Question

a fraction of ship iron has to replace every year why

Answer 1

 First, the ship is made of steel, not iron. It is not "replaced every year." The ship is built with extra steel so it can rust a little during it's expected lifetime and still be useable. If it rusts too fast, it's useful life is shortened. Every attempt is made to keep it from rusting including several coats of paint and using sacrificial zinc anodes

Answer 2

Explanation:

a fraction of a ship's iron part has to be replaced every year. Answer: Ships floats in the sea, when iron parts of ships come in contact with the sea water, it gets corroded because of action of sea water and air on them. Therefore for durability of the ships such corroded parts are replaced every year.