then UA will rebook you on their next available flight with no fee. (They will also allow you to standby on flights that don't have any available seats, but that's your choice.) If you have to overnight at IAD, however, then you will be responsible for the cost of a hotel room unless the delay is caused by a mechanical problem. In that case, UA will pay for the room.