How the corrosion happen?

Answer 1

Most people generally associate the word "Corrosion" as something which occurs to metals. The most commonly recognized type is rust.

However, in the sense used here corrosion is another name for dissolution, whereby the chemicals in the water corrode (dissolve) away the river bed and bank, changing the passage of the river.

NOTE: This process happens very slowly.

Answer 2

There are different types of corrosion - but the most common one is rusting which happens when Iron is exposed to Oxygen andWater.

The surface of a piece of Iron in the presence of distilled de-oxygenated water will get wet - there is no chemical reaction.

The surface of a piece of Iron in the presence of Oxygen will react to produce a layer of Iron Oxide (the passivation layer) - this is quite stable and impermeable to Oxygen stopping further corrosion.

However in the presence of both Oxygen and Water the surface of a piece of Iron will react to produce Iron Hydroxide, this is a flaky material that pushes away from the pure Iron below, allowing Oxygen and Water to penetrate further into the Iron - this causes the sample to corrode - eventually all of the Iron will convert to Iron Hydroxide and the sample will lose a lot of it's structural integrity and will collapse.

Extra Credit

Older cars tend to rust more quickly by the coast and in winter. This is due to the presence of salt (from the sea or used gritting roads).

Rusting is an electrochemical process that involves the transfer of electrons from Iron to Oxygen, in solution the Na+ and Cl- ions help to catalyse the reaction.