I've bent over many nails in a salt water environment, some of them I've been able to observe for as long as 40 years. I've never noticed an increased corrosion rate, except when I damaged the zinc coating.
Yes and no. Like many other inventions, they were in common use in China long before Europe. They came to the West about 1000 CE. But at the beginning of the IR, they and other innovations were used to greatly increase farm productivity. That freed up a massive supply of labor, without which...
The steam engine is a good choice because it powered the many machines that provided the great multiplication of human labor that was the Industrial Revolution. But too many people today forget that the IR happened only on the backs of a huge labor force that had never before been available...
The horse collar, which enabled farmers to plow with a horse instead of the much slower ox. That freed up the non-farming labor to run all those new factories.