Why is metal rusty? what reaction?

Rust occurs when iron or its alloys, such as steel, corrode. The surface of a piece of iron will corrode first in the presence of oxygen and water. ... The process of rusting is a combustion reaction, similar to fire. Left in contact with oxygen, iron will react with the oxygen to form rust.

Ch4 and ph3 are both covalent since they are composed both of non-metals.    ch4 is non- polar because of  london forces and has a geometry of tetrahedral  ph3 , on the other hand, is umbrella shaped polar and has dipole-dipole intermolecular force (imf) . since ph3 is polar, it has a stronger imf