I think corrosion is basically down to the quality of the steel and not the corrosion protection. I spent my whole working life in the construction industry (now well re-tyred) and had much to do with steel and corrosion protection. We used steel sheet piles in marine environments often with minimal corrosion protection but the steel quality was "high". They are expected to have a working life of decades. We used Corten grade steel for fence posts in a motorway environment, bare steel which gets a surface rust and no more, it will last forever. 20 odd years ago I was in charge of a job shot blasting and painting the main rail bridge over the Thames into Victoria, a job which has to be done every 15 years, and never saw any rust on the structure, even inside the bare steel hollow members. Again, top quality steel originally used. Ever seen a rusty manhole lid in the road? again high quality ferrous material. But cars are made out of much lower grade steel, it only has to last a few years before the car ends up in the scrapyard. The steel used in automotive applications will corrode from the inside out no matter what corrosion protection it is given, it breaks down chemically. I don't understand the galvanising chemistry but do know that it will massively extend the life of lower grade steels. In a nutshell, car grade steel has a relatively short design life no matter what corrosion protection you give it. (Galvanising excepted).