Depending on the corrosion, the contact, the position, there may not have been enough current to trip the breaker. It's all about how much resistance there is between the power and a return path. Keep in mind that the heating elements can produce a lot of heat and, if they weren't covered in water, would burn out in seconds. A fault out in the open could, over time, melt the wire without exceeding the current available and trip the breaker.
Since breakers can wear out or fail, you might consider replacing the breaker when you replace the WH, but it's probably not necessary. It's not expensive. Don't do it yourself unless you're familiar with the process, but in reality, it's fairly simple...doing it wrong, could kill you, though!
If there's no scorched wire or evidence of overheating at the junction box where the supply wires are connected, or back at the breaker, you're probably okay.
