Why is there more rust in a solution with less salt?

[prime-factor]

more rust in less salt??

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]

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.

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]

That makes me think... I would expect quite the opposite

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

The oxygen idea seems plausible though - it http://www.engineeringtoolbox.com/oxygen-solubility-water-d_841.html" oxygen solubility does decrease with salinity. Corrosion in water is likely oxygen-limited (unlike in air).