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Why citric acid changes the polarity of the tin-iron couple in NaCl?

 

When a mild steel plate and a tin plate are inserted in a solution of NaCl (1 M), the natural polarity of the cell is "+steel and -tin" (i.e. positive on steel and negative on tin).

If a small amount of citric acid ⇦this on eBay or Amazon [affil links] powder is added into the solution, the polarity of the cell changes to "-steel and +tin". One suggested explanation is that this is due to the pH change of the solution. However, I think there may be other reasons. Can someone of this discussion list explain this phenomenon for me?

George Chen
- Cambridge, UK

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I was not aware of the reaction you describe, but if it is true I'd credit the Nernst equation and the complexing power of citric acid .

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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I suspect the answer involves the structure of tinplate. Most tinplate is manufactured by electrodepositing metallic tin onto the steel and then flash melting it to give the bright colour. During the melting, some of the molten tin migrates into the steel to form an grey intermetallic compound of composition FeSn2. There is a whole range of these intermetallics, but FeSn2 is the only stable one under normal conditions. This intermetallic can often be seen if you leave an open tin of,say, peaches in the fridge for a few days; - you will see a dark grey ring around the inside of the tin where the tin has been dissolved to expose the intermetallic. Obviously this works better if the inside of the tin has not been coated with a protective lacquer!

Hence, you are correct in thinking that the citric acid alters the pH of the brine solution, but furthermore, the chloride ion is notorious for corroding metal (just think of the corrosive powers of seawater). As has already been stated, citric acid can also form complexes with many metals,albeit generally only weak ones. This acidic solution dissolves the free tin to reveal the intermetallic layer. The intermetallic is cathodic to the steel, thereby imparting corrosion resistance to the steel substrate; it is just another form of cathodic protection, as so widely used with many metal structures in corrosive environments (e.g., cars, pipe lines, drilling rigs etc). The intermetallic is chemically very inert and difficult to break down even though it is very thin (typically about 5-10% the thickness of the tin layer, depending on how much tin is originally present)

In hot dip tinplate manufacture, if it is still done, the amount of FeSn2 is much greater because of the extra amount of thermal energy in the system.

The rate of FeSn2 formation follows Ficks 3rd law of diffusion if you want to do some modeling! The energy of formation of the intermetallics is available in the technical literature; there as some interesting work done in the mid-late 1970's with both Fe-Sn and Cu-Sn systems. I personally found the energies of formation to be slightly lower than those quoted. I also found the rates and energies of formation of FeSn2 to change significantly at 232C, which just happens to be the melting point of tin (...surprise...surprise!). However, I never published my results. Also of interest was an observation that during the formation of other FeSn compounds, only FeSn2 was formed whilst there was a reservoir of free tin available; only when there was no free tin did I get any other alloys (FeSn, Fe2Sn3, Fe3Sn2, Fe2Sn).

I hope this helps you.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK