MadSci Network: Chemistry

Re: Steel corrosion process in salt Water

Date: Sat Oct 17 23:56:35 1998
Posted By: Charlie Crutchfield, , Retired, Retired
Area of science: Chemistry
ID: 903568488.Ch

Dear Mr. Gray
Your question is in the field of corrosion.which is a branch of chemistry. 
You are correct, electrons are involved in the rusting "corrosion" of iron. 
In your situation, there are three different chemical reactions occurring 
at three different areas on the piece of Iron. I am assuming that you have 
not had a course in chemistry, My apologies if I am wrong.

 The explanation involves chemical symbols, equations, etc.. My eMail cant 
[well, I cant] type the necessary "correct" superscripts and subscripts 
needed to show chemical reactions correctly, but I will try.
 For example:  The metal Iron is Fe , the ionized iron,called a Ferrous ion 
[which is an iron atom [an "ion"] with an electric charge of plus two] I 
will show as Fe<+2> [the +2 should be a superscript].
The number of atoms, etc. in a chemical formula should be shown as a 
subscript, I will show the formula for water [which is two atoms of 
Hydrogen and one of Oxygen] as H2O. 
The electron [symbol "e"] has an electric charge of -1, which I will type 
as e<-1>. So, if two electrons are involved in a reaction it will be typed 
2e<-1>. The equal sign [=] means that what is on the left side chemically 
changes or reacts to form what is on the right side.
     Now to begin.
The major substances involved in the rusting and pitting of your pilings 
are: The metal Iron [Fe], water [H2O], the oxygen in the air {O2], and  the 
salt in the seawater [NaCl]. Formed in the corrosion process are the 
Ferrous ion [Fe<+2>, electrons [e<-1>], Hydroxyl [or Hydroxide] ions 
[OH<-1>] and Ferrous [Iron]Hydroxide ,Fe[OH]2 which is insoluble in water
 The salt is involved only to make the water a good conductor of 
electricity, so it will not appear in the reaction below

[1] All metals when in contact with a liquid that conducts electricity have 
a tendency to give up electrons to that liquid and become a metal ion. 
Here, Fe loses two electrons and forms a Ferrous ion. [This is called 
"oxidation", the site where this happens is called the Anode, but dont 
worry about these terms.]
     Fe =  Fe<+2>  + 2 e<-1>  This happens on the metal surface.
[2]Since an atom of Fe has oxidized i.e. lost two electrons, somewhere 
something must take up these electrons, i.e."reduction" must occur.
The Oxygen dissolved in the water takes up these electron, reacts with a 
molecule of water to form 2 Hydroxide ions.
     O  +  2e<-1>   +   H2O      =     2OH<-1>  
The oxygen is most available where the air, iron, and water meet. For 
pilings This happens at the "splash" zone, just below the surface of the 

[3]  The Ferrous ion and the Hydroxide ions being of opposite electrical 
charge are attracted and they tend to meet somewhere between Nos 1 and  2 
above. They combine to form Ferrous Hydroxide which is insoluble in water. 
It is at first white, but very quickly turns green, yellow, orange and 
finally Rust red in color,         
   Fe<+2>   +   2OH<-1>      =   Fe[OH]2

This is a very simple, idealized description of what happens to plain steel 
in salt water exposed to air. Why and how it rusts.
I will write the equation of the reactions again to help clarify what is 
happening and add them together

   [1]                         Fe   =   Fe<+2>   +  2e<-1>
  +[2]   O   +    2e<-1>   +   H2O  =   2OH<-1>
  +[3]        Fe<+2>   +  2OH<-1>   =   Fe[OH]2
 Sum of 1,2,3     Fe  +  O +  H2O   =   Fe[OH]2

 Holes are really very deep pits.  The actual corrosion of the Fe [No 1 
above]is not uniform over the Fe surface since the surface itself is not 
perfectly uniform. Corrosion is a surface phenomonenon, therefore the 
condition of the surface is crucially important. The Fe will corrode faster 
at some points than others, due to small imperfections, impurities, 
scratches, debris, etc. on the surface. This will cause pits to be formed 
at these fast-corroding sites. The pits will tend to trap the Fe<+2> ions 
and the interior of these pits may become acidic, which makes the pits grow 
even deeper.   

This is a very simplified description of a fairly complex topic. I hope it 
is of use to you, if you need more information, contact me again. For more 
detailed information on corrosion I suggest a good encyclopedia.

Hwere is a simple experiment to try. Get a clean nail [not galvanized or 
coated], ome sea-water,a tall narrow glass container, such as atest rube or 
an empty olive jar. CLEAN the nail well, abrade the surface with fine steel 
wool or emery paper. Place the nail in the container, add enough sea water 
to the jar so that some of the nail is exposed to air. Set this all aside 
and observe it from time to time.
                   Charlie Crutchfield

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