Corrosion process?

Question

Whwn corrosion occurs, wat happenes? please brief simply...i am confused that there is oxidation and reduction:
Fe...>Fe+2 +2e- and 2h+2e...>h2 {if there is no h available then? where does h2 goes? and always oxidation will occur?}

Answer 1

Corrosion is almost always oxidation of a metal. Oxidation is losing elections, so you got it right.

Since it typically happens in water (even damp air) there is almost always hydrogen around... from the water.

Remember, the Fe2+ combines w/ O2- (thus the name "oxidation") from the water, leaving the hydrogens to be reduced (as you wrote).

Any H2 formed here will pass thru the water and float away.

Oxidation (corrosion) can occur in a dry environment w/ the O2 from air being reduced as the metal becomes oxidized. (This is typically slow).

Answer 2

You are correct in that you are confused. And Iridium190 above is making it more confusing for you!!!

Fact: If a clean test tube is filled with recently-boiled water and a brand new iron nail is placed in that water, and the test tube is tightly covered with a rubber bung, THE NAIL WILL NOT CORRODE, EVEN IF THE SYSTEM IS LEFT TO STAND FOR 30 DAYS! Water does NOT cause corrosion. It facilitates it.

Water has oxygen dissolved in it. This is the oxygen that fishes extract as they force water across their gills. This is not to be confused with the oxygen that is part of the compound water, H2O.

The iron does undergo oxidation (a loss of electron or an increase in oxidation number):

Fe -------> Fe^3+ + 3e^ 1-

The free oxygen undergo reduction (a gain in electron or a decrease in oxidation number):

O + 2e^1- ------> O^2- [since oxygen exists as O2, the actual equation is O2 + 4e^1- ------> 2 O^2- ]

Final compound : Fe2O3 (rust)

Hydrogen as part of the water molecule has nothing to do with corrosion process. And yes, when there is both oxidation and reduction, the reaction is called a redox reaction.

Answer 3

I specialize in corrosion for my job. you basically got the answer right.
What your talking about is submerged corrosion opposed to atmospheric corrosion. In submerged corrosion there is an anode and a cathode. Both have a different electrical polarity. Oxygen is more attracted to the anode which causes oxidation (corrosion) Hydrogen is attracted to the cathode. In atmospheric corrosion the air around the object, like iron, is the cathode.