Corrosion is the adverse interaction of materials, typically metals, with the surrounding environment. This is driven by redox electrochemical reactions in aqueous environments. The corrosion process converts the base metal to oxides, hydroxides, sulphides/sulphates or chlorides, which can be soluble.
Corrosion can affect the whole surface uniformly, as shown by the image of the rusty chain, but it can also be more localised where the local environment can be much more aggressive leading to pitting, as shown by the damage on the flange.
The situation can become dramatically worst if corrosion occurs in the presence of tribological, this is covered in more detail in tribocorrosion .