Re: Why would corroded nails with rust removed gain weight?

Emily, you asked a number of good questions, and I'll try to answer them 
and provide references for more information.  First of all, consider the 
process of weighing.  The precision of a measurement relates to how many 
decimal places can be measured, but the accuracy of a measurement is how 
close you come to the true value using a certain measuring instrument.  To 
test the repeatability of your balance, take a fresh clean nail and 
(handling it with cotton gloves or a tissue) weigh it 10 times, removing 
it completely from the scale each time, putting it down, picking it up, 
and remeasuring it. Check the difference between measurements. Does the 
weight fluctuate?  Is the fluctuation smaller than or equal to the 
measurements in your experiments? The percent difference among the 
measurements relates to how much error you might expect to observe when 
using your electronic balance.  If the differences you measure in 
experiments are much greater than the repeatability error of the balance, 
then you can have more confidence that the weight changes are real.

Next, the nail gains weight because corrosion products, such as iron 
oxides and hydroxides are formed. The composition of the corrosion 
products depends on the chemistry and pH of the solution.  Establishing an 
effective cleaning method for the test specimens is a critical part of 
conducting corrosion tests.  In fact, the American Society for Testing and 
Materials (ASTM) has prepared a special standard practice for preparing, 
cleaning, and evaluating corrosion test specimens.  It is called ASTM G-1 
(Annual Book of ASTM Standards, volume 03.02) and this document can be 
obtained from a good technical library, or from ASTM at their website 
  It involves multiple cleaning steps to determine 
when the specimens have been satisfactorily cleaned after testing.

Electrolytes can affect the driving force for the corrosion reactions, 
especially when external currents (that is, electrical circuits between 
specimens and other materials) are present.  The amount of oxygen and 
hydrogen at the surface to react with the metals is important, as is the 
temperature.  That is why the type of corrosion can change from pitting to 
scale formation and other forms when the chemical conditions change.  The 
form of corrosion even under the same exposure conditions is also affected 
by the chemistry of the metal itself (so make sure you use the same source 
for the nails).

The melted snow at pH 4.5 is slightly acidic, but NaCl is slightly 
alkaline and would tend to drive the pH upward.  At low pH (< 3, acidic) 
the corrosion scales and the metal itself tends to dissolve away.  Above 
about pH = 9, the corrosion rate increases with the formation of iron 
hydroxides Fe(OH)3- and Fe(OH)4-  The extent to which corrosion occurs 
depends on whether the scales on the surface are porous or not -- tight 
scales can sometimes serve as a barrier to protect the underlying metal 
from the attacking liquid.  Chromium oxide scale, for example, protects 
stainless steels from attack.

In solutions around pH 5, a complex structure of iron oxide and hydroxide 
can form as adherent scales. The exact composition varies with small 
changes in conditions and has been the subject of much scientific study.  
A good reference for corrosion of metals is the ASM International, Metals 
Handbook, Volume 13 "Corrosion."  A good textbook is "Corrosion" by 
Fontana and Greene.  Both books describe the reactions that can occur in 
iron and methods that can be used to accurately measure corrosion rates of 
metals.

Sodium silico-aluminate prevents caking so the salt flows freely in 
shakers.