You don't always need a tetanus injection after getting cut by iron.
However if you haven't had a tetanus injection within 10 years and you get cut by iron then it's best to get a tetanus vaccine.
Tetanus is a rare but often fatal infection that occurs because of bacteria that the iron or other metal can contain.
Rust does not give you tetanus.
However rust can contain the bacteria that causes tetanus.
So if you get cut by rusty metal or step on a rusty nail the rusty nail or metal can contain the tetanus bacteria which is actually what gives you tetanus.
You can even get tetanus from stepping on a clean nail or getting cut with a clean piece of metal.
However you're more likely to get tetanus when stepping on a rusty nail or cutting yourself with a rusty piece of metal.
To prevent tetanus you should get a tetanus shot every 10 years and if you haven't had a tetanus shot within 10 years and you got cut especially with rusty metal or a rusty nail you need to get a tetanus shot.
Tetanus can be deadly if you're not vaccinated for it and you get it and leave it untreated.