Friday, December 11, 2009

How does acid rain affect metals?

Although the effects of acid deposition on forest and aquatic ecosystems are much greater, acid rain definitely has an effect on metals. The effect of acid deposition is basically the effect any acid has on metal: it tends to corrode it. Corrosion is the electrochemical degradation of a material. That means that as the metal breaks down, it loses electrons to become a different form . In a typical corrosion reaction, solid iron (Fe) loses electrons to become a soluble form:

Fe(s) --> Fe2+(in solution) + electrons

These electrons are taken up by oxygen (O2), which reacts with hydrogen ions (H+) to form water:

O2 + H+ + electrons --> H2O

The overall reaction (unbalanced) looks something like this:

Fe(s) + O2 + H+ --> Fe2+ + H20

To get this reaction to run you need three things: iron, oxygen and acid (to provide the H+). Normally, the H+ can come from the acid that forms naturally as CO2 dissolves in water. However, acid rain provides a lot more H+ (over a hundred times more in many cases) this makes it much easier for the reaction to run because there is plenty of everything needed. Once the iron is in solution it can react further to form iron oxide, which is also known as rust. The result of all this chemistry is that acid deposition can corrode bronze (which is mostly copper), iron, steel (which is mostly iron), and copper much more quickly than under natural conditions. Acid rain has even been seen to damage metallic auto paint.

One experiment you might do to test these ideas would be to expose metals to acid rain. Some researchers in Britain did exactly that, in what was called the National Materials Exposure Program. They placed many different types of metals and rocks (which are also vulnerable to damage) outdoors at sites all over the country and very carefully measured the changes that occurred in size, weight, surface composition, etc. Technically, this experiment tested the effects of air pollution in general but you would expect a significant portion of that effect to be due to acid deposition. In a preliminary report I was able to track down, they found that “mild steel” was corroded the most, followed by galvanized steel, copper and then aluminum, which was hardly affected. It is worth pointing out that all these reactions are still pretty slow – it can take years for visible effects to accumulate. Some rocks, like limestone (which dissolves in acid), are damaged more quickly – huge amounts of damage to sculptures and buildings have occurred throughout areas affected by acid rain.

Another effect acid deposition has on metals occurs in the soils of forests. The changes in pH (one measure of acidity), which occur as a result of prolonged acid deposition, increase the mobility of some metals in the soil. This means that toxic forms of metals like aluminum, copper and mercury can leach into rivers and streams where they might hurt aquatic life or even get into drinking water.

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