Yes, the pattern that you mention is true. The reason for the difference is that "iron" is considered to be a so-called mass noun, or "uncountable".
When you say something like "The dog is an animal" or "The corkscrew is a useful invention" etc, what you are basically saying is "Any prototypical example of a dog/corkscrew is...". In other words, for the sentence to work, you have to be able to conceptualise a "single example instance" of the thing/animal etc in question. For nouns that represent non-specific quantities of things, the construction isn't possible.
(For what it's worth, this actually differs from the 'generic' use of the definite article in various other languages such as French.)
So, compare the difference between:
(a) *The sugar makes any coffee taste sweeter. [When "sugar" is meant to mean "sugar in general".]
(b) The sugar cube makes any coffee taste sweeter.
(a) *The milk is a nutritious drink.
(b) The typical glass of milk contains 200 calories.
Note that sometimes, words like "sugar", "milk" and "iron" can actually be used countably in cases where the context makes it clear that e.g. by "two sugars" you mean "two teaspoons or sugar" and by "two irons" you mean "two atoms of iron". But despite this, such nouns aren't "countable enough by default" for "the sugar", "the milk", "the iron" etc to work as meaning "any prototypical example of sugar/milk/iron", and so sentences such as those above don't work.