There's some different theories and approaches to this in the medical field and it's generally believed that after the first tetanus shot and 3 subsequent boosters at 7-10 year intervals, you're pretty-much set for life. On the other hand, continuing the boosters past the 3rd one has no known side effect. Medical practitioners have known this for years but what they don't know is if an individual had 3 boosters at 7-10 year intervals so, an additional booster is usually administered proactively/gratuitously...
Anyhow, I've had a whole bunch of boosters -more than I can count. When I cut my fingers last year, the ER asked if I got regular boosters, I said yes, and to my surprise, they didn't bother with another.
Anyhow, tetanus has little to do with rust or metal. The bacteria is everywhere and is rampant on most farms, zoos etc. To get infected, it has to enter the bloodstream directly and since punctures don't bleed like an open wound, stepping on nails (especially porus rusty ones) tend to be a common cause.
My father in law treated many cases of tetanus while practicing medicine in the rural south -usually happened on a farm and usually from a puncture wound...
FWIW, the next time I happen to be in the Dr's office, I'll probably have him hit me with a dose of booster.
Ray