Question

Hi experts,

I designed an experiment to test the effect of increasing the saltconcentrations on the corrosion rate of iron nail. I had thecontrol (no salt), 0.63%, 6.3%, 14.5% and 29% (% mass/mass). After2 weeks when I got the nails out and wiped all of them with papertowel, I found that there is a black substance sticking on thenails immersed in the 0.63% and 6.3%, where in 6.3% salt solution,the black was the most. After the 3rd week, the 6.3% had less thanthe 0.63%. Note that in the 14.5 % and 29 %, no black substance wassticking. I found that as the salt concentrations increase, thereis more red rust.

My question is:

1) Why is less salt causing more black substance sticking? when Iresearched on the Internet, I found that the black substance wasmost likely to be FeO (as it is sticky) and they say that it formsin anaerobic conditions and that red rust (Fe2O3) forms when oxygenavailability is more and I also found that more salt decrease theoxygen solubility. If this is so, shouldn't the least saltconcentrations have the least black rust and the most red rust asthere is the maximum amount of Oxygen? (i.e. black forms inanaerobic or when oxygen amounts are little and red forms whenthere is more oxygen).

Please help me, I have to find trends in my data and explain themin details. Any help will be appreciated.

Thank you in advance
Best Regards