Rusting of Nails in Salt Solutions

[dazza95]

Three tests were conducted where a
nail was placed in water, 10%salt saturated, 20% salt saturated, 30% salt saturated etc to 100%
a Nail + Cu placed in water (same solutions above^)
and A Nail + zinc in water.. (same solutions)
All left over the same amount of days
The results are attached, the graphs show mass gained against salt solution.
Can anyone explain the graphs as to what happened/caused it to look like this?
And what should the graphs look like?
Thanks

 

[Borek]

And what should the graphs look like?

This is an experimental research, there is no "correct way" the graphs should look like. They look the way they look, it is up to researcher (you in this case) to explain why.

 

[dazza95]

So does it show what's supposed to happen?

the more salt that is added the faster the nail rusts, so 100% showed the most because its happening faster?

it was originally stated that a nail is observed to rust slower in salt water; it is believed to be due to the lower oxygen solubility in the saltier water
Why did it put on so must mass at 100% saturated if it has the least oxygen in it?

 

[Borek]

Apparently presence of the oxygen is not the only important factor.

 

[cmb]

Can anyone explain the graphs as to what happened/caused it to look like this?
And what should the graphs look like?
Thanks

Give us one (very obvious) reason why the graph of 'mass gained' might go negative?

 

[dazza95]

Give us one (very obvious) reason why the graph of 'mass gained' might go negative?

Because it is oxidising,
In the nail and zn case, the nail is sacrificial in order for the zn to gain electrons and put on mass

 

[dazza95]

Apparently presence of the oxygen is not the only important factor.

What other important factors would effect it?
o2, temperature, pressure...

 

[cmb]

..and why might it go negative, then gain mass, then loose it again [with increased concentration]?

 

[Borek]

What other important factors would effect it?
o2, temperature, pressure...

There is more. Those listed still don't explain why presence of NaCl speeds up corrosion.

What kind of reaction it is?

 

[dazza95]

There is more. Those listed still don't explain why presence of NaCl speeds up corrosion.

What kind of reaction it is?

Corrosion of metals?

 

[dazza95]

..and why might it go negative, then gain mass, then loose it again [with increased concentration]?

Something to do with the salt water being more polar, so it is conducting the electrons faster speeding up the rust?

 

[cmb]

Something to do with the salt water being more polar, so it is conducting the electrons faster speeding up the rust?

Best not to approach scientific results as if they are so precise that you can always extract some fundamental 'scientificy' principle at work. I recommend to always ponder on the mundane before assuming the complex.

How about bits of rust are falling off the nail, as an explanation for the variable results in the last graph?

 

[Borek]

Corrosion of metals?

What kind (type) of reaction is corrosion? What happens to the metal?

 

[dazza95]

Best not to approach scientific results as if they are so precise that you can always extract some fundamental 'scientificy' principle at work. I recommend to always ponder on the mundane before assuming the complex.

How about bits of rust are falling off the nail, as an explanation for the variable results in the last graph?

Ok i see what you mean!,
yeah bits did fall off the nail but we filtered each solution and added it on the mass of nail! haha

 

[dazza95]

What kind (type) of reaction is corrosion? What happens to the metal?

redox reation?
the metal is oxidising/reducting?

 

[Borek]

redox reation?
the metal is oxidising/reducting?

Good. That means charge must be transferred.

 

[dazza95]

Good. That means charge must be transferred.

the NaCl speeds up corrosion because the sodium and chloride ions are able to carry a charge (+) and (-) so they will conduct electricity which means they conduct electrons which means since the electrons have to get to oxygen from the iron, they can through the medium of nthe salt solution. In a mix that can't conduct, they have to have the oxygen come in contact with the nail.

 

[frozonecom]

I'm sorry if I'm bringing up to life a thread that has been long dead. Then, does that mean any solution, no matter acidic, neutral, or basic, that dissociates into ions (that is with electric charge) speeds up corrosion of iron?

 

[Borek]

I'm sorry if I'm bringing up to life a thread that has been long dead. Then, does that mean any solution, no matter acidic, neutral, or basic, that dissociates into ions (that is with electric charge) speeds up corrosion of iron?

Yes.

 

[ajkoer]

Galvanic Corrosion - a contributing factor

To quote from Wikipedia on Galvanic Corrosion (Please see http://en.wikipedia.org/wiki/Galvanic_corrosion):

"In other cases, such as mixed metals in piping (for example, copper and cast iron), galvanic corrosion will contribute to accelerated corrosion of the system."

So part of the corrosion is chemically based, and part is galvanic in nature.

In the chemical part, also note in highly ionic and concentrated salt solutions, the 'activity level' of acids is increased (even for H2CO3). This is the 'salt effect' (see Wikipedia on Solubility Equilibrium at http://en.wikipedia.org/wiki/Solubility_equilibrium ), or also called, 'the non-common ion effect'.

Some factors to be incorporated include the ionic strength of the solution (as the ionic strength increases, the Ka increases, leading to a greater degree of dissociation for the acid and a lower pH of the solution), the charge of the ion, the size of the ion (or, the “effective diameter of the hydrated ion”), temperature and the particular solvent used (see page 137 at https://docs.google.com/viewer?a=v&...GTqbWl&sig=AHIEtbRsI5y0wP2om1zDaAyVq6bLgadamw )

Bottom line, this seemingly simple experiment is far from simple in level of knowledge require to explain the actual observed results.