MadSci Network: Chemistry

Re: What happens to the weight of an iron bar when it rusts?

Date: Tue Jan 18 13:32:50 2000
Posted By: Joseph Weeks, President, Thermal Products, Inc.
Area of science: Chemistry
ID: 947765420.Ch

Interesting question.  When iron or any other metal oxidizes, oxygen from 
the air combines with the iron to make iron oxide or rust.  So the first 
part of the answer is that the weight of the bar increases, due to the 
increased weight of oxygen which has combined with the iron.

The second part of the answer has to do with the nature of the oxide that 
is formed.  Iron oxide is non-adherent; either the volume of the oxide is 
greater or less than the volume of iron it replaced.  So over time the rust 
will fall off of the bar.  So if you look at something which has rusted for 
a long time (like my 75 Chevy pickup truck), the ground surrounding the 
iron part is coated with a layer of rust which has blown or washed off the 
iron part.  So, over time, the weight of the iron bar will tend to 
decrease, due to factors related more to erosion than to chemistry.

Some metals form adherent oxides, such as aluminum and chromium.  If you 
were able to expose a perfectly clean surface of either to the air, it 
would initially gain weight (although the increase would be very small 
since we are only talking about a couple molecules thick oxide coating) and 
then the weight would remain essentially constant.

So, my 75 Chevy pickup ought to gradually get better gas milage as I leave 
pieces of fender, truck bed, and floorboard on the sides of the road in the 
form of iron powder.

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