more rust in less salt??

prime-factor

As part of my chemistry experiments on corrosion, I am noticing that the steel nails are rusting more in solutions of 3.5g/L salt, than 35g/L salt. What could be some reasons to explain this??

I would have thought that the 35g/L solutions would have lead to more rust, not less.
Perhaps there is another reaction going on.

Borek

No idea, but I would start checking oxygen solubility and difusion coefficient in the concentrated NaCl solution.

alxm

Reminds me that I really should brush up on my electrochemistry. :(

I'm guessing that corrosion is being inhibited due to to reduced solubility of the resultant ions, due to the high ionic strength of the solution. Although Borek's suggestions are good too.

Borek

That makes me think... I would expect quite the opposite, increase of the solubility. Fe³⁺ is easily complexed by the Cl^-. But a lot depends on the solution pH, as Cl^- can be too weak complexing agent to prevent hydrolysis and Fe(OH)₃ precipitation.

Could be nails are corroding more, just there is less rust visible.

prime-factor

In the low saline solution, there is both more visible surface rust on the nails, as well as flaked off rust. The high saline solutions have almost no surface rust, but a little less flaked off rust than the low saline solutions.

alxm

Yeah, no you're probably right. Corrosion should normally increase with ionic strength.

The oxygen idea seems plausible though - it appears oxygen solubility does decrease with salinity. Corrosion in water is likely oxygen-limited (unlike in air).