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Name: Derrick
Status: student
Grade: 12
Location: MD
Country: USA
Date: July 2006


Question:
I cannot find any information on wether sodium bicarbonate (baking soda) will effect metal (corrode or rust). I wonder if it will because I see the "sodium" part of "sodium bicarbonate" and I know sodium is salt which will corrode or rust metal.



Replies:
Derrick,

We need to be very careful of the term "corrosion". In chemistry, it has a very specific meaning which is not quite as loose as it is used in every-day English. Corrosion (whether it be the rusting of iron, or the tarnish formed on precious metals) means that the substance (usually a metal) has been oxidized. Recall that oxidation means that the substance has lost at least one electron. When applied to metals, corrosion usually means that the metal with an oxidation state of zero, becomes positively charged. For example: Fe goes to Fe(2+) or Fe(3+), Cu becomes Cu(+) or Cu(2+) and so on.

Now, let us look at your specific question. Can sodium bicarbonate, NaHCO3, cause the corrosion of metals? We should change that question to: can any ion of sodium bicarbonate (Na(+), H(+), or CO3(2-)) -by itself- cause any metal to lose an electron? We probably should not expect the carbonate ion (CO3(2-)) to initiate a loss of electron since it is already quite negative. We also should not expect Na(+) to cause a loss of electron from a metal because if a metal were to give up an electron, that electron has to go somewhere, and in this case, it would go to Na(+) to convert it to Na (we say that Na(+) is "reduced" to Na). However the reduction of Na(+) to Na requires a lot of energy and so it usually does not happen (it is more likely that Na will become Na(+) then the other way around).

So that leaves us with H(+) causing the oxidation and in turn becoming hydrogen gas (2H(+) + 2electrons --> H2(gas)). Because H(+) can receive the electrons produced in oxidation, we can imagine that sodium hydrogen carbonate can cause corrosion. However, since sodium bicarbonate solutions actually have a high pH, this means that the amount of H(+) actually present is very low. Thus, although corrosion can happen due to NaHCO3 alone, it would be very slow.

Be careful though. A solution of NaHCO3, may by itself only cause slow corrosion. However, remember that by our definition, corrosion can mean any kind of oxidation of a metal. So if NaHCO3 helps to speed up other reactions (such as the reaction of ambient oxygen (O2gas) with the metal, then we might say that while NaHCO3 only slowly corrodes metals, NaHCO3 may actually assist the faster corrosion of metals by speeding up the adsorption of O2 and cause corrosion that way.

Hope this helped your thinking.

Greg (Roberto Gregorius)



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