Rusty fuel tanks, filler pipes and the fuel lines themselves, all the way from the tank to the engine, are common occurrence. Yet plastic fuel tanks that will never rust have been around for more than twenty years.
An example of what can occur is a client showing up at a service garage complaining that he had a terrific smell of gasoline in his car. The car was brought into the shop where it was immediately discovered that the carpets in the car were wet with gasoline. All that was needed was one spark and the client would have become front page news. The fuel tank in this car had a rust hole the size of a soup plate on the upper surface that cannot be seen from below. The gasoline smell must have been slowly increasing over the past months. Every time the driver applied the brakes with a full tank of fuel, a tidal wave of gas slopped over, out of the tank, infiltrated below the rear seat and entered the car. The galvanized steel tank had given up the ghost entirely. (reference 4)
Another part of the fuel system that designers do not pay much attention to is the fuel filler pipe. This pipe is very expensive to buy and install and is made of carbon steel. It's true that this steel is treated with some sort of anti corrosion plating, but that seems to last just about the length of the warranty! Then this pipe starts to corrode as it is continuously sprayed with road grit and salt from the rear wheels. In some cases a splash shield has been provided, but this rapidly fills up with sand and other debris and provides a constantly wet source for corrosion to continue all year round.
Eventually, a hole appears in the pipe and now all the water and residue from the road enters the fuel tank. This is a huge problem indeed. Because not only does the tank have to be cleaned and the filler pipe replaced, but all the fuel lines and the injectors have to be overhauled.
The same situation applies to the fuel lines themselves. Pieces of new steel tubing have to be spliced with high pressure fittings into an existing line, to overcome a dripping fuel leak from below the vehicle. Before a job is finished, all the lines, including the brake lines, should be treated by a good soaking of anti corrosion liquid. 'Rust proofing alone does not seem to overcome most of these problems.
Modern cars have plastic fuel tanks, which cost about five times more than a steel tank, but in the long run these tanks are worth it. However, the fuel filler pipe is still vulnerable to corrosion. Even though a check engine light will now display itself if the evaporative system is not sealed, it's too late for the tank, because plastic tanks cannot be cleaned and have to be replaced. One solution would be to use stainless steel filler pipes and fuel lines. Then plastic tanks would last the life of the car.
A last word of advice, if you're buying a used car, this is one aspect of an inspection to which we apply particular attention and so should you.